Cold pools(CPs)significantly influence coastal heavy rainfall,but detailed observations of them are limited due to the lack of vertical measurement instruments.This study statistically characterizes CPs in the coastal...Cold pools(CPs)significantly influence coastal heavy rainfall,but detailed observations of them are limited due to the lack of vertical measurement instruments.This study statistically characterizes CPs in the coastal monsoon region of South China using unique data from the 356-m-high Shenzhen Meteorological Tower.CP occurrence correlates with convective activities influenced by the summer monsoon in the seasonal variations and land–sea breeze activities in the diurnal cycle.The CPs predominantly dry the atmosphere,highlighting the dominant role of dry entrainment through convective downdrafts in their formation,with a minor role of hydrometeor evaporation.The average CP depth is estimated at 668.0 m,deeper than tropical CPs but shallower than midlatitude counterparts.The CP properties exhibit diurnal variability,largely influenced by mesoscale convective system(MCS)activities.MCS-induced CPs are deeper and more intense than those from individual convective cells,while linear-MCS-produced CPs are the most intense.These observations from the coastal monsoon region contribute to a comprehensive global understanding of CP characteristics,complementing existing studies from midlatitude and tropical regions.展开更多
Cold pools and associated wind storms are frequent occurrences in Southwestern Nigeria, especially during the early monsoon phase. The associated surface wind gust frequently destroys properties resulting in economic ...Cold pools and associated wind storms are frequent occurrences in Southwestern Nigeria, especially during the early monsoon phase. The associated surface wind gust frequently destroys properties resulting in economic losses. Two case events were investigated in this study;one event occurred in May 2019 and the other occurred in March 2020, both in southwestern Nigeria. The National Oceanic and Atmospheric Administration (NOAA) National Center for Environmental Prediction (NCEP)/Climate Prediction Center (CPC) Infrared brightness temperatures and CPC Morphing technique (CMORPH) rainfall products were analysed alongside in-situ observations from the Nigerian Meteorological Agency (NiMET). Other data sources analysed are the National Aeronautics and Space Administration (NASA) Prediction Of Worldwide Energy Resources (POWER) and the World Wide Lightning Location Network (WWLLN). Cold pools were identified in the impacted communities as indicated by surface characteristics investigated from the in-situ observations. There was a sudden change in wind direction, with a simultaneous drop in temperature accompanied by increasing wind speed. Pressure and humidity were observed to change in the same period. Thunderstorms were also present in the impacted communities, as observed by the in-situ observations, in both case events. The presence of lightning as observed by WWLLN agrees with the in-situ thunderstorms. The cloud characteristics showed the presence of cloud shields, by their brightness temperature, over the impacted communities during the period of the cold pools in both case events. The systems were raining at the time of the observations in both cases, consistent with the in-situ thunderstorm observations. The communities were heavily impacted with several properties destroyed in the events. These early monsoon seasonal windstorms require a forecasting tool for their prediction and this study presents an eye-opener for further investigation and innovative research to address the menace.展开更多
CHEMTAX analysis of high-performance liquid chromatography （HPLC） pigment was conducted to study phytoplankton community structure in the northern Bering Sea shelf, where a seasonal subsurface cold pool emerges. The...CHEMTAX analysis of high-performance liquid chromatography （HPLC） pigment was conducted to study phytoplankton community structure in the northern Bering Sea shelf, where a seasonal subsurface cold pool emerges. The results showed that fucoxanthin （Fuco） and chlorophyll a （Chl a） were the most abundant diagnostic pigments, with the integrated water column values ranging from 141 to 2160 μg/m2 and 477 to 5 535 μg/m2, respectively. Moreover, a diatom bloom was identified at Sta. BB06 with the standing stock of Fuco up to 9214 μg/m3. The results of CHEMTAX suggested that the phytoplankton community in the northern Bering Sea shelf was dominated by diatoms and chrysophytes with an average relative contribu- tion to Chl a of 80% and 12%, respectively, followed by chlorophytes, dinoflagellates, and cryptophytes. Dia- toms were the absolutely dominant algae in the subsurface cold pool with a relative contribution exceeding 90%, while the contribution of chrysophytes was generally higher in oligotrophic upper water. Additionally, the presence of a cold pool would tend to favor accumulation of diatom biomass and a bloom that occurred beneath the halocline would be beneficial to organic matter sinks, which suggests that a large part of the phytoplankton biomass would settle to the seabed and support a rich benthic biomass.展开更多
During the summer monsoon season, strong coastal upwelling occurs along the southwest coast of India and at the southern tip of India, which cools the surface temperature of the waters around these regions. The summer...During the summer monsoon season, strong coastal upwelling occurs along the southwest coast of India and at the southern tip of India, which cools the surface temperature of the waters around these regions. The summer monsoon current carries the upwelled cold waters into the Bay of Bengal and forms the ‘cold pool of the Bay of Bengal', with its core south of Sri Lanka and over the southcentral Bay of Bengal. The present study focuses on the intrusion of these cold waters into the south of the Bay of Bengal, its interannual variability, and its association with the surface wind during the break phase of the summer monsoon, when strong westerly surface winds flow south of 10°N.The authors hypothesize that the enhanced cooling in the cold pool region during monsoon spells is associated with the strong westerly wind stress there during the break spells of the monsoon.Seven cases of long break monsoon spells that occurred during the nine years from 2001 to 2009 are analyzed, and the results confirm our hypothesis.展开更多
Near-surface PM2.5 and meteorological observations were performed in three rural communities in the high latitude Yukon Flats valley at various times during the cold season (October to April). These data were synthesi...Near-surface PM2.5 and meteorological observations were performed in three rural communities in the high latitude Yukon Flats valley at various times during the cold season (October to April). These data were synthesized with data from other meteorological sites, NCEP reanalysis and MAIAC retrieved aerosol optical depths data to analyze the role of mesoscale processes and radiation on air quality. Under weak large-scale forcing mountain-valley circulations develop that are driven by the differences in insolation. During the long dark nights, radiative cooling occurs in the near-surface layer of the mountain slopes of the Brooks, Ogilvie and White Mountains Ranges and at the bottom of the valley. Here surface-based inversions (SBI)—known as roof-top inversions—forms, while the cold air drains from the slopes. A frontal wedge forms when the cold air slides over the relatively colder air in the valley. Drainage of cold air from the Brooks Range governed the circulation and cold air pooling in the valley. Concentrations during times with and without SBI differed significantly (at 95% confidence) at two sites indicating that local emissions were the major contributor. At the site, which is closest to the mountains, concentrations marginally changed in the presence of inversions. At all sites, 24-h mean PM2.5 remained below the National Ambient Air Quality Standard.展开更多
