2011 Vol. 17, No. 3
2011, 17(3): 193-201.
Abstract:
As one of the most severe typhoons in the year 2005, Typhoon Longwang is chosen as a case study in this article. Throughout its life, two surveillance flights are carried out on it. Different from previous studies, GPS (global positioning system) Dropwinsonde data collected from the Dropwinsonde Observations for Typhoon Surveillance near the Taiwan Region is chosen to present the thermodynamic and kinetic structure at its two different stages of development. This study suggests that not only kinetic structure but also thermodynamic structure of Longwang are more robust in the second surveillance than the first surveillance, with stronger and larger circulation and a warmer core. Further research shows that the environmental vertical wind shear mainly contributes to the asymmetric structure of the typhoon. The strong vertical wind shear not only results in the distinct asymmetric structure, but also restrains the development of the typhoo.
As one of the most severe typhoons in the year 2005, Typhoon Longwang is chosen as a case study in this article. Throughout its life, two surveillance flights are carried out on it. Different from previous studies, GPS (global positioning system) Dropwinsonde data collected from the Dropwinsonde Observations for Typhoon Surveillance near the Taiwan Region is chosen to present the thermodynamic and kinetic structure at its two different stages of development. This study suggests that not only kinetic structure but also thermodynamic structure of Longwang are more robust in the second surveillance than the first surveillance, with stronger and larger circulation and a warmer core. Further research shows that the environmental vertical wind shear mainly contributes to the asymmetric structure of the typhoon. The strong vertical wind shear not only results in the distinct asymmetric structure, but also restrains the development of the typhoo.
2011, 17(3): 202-208.
Abstract:
Since the South China Sea (SCS) summer monsoon (SCSSM) is pronouncedly featured by abruptly intensified southwesterly and obviously increased precipitation over the SCS, the lower-tropospheric winds and/or convection intensities are widely used to determine the SCSSM onset. The methods can be used successfully in most of the years but not in 2006. Due to the intrusion of Typhoon Chanchu (0601) that year, the usual method of determining SCSSM onset date by utilizing the SCS regional indices is less capable of pinpointing the real onset date. In order to solve the problem, larger-scale situations have to be taken into account. Zonal and meridional circulations would be better to determine the break-out date of SCSSM in 2006. The result indicates that its onset date is May 16. Moreover, similar onset dates for other years can be obtained using various methods, implying that large-scale zonal and meridional circulations can be used as an alternative method for determining the SCSSM onset date.
Since the South China Sea (SCS) summer monsoon (SCSSM) is pronouncedly featured by abruptly intensified southwesterly and obviously increased precipitation over the SCS, the lower-tropospheric winds and/or convection intensities are widely used to determine the SCSSM onset. The methods can be used successfully in most of the years but not in 2006. Due to the intrusion of Typhoon Chanchu (0601) that year, the usual method of determining SCSSM onset date by utilizing the SCS regional indices is less capable of pinpointing the real onset date. In order to solve the problem, larger-scale situations have to be taken into account. Zonal and meridional circulations would be better to determine the break-out date of SCSSM in 2006. The result indicates that its onset date is May 16. Moreover, similar onset dates for other years can be obtained using various methods, implying that large-scale zonal and meridional circulations can be used as an alternative method for determining the SCSSM onset date.
2011, 17(3): 209-220.
Abstract:
Using the NCEP/DOE AMIP-II reanalysis data from 1979 to 2005 and the characteristics of monsoon troughs in the western North Pacific, we established an intensity index and a location index to describe the activity of the monsoon troughs in three different regions and their impacts on tropical cyclones generated therein (MTTCs). The monsoon troughs was analyzed. (1) The established monsoon trough intensity index is positively correlated to the location index. (2) Monsoon trough intensity exhibits significant interannual variation, with obvious periods of 4–C5 years prior to 1994 and 2–C3 years afterwards. (3) The affecting factors are different with areas. The preceding SST anomaly results in anomalous atmospheric circulation, leading to the anomaly of monsoon trough intensity in different areas. (4) The frequency of cyclogenesis and location anomalies of the MTTC are closely related to the intensity and location of the monsoon trough. Most of the anomalously less MTTC years coincide with the years with a weak general monsoon trough and weak regional monsoon troughs. The anomalously more MTTC years are associated with both a strong general monsoon trough and a weak general monsoon trough combined with a strong one over the South China Sea. (5) The interseasonal variation of the intensity of monsoon troughs provides favorable conditions for TC generation and development. The monsoon trough is in the active periods of both quasi-biweekly 10 to 20 day and 30 to 60 day oscillations, being favorable for MTTC occurrence.
