The Sichuan Basin(SCB),China has a high incidence of extremely persistent heavy rainfall(EPHR)events.The EPHR events from 2009 to 2019 in the SCB were mainly concentrated over the northern and northwestern windward sl...The Sichuan Basin(SCB),China has a high incidence of extremely persistent heavy rainfall(EPHR)events.The EPHR events from 2009 to 2019 in the SCB were mainly concentrated over the northern and northwestern windward slopes and the central basin.They occurred from June to September,but especially in July,and peaked at 0300 LST.ERA5 reanalysis data and objective classification were used to investigate the synoptic patterns and their effects.There were three synoptic patterns during EPHR events,all accompanied by a Southwest Vortex.The location and intensity of the Southwest Vortex,thermal forcing of the Tibetan Plateau(TP),and low-level winds can greatly affect the intensity and spatial distribution of EPHR.When the Southwest Vortex was located in the western SCB and there were southerly low-level jets(LLJs),convergence and upslope wind would lead to EPHR over the northwestern or northern windward slopes.If there was no LLJ and the whole SCB was under the center of the Southwest Vortex,nocturnal EPHR was controlled by the internal circulation of the Southwest Vortex and the updraft generated by the thermal forcing of the TP,and the rainfall was weaker.The southeastern entrance of the SCB was a key area where the low-level wind dominated the nocturnal peak of EPHR.The nocturnal strengthened southeasterly wind in the key area is attributable to inertial oscillation,and the topographic friction plays an essential role in transporting momentum and moisture into the basin by generating easterly and northeasterly ageostrophic winds.展开更多
Cloud-to-ground(CG)lightning data and the ECMWF ERA-Interim reanalysis dataset are analyzed to gain insight into the spatiotemporal distribution and synoptic background of winter-season CG flashes between December 201...Cloud-to-ground(CG)lightning data and the ECMWF ERA-Interim reanalysis dataset are analyzed to gain insight into the spatiotemporal distribution and synoptic background of winter-season CG flashes between December 2010 and February 2020 in China.We identify three Winter Lightning Frequent Areas(WLAs):the southwest side of the Yunnan-Guizhou Plateau(WLA1),the east side of the Yunnan-Guizhou Plateau(WLA2),and the Poyang Lake Plain(WLA3).The CG lightning flashes most frequently occur at local midnight and have a monthly peak in February.The CG lightning in WLA1 is mostly generated in non-frontal weather;however,the lightning in WLA2 and WLA3 mostly occurs in frontal systems.The frontal circulation situation is divided into four typical types:transversal trough after high pressure,low vortex,confrontational convergence,and asymptotic convergence.In all typical weather patterns,the lightning occurs downstream of a 500 hPa trough and is accompanied by a southwesterly low-level jet.The convective parameters of winter thunderstorms differ greatly from those of summer thunderstorms.The maximum convective available potential energy(MCAPE)and K-index(KI)are more useful metrics than convective available potential energy(CAPE)and Showalter index(SI)during winter.This study further deepens the understanding of the distribution characteristics of winter CG lightning in China,which motivates further research to improve the ability of winter thunderstorm prediction.展开更多
Forecasts of the intensity and quantitative precipitation of tropical cyclones（TCs） are generally inaccurate, because the strength and structure of a TC show a complicated spatiotemporal pattern and are affected by ...Forecasts of the intensity and quantitative precipitation of tropical cyclones（TCs） are generally inaccurate, because the strength and structure of a TC show a complicated spatiotemporal pattern and are affected by various factors. Among these, asymmetric convection plays an important role. This study investigates the asymmetric distribution of convection in TCs over the western North Pacific during the period 2005–2012, based on data obtained from the Feng Yun 2（FY2）geostationary satellite. The asymmetric distributions of the incidence, intensity and morphology of convections are analyzed.Results show that the PDFs of the convection occurrence curve to the azimuth are sinusoidal. The rear-left quadrant relative to TC motion shows the highest occurrence rate of convection, while the front-right quadrant has the lowest. In terms of intensity, weak convections are favored in the front-left of a TC at large distances, whereas strong convections are more likely to appear to the rear-right of a TC within a 300 km range. More than 70% of all MCSs examined here are elongated systems, and meso-β enlongated convective systems（MβECSs） are the most dominant type observed in the outer region of a TC. Smaller MCSs tend to be more concentrated near the center of a TC. While semi-circular MCSs [MβCCSs, MCCs（mesoscale convective complexes）] show a high incidence rate to the rear of a TC, elongated MCSs [MβECSs, PECSs（persistent elongated convective systems）] are more likely to appear in the rear-right quadrant of a TC within a range of 400 km.展开更多