This study presents finely resolved radar signatures of multiple cyclonic vortices associated with an EF2 tornadic supercell that occurred in Guangzhou on 16 June 2022 and discusses how the mesocyclone formed on the l...This study presents finely resolved radar signatures of multiple cyclonic vortices associated with an EF2 tornadic supercell that occurred in Guangzhou on 16 June 2022 and discusses how the mesocyclone formed on the lee side of mountain.A nearby X-band phased-array radar provides evidence that the mesocyclone was shallow,with a depth generally confined to less than 3 km.The mesocyclonic feature was observed to initiate from near-ground level,driven by the interaction between intensifying cold pool surges and shallow lee-side ambient flows.It was first recognized shortly after the presence of near-ground cyclonic convergence signatures over the leading edges of cold pool outflows.Over the subsequent 17 min,the mesocyclone developed upward,reaching a maximum height of 3 km,and produced a tornado 8min later.Nearly coinciding with the time of tornadogenesis,a noticeable separation of the low-level tornado cyclone from the midlevel mesocyclone was observed.This shift in the vertically oriented vortex tube was likely caused by modifications to the low-level flow due to the complex hilly terrain or by occlusions associated with rear-flank downdrafts.After tornadogenesis,high-resolution X-PAR observations revealed that the lowest-level mesocyclonic signature contracted into a gate-to-gate tornadic vortex signature(TVS)at the tip of hook echoes.Compared to conventional S-band operational weather radars,rapid-scan X-PAR observations indicate that a core diameter threshold of 1.5–2 km could be employed to identify a cyclonically sheared radial velocity couplet as a TVS,potentially extending the lead time for Doppler-based tornado warnings.展开更多
Observational analyses and convection-permitting simulations are conducted to study an extreme event of damaging surface winds within a bow-shaped squall line over South China.Prevailing explanations for the formation...Observational analyses and convection-permitting simulations are conducted to study an extreme event of damaging surface winds within a bow-shaped squall line over South China.Prevailing explanations for the formation of damaging surface winds were applied to investigate the detailed processes at the mature stage of a squall line in a complicated real-world scenario.It is shown that environmental vertical wind shear was adequately strong to cause a shearing vorticity,and the shearing vorticity was comparable to the baroclinic vorticity by a cold pool.The balance led to strong upward motions at the leading edge of the squall line and brought plenty of rainfall.The descending rainfall cooled the surrounding air and entrained the upper-level cold air downward to the lower level,strengthening the cold pool by excessive evaporation and melting.The cold pool accelerated the propagation speed of the squall line and caused extensive wind damage at the surface.Meanwhile,the horizontal vortex lines at the leading edge of the cold pool were lifted by frontal updrafts and tilted to form the anti-cyclonic vortex at the middle level.This maintained and intensified the rearinflow jet behind the apex of bow echo in association with a notable midlevel pressure gradient.The rear-inflow jet was then transported downward to the surface by descending condensate,leading to damaging winds there.Other factors,such as environmental flows,however,contributed less to the damaging surface winds.展开更多
A squall line on 14 June 2009 in the provinces of Jiangsu and Anhui was well simulated using the Advanced Regional Prediction System （ARPS） model. Based on high resolution spatial and temporal data, a detailed analy...A squall line on 14 June 2009 in the provinces of Jiangsu and Anhui was well simulated using the Advanced Regional Prediction System （ARPS） model. Based on high resolution spatial and temporal data, a detailed analysis of the structural features and propagation mechanisms of the squall line was conducted. The dynamic and thermodynamic structural charac- teristics and their causes were analyzed in detail. Unbalanced flows were found to play a key role in initiating gravity waves during the squall line＇s development. The spread and development of the gravity waves were sustained by convection in the wave-CISK process. The squall line＇s propagation and development mainly relied on the combined effect of gravity waves at the midlevel and cold outflow along the gust front. New cells were continuously forced by the cold pool outflow and were enhanced and lifted by the intense upward motion. At a particular phase, the new cells merged with the updraft of the gravity waves, leading to an intense updraft that strengthened the squall line.展开更多
ABSTRACT Numerical simulations with the Advanced Regional Prediction System （ARPS） model were performed to investigate the impact of microphysical drop size distribution （DSD） on tornadogenesis in a subtropical s...ABSTRACT Numerical simulations with the Advanced Regional Prediction System （ARPS） model were performed to investigate the impact of microphysical drop size distribution （DSD） on tornadogenesis in a subtropical supercell thunderstorm over Anhui Province, eastern China. Sensitivity experiments with different intercept parameters of rain, hail and snow DSDs in a Lin-type microphysics scheme were conducted. Results showed that rain and hail DSDs have a significant impact on the simulated storm both microphysically and dynamically. DSDs characterized by larger （smaller） intercepts have a smaller （larger） particle size and a lower （higher） mass-weighted mean fall velocity, and produce relatively stronger （weaker） and wider （narrower） cold pools through enhanced （reduced） rain evaporation and hail melting processes, which are then less favorable （favorable） for tornadogenesis. However, tornadogenesis will also be suppressed by the weakened mid-level mesocyclone when the cold pool is too weak. When compared to a U.S. Great Plain case, the two microphysical processes are more sensitive to DSD variations in the present case with a higher melting level and deeper warm layer. This suggests that DSD-related cloud microphysics has a stronger influence on tornadogenesis in supercells over the subtropics than the U.S. Great Plains.展开更多
An extreme rainfall event occurred over Hangzhou,China,during the afternoon hours on 24 June 2013.This event occurred under suitable synoptic conditions and the maximum 4-h cumulative rainfall amount was over 150 mm.T...An extreme rainfall event occurred over Hangzhou,China,during the afternoon hours on 24 June 2013.This event occurred under suitable synoptic conditions and the maximum 4-h cumulative rainfall amount was over 150 mm.This rainfall event had two major rainbands.One was caused by a quasi-stationary convective line,and the other by a backbuilding convective line related to the interaction of the outflow boundary from the first rainband and an existing low-level mesoscale convergence line associated with a mei-yu frontal system.The rainfall event lasted 4 h,while the back-building process occurred in 2 h when the extreme rainfall center formed.So far,few studies have examined the back-building processes in the mei-yu season that are caused by the interaction of a mesoscale convergence line and a convective cold pool.The two rainbands are successfully reproduced by the Weather Research and Forecasting(WRF)model with fourlevel,two-way interactive nesting.In the model,new cells repeatedly occur at the west side of older cells,and the backbuilding process occurs in an environment with large CAPE,a low LFC,and plenty of water vapor.Outflows from older cells enhance the low-level convergence that forces new cells.High precipitation efficiency of the back-building training cells leads to accumulated precipitation of over 150 mm.Sensitivity experiments without evaporation of rainwater show that the convective cold pool plays an important role in the organization of the back-building process in the current extreme precipitation case.展开更多
The impacts of different moisture profiles on the structure and vertical motion of squall lines were investigated by conducting a set of numerical simulations.The base state was determined by an observational sounding...The impacts of different moisture profiles on the structure and vertical motion of squall lines were investigated by conducting a set of numerical simulations.The base state was determined by an observational sounding,with high precipitable water representing moist environmental conditions in the East Asian monsoon region.To reveal the impact of moisture at different levels,the moisture content at the middle and low levels were changed in the numerical simulations.The numerical results showed that more convective cells developed and covered a larger area in the high moisture experiments,which was characteristic of the convection during the Meiyu season in China.In addition,high moisture content at low levels favored the development of updrafts and triggered convection of greater intensity.This was demonstrated by the thermodynamic parameters,including Convective Available Potential Energy(CAPE),Lifted Index(LI),Lift Condensation Level(LCL),and Level of Free Convection(LFC).Dry air at middle levels led to strong downdrafts in the environment and updrafts in clouds.This could be because dry air at middle levels favors the release of latent heat,thereby promoting updrafts in clouds and downdrafts in the environment.Therefore,high relative humidity(RH)at low levels and low RH at middle levels favors updrafts in the cloud cores.Additionally,moist air at low levels and dry air at middle levels promotes the development of convective cells and the intensification of cold pool.The squall line can be organized by the outflow boundary induced by cold pool.The balance of cold pool and environmental wind shear is favorable for the maintenance and strengthening of squall lines.展开更多