Using the NCEP/DOE AMIP-II reanalysis data from 1979 to 2005 and the characteristics of monsoon troughs in the western North Pacific, we established an intensity index and a location index to describe the activity of the monsoon troughs in three different regions and their impacts on tropical cyclones generated therein (MTTCs). The monsoon troughs was analyzed. (1) The established monsoon trough intensity index is positively correlated to the location index. (2) Monsoon trough intensity exhibits significant interannual variation, with obvious periods of 4–C5 years prior to 1994 and 2–C3 years afterwards. (3) The affecting factors are different with areas. The preceding SST anomaly results in anomalous atmospheric circulation, leading to the anomaly of monsoon trough intensity in different areas. (4) The frequency of cyclogenesis and location anomalies of the MTTC are closely related to the intensity and location of the monsoon trough. Most of the anomalously less MTTC years coincide with the years with a weak general monsoon trough and weak regional monsoon troughs. The anomalously more MTTC years are associated with both a strong general monsoon trough and a weak general monsoon trough combined with a strong one over the South China Sea. (5) The interseasonal variation of the intensity of monsoon troughs provides favorable conditions for TC generation and development. The monsoon trough is in the active periods of both quasi-biweekly 10 to 20 day and 30 to 60 day oscillations, being favorable for MTTC occurrence.
2011, 17(3): 221-230.
Abstract:
Although satellite observations provide large amount of information of clouds and precipitation and play an important role in the forecast of heavy rainfall, they have not been fully taken advantage of in data assimilation of numerical weather predictions, especially those in infrared channels. Assimilating radiances is common under clear-sky conditions since it is extremely difficult to simulate infrared transmittance in cloudy sky. Based on the Global and Regional Assimilation and Prediction Enhanced System 3-dimensional variance (GRAPES-3DVar), cloud liquid water content, ice-water content and cloud cover are employed as governing variables in the assimilation system. This scheme can improve the simulation of infrared transmittance by a fast radiative transfer model for TOVS (RTTOV) and adjust the atmospheric and cloud parameters based on infrared radiance observations. In this paper, we investigate a heavy rainfall over Guangdong province on May 26, 2007, which is right after the onset of a South China Sea monsoon. In this case, channels of the Moderate Resolution Imaging Spectroradiometer (MODIS) for observing water vapor (Channel 27) and cloud top altitude (Channel 36) are selected for the assimilation. The process of heavy rainfall is simulated by the Weather Research and Forecasting (WRF) model. Our results show that the assimilated MODIS data can improve the distribution of water vapor and temperature in the first guess field and indirectly adjust the upper-level wind field. The tendency of adjustment agrees well with the satellite observations. The assimilation scheme has positive impacts on the short-range forecasting of rainstorm.
Although satellite observations provide large amount of information of clouds and precipitation and play an important role in the forecast of heavy rainfall, they have not been fully taken advantage of in data assimilation of numerical weather predictions, especially those in infrared channels. Assimilating radiances is common under clear-sky conditions since it is extremely difficult to simulate infrared transmittance in cloudy sky. Based on the Global and Regional Assimilation and Prediction Enhanced System 3-dimensional variance (GRAPES-3DVar), cloud liquid water content, ice-water content and cloud cover are employed as governing variables in the assimilation system. This scheme can improve the simulation of infrared transmittance by a fast radiative transfer model for TOVS (RTTOV) and adjust the atmospheric and cloud parameters based on infrared radiance observations. In this paper, we investigate a heavy rainfall over Guangdong province on May 26, 2007, which is right after the onset of a South China Sea monsoon. In this case, channels of the Moderate Resolution Imaging Spectroradiometer (MODIS) for observing water vapor (Channel 27) and cloud top altitude (Channel 36) are selected for the assimilation. The process of heavy rainfall is simulated by the Weather Research and Forecasting (WRF) model. Our results show that the assimilated MODIS data can improve the distribution of water vapor and temperature in the first guess field and indirectly adjust the upper-level wind field. The tendency of adjustment agrees well with the satellite observations. The assimilation scheme has positive impacts on the short-range forecasting of rainstorm.
2011, 17(3): 231-247.