In operational data assimilation systems, observation-error covariance matrices are commonly assumed to be diagonal.However, inter-channel and spatial observation-error correlations are inevitable for satellite radian...In operational data assimilation systems, observation-error covariance matrices are commonly assumed to be diagonal.However, inter-channel and spatial observation-error correlations are inevitable for satellite radiances. The observation errors of the Microwave Temperature Sounder(MWTS) and Microwave Humidity Sounder(MWHS) onboard the FengYun-3A(FY-3A) and FY-3B satellites are empirically assigned and considered to be uncorrelated when they are assimilated into the WRF model's Community Variational Data Assimilation System(WRFDA). To assimilate MWTS and MWHS measurements optimally, a good characterization of their observation errors is necessary. In this study, background and analysis residuals were used to diagnose the correlated observation-error characteristics of the MWTS and MWHS. It was found that the error standard deviations of the MWTS and MWHS were less than the values used in the WRFDA. MWTS had small inter-channel errors, while MWHS had significant inter-channel errors. The horizontal correlation length scales of MWTS and MWHS were about 120 and 60 km, respectively. A comparison between the diagnosis for instruments onboard the two satellites showed that the observation-error characteristics of the MWTS or MWHS were different when they were onboard different satellites. In addition, it was found that the error statistics were dependent on latitude and scan positions.The forecast experiments showed that using a modified thinning scheme based on diagnosed statistics can improve forecast accuracy.展开更多
Oceanic mesoscale circulations, primarily manifesting as eddies, typically span around 100 km horizontally and persist for about a month. These circulations represent the main component of oceanic kinetic energy and a...Oceanic mesoscale circulations, primarily manifesting as eddies, typically span around 100 km horizontally and persist for about a month. These circulations represent the main component of oceanic kinetic energy and are referred to as the“weather” of the ocean. Forecasting efforts, focused on periods from a day to a month, target these mesoscale circulations.Advances in eddy-resolving models, observing systems such as Argo floats and satellite altimeters, and data assimilation techniques have enabled systems like HYCOM in the United States and Mercator-Ocean in Europe to predict these features effectively. To further enhance mesoscale forecasting, the Pacific Regional Ocean Forecast System(RPOFS) was developed for the pan-western Pacific(100°E–178°E, 20°S–50°N). Covering the dynamic Kuroshio and Kuroshio Extension regions, RPOFS features a 3 km horizontal grid and 67 vertical levels, with a multi-scale three-dimensional variational data assimilation(MS-3DVAR) scheme implemented. This system assimilates data from various sources, including satellite sea surface height and temperature, as well as subsurface profiles. In particular, an enhanced altimeter data assimilation methodology allows explicitly constraining both barotropic and baroclinic components of sea surface height. RPOFS demonstrates reliable forecasting of mesoscale circulations, with five-day forecasts accurately predicting eddies larger than 150 km and capturing features of the Kuroshio large meander. Forecasting errors are comparable to or smaller than those of HYCOM and Mercator-Ocean systems.展开更多
This study first utilizes four well-performing pre-trained convolutional neural networks(CNNs) to gauge the intensity of tropical cyclones(TCs) using geostationary satellite infrared(IR) imagery.The models are trained...This study first utilizes four well-performing pre-trained convolutional neural networks(CNNs) to gauge the intensity of tropical cyclones(TCs) using geostationary satellite infrared(IR) imagery.The models are trained and tested on TC cases spanning from 2004 to 2022 over the western North Pacific Ocean.To enhance the models performance,various techniques are employed,including fine-tuning the original CNN models,introducing rotation augmentation to the initial dataset,temporal enhancement via sequential imagery,integrating auxiliary physical information,and adjusting hyperparameters.An optimized CNN model,i.e.,visual geometry group network(VGGNet),for TC intensity estimation is ultimately obtained.When applied to the test data,the model achieves a relatively low mean absolute error(MAE) of 4.05 m s~(-1).To improve the interpretability of the model,the SmoothGrad combined with the Integrated Gradients approach is employed.The analyses reveal that the VGGNet model places significant emphasis on the distinct inner core region of a TC when estimating its intensity.Additionally,it partly takes into account the configuration of cloud systems as input features for the model,aligning well with meteorological principles.The several improvements made to this model's performance offer valuable insights for enhancing TC intensity forecasts through deep learning.展开更多