A mesoscale convective system(MCS) occurred over the East China coastal provinces and the East China Sea on 30April 2021, producing damaging surface winds near the coastal city Nantong with observed speeds reaching 45...A mesoscale convective system(MCS) occurred over the East China coastal provinces and the East China Sea on 30April 2021, producing damaging surface winds near the coastal city Nantong with observed speeds reaching 45 m s^(–1). A simulation using the Weather Research and Forecasting model with a 1.5-km grid spacing generally reproduces the development and subsequent organization of this convective system into an MCS, with an eastward protruding bow segment over the sea. In the simulation, an east-west-oriented high wind swath is generated behind the gust front of the MCS. Descending dry rear-to-front inflows behind the bow and trailing gust front are found to feed the downdrafts in the main precipitation regions. The inflows help to establish spreading cold outflows and enhance the downdrafts through evaporative cooling. Meanwhile, front-to-rear inflows from the south are present, associated with severely rearward-tilted updrafts initially forming over the gust front. Such inflows descend behind(north of) the gust front, significantly enhancing downdrafts and near-surface winds within the cold pool. Consistently, calculated trajectories show that these parcels that contribute to the derecho originate primarily from the region ahead(south) of the east-west-oriented gust front, and dry southwesterly flows in the low-to-middle levels contribute to strong downdrafts within the MCS. Moreover, momentum budget analyses reveal that a large westward-directed horizontal pressure gradient force within the simulated cold pool produced rapid flow acceleration towards Nantong. The analyses enrich the understanding of damaging wind characteristics over coastal East China and will prove helpful to operational forecasters.展开更多
The occurrence and evolution of an oceanic tropical squall line observed on 22 February 1993 during TOGA-COARE over the equatorial Pacific Ocean were simulated by use of a three-dimensional, nonhydrostatic storm-scale...The occurrence and evolution of an oceanic tropical squall line observed on 22 February 1993 during TOGA-COARE over the equatorial Pacific Ocean were simulated by use of a three-dimensional, nonhydrostatic storm-scale numerical model ARPS. The capacity of ARPS to simulate such tropical squall line was verified. The structure and dynamic mechanism of the squall line were discussed in details as well The impacts of the different microphysical process that including the ice phase and warm rain schemes on structure and evolution of the squall line were investigated by the sensitive experiment. The simulations of the three-dimensional structure and evolution of the squall line are closely related with the observations when the proper microphysical processes were employed. The more latent heating released in the ice phase processes associated with the freezing process leads to strengthening deep convection due to the vertical gradient of buoyancy, which results in a long life of the convective system. In contrast, the warm rain process is characterized by short life period, more pronounced rearward tilt structure and extension of stratiform cloud.展开更多
In the present study, a severe squall line(SL) was analyzed by using intensive observational surface data and radar monitoring products. In this process, mesoscale convergence lines, such as the sea breeze front(SBF),...In the present study, a severe squall line(SL) was analyzed by using intensive observational surface data and radar monitoring products. In this process, mesoscale convergence lines, such as the sea breeze front(SBF), gust front and dry line, served as the main triggering and strengthening factors. The transition from convection triggering to the formation of the initial shape was mainly affected by the convergence line of the SBF, which combined with thermal convection to form the main parts of the SL. In the later stage, the convergence line of the gust front merged with other convergence lines to form a series of strong convective cells. The SBF had good indicative significance in terms of severe convective weather warnings. The suitable conditions of heat, water vapor and vertical wind shear on the Shandong Peninsula were beneficial to the maintenance of the SL. Before SL occurrence, tropopause folding strengthened, which consequently enhanced the baroclinic property in the middle and upper troposphere. The high sensible heat flux at the surface easily produced a positive potential vorticity anomaly in the low layer, resulting in convective instability, which was conducive to the maintenance of these processes. In the system, when precipitation particles passed through the unsaturated air layer, they underwent strong evaporation, melting or sublimation, and the cooling effect formed negative buoyancy, which accelerated the sinking of the air and promoted the sustained development of the surface gale. Together with the development of lowlevel mesocyclones, the air pressure decreased rapidly, which was conducive to gale initiation.展开更多
The characteristics and influence of the dynamical, thermal, and microphysical processes of a squall line are studied using the WRF model. This squall line occurred in the Yangtze and Huaihe river basins in Anhui Prov...The characteristics and influence of the dynamical, thermal, and microphysical processes of a squall line are studied using the WRF model. This squall line occurred in the Yangtze and Huaihe river basins in Anhui Province on 5 July 2013. The active cold pool and the downdraft caused by the rear inflow contribute to the development of the squall line. The cold pool develops strongly, moves ahead of the storm, and then cuts off the supply of the warm air to the updraft, leading to the storm＇s demise. Analysis of the microphysics budget shows that snow crystals contribute to the rainfall rate most during the evolution of the squall line. The heating effect is predominant at the high level,whereas cooling plays an important role at the low level. A typical stratified ‘seeder–feeder＇ structure of the cloud exists primarily in the convective region of the squall line. The heights of the cloud ice and snow layers decrease continuously with time, and both the seeder and feeder cloud decrease dramatically in the dissipation stage, which leads to the reduced rainfall rate.展开更多
A second rain belt sometimes occurs ahead of a frontal rain belt in the warm sector over coastal South China,leading to heavy precipitation.We examined the differences in the mesoscale characteristics and microphysics...A second rain belt sometimes occurs ahead of a frontal rain belt in the warm sector over coastal South China,leading to heavy precipitation.We examined the differences in the mesoscale characteristics and microphysics of the frontal and warm sector rain belts that occurred in South China on May 10–13,2022.The southern rain belt occurred in an environment with favorable mesoscale conditions but weak large-scale forcing.In contrast,the northern rain belt was related to low-level horizontal shear and the surface-level front.The interaction between the enhanced southeasterly winds and the rainfall-induced cold pool promoted the persistent growth of convection along the southern rain belt.The convective cell propagated east over the coastal area,where there was a large temperature gradient.The bow-shaped echo in this region may be closely related to the rear-inflow jet.By contrast,the initial convection of the northern rain belt was triggered along the front and the region of low-level horizontal shear,with mesoscale interactions between the enhanced warm-moist southeasterly airflow and the cold dome associated with the earlier rain.The terrain blocked the movement of the cold pool,resulting in the stagnation of the frontal convective cell at an early stage.Subsequently,a meso-γ-scale vortex formed during the rapid movement of the convective cell,corresponding to an enhancement of precipitation.The representative raindrop spectra for the southern rain belt were characterized by a greater number and higher density of raindrops than the northern rain belt,even though both resulted in comparable hourly rainfalls.These results help us better understand the characteristics of double rain belts over South China.展开更多