Abstract:
A rainfall that occurred during 0200–C1400 Beijing Standard Time (BST) 25 August 2008 shows the rapid development of a convective system, a short life span, and a record rate of 117.5 mm h-1 for Xujiahui station since 1872. To study this torrential rainfall process, the partitioning method of Q vector is developed, in which a moist Q vector is first separated into a dry ageostrophic Q vector ( ) and a diabatic-heating component. The dry ageostrophic Q vector is further partitioned along isothermal lines in the natural coordinate to identify different scale forcing in adiabatic atmosphere, and the large-scale and convective condensational heating in non-uniform saturated atmosphere, convective condensational heating, and Laplace of diabatic heating that includes radiative heating and other heating and cooling processes, are calculated to study the forcing from diabatic heating. The effects of the environmental conditions on the development of the rainfall processes are diagnosed by performing the partitioning of Q vector based on 6-hourly NCEP/NCAR Final Analysis (FNL) data with the horizontal resolution of 1° 1° . The results include the following: (1) a low-pressure inverted trough associated with the landfall of Typhoon Nuri (2008), a strong southwesterly jet along the western side of the subtropical high, and an eastward-propagating westerly low-pressure trough provide favorable synoptic conditions for the development of torrential rainfall; (2) the analysis of vector showed that the upward motions forced by the convergence of vector in the lower troposphere (1000–C600 hPa) favor the development of torrential rainfall. When vector converges in the upper troposphere (500–C100 hPa), upward motions in the whole air column intensify significantly to accelerate the development of torrential rainfall; (3) the partitioning analysis of vector reveals that large-scale forcing persistently favors the development of torrential rainfall whereas the mesoscale forcing speeds up the torrential rainfall; (4) the calculations of large-scale condensational heating in non-uniform saturated atmosphere, convective condensational heating, and Laplace of diabatic heating showed that the forcing related to diabatic heating has the positive feedback on the convective development, and such positive feedback decays and dissipates when the convective system propagates eastward and weakens.
A rainfall that occurred during 0200–C1400 Beijing Standard Time (BST) 25 August 2008 shows the rapid development of a convective system, a short life span, and a record rate of 117.5 mm h-1 for Xujiahui station since 1872. To study this torrential rainfall process, the partitioning method of Q vector is developed, in which a moist Q vector is first separated into a dry ageostrophic Q vector ( ) and a diabatic-heating component. The dry ageostrophic Q vector is further partitioned along isothermal lines in the natural coordinate to identify different scale forcing in adiabatic atmosphere, and the large-scale and convective condensational heating in non-uniform saturated atmosphere, convective condensational heating, and Laplace of diabatic heating that includes radiative heating and other heating and cooling processes, are calculated to study the forcing from diabatic heating. The effects of the environmental conditions on the development of the rainfall processes are diagnosed by performing the partitioning of Q vector based on 6-hourly NCEP/NCAR Final Analysis (FNL) data with the horizontal resolution of 1° 1° . The results include the following: (1) a low-pressure inverted trough associated with the landfall of Typhoon Nuri (2008), a strong southwesterly jet along the western side of the subtropical high, and an eastward-propagating westerly low-pressure trough provide favorable synoptic conditions for the development of torrential rainfall; (2) the analysis of vector showed that the upward motions forced by the convergence of vector in the lower troposphere (1000–C600 hPa) favor the development of torrential rainfall. When vector converges in the upper troposphere (500–C100 hPa), upward motions in the whole air column intensify significantly to accelerate the development of torrential rainfall; (3) the partitioning analysis of vector reveals that large-scale forcing persistently favors the development of torrential rainfall whereas the mesoscale forcing speeds up the torrential rainfall; (4) the calculations of large-scale condensational heating in non-uniform saturated atmosphere, convective condensational heating, and Laplace of diabatic heating showed that the forcing related to diabatic heating has the positive feedback on the convective development, and such positive feedback decays and dissipates when the convective system propagates eastward and weakens.
2011, 17(3): 248-256.
Abstract:
Using the daily average outgoing longwave radiation and NCEP/NCAR reanalysis data in boreal summer (Mays to Octobers) from 1979 to 2007, the propagating characteristics of convection intraseasonal oscillations (ISOs) in the Asian-western Pacific (AWP) region and the relationship between tropical synoptic waves and ISOs are examined by means of finite-domain wavenumber-frequency energy spectrum analysis and lagged linear regression technique. The results are shown as follows. (1) The AWP ISOs propagate both eastward and westward, showing seasonality and regionality. The ISOs propagate eastward with a period of 30 to 60 days over equatorial regions in the whole AWP region, while the westward propagation occurs over 10 to 20? N western Pacific or in the late summers (August, September and October) with periods of 20 to 40 days. The ISOs eastward propagation mainly occurs in primary summers while the westward propagation enhances in late summers. (2) Deep ISO convections associate with westerly and cyclonic circulation anomalies that first form in the Indian Ocean, propagate eastward to the dateline in the Pacific and then turn northwestward. The ISOs convections show northwestward propagating characteristics in the western North Pacific. (3) The ISOs link with the tropical synoptic waves closely. Both convection signals, though with different spatio-temporal scale, enhance simutaneously in the northwestern Pacific, and the ISOs facilitate the forming of a cluster of tropical cyclones (TCs), while a cluster of TCs convection becomes one portion of the northwestward ISOs.