Capture of a strong elevated ducting event,especially its maintenance and sudden change,is of great value to airborne radar to achieve its beyond-the-line-of-sight detection.However,the knowledge is not easily accessi...Capture of a strong elevated ducting event,especially its maintenance and sudden change,is of great value to airborne radar to achieve its beyond-the-line-of-sight detection.However,the knowledge is not easily accessible over the open ocean and hence very rare.During the Air–Sea Interaction Survey(ASIS)over the western North Pacific(WNP)in May 2016,a strong elevated ducting event with a long-life period and sudden change in its evolution was observed.Measurements from the ASIS,images from the Himawari-8 satellite,reanalysis data from the ECMWF,and Weather Research and Forecasting(WRF)model,were used to analyze the maintenance and sudden change of this strong ducting event,together with the model performance on simulating it.The results showed that the maintenance of strong elevated ducts,with their tops ranging from 750 to 1050 m and average strength of approximately 38 M units,was caused by a strong dry air mass capping over the wet marine atmospheric boundary layer(MABL),together with the subsidence inversion associated with high pressure.The WRF model performs well in simulating them.However,a sudden increase in duct height with a slight decrease of strength was recorded by the subsequent GPS radiosonde,which was finally contributed to the mechanical turbulent inversion and hydrolapse associated with the marine low-level jet(MLLJ).The height of the maximum horizontal wind speed(Umh)of the MLLJ corresponds well with the bottom of the trapping layer.However,these jet-relevant ducts are generally weak and it is difficult to accurately simulate them by using the mesoscale numerical model,since the wind-shear produced eddies are too small to be properly parameterized.展开更多
As a typhoon approaches the continent,the position where anthropogenic aerosols penetrate,the convection competition between the eyewall and peripheral rainbands,and the separate contributions of direct aerosol-radiat...As a typhoon approaches the continent,the position where anthropogenic aerosols penetrate,the convection competition between the eyewall and peripheral rainbands,and the separate contributions of direct aerosol-radiation interactions(ARI)and indirect aerosol-cloud interactions(ACI),yield uncertainties in the convection intensification area and hence the typhoon intensity.Typhoon Lupit(2009)was simulated using the Weather Research and Forecasting Model with Chemistry(WRF-Chem)to investigate and isolate the direct and indirect effects of aerosols on the intensity,convection,and precipitation of the typhoon.Three simulations(CTL,CLEAN,and CTLARIOFF)were designed,representing a polluted case(CTL,considering the ingestion of anthropogenic aerosols with ARI and ACI),a clean maritime case(CLEAN,mainly with sea salt aerosols),and a polluted case without aerosol radiative forcing(CTLARIOFF,as per CTL but without ARI).The results showed that anthropogenic aerosols could penetrate into both the peripheral rainbands and the eyewall when the typhoon was approaching the Asian continent.Owing to the representation of the real aerosol scenario,the simulated typhoon intensity weakened and was closer to observed values in the CTL experiment.The ARI dominated over ACI with the opposite effects.Specifically,the ACI mainly enhanced the formation of ice-phase hydrometeors within the upper level of the eyewall with more freezing latent heat releases,leading to an invigoration of eyewall convection.These excess ice-phase particles melted after they descended into the warm layer below the 0°C level,which accelerated the accretion of cloud droplets by raindrops(Pcacr)and hence the mixed phase precipitation process in the eyewall.The dynamic feedback induced by the ACI enhanced the boundary layer inflow and the upper layer outflow,supporting the maintenance of strong eyewall convection and intensification of the typhoon.Inversely,the ARI heated the distant periphery low-level atmosphere at an altitude of 1-2 km by the absorbing polluted aerosols.The heated air,driven by the radial inflow,firstly went through the periphery rainbands of the typhoon and invigorated convection there due to the low-level warming.Then,the enhanced periphery convection inhibited the further transport of warm moist air into the eyewall,resulting in weakening of the eyewall convection and hence typhoon intensity.In sum,for the polluted scenario,as the typhoon approached the continent,ARI played a dominant role over ACI.The WRF-Chem model with full consideration of aerosol-cloud-radiation interactions is advantageous in terms of reliably simulating typhoon intensity and precipitation distribution.展开更多