Phase changes in the precipitation processes of early winter and late spring in midlatitude regions represent challenges when forecasting the timing and magnitude of snowfall.On 4 April 2018,a heavy snow process occur...Phase changes in the precipitation processes of early winter and late spring in midlatitude regions represent challenges when forecasting the timing and magnitude of snowfall.On 4 April 2018,a heavy snow process occurred in Beijing and northwestern Hebei Province,becoming the most delayed occurrence of heavy spring snow ever recorded over Beijing in the last 30 years.This paper uses observational and numerical simulation data to investigate the causes for the rapid rain-to-snow(RRTS)phase transition during this process.The following results are obtained.(1)Return flows(RFs),an interesting type of easterly wind,including those at 1000,925,and 800 hPa,played an important role in this heavy snow process and presented a characteristic"sandwich"structure.The RFs,complex topography,and snow particles that dominated the clouds,were the three key factors for the RRTS transition.(2)The RRTS transition in the plains was directly related to the RF at 925 hPa,which brought about advective cooling initiated approximately 4-6 h before the onset of precipitation.Then,the RF played a role of diabatic cooling when snow particles began to fall at the onset of precipitation.(3)The RRTS transition in the northern part of the Taihang Mountains was closely related to the relatively high altitude that led to a lower surface temperature owing to the vertical temperature lapse rate.Both immediately before and after the onset of precipitation,the snow particles in clouds entrained the middle-level cold air downward,causing the melting layer(from surface to the 0℃-isotherm level)to become very thin;and thus the snow particles did not have adequate time to melt before falling to the ground.(4)The rapid RRTS over the Yanqing mountainous area in the northwest of Beijing could have involved all the three concurrent mechanisms:the advective cooling of RF,the melting cooling of cloud snow particles,and the high-altitude effect.Compared with that in the plain area with less urbanization the duration of the RRTS in the plain area with significant urbanization was extended by approximately 2 h.展开更多
In this study, based on simulations of a two-dimensional multicell storm under a ground-layer upshear （Uz〈0） by a mesoscale numerical model, a new mechanism of cell regeneration and development within the multicell...In this study, based on simulations of a two-dimensional multicell storm under a ground-layer upshear （Uz〈0） by a mesoscale numerical model, a new mechanism of cell regeneration and development within the multicell storm at the ＂less than optimal shear＂ state.is proposed. In the presence of a ground-layer upshear, the circulation associated with the surface cold pool is not counteracted by that associated with the ambient wind sl^ear, and the density current extends out faster, making the multicell storm stay at the ＂less than optimal shear＂ state. As a result, a new cell is triggered by the strong vertical perturbation ahead of the mature convection, rather than by the split-up from the updraft at the leading edge of the surface cold pool as well as the gust front. The latter is the mechanism at the ＂optimal＂ state proposed by Lin et al. in 1998. In the new mechanism, the regenerated cell grows fast with the incident warm moist air from the upstream of the multicell storm, and tends to cut off the moist airflow into the mature convection at its western sector. Consequently, the mature convection would weaken, be replaced, and eventually decay. Actually, these two different mechanisms come into play in a way depending on the relationship between the circulation of the low-level shear and that of the cold pool. When the circulation of the cold pool is stronger than that of the wind shear, the multicell storm is at the ＂less than optimal shear＂ state, and the new convective cell is produced by the disturbance ahead of the mature cell. When the circulation of the cold pool is weaker, the cell regeneration is dominated by the mechanism at the ＂optimal＂ state, and the new cell is split from the gust front updraft. Therefore, these two mechanisms are not contradictive. With a moderate ground-layer upshear, they can alternately operate within a multicell storm.展开更多
Weather forecasting for the Zhangjiakou competition zone of the Beijing 2022 Winter Olympic Games is a challenging task due to its complex terrain.Numerical weather prediction models generally perform poorly for cold ...Weather forecasting for the Zhangjiakou competition zone of the Beijing 2022 Winter Olympic Games is a challenging task due to its complex terrain.Numerical weather prediction models generally perform poorly for cold air pools and winds over complex terrains,due to their low spatiotemporal resolution and limitations in the description of dynamics,thermodynamics,and microphysics in mountainous areas.This study proposes an ensemble-learning model,named ENSL,for surface temperature and wind forecasts at the venues of the Zhangjiakou competition zone,by integrating five individual models—linear regression,random forest,gradient boosting decision tree,support vector machine,and artificial neural network(ANN),with a ridge regression as meta model.The ENSL employs predictors from the high-resolution ECMWF model forecast(ECMWF-HRES) data and topography data,and targets from automatic weather station observations.Four categories of predictors(synoptic-pattern related fields,surface element fields,terrain,and temporal features) are fed into ENSL.The results demonstrate that ENSL achieves better performance and generalization than individual models.The root-mean-square error(RMSE) for the temperature and wind speed predictions is reduced by 48.2% and 28.5%,respectively,relative to ECMWF-HRES.For the gust speed,the performance of ENSL is consistent with ANN(best individual model) in the whole dataset,whereas ENSL outperforms on extreme gust samples(42.7% compared with 38.7% obtained by ECMWF-HRES in terms of RMSE reduction).Sensitivity analysis of predictors in the four categories shows that ENSL fits their feature importance rankings and physical explanations effectively.展开更多
基金supported by the National Key Research and Development Program of China (Grant No. 2024YFC3013003)the National Natural Science Foundation of China (Grant No. 42475002)+2 种基金the Innovation Group Project of Southern Marine Science and Engineering Guangdong Laboratory (Zhuhai) (Grant Nos. SML2024SP035, SML2024SP012, and 311024001)the Guangdong Basic and Applied Basic Research Foundation (Grant Nos. 2025A1515011974, 2024A1515510005 and 2020B0301030004)the Key Innovation Team of China Meteorological Administration (Grant No. CMA2023ZD08)
文摘Cold pools(CPs)significantly influence coastal heavy rainfall,but detailed observations of them are limited due to the lack of vertical measurement instruments.This study statistically characterizes CPs in the coastal monsoon region of South China using unique data from the 356-m-high Shenzhen Meteorological Tower.CP occurrence correlates with convective activities influenced by the summer monsoon in the seasonal variations and land–sea breeze activities in the diurnal cycle.The CPs predominantly dry the atmosphere,highlighting the dominant role of dry entrainment through convective downdrafts in their formation,with a minor role of hydrometeor evaporation.The average CP depth is estimated at 668.0 m,deeper than tropical CPs but shallower than midlatitude counterparts.The CP properties exhibit diurnal variability,largely influenced by mesoscale convective system(MCS)activities.MCS-induced CPs are deeper and more intense than those from individual convective cells,while linear-MCS-produced CPs are the most intense.These observations from the coastal monsoon region contribute to a comprehensive global understanding of CP characteristics,complementing existing studies from midlatitude and tropical regions.
