A series of mesoscale convective systems (MCSs) occurred daily over the Qinghai-Xizang Plateau during 25–28 July 1995. In this paper, their physical characteristics and evolutions based on infrared satellite imagery,...A series of mesoscale convective systems (MCSs) occurred daily over the Qinghai-Xizang Plateau during 25–28 July 1995. In this paper, their physical characteristics and evolutions based on infrared satellite imagery, their largescale meteorological conditions, and convective available potential energy (CAPE) are analyzed. It is found that similar diurnal evolution is present in all these MCSs. Their initial convective activities became active at noon LST by solar heating, and then built up rapidly. They formed and reached a peak in the early evening hours around 1800 LST and then abated gradually. Among them, the strongest and largest is the MCS on 26 July, which developed under the conditions of the great upper-level nearly-circular Qinghai-Xizang anticyclonic high and driven by the strong low-level thermal forcing and conditional instability. All these conditions are intimately linked with the thermal effects of the plateau itself. So its development was mainly associated with the relatively pure thermal effects peculiar to the Qinghai-Xizang Plateau. The next strongest one is the MCS on 28 July, which was affected notably by the baroclinic zone linked with the westerly trough. There are different features and development mechanisms between these two strongest MCSs.展开更多
A mesoscale convective system (MCS) developing over the Qinghai-Xizang Plateau on 26 July 1995 is simulated using the fifth version of the Penn State-NCAR nonhydrostatic mesoscale model (MM5). The results obtained are...A mesoscale convective system (MCS) developing over the Qinghai-Xizang Plateau on 26 July 1995 is simulated using the fifth version of the Penn State-NCAR nonhydrostatic mesoscale model (MM5). The results obtained are inspiring and are as follows. (1) The model simulates well the largescale conditions in which the MCS concerned is embedded, which are the well-known anticyclonic Qinghai-Xizang Plateau High in the upper layers and the strong thermal forcing in the lower layers. In particular, the model captures the meso-α scale cyclonic vortex associated with the MCS, which can be analyzed in the 500 hPa observational winds; and to some degree, the model reproduces even its meso-β scale substructure similar to satellite images, reflected in the model-simulated 400 hPa rainwater. On the other hand, there are some distinct deficiencies in the simulation; for example, the simulated MCS occurs with a lag of 3 hours and a westward deviation of 3–5° longitude. (2) The structure and evolution of the meso-α scale vortex associated with the MCS are undescribable for upper-air sounding data. The vortex is confined to the lower troposphere under 450 hPa over the plateau and shrinks its extent with height, with a diameter of 4° longitude at 500 hPa. It is within the updraft area, but with an upper-level anticyclone and downdraft over it. The vortex originates over the plateau, and does not form until the mature stage of the MCS. It lasts for 3–6 hours. In its processes of both formation and decay, the change in geopotential height field is prior to that in the wind field. It follows that the vortex is closely associated with the thermal effects over the plateau. (3) A series of sensitivity experiments are conducted to investigate the impact of various surface thermal forcings and other physical processes on the MCS over the plateau. The results indicate that under the background conditions of the upper-level Qinghai-Xizang High, the MCS involved is mainly dominated by the low-level thermal forcing. The simulation described here is a good indication that it may be possible to reproduce the MCS over the plateau under certain large-scale conditions and with the incorporation of proper thermal physics in the lower layers.展开更多
The approach to remote sensing of water vapor by using global positioning systems(GPS)is discussed.In order to retrieve the vertical integrated water vapor(IWV)or the precipitable water (PW),the weighted“mean tempera...The approach to remote sensing of water vapor by using global positioning systems(GPS)is discussed.In order to retrieve the vertical integrated water vapor(IWV)or the precipitable water (PW),the weighted“mean temperature”of the atmosphere,T_m would be estimated to the specific area and season.T_m depends on surface temperature,tropospheric temperature profile,and the vertical distribution of water vapor.The surface temperature dependence is borne out by a comparison of T_m and the values of surface temperature T_s using radiosonde profiles of Beijing Station(No.54511)throughout 1992.The analysis of radiosonde profiles spanning a one-year interval(1992)from sites in eastern region of China with a latitude range of 20-50°N and a longitude range of 100-130°E yields the coefficients α and b of a linear regression equation T_m= α+bT_s.展开更多