At kilometer and sub-kilometer resolutions,known as the numerical gray zone for boundary layer turbulence,the atmospheric boundary layer turbulence becomes partially resolved and partially subgrid-scale(SGS) in a nume...At kilometer and sub-kilometer resolutions,known as the numerical gray zone for boundary layer turbulence,the atmospheric boundary layer turbulence becomes partially resolved and partially subgrid-scale(SGS) in a numerical model,thus requiring scale-adaptive turbulence schemes.Such schemes are often built by modifying the existing parameterizations,either the planetary boundary layer(PBL) schemes or the large-eddy simulation(LES) closures,to produce the right SGS turbulent fluxes at gray zone resolutions.However,the underlying forcings responsible for the changes in the vertical turbulent fluxes are largely ignored in these approaches.This study follows the original approach of Wyngaard(2004) and analyzes the turbulent buoyancy and momentum flux budgets,to gain a better understanding of the variations of flux forcings at gray zone resolutions.The investigation focuses on the pressure covariance term,which is one of the most dominant terms in the budget equations.By using the coarse-grained LES of a dry convective boundary layer(CBL) case as reference,two widely-used pressure covariance models are evaluated and optimized across the gray zone resolution range.The optimized linear model is further evaluated a priori against another dry CBL case with a different bulk stability,and a shallow-cumulus-topped boundary layer case.The model applies well to both cases,and notably shows good performance for the cloud layer.Based on the analysis of the flux forcings and the optimized pressure model,a scale-adaptive turbulence model for the gray zone is derived from the steady-state flux budgets.展开更多
Due to the coarse temporal resolution of the best track databases for typhoons,some small-scale characteristics of typhoon motion have been neglected.In order to reveal the fine features of typhoon motion,the dense cl...Due to the coarse temporal resolution of the best track databases for typhoons,some small-scale characteristics of typhoon motion have been neglected.In order to reveal the fine features of typhoon motion,the dense cloud within the radius of maximum wind(RMW)of mature typhoons,defined as a“cloud gyro”that simultaneously spins around its own rotational axis and precesses around the vertical axis,is approximated to a rigid body.Based on the principles of gyrodynamics,the derived mathematical model of a typhoon track indicates that typhoon movement is composed of translational motion governed by the steering flow and superimposed precessional motion that manifests as the inherent characteristics of typhoon tracks,especially with the external torque.High temporal resolution numerical simulation of a real case verifies the conclusion of this study,which is that the smaller the RMW,the larger the tangential wind speed or the larger the external torque,and the more obviously precessional motion manifests.展开更多
基金supported by the National Natural Science Foundation of China(Grant Nos.42330610 and 42075010)。
文摘The Sichuan Basin(SCB),China has a high incidence of extremely persistent heavy rainfall(EPHR)events.The EPHR events from 2009 to 2019 in the SCB were mainly concentrated over the northern and northwestern windward slopes and the central basin.They occurred from June to September,but especially in July,and peaked at 0300 LST.ERA5 reanalysis data and objective classification were used to investigate the synoptic patterns and their effects.There were three synoptic patterns during EPHR events,all accompanied by a Southwest Vortex.The location and intensity of the Southwest Vortex,thermal forcing of the Tibetan Plateau(TP),and low-level winds can greatly affect the intensity and spatial distribution of EPHR.When the Southwest Vortex was located in the western SCB and there were southerly low-level jets(LLJs),convergence and upslope wind would lead to EPHR over the northwestern or northern windward slopes.If there was no LLJ and the whole SCB was under the center of the Southwest Vortex,nocturnal EPHR was controlled by the internal circulation of the Southwest Vortex and the updraft generated by the thermal forcing of the TP,and the rainfall was weaker.The southeastern entrance of the SCB was a key area where the low-level wind dominated the nocturnal peak of EPHR.The nocturnal strengthened southeasterly wind in the key area is attributable to inertial oscillation,and the topographic friction plays an essential role in transporting momentum and moisture into the basin by generating easterly and northeasterly ageostrophic winds.