文摘Cold pools and associated wind storms are frequent occurrences in Southwestern Nigeria, especially during the early monsoon phase. The associated surface wind gust frequently destroys properties resulting in economic losses. Two case events were investigated in this study;one event occurred in May 2019 and the other occurred in March 2020, both in southwestern Nigeria. The National Oceanic and Atmospheric Administration (NOAA) National Center for Environmental Prediction (NCEP)/Climate Prediction Center (CPC) Infrared brightness temperatures and CPC Morphing technique (CMORPH) rainfall products were analysed alongside in-situ observations from the Nigerian Meteorological Agency (NiMET). Other data sources analysed are the National Aeronautics and Space Administration (NASA) Prediction Of Worldwide Energy Resources (POWER) and the World Wide Lightning Location Network (WWLLN). Cold pools were identified in the impacted communities as indicated by surface characteristics investigated from the in-situ observations. There was a sudden change in wind direction, with a simultaneous drop in temperature accompanied by increasing wind speed. Pressure and humidity were observed to change in the same period. Thunderstorms were also present in the impacted communities, as observed by the in-situ observations, in both case events. The presence of lightning as observed by WWLLN agrees with the in-situ thunderstorms. The cloud characteristics showed the presence of cloud shields, by their brightness temperature, over the impacted communities during the period of the cold pools in both case events. The systems were raining at the time of the observations in both cases, consistent with the in-situ thunderstorm observations. The communities were heavily impacted with several properties destroyed in the events. These early monsoon seasonal windstorms require a forecasting tool for their prediction and this study presents an eye-opener for further investigation and innovative research to address the menace.
基金The National Natural Science Foundation of China under contract Nos 41276198,41076135 and 41003036the Scientific Research Fund of Second Institute of Oceanography,SOA under contract Nos JG1323 and JG1023the Chinese Polar Environment Comprehensive Investiga-tion and Assessment Programs under contract Nos 20130403 and 20130304
文摘CHEMTAX analysis of high-performance liquid chromatography （HPLC） pigment was conducted to study phytoplankton community structure in the northern Bering Sea shelf, where a seasonal subsurface cold pool emerges. The results showed that fucoxanthin （Fuco） and chlorophyll a （Chl a） were the most abundant diagnostic pigments, with the integrated water column values ranging from 141 to 2160 μg/m2 and 477 to 5 535 μg/m2, respectively. Moreover, a diatom bloom was identified at Sta. BB06 with the standing stock of Fuco up to 9214 μg/m3. The results of CHEMTAX suggested that the phytoplankton community in the northern Bering Sea shelf was dominated by diatoms and chrysophytes with an average relative contribu- tion to Chl a of 80% and 12%, respectively, followed by chlorophytes, dinoflagellates, and cryptophytes. Dia- toms were the absolutely dominant algae in the subsurface cold pool with a relative contribution exceeding 90%, while the contribution of chrysophytes was generally higher in oligotrophic upper water. Additionally, the presence of a cold pool would tend to favor accumulation of diatom biomass and a bloom that occurred beneath the halocline would be beneficial to organic matter sinks, which suggests that a large part of the phytoplankton biomass would settle to the seabed and support a rich benthic biomass.
基金a donation from Trond Mohn,c/o Frank Mohn AS,for financing part of the workGrants from the Nansen Scientific Society,EU-FP7[project number 295092],INDOMARECLIM
文摘During the summer monsoon season, strong coastal upwelling occurs along the southwest coast of India and at the southern tip of India, which cools the surface temperature of the waters around these regions. The summer monsoon current carries the upwelled cold waters into the Bay of Bengal and forms the ‘cold pool of the Bay of Bengal', with its core south of Sri Lanka and over the southcentral Bay of Bengal. The present study focuses on the intrusion of these cold waters into the south of the Bay of Bengal, its interannual variability, and its association with the surface wind during the break phase of the summer monsoon, when strong westerly surface winds flow south of 10°N.The authors hypothesize that the enhanced cooling in the cold pool region during monsoon spells is associated with the strong westerly wind stress there during the break spells of the monsoon.Seven cases of long break monsoon spells that occurred during the nine years from 2001 to 2009 are analyzed, and the results confirm our hypothesis.
基金the Tribal Resilience Program,NASA grant#80NSSC19K0981 and the State of Alaska for financial support of this study
文摘Near-surface PM2.5 and meteorological observations were performed in three rural communities in the high latitude Yukon Flats valley at various times during the cold season (October to April). These data were synthesized with data from other meteorological sites, NCEP reanalysis and MAIAC retrieved aerosol optical depths data to analyze the role of mesoscale processes and radiation on air quality. Under weak large-scale forcing mountain-valley circulations develop that are driven by the differences in insolation. During the long dark nights, radiative cooling occurs in the near-surface layer of the mountain slopes of the Brooks, Ogilvie and White Mountains Ranges and at the bottom of the valley. Here surface-based inversions (SBI)—known as roof-top inversions—forms, while the cold air drains from the slopes. A frontal wedge forms when the cold air slides over the relatively colder air in the valley. Drainage of cold air from the Brooks Range governed the circulation and cold air pooling in the valley. Concentrations during times with and without SBI differed significantly (at 95% confidence) at two sites indicating that local emissions were the major contributor. At the site, which is closest to the mountains, concentrations marginally changed in the presence of inversions. At all sites, 24-h mean PM2.5 remained below the National Ambient Air Quality Standard.