Using the daily average outgoing longwave radiation and NCEP/NCAR reanalysis data in boreal summer (Mays to Octobers) from 1979 to 2007, the propagating characteristics of convection intraseasonal oscillations (ISOs) in the Asian-western Pacific (AWP) region and the relationship between tropical synoptic waves and ISOs are examined by means of finite-domain wavenumber-frequency energy spectrum analysis and lagged linear regression technique. The results are shown as follows. (1) The AWP ISOs propagate both eastward and westward, showing seasonality and regionality. The ISOs propagate eastward with a period of 30 to 60 days over equatorial regions in the whole AWP region, while the westward propagation occurs over 10 to 20? N western Pacific or in the late summers (August, September and October) with periods of 20 to 40 days. The ISOs eastward propagation mainly occurs in primary summers while the westward propagation enhances in late summers. (2) Deep ISO convections associate with westerly and cyclonic circulation anomalies that first form in the Indian Ocean, propagate eastward to the dateline in the Pacific and then turn northwestward. The ISOs convections show northwestward propagating characteristics in the western North Pacific. (3) The ISOs link with the tropical synoptic waves closely. Both convection signals, though with different spatio-temporal scale, enhance simutaneously in the northwestern Pacific, and the ISOs facilitate the forming of a cluster of tropical cyclones (TCs), while a cluster of TCs convection becomes one portion of the northwestward ISOs.
Application of WRF/UCM in the simulation of a heat wave event and urban heat island around Guangzhou
2011, 17(3): 257-267.
Abstract:
This paper evaluated the performance of a coupled modeling system, Weather Research and Forecasting (WRF)/Urban Canopy Model (UCM), in the simulation of a heat wave event which occurred around Guangzhou during late June through early July, 2004. Results from three experiments reveal that the UCM with new land data (E-UCM) reproduces the best 2-m temperature evolution and the smallest minimum absolute average error as compared with the other two experiments, the BPA-Bulk Parameterization Approach with new land data (E-BPA) and the UCM with original U.S. Geological Survey land data (E-NOU). The E-UCM is more useful in capturing the temporal and spatial distribution of the nighttime Urban Heat Island (UHI). Differences in surface energy balance between the urban and suburban areas show that low daytime albedo causes more absorption of solar radiation by urban areas. Due to the lack of vegetation which inhibits cooling by evapotranspiration, most of the incoming energy over urban areas is partitioned into sensible heat flux and therefore heats the surface and enhances the heat wave. During nighttime, the energy in the urban area is mainly from soil heat flux. Although some energy is partitioned as outgoing long wave radiation, most of the soil heat flux is partitioned into sensible heat flux due to the small latent heat flux at night. This leads to the development of nighttime UHI and the increase of the magnitude and duration of heat waves within the municipality.
This paper evaluated the performance of a coupled modeling system, Weather Research and Forecasting (WRF)/Urban Canopy Model (UCM), in the simulation of a heat wave event which occurred around Guangzhou during late June through early July, 2004. Results from three experiments reveal that the UCM with new land data (E-UCM) reproduces the best 2-m temperature evolution and the smallest minimum absolute average error as compared with the other two experiments, the BPA-Bulk Parameterization Approach with new land data (E-BPA) and the UCM with original U.S. Geological Survey land data (E-NOU). The E-UCM is more useful in capturing the temporal and spatial distribution of the nighttime Urban Heat Island (UHI). Differences in surface energy balance between the urban and suburban areas show that low daytime albedo causes more absorption of solar radiation by urban areas. Due to the lack of vegetation which inhibits cooling by evapotranspiration, most of the incoming energy over urban areas is partitioned into sensible heat flux and therefore heats the surface and enhances the heat wave. During nighttime, the energy in the urban area is mainly from soil heat flux. Although some energy is partitioned as outgoing long wave radiation, most of the soil heat flux is partitioned into sensible heat flux due to the small latent heat flux at night. This leads to the development of nighttime UHI and the increase of the magnitude and duration of heat waves within the municipality.
2011, 17(3): 268-275.