基金supported by the National Natural Science Foundation of China(Grant No.42075010)the National Key R&D Program of China(Grant No.2018YFC1507304,2018YFC1507402)。
文摘Cloud-to-ground(CG)lightning data and the ECMWF ERA-Interim reanalysis dataset are analyzed to gain insight into the spatiotemporal distribution and synoptic background of winter-season CG flashes between December 2010 and February 2020 in China.We identify three Winter Lightning Frequent Areas(WLAs):the southwest side of the Yunnan-Guizhou Plateau(WLA1),the east side of the Yunnan-Guizhou Plateau(WLA2),and the Poyang Lake Plain(WLA3).The CG lightning flashes most frequently occur at local midnight and have a monthly peak in February.The CG lightning in WLA1 is mostly generated in non-frontal weather;however,the lightning in WLA2 and WLA3 mostly occurs in frontal systems.The frontal circulation situation is divided into four typical types:transversal trough after high pressure,low vortex,confrontational convergence,and asymptotic convergence.In all typical weather patterns,the lightning occurs downstream of a 500 hPa trough and is accompanied by a southwesterly low-level jet.The convective parameters of winter thunderstorms differ greatly from those of summer thunderstorms.The maximum convective available potential energy(MCAPE)and K-index(KI)are more useful metrics than convective available potential energy(CAPE)and Showalter index(SI)during winter.This study further deepens the understanding of the distribution characteristics of winter CG lightning in China,which motivates further research to improve the ability of winter thunderstorm prediction.
基金supported by the National Natural Science Foundation of China (Grant No. 41230421)the "973" project (Grant Nos. 2015CB452802 and 2013CB430101) of the Ministry of Science and Technology of China
文摘Forecasts of the intensity and quantitative precipitation of tropical cyclones（TCs） are generally inaccurate, because the strength and structure of a TC show a complicated spatiotemporal pattern and are affected by various factors. Among these, asymmetric convection plays an important role. This study investigates the asymmetric distribution of convection in TCs over the western North Pacific during the period 2005–2012, based on data obtained from the Feng Yun 2（FY2）geostationary satellite. The asymmetric distributions of the incidence, intensity and morphology of convections are analyzed.Results show that the PDFs of the convection occurrence curve to the azimuth are sinusoidal. The rear-left quadrant relative to TC motion shows the highest occurrence rate of convection, while the front-right quadrant has the lowest. In terms of intensity, weak convections are favored in the front-left of a TC at large distances, whereas strong convections are more likely to appear to the rear-right of a TC within a 300 km range. More than 70% of all MCSs examined here are elongated systems, and meso-β enlongated convective systems（MβECSs） are the most dominant type observed in the outer region of a TC. Smaller MCSs tend to be more concentrated near the center of a TC. While semi-circular MCSs [MβCCSs, MCCs（mesoscale convective complexes）] show a high incidence rate to the rear of a TC, elongated MCSs [MβECSs, PECSs（persistent elongated convective systems）] are more likely to appear in the rear-right quadrant of a TC within a range of 400 km.
基金funded by the National Basic Research (973) Program of China (Grant No. 2015CB452802)the National Natural Science Foundation of China (Grant Nos. 41230421, 41605075, and 41675058)
文摘In operational data assimilation systems, observation-error covariance matrices are commonly assumed to be diagonal.However, inter-channel and spatial observation-error correlations are inevitable for satellite radiances. The observation errors of the Microwave Temperature Sounder(MWTS) and Microwave Humidity Sounder(MWHS) onboard the FengYun-3A(FY-3A) and FY-3B satellites are empirically assigned and considered to be uncorrelated when they are assimilated into the WRF model's Community Variational Data Assimilation System(WRFDA). To assimilate MWTS and MWHS measurements optimally, a good characterization of their observation errors is necessary. In this study, background and analysis residuals were used to diagnose the correlated observation-error characteristics of the MWTS and MWHS. It was found that the error standard deviations of the MWTS and MWHS were less than the values used in the WRFDA. MWTS had small inter-channel errors, while MWHS had significant inter-channel errors. The horizontal correlation length scales of MWTS and MWHS were about 120 and 60 km, respectively. A comparison between the diagnosis for instruments onboard the two satellites showed that the observation-error characteristics of the MWTS or MWHS were different when they were onboard different satellites. In addition, it was found that the error statistics were dependent on latitude and scan positions.The forecast experiments showed that using a modified thinning scheme based on diagnosed statistics can improve forecast accuracy.