A numerical model has been developed for simulating land-surface processes and atmospheric boundary layer climate of vegetation and desert in semi-arid region.Dynamically,thermal and hydrological processes take place ...A numerical model has been developed for simulating land-surface processes and atmospheric boundary layer climate of vegetation and desert in semi-arid region.Dynamically,thermal and hydrological processes take place in the atmospheric boundary layer.Vegetation and surface layer of soil are included in the soil-vegetation-atmosphere coupled system,in which,vegetation is considered as a horizontally uniform layer,soil is divided into 13 layers and the horizontal differences of variables in the system are neglected.The influence of local boundary layer climate by vegetation cover factor is simulated with the coupled model in the semi-arid region of Northwest China (around 38°N,105°E).Results indicate that due to significant differences of water and energy budgets in vegetation and desert region,the air is colder and wetter over the vegetation and correspondingly an obvious local circulation in the lower atmosphere is formed. Simulating results also show that maximum updraft and downdraft occur around the vegetation-desert marginal area,where the dynamical and thermodynamical properties of PBL (Planetary Boundary Layer) are uncontinuous.It is stronger at daytime,weaker and reverse at nighttime.In the simulation,the moisture inversion phenomena are analyzed.Finally.the influences of vegetation cover factor exchange on local boundary layer climate are simulated.The simulating results bring to light that water may be conserved and improved by developing tree planting and afforestation,and improving cover factor of vegetation in local ecoenvironment,and this is an important way of transforming local climate in arid and semi-arid area.Results indicate that the coupled model can be used to study the soil-vegetation-atmosphere interaction and local boundary layer climate.展开更多
In this paper,two schemes proposed by Julian(1984)and Krishnamauti(1986)are used to retrieve the tropical divergent wind at 200 hPa and 850 hPa from the satellite observational Outgoing Long-wave Radiation(OLR).The co...In this paper,two schemes proposed by Julian(1984)and Krishnamauti(1986)are used to retrieve the tropical divergent wind at 200 hPa and 850 hPa from the satellite observational Outgoing Long-wave Radiation(OLR).The comparison study has been conducted among the OLR-derived divergent wind field and those directly from wind fields of ECMWF and CAC tropical analysis,and NCEP/NCAR reanalysis for examining their reliability.Then,the divergent wind retrieved from OLR by using Julian's scheme is used to composite the diagrams of the Walker circulation and the local Hadley circulation during 1982—1983 ENSO event.The possible linkage between the anomalies of summer rainfall in East China during this period and the anomalous Walker and local Hadley circulations is discussed. It is shown that it is practically feasible to use the satellite observed OLR data in the estimation of the tropical divergent wind.It is aiso indicated that NCEP/NCAR reanalysis has made a progress for improving the reliability of the tropical divergent wind,though some biases still exist in the description of the intensity and position of the divergence(convergence)maximum centers over Asian monsoon region.The application of Julian's method to a diagnosis on the evolutions of the anomalous Walker and Hadley circulations during 1982—1983 ENSO event shows that the development of this ENSO event is not companied with the sudden reversal of the Walker circulation,but the propagation of the ascending branch over the western Pacific to the central- eastern Pacific and crossing through the date line,which results in a significant displacement of the vertical circulation over the West Pacific(WP)and the central-east Pacific(CEP).It is also indicated that there exists a close linkage between the change of local Hadley circulation in the WP and the Walker circulation in the CEP,implying that the Walker circulation possibly severs as a bridge between the anomalies of the SST in the CEP and the change of local Hadley circulation in Northwest Pacific.The latter is responsible for the climate anomaly over eastern China during this period.展开更多
基金the Chinese National Climbing Project"The Tibetan Plateau Meteorological Experiment"and in part by the Naltional Natural Science Foundation of China under Grant No.49675296.
文摘A series of mesoscale convective systems (MCSs) occurred daily over the Qinghai-Xizang Plateau during 25–28 July 1995. In this paper, their physical characteristics and evolutions based on infrared satellite imagery, their largescale meteorological conditions, and convective available potential energy (CAPE) are analyzed. It is found that similar diurnal evolution is present in all these MCSs. Their initial convective activities became active at noon LST by solar heating, and then built up rapidly. They formed and reached a peak in the early evening hours around 1800 LST and then abated gradually. Among them, the strongest and largest is the MCS on 26 July, which developed under the conditions of the great upper-level nearly-circular Qinghai-Xizang anticyclonic high and driven by the strong low-level thermal forcing and conditional instability. All these conditions are intimately linked with the thermal effects of the plateau itself. So its development was mainly associated with the relatively pure thermal effects peculiar to the Qinghai-Xizang Plateau. The next strongest one is the MCS on 28 July, which was affected notably by the baroclinic zone linked with the westerly trough. There are different features and development mechanisms between these two strongest MCSs.
基金the Chinese National Climbing Project"The Tibetan PlateauMeteorological Experiment"and in part by the NationalNatural Science Foundation of China under Grant No.49675296.