基金supported by the National Key R&D Program of China(2022YFC3004101)the National Natural Science Foundation of China(Grant No.42275006)+2 种基金the Guangdong Basic and Applied Basic Research Foundation(Grant No.2022A1515011814)the China Meteorological Administration Tornado Key Laboratory(Grant No.TKL202302)the Science and Technology Research Project of Guangdong Meteorological Service(Grant No.GRMC2023Q35)。
文摘This study presents finely resolved radar signatures of multiple cyclonic vortices associated with an EF2 tornadic supercell that occurred in Guangzhou on 16 June 2022 and discusses how the mesocyclone formed on the lee side of mountain.A nearby X-band phased-array radar provides evidence that the mesocyclone was shallow,with a depth generally confined to less than 3 km.The mesocyclonic feature was observed to initiate from near-ground level,driven by the interaction between intensifying cold pool surges and shallow lee-side ambient flows.It was first recognized shortly after the presence of near-ground cyclonic convergence signatures over the leading edges of cold pool outflows.Over the subsequent 17 min,the mesocyclone developed upward,reaching a maximum height of 3 km,and produced a tornado 8min later.Nearly coinciding with the time of tornadogenesis,a noticeable separation of the low-level tornado cyclone from the midlevel mesocyclone was observed.This shift in the vertically oriented vortex tube was likely caused by modifications to the low-level flow due to the complex hilly terrain or by occlusions associated with rear-flank downdrafts.After tornadogenesis,high-resolution X-PAR observations revealed that the lowest-level mesocyclonic signature contracted into a gate-to-gate tornadic vortex signature(TVS)at the tip of hook echoes.Compared to conventional S-band operational weather radars,rapid-scan X-PAR observations indicate that a core diameter threshold of 1.5–2 km could be employed to identify a cyclonically sheared radial velocity couplet as a TVS,potentially extending the lead time for Doppler-based tornado warnings.
基金Research(2020B0301030004)National Natural Science Foundation of China(42275002)+3 种基金Natural Science Foundation of Chongqing(CSTB2022NSCQ-MSX0890)Scientific and Technological Project of Chongqing Meteorological Service(YWJSGG-202124)Key Innovation Team of China Meteorological Administration(CMA2022ZD09)Southern Marine Science and Engineering Guangdong Laboratory(Zhuhai)(SML2023SP209)。
文摘Observational analyses and convection-permitting simulations are conducted to study an extreme event of damaging surface winds within a bow-shaped squall line over South China.Prevailing explanations for the formation of damaging surface winds were applied to investigate the detailed processes at the mature stage of a squall line in a complicated real-world scenario.It is shown that environmental vertical wind shear was adequately strong to cause a shearing vorticity,and the shearing vorticity was comparable to the baroclinic vorticity by a cold pool.The balance led to strong upward motions at the leading edge of the squall line and brought plenty of rainfall.The descending rainfall cooled the surrounding air and entrained the upper-level cold air downward to the lower level,strengthening the cold pool by excessive evaporation and melting.The cold pool accelerated the propagation speed of the squall line and caused extensive wind damage at the surface.Meanwhile,the horizontal vortex lines at the leading edge of the cold pool were lifted by frontal updrafts and tilted to form the anti-cyclonic vortex at the middle level.This maintained and intensified the rearinflow jet behind the apex of bow echo in association with a notable midlevel pressure gradient.The rear-inflow jet was then transported downward to the surface by descending condensate,leading to damaging winds there.Other factors,such as environmental flows,however,contributed less to the damaging surface winds.
基金supported by the National Basic Research Program of China (Grant No. 2013CB 430105)the Key Program of the Chinese Academy of Sciences (Grant No. KZZD-EW-05)+1 种基金the project of CAMS (Chinese Academy of Meteorological Sciences) (Grant No. 2011LASWB15)the National Natural Sciences Foundation of China (Grant No. 41175060)
文摘A squall line on 14 June 2009 in the provinces of Jiangsu and Anhui was well simulated using the Advanced Regional Prediction System （ARPS） model. Based on high resolution spatial and temporal data, a detailed analysis of the structural features and propagation mechanisms of the squall line was conducted. The dynamic and thermodynamic structural charac- teristics and their causes were analyzed in detail. Unbalanced flows were found to play a key role in initiating gravity waves during the squall line＇s development. The spread and development of the gravity waves were sustained by convection in the wave-CISK process. The squall line＇s propagation and development mainly relied on the combined effect of gravity waves at the midlevel and cold outflow along the gust front. New cells were continuously forced by the cold pool outflow and were enhanced and lifted by the intense upward motion. At a particular phase, the new cells merged with the updraft of the gravity waves, leading to an intense updraft that strengthened the squall line.
基金jointly supported by the National Natural Science Foundation of China (Grant Nos.41175118, 40775005 and 41175043)the National Basic Research Program of China (Grant No.2013CB430105)partially supported by the China Special Fund for Meteorological Research in the Public Interest (Grant Nos.GYHY200906003 and GYHY201306040)
文摘ABSTRACT Numerical simulations with the Advanced Regional Prediction System （ARPS） model were performed to investigate the impact of microphysical drop size distribution （DSD） on tornadogenesis in a subtropical supercell thunderstorm over Anhui Province, eastern China. Sensitivity experiments with different intercept parameters of rain, hail and snow DSDs in a Lin-type microphysics scheme were conducted. Results showed that rain and hail DSDs have a significant impact on the simulated storm both microphysically and dynamically. DSDs characterized by larger （smaller） intercepts have a smaller （larger） particle size and a lower （higher） mass-weighted mean fall velocity, and produce relatively stronger （weaker） and wider （narrower） cold pools through enhanced （reduced） rain evaporation and hail melting processes, which are then less favorable （favorable） for tornadogenesis. However, tornadogenesis will also be suppressed by the weakened mid-level mesocyclone when the cold pool is too weak. When compared to a U.S. Great Plain case, the two microphysical processes are more sensitive to DSD variations in the present case with a higher melting level and deeper warm layer. This suggests that DSD-related cloud microphysics has a stronger influence on tornadogenesis in supercells over the subtropics than the U.S. Great Plains.
基金supported by the National Natural Science Foundation of China (Grant Nos.41730965, U2242204, and 41175047)the National Key Basic Research and Development Project of China (Grant No.2013CB430104)+2 种基金the Key Project of the Joint Funds of the Natural Science Foundation of Zhejiang Province (Grant No.LZJMZ23D050003financial support from the China Scholarship Council for her visit to CAPSUniversity of Oklahoma
文摘An extreme rainfall event occurred over Hangzhou,China,during the afternoon hours on 24 June 2013.This event occurred under suitable synoptic conditions and the maximum 4-h cumulative rainfall amount was over 150 mm.This rainfall event had two major rainbands.One was caused by a quasi-stationary convective line,and the other by a backbuilding convective line related to the interaction of the outflow boundary from the first rainband and an existing low-level mesoscale convergence line associated with a mei-yu frontal system.The rainfall event lasted 4 h,while the back-building process occurred in 2 h when the extreme rainfall center formed.So far,few studies have examined the back-building processes in the mei-yu season that are caused by the interaction of a mesoscale convergence line and a convective cold pool.The two rainbands are successfully reproduced by the Weather Research and Forecasting(WRF)model with fourlevel,two-way interactive nesting.In the model,new cells repeatedly occur at the west side of older cells,and the backbuilding process occurs in an environment with large CAPE,a low LFC,and plenty of water vapor.Outflows from older cells enhance the low-level convergence that forces new cells.High precipitation efficiency of the back-building training cells leads to accumulated precipitation of over 150 mm.Sensitivity experiments without evaporation of rainwater show that the convective cold pool plays an important role in the organization of the back-building process in the current extreme precipitation case.