基金supported by the National Key R&D Program of China (Grant No. 2022YFF0801404)the National Natural Science Foundation of China (Grant No. 42192552)。
文摘Oceanic mesoscale circulations, primarily manifesting as eddies, typically span around 100 km horizontally and persist for about a month. These circulations represent the main component of oceanic kinetic energy and are referred to as the“weather” of the ocean. Forecasting efforts, focused on periods from a day to a month, target these mesoscale circulations.Advances in eddy-resolving models, observing systems such as Argo floats and satellite altimeters, and data assimilation techniques have enabled systems like HYCOM in the United States and Mercator-Ocean in Europe to predict these features effectively. To further enhance mesoscale forecasting, the Pacific Regional Ocean Forecast System(RPOFS) was developed for the pan-western Pacific(100°E–178°E, 20°S–50°N). Covering the dynamic Kuroshio and Kuroshio Extension regions, RPOFS features a 3 km horizontal grid and 67 vertical levels, with a multi-scale three-dimensional variational data assimilation(MS-3DVAR) scheme implemented. This system assimilates data from various sources, including satellite sea surface height and temperature, as well as subsurface profiles. In particular, an enhanced altimeter data assimilation methodology allows explicitly constraining both barotropic and baroclinic components of sea surface height. RPOFS demonstrates reliable forecasting of mesoscale circulations, with five-day forecasts accurately predicting eddies larger than 150 km and capturing features of the Kuroshio large meander. Forecasting errors are comparable to or smaller than those of HYCOM and Mercator-Ocean systems.
基金Supported by the National Natural Science Foundation of China (42192552)。
文摘This study first utilizes four well-performing pre-trained convolutional neural networks(CNNs) to gauge the intensity of tropical cyclones(TCs) using geostationary satellite infrared(IR) imagery.The models are trained and tested on TC cases spanning from 2004 to 2022 over the western North Pacific Ocean.To enhance the models performance,various techniques are employed,including fine-tuning the original CNN models,introducing rotation augmentation to the initial dataset,temporal enhancement via sequential imagery,integrating auxiliary physical information,and adjusting hyperparameters.An optimized CNN model,i.e.,visual geometry group network(VGGNet),for TC intensity estimation is ultimately obtained.When applied to the test data,the model achieves a relatively low mean absolute error(MAE) of 4.05 m s~(-1).To improve the interpretability of the model,the SmoothGrad combined with the Integrated Gradients approach is employed.The analyses reveal that the VGGNet model places significant emphasis on the distinct inner core region of a TC when estimating its intensity.Additionally,it partly takes into account the configuration of cloud systems as input features for the model,aligning well with meteorological principles.The several improvements made to this model's performance offer valuable insights for enhancing TC intensity forecasts through deep learning.
基金Supported by the National Natural Science Foundation of China(41775017 and 41675058)。
文摘Capture of a strong elevated ducting event,especially its maintenance and sudden change,is of great value to airborne radar to achieve its beyond-the-line-of-sight detection.However,the knowledge is not easily accessible over the open ocean and hence very rare.During the Air–Sea Interaction Survey(ASIS)over the western North Pacific(WNP)in May 2016,a strong elevated ducting event with a long-life period and sudden change in its evolution was observed.Measurements from the ASIS,images from the Himawari-8 satellite,reanalysis data from the ECMWF,and Weather Research and Forecasting(WRF)model,were used to analyze the maintenance and sudden change of this strong ducting event,together with the model performance on simulating it.The results showed that the maintenance of strong elevated ducts,with their tops ranging from 750 to 1050 m and average strength of approximately 38 M units,was caused by a strong dry air mass capping over the wet marine atmospheric boundary layer(MABL),together with the subsidence inversion associated with high pressure.The WRF model performs well in simulating them.However,a sudden increase in duct height with a slight decrease of strength was recorded by the subsequent GPS radiosonde,which was finally contributed to the mechanical turbulent inversion and hydrolapse associated with the marine low-level jet(MLLJ).The height of the maximum horizontal wind speed(Umh)of the MLLJ corresponds well with the bottom of the trapping layer.However,these jet-relevant ducts are generally weak and it is difficult to accurately simulate them by using the mesoscale numerical model,since the wind-shear produced eddies are too small to be properly parameterized.
基金This work was supported by the National Natural Science Foundation of China(Grant Nos.41775017&41675058).