文摘A mesoscale convective system (MCS) developing over the Qinghai-Xizang Plateau on 26 July 1995 is simulated using the fifth version of the Penn State-NCAR nonhydrostatic mesoscale model (MM5). The results obtained are inspiring and are as follows. (1) The model simulates well the largescale conditions in which the MCS concerned is embedded, which are the well-known anticyclonic Qinghai-Xizang Plateau High in the upper layers and the strong thermal forcing in the lower layers. In particular, the model captures the meso-α scale cyclonic vortex associated with the MCS, which can be analyzed in the 500 hPa observational winds; and to some degree, the model reproduces even its meso-β scale substructure similar to satellite images, reflected in the model-simulated 400 hPa rainwater. On the other hand, there are some distinct deficiencies in the simulation; for example, the simulated MCS occurs with a lag of 3 hours and a westward deviation of 3–5° longitude. (2) The structure and evolution of the meso-α scale vortex associated with the MCS are undescribable for upper-air sounding data. The vortex is confined to the lower troposphere under 450 hPa over the plateau and shrinks its extent with height, with a diameter of 4° longitude at 500 hPa. It is within the updraft area, but with an upper-level anticyclone and downdraft over it. The vortex originates over the plateau, and does not form until the mature stage of the MCS. It lasts for 3–6 hours. In its processes of both formation and decay, the change in geopotential height field is prior to that in the wind field. It follows that the vortex is closely associated with the thermal effects over the plateau. (3) A series of sensitivity experiments are conducted to investigate the impact of various surface thermal forcings and other physical processes on the MCS over the plateau. The results indicate that under the background conditions of the upper-level Qinghai-Xizang High, the MCS involved is mainly dominated by the low-level thermal forcing. The simulation described here is a good indication that it may be possible to reproduce the MCS over the plateau under certain large-scale conditions and with the incorporation of proper thermal physics in the lower layers.
文摘The approach to remote sensing of water vapor by using global positioning systems(GPS)is discussed.In order to retrieve the vertical integrated water vapor(IWV)or the precipitable water (PW),the weighted“mean temperature”of the atmosphere,T_m would be estimated to the specific area and season.T_m depends on surface temperature,tropospheric temperature profile,and the vertical distribution of water vapor.The surface temperature dependence is borne out by a comparison of T_m and the values of surface temperature T_s using radiosonde profiles of Beijing Station(No.54511)throughout 1992.The analysis of radiosonde profiles spanning a one-year interval(1992)from sites in eastern region of China with a latitude range of 20-50°N and a longitude range of 100-130°E yields the coefficients α and b of a linear regression equation T_m= α+bT_s.
文摘A numerical model has been developed for simulating land-surface processes and atmospheric boundary layer climate of vegetation and desert in semi-arid region.Dynamically,thermal and hydrological processes take place in the atmospheric boundary layer.Vegetation and surface layer of soil are included in the soil-vegetation-atmosphere coupled system,in which,vegetation is considered as a horizontally uniform layer,soil is divided into 13 layers and the horizontal differences of variables in the system are neglected.The influence of local boundary layer climate by vegetation cover factor is simulated with the coupled model in the semi-arid region of Northwest China (around 38°N,105°E).Results indicate that due to significant differences of water and energy budgets in vegetation and desert region,the air is colder and wetter over the vegetation and correspondingly an obvious local circulation in the lower atmosphere is formed. Simulating results also show that maximum updraft and downdraft occur around the vegetation-desert marginal area,where the dynamical and thermodynamical properties of PBL (Planetary Boundary Layer) are uncontinuous.It is stronger at daytime,weaker and reverse at nighttime.In the simulation,the moisture inversion phenomena are analyzed.Finally.the influences of vegetation cover factor exchange on local boundary layer climate are simulated.The simulating results bring to light that water may be conserved and improved by developing tree planting and afforestation,and improving cover factor of vegetation in local ecoenvironment,and this is an important way of transforming local climate in arid and semi-arid area.Results indicate that the coupled model can be used to study the soil-vegetation-atmosphere interaction and local boundary layer climate.
基金This research is supported by the Chinese Ninth Five-Year National Project"Short-Range Climate Prediction System" the National Natural Science Foundation of China project for the young scientists,Grant No.49705061.
文摘In this paper,two schemes proposed by Julian(1984)and Krishnamauti(1986)are used to retrieve the tropical divergent wind at 200 hPa and 850 hPa from the satellite observational Outgoing Long-wave Radiation(OLR).The comparison study has been conducted among the OLR-derived divergent wind field and those directly from wind fields of ECMWF and CAC tropical analysis,and NCEP/NCAR reanalysis for examining their reliability.Then,the divergent wind retrieved from OLR by using Julian's scheme is used to composite the diagrams of the Walker circulation and the local Hadley circulation during 1982—1983 ENSO event.The possible linkage between the anomalies of summer rainfall in East China during this period and the anomalous Walker and local Hadley circulations is discussed. It is shown that it is practically feasible to use the satellite observed OLR data in the estimation of the tropical divergent wind.It is aiso indicated that NCEP/NCAR reanalysis has made a progress for improving the reliability of the tropical divergent wind,though some biases still exist in the description of the intensity and position of the divergence(convergence)maximum centers over Asian monsoon region.The application of Julian's method to a diagnosis on the evolutions of the anomalous Walker and Hadley circulations during 1982—1983 ENSO event shows that the development of this ENSO event is not companied with the sudden reversal of the Walker circulation,but the propagation of the ascending branch over the western Pacific to the central- eastern Pacific and crossing through the date line,which results in a significant displacement of the vertical circulation over the West Pacific(WP)and the central-east Pacific(CEP).It is also indicated that there exists a close linkage between the change of local Hadley circulation in the WP and the Walker circulation in the CEP,implying that the Walker circulation possibly severs as a bridge between the anomalies of the SST in the CEP and the change of local Hadley circulation in Northwest Pacific.The latter is responsible for the climate anomaly over eastern China during this period.