基金National Natural Science Foundation of China(41705029,41675045)Strategic Priority Research Program of Chinese Academy of Sciences(XDA17010105)Key R&D Projects of Anhui Province(201904a07020099)。
文摘The impacts of different moisture profiles on the structure and vertical motion of squall lines were investigated by conducting a set of numerical simulations.The base state was determined by an observational sounding,with high precipitable water representing moist environmental conditions in the East Asian monsoon region.To reveal the impact of moisture at different levels,the moisture content at the middle and low levels were changed in the numerical simulations.The numerical results showed that more convective cells developed and covered a larger area in the high moisture experiments,which was characteristic of the convection during the Meiyu season in China.In addition,high moisture content at low levels favored the development of updrafts and triggered convection of greater intensity.This was demonstrated by the thermodynamic parameters,including Convective Available Potential Energy(CAPE),Lifted Index(LI),Lift Condensation Level(LCL),and Level of Free Convection(LFC).Dry air at middle levels led to strong downdrafts in the environment and updrafts in clouds.This could be because dry air at middle levels favors the release of latent heat,thereby promoting updrafts in clouds and downdrafts in the environment.Therefore,high relative humidity(RH)at low levels and low RH at middle levels favors updrafts in the cloud cores.Additionally,moist air at low levels and dry air at middle levels promotes the development of convective cells and the intensification of cold pool.The squall line can be organized by the outflow boundary induced by cold pool.The balance of cold pool and environmental wind shear is favorable for the maintenance and strengthening of squall lines.
基金primarily supported by the Ministry of Science and Technology of the People's Republic of China (MOST)(Grant No. 2018YFC1507303)National Natural Science Foundation of China (Grant Nos. 419505044,41941007, and 42230607)+1 种基金by the Talent Research Start-Up Fund of Nanjing University of Aeronautics and Astronautics(Grant No. 1007-90YAH22046)supported by The High Performance Computing Platform of Nanjing University of Aeronautics and Astronautics。
文摘A mesoscale convective system(MCS) occurred over the East China coastal provinces and the East China Sea on 30April 2021, producing damaging surface winds near the coastal city Nantong with observed speeds reaching 45 m s^(–1). A simulation using the Weather Research and Forecasting model with a 1.5-km grid spacing generally reproduces the development and subsequent organization of this convective system into an MCS, with an eastward protruding bow segment over the sea. In the simulation, an east-west-oriented high wind swath is generated behind the gust front of the MCS. Descending dry rear-to-front inflows behind the bow and trailing gust front are found to feed the downdrafts in the main precipitation regions. The inflows help to establish spreading cold outflows and enhance the downdrafts through evaporative cooling. Meanwhile, front-to-rear inflows from the south are present, associated with severely rearward-tilted updrafts initially forming over the gust front. Such inflows descend behind(north of) the gust front, significantly enhancing downdrafts and near-surface winds within the cold pool. Consistently, calculated trajectories show that these parcels that contribute to the derecho originate primarily from the region ahead(south) of the east-west-oriented gust front, and dry southwesterly flows in the low-to-middle levels contribute to strong downdrafts within the MCS. Moreover, momentum budget analyses reveal that a large westward-directed horizontal pressure gradient force within the simulated cold pool produced rapid flow acceleration towards Nantong. The analyses enrich the understanding of damaging wind characteristics over coastal East China and will prove helpful to operational forecasters.
基金the National Natural Science Foundation of China under the grant !Nos. 49605064 and 49735180by State Key Basic Research Prog
文摘The occurrence and evolution of an oceanic tropical squall line observed on 22 February 1993 during TOGA-COARE over the equatorial Pacific Ocean were simulated by use of a three-dimensional, nonhydrostatic storm-scale numerical model ARPS. The capacity of ARPS to simulate such tropical squall line was verified. The structure and dynamic mechanism of the squall line were discussed in details as well The impacts of the different microphysical process that including the ice phase and warm rain schemes on structure and evolution of the squall line were investigated by the sensitive experiment. The simulations of the three-dimensional structure and evolution of the squall line are closely related with the observations when the proper microphysical processes were employed. The more latent heating released in the ice phase processes associated with the freezing process leads to strengthening deep convection due to the vertical gradient of buoyancy, which results in a long life of the convective system. In contrast, the warm rain process is characterized by short life period, more pronounced rearward tilt structure and extension of stratiform cloud.
基金Natural Science Foundation of Shandong Province (ZR2021MD062, ZR2021MD010, ZR2023MD118)National Natural Science Foundation of China (42275001)+2 种基金Shandong Meteorological Bureau Innovation Team Project (SDCXTD2023-1)Huaihe River Meteorology Open Research Fund (HRM201807)Key Special Project of Qingdao Meteorological Bureau (2023qdqxz02)。
文摘In the present study, a severe squall line(SL) was analyzed by using intensive observational surface data and radar monitoring products. In this process, mesoscale convergence lines, such as the sea breeze front(SBF), gust front and dry line, served as the main triggering and strengthening factors. The transition from convection triggering to the formation of the initial shape was mainly affected by the convergence line of the SBF, which combined with thermal convection to form the main parts of the SL. In the later stage, the convergence line of the gust front merged with other convergence lines to form a series of strong convective cells. The SBF had good indicative significance in terms of severe convective weather warnings. The suitable conditions of heat, water vapor and vertical wind shear on the Shandong Peninsula were beneficial to the maintenance of the SL. Before SL occurrence, tropopause folding strengthened, which consequently enhanced the baroclinic property in the middle and upper troposphere. The high sensible heat flux at the surface easily produced a positive potential vorticity anomaly in the low layer, resulting in convective instability, which was conducive to the maintenance of these processes. In the system, when precipitation particles passed through the unsaturated air layer, they underwent strong evaporation, melting or sublimation, and the cooling effect formed negative buoyancy, which accelerated the sinking of the air and promoted the sustained development of the surface gale. Together with the development of lowlevel mesocyclones, the air pressure decreased rapidly, which was conducive to gale initiation.
基金supported by the National Basic Research Program of China[grant numbers 2013CB430100,2015CB453201]the National Natural Science Foundation of China[grant numbers41375058,41530427]
文摘The characteristics and influence of the dynamical, thermal, and microphysical processes of a squall line are studied using the WRF model. This squall line occurred in the Yangtze and Huaihe river basins in Anhui Province on 5 July 2013. The active cold pool and the downdraft caused by the rear inflow contribute to the development of the squall line. The cold pool develops strongly, moves ahead of the storm, and then cuts off the supply of the warm air to the updraft, leading to the storm＇s demise. Analysis of the microphysics budget shows that snow crystals contribute to the rainfall rate most during the evolution of the squall line. The heating effect is predominant at the high level,whereas cooling plays an important role at the low level. A typical stratified ‘seeder–feeder＇ structure of the cloud exists primarily in the convective region of the squall line. The heights of the cloud ice and snow layers decrease continuously with time, and both the seeder and feeder cloud decrease dramatically in the dissipation stage, which leads to the reduced rainfall rate.