文摘As a typhoon approaches the continent,the position where anthropogenic aerosols penetrate,the convection competition between the eyewall and peripheral rainbands,and the separate contributions of direct aerosol-radiation interactions(ARI)and indirect aerosol-cloud interactions(ACI),yield uncertainties in the convection intensification area and hence the typhoon intensity.Typhoon Lupit(2009)was simulated using the Weather Research and Forecasting Model with Chemistry(WRF-Chem)to investigate and isolate the direct and indirect effects of aerosols on the intensity,convection,and precipitation of the typhoon.Three simulations(CTL,CLEAN,and CTLARIOFF)were designed,representing a polluted case(CTL,considering the ingestion of anthropogenic aerosols with ARI and ACI),a clean maritime case(CLEAN,mainly with sea salt aerosols),and a polluted case without aerosol radiative forcing(CTLARIOFF,as per CTL but without ARI).The results showed that anthropogenic aerosols could penetrate into both the peripheral rainbands and the eyewall when the typhoon was approaching the Asian continent.Owing to the representation of the real aerosol scenario,the simulated typhoon intensity weakened and was closer to observed values in the CTL experiment.The ARI dominated over ACI with the opposite effects.Specifically,the ACI mainly enhanced the formation of ice-phase hydrometeors within the upper level of the eyewall with more freezing latent heat releases,leading to an invigoration of eyewall convection.These excess ice-phase particles melted after they descended into the warm layer below the 0°C level,which accelerated the accretion of cloud droplets by raindrops(Pcacr)and hence the mixed phase precipitation process in the eyewall.The dynamic feedback induced by the ACI enhanced the boundary layer inflow and the upper layer outflow,supporting the maintenance of strong eyewall convection and intensification of the typhoon.Inversely,the ARI heated the distant periphery low-level atmosphere at an altitude of 1-2 km by the absorbing polluted aerosols.The heated air,driven by the radial inflow,firstly went through the periphery rainbands of the typhoon and invigorated convection there due to the low-level warming.Then,the enhanced periphery convection inhibited the further transport of warm moist air into the eyewall,resulting in weakening of the eyewall convection and hence typhoon intensity.In sum,for the polluted scenario,as the typhoon approached the continent,ARI played a dominant role over ACI.The WRF-Chem model with full consideration of aerosol-cloud-radiation interactions is advantageous in terms of reliably simulating typhoon intensity and precipitation distribution.
基金Supported by the Joint Funds of the National Natural Science Foundation of China (U2142209)Major Program of the National Natural Science Foundation of China (42192552)。
文摘At kilometer and sub-kilometer resolutions,known as the numerical gray zone for boundary layer turbulence,the atmospheric boundary layer turbulence becomes partially resolved and partially subgrid-scale(SGS) in a numerical model,thus requiring scale-adaptive turbulence schemes.Such schemes are often built by modifying the existing parameterizations,either the planetary boundary layer(PBL) schemes or the large-eddy simulation(LES) closures,to produce the right SGS turbulent fluxes at gray zone resolutions.However,the underlying forcings responsible for the changes in the vertical turbulent fluxes are largely ignored in these approaches.This study follows the original approach of Wyngaard(2004) and analyzes the turbulent buoyancy and momentum flux budgets,to gain a better understanding of the variations of flux forcings at gray zone resolutions.The investigation focuses on the pressure covariance term,which is one of the most dominant terms in the budget equations.By using the coarse-grained LES of a dry convective boundary layer(CBL) case as reference,two widely-used pressure covariance models are evaluated and optimized across the gray zone resolution range.The optimized linear model is further evaluated a priori against another dry CBL case with a different bulk stability,and a shallow-cumulus-topped boundary layer case.The model applies well to both cases,and notably shows good performance for the cloud layer.Based on the analysis of the flux forcings and the optimized pressure model,a scale-adaptive turbulence model for the gray zone is derived from the steady-state flux budgets.
基金supported by the National Natural Science Foundation of China(Grant Nos.41675058&41775055).
文摘Due to the coarse temporal resolution of the best track databases for typhoons,some small-scale characteristics of typhoon motion have been neglected.In order to reveal the fine features of typhoon motion,the dense cloud within the radius of maximum wind(RMW)of mature typhoons,defined as a“cloud gyro”that simultaneously spins around its own rotational axis and precesses around the vertical axis,is approximated to a rigid body.Based on the principles of gyrodynamics,the derived mathematical model of a typhoon track indicates that typhoon movement is composed of translational motion governed by the steering flow and superimposed precessional motion that manifests as the inherent characteristics of typhoon tracks,especially with the external torque.High temporal resolution numerical simulation of a real case verifies the conclusion of this study,which is that the smaller the RMW,the larger the tangential wind speed or the larger the external torque,and the more obviously precessional motion manifests.