基金National Natural Science Foundation of China(41930972,52239006,41975001)。
文摘A second rain belt sometimes occurs ahead of a frontal rain belt in the warm sector over coastal South China,leading to heavy precipitation.We examined the differences in the mesoscale characteristics and microphysics of the frontal and warm sector rain belts that occurred in South China on May 10–13,2022.The southern rain belt occurred in an environment with favorable mesoscale conditions but weak large-scale forcing.In contrast,the northern rain belt was related to low-level horizontal shear and the surface-level front.The interaction between the enhanced southeasterly winds and the rainfall-induced cold pool promoted the persistent growth of convection along the southern rain belt.The convective cell propagated east over the coastal area,where there was a large temperature gradient.The bow-shaped echo in this region may be closely related to the rear-inflow jet.By contrast,the initial convection of the northern rain belt was triggered along the front and the region of low-level horizontal shear,with mesoscale interactions between the enhanced warm-moist southeasterly airflow and the cold dome associated with the earlier rain.The terrain blocked the movement of the cold pool,resulting in the stagnation of the frontal convective cell at an early stage.Subsequently,a meso-γ-scale vortex formed during the rapid movement of the convective cell,corresponding to an enhancement of precipitation.The representative raindrop spectra for the southern rain belt were characterized by a greater number and higher density of raindrops than the northern rain belt,even though both resulted in comparable hourly rainfalls.These results help us better understand the characteristics of double rain belts over South China.
基金Supported by the National Natural Science Foundation of China(41475051 and 42075008)Beijing Natural Science Foundation(8192019)Civil Aviation Administration of China Security Capacity Building Project(20600822)。
文摘Phase changes in the precipitation processes of early winter and late spring in midlatitude regions represent challenges when forecasting the timing and magnitude of snowfall.On 4 April 2018,a heavy snow process occurred in Beijing and northwestern Hebei Province,becoming the most delayed occurrence of heavy spring snow ever recorded over Beijing in the last 30 years.This paper uses observational and numerical simulation data to investigate the causes for the rapid rain-to-snow(RRTS)phase transition during this process.The following results are obtained.(1)Return flows(RFs),an interesting type of easterly wind,including those at 1000,925,and 800 hPa,played an important role in this heavy snow process and presented a characteristic"sandwich"structure.The RFs,complex topography,and snow particles that dominated the clouds,were the three key factors for the RRTS transition.(2)The RRTS transition in the plains was directly related to the RF at 925 hPa,which brought about advective cooling initiated approximately 4-6 h before the onset of precipitation.Then,the RF played a role of diabatic cooling when snow particles began to fall at the onset of precipitation.(3)The RRTS transition in the northern part of the Taihang Mountains was closely related to the relatively high altitude that led to a lower surface temperature owing to the vertical temperature lapse rate.Both immediately before and after the onset of precipitation,the snow particles in clouds entrained the middle-level cold air downward,causing the melting layer(from surface to the 0℃-isotherm level)to become very thin;and thus the snow particles did not have adequate time to melt before falling to the ground.(4)The rapid RRTS over the Yanqing mountainous area in the northwest of Beijing could have involved all the three concurrent mechanisms:the advective cooling of RF,the melting cooling of cloud snow particles,and the high-altitude effect.Compared with that in the plain area with less urbanization the duration of the RRTS in the plain area with significant urbanization was extended by approximately 2 h.
基金Supported jointly by the State Key and Basic Research and Development Project (2004CB418301)the key Project of Ministry of Education of China (No. 02109)+1 种基金the Specialized Research Fund for the Doctoral Program of Higher Education (20080284019)the National Special Funding Project for Meteorology (GYHY200706033 and GYHY200706020)
文摘In this study, based on simulations of a two-dimensional multicell storm under a ground-layer upshear （Uz〈0） by a mesoscale numerical model, a new mechanism of cell regeneration and development within the multicell storm at the ＂less than optimal shear＂ state.is proposed. In the presence of a ground-layer upshear, the circulation associated with the surface cold pool is not counteracted by that associated with the ambient wind sl^ear, and the density current extends out faster, making the multicell storm stay at the ＂less than optimal shear＂ state. As a result, a new cell is triggered by the strong vertical perturbation ahead of the mature convection, rather than by the split-up from the updraft at the leading edge of the surface cold pool as well as the gust front. The latter is the mechanism at the ＂optimal＂ state proposed by Lin et al. in 1998. In the new mechanism, the regenerated cell grows fast with the incident warm moist air from the upstream of the multicell storm, and tends to cut off the moist airflow into the mature convection at its western sector. Consequently, the mature convection would weaken, be replaced, and eventually decay. Actually, these two different mechanisms come into play in a way depending on the relationship between the circulation of the low-level shear and that of the cold pool. When the circulation of the cold pool is stronger than that of the wind shear, the multicell storm is at the ＂less than optimal shear＂ state, and the new convective cell is produced by the disturbance ahead of the mature cell. When the circulation of the cold pool is weaker, the cell regeneration is dominated by the mechanism at the ＂optimal＂ state, and the new cell is split from the gust front updraft. Therefore, these two mechanisms are not contradictive. With a moderate ground-layer upshear, they can alternately operate within a multicell storm.
基金Supported by the National Key Research and Development Program of China (2018YDD0300104)Key Research and Development Program of Hebei Province of China (21375404D)After-Action-Review Project of China Meteorological Administration(FPZJ2023-014)。
文摘Weather forecasting for the Zhangjiakou competition zone of the Beijing 2022 Winter Olympic Games is a challenging task due to its complex terrain.Numerical weather prediction models generally perform poorly for cold air pools and winds over complex terrains,due to their low spatiotemporal resolution and limitations in the description of dynamics,thermodynamics,and microphysics in mountainous areas.This study proposes an ensemble-learning model,named ENSL,for surface temperature and wind forecasts at the venues of the Zhangjiakou competition zone,by integrating five individual models—linear regression,random forest,gradient boosting decision tree,support vector machine,and artificial neural network(ANN),with a ridge regression as meta model.The ENSL employs predictors from the high-resolution ECMWF model forecast(ECMWF-HRES) data and topography data,and targets from automatic weather station observations.Four categories of predictors(synoptic-pattern related fields,surface element fields,terrain,and temporal features) are fed into ENSL.The results demonstrate that ENSL achieves better performance and generalization than individual models.The root-mean-square error(RMSE) for the temperature and wind speed predictions is reduced by 48.2% and 28.5%,respectively,relative to ECMWF-HRES.For the gust speed,the performance of ENSL is consistent with ANN(best individual model) in the whole dataset,whereas ENSL outperforms on extreme gust samples(42.7% compared with 38.7% obtained by ECMWF-HRES in terms of RMSE reduction).Sensitivity analysis of predictors in the four categories shows that ENSL fits their feature importance rankings and physical explanations effectively.