Abstract:
According to the national standard (2006) on tropical cyclone (TC) intensity, TCs are categorized into six intensity types, namely, tropical depression (TD), tropical storm (TS), severe tropical storm (STS), typhoon (TY), severe typhoon (STY), and super severe typhoon (SSTY). Fifty-eight years (1949–C2006) of the datasets from the Yearbook of Typhoons and Yearbook of Tropical Cyclones were used to study the variation characteristics of TCs making landfalls in mainland China, Hainan and Taiwan islands. The main results are as follows. First, interannual or interdecadal variations in the number of landfalling TCs at different intensities exist. As far as long-term trends are concerned, the TD and TS frequencies show a significant linearly decreasing trend while those of STY show a significant linearly increasing trend. Second, a significant period of 6–C8 years exist in the variations of annual landfalling TD, TS, and STS frequencies while quasi-16-year periods are found in the annual TY frequency. Third, TD and TS are generated mostly over the South China Sea, while TY, STY, and SSTY mostly over the waters southeast of the Bashi Channel and the ocean to the east of the Philippines. Fourth, as far as interdecadal trends are concerned, the frequencies of landfalling TD and TS generated over the South China Sea show significant linearly decreasing trends. However, TY and STY show significant linearly increasing trends.
According to the national standard (2006) on tropical cyclone (TC) intensity, TCs are categorized into six intensity types, namely, tropical depression (TD), tropical storm (TS), severe tropical storm (STS), typhoon (TY), severe typhoon (STY), and super severe typhoon (SSTY). Fifty-eight years (1949–C2006) of the datasets from the Yearbook of Typhoons and Yearbook of Tropical Cyclones were used to study the variation characteristics of TCs making landfalls in mainland China, Hainan and Taiwan islands. The main results are as follows. First, interannual or interdecadal variations in the number of landfalling TCs at different intensities exist. As far as long-term trends are concerned, the TD and TS frequencies show a significant linearly decreasing trend while those of STY show a significant linearly increasing trend. Second, a significant period of 6–C8 years exist in the variations of annual landfalling TD, TS, and STS frequencies while quasi-16-year periods are found in the annual TY frequency. Third, TD and TS are generated mostly over the South China Sea, while TY, STY, and SSTY mostly over the waters southeast of the Bashi Channel and the ocean to the east of the Philippines. Fourth, as far as interdecadal trends are concerned, the frequencies of landfalling TD and TS generated over the South China Sea show significant linearly decreasing trends. However, TY and STY show significant linearly increasing trends.
2011, 17(3): 276-284.
Abstract:
Using the WRF (Weather Research Forecast) model, this work performed analysis and simulation on the rainband change during the landfall of Typhoon Haitang (2005) and found that breaking may occur over land and oceans leads to distinct asymmetric precipitation. The breaking is related to the topographic effect as well as interactions between the typhoon and midlatitude systems at upper levels. During the landfall, divergent flows at the 200-hPa level of the South-Asian high combined with divergent flows at the periphery of the typhoon to form a weak, inverted trough in the northwest part of the storm, with the mid- and low-level divergence fields on the west and northwest side of the typhoon center maintaining steadily. It intensifies the upper-level cyclonic flows, in association with positive vorticity rotating counterclockwise together with air currents that travel stepwise into a vorticity zone in the vicinity of the typhoon core, thereby forming a vorticity transfer belt in 22–C25? N that extends to the eastern part of the storm. It is right here that the high-level vorticity band is subsiding so that rainfall is prevented from developing, resulting in the rainbelt breaking, which is the principal cause of asymmetric precipitation occurrence. Migrating into its outer region, the banded vorticity of Haitang at high levels causes further amplification of the cyclonic circulation in the western part and transfer of positive vorticity into the typhoon such that the rainband breaking is more distinct.
Using the WRF (Weather Research Forecast) model, this work performed analysis and simulation on the rainband change during the landfall of Typhoon Haitang (2005) and found that breaking may occur over land and oceans leads to distinct asymmetric precipitation. The breaking is related to the topographic effect as well as interactions between the typhoon and midlatitude systems at upper levels. During the landfall, divergent flows at the 200-hPa level of the South-Asian high combined with divergent flows at the periphery of the typhoon to form a weak, inverted trough in the northwest part of the storm, with the mid- and low-level divergence fields on the west and northwest side of the typhoon center maintaining steadily. It intensifies the upper-level cyclonic flows, in association with positive vorticity rotating counterclockwise together with air currents that travel stepwise into a vorticity zone in the vicinity of the typhoon core, thereby forming a vorticity transfer belt in 22–C25? N that extends to the eastern part of the storm. It is right here that the high-level vorticity band is subsiding so that rainfall is prevented from developing, resulting in the rainbelt breaking, which is the principal cause of asymmetric precipitation occurrence. Migrating into its outer region, the banded vorticity of Haitang at high levels causes further amplification of the cyclonic circulation in the western part and transfer of positive vorticity into the typhoon such that the rainband breaking is more distinct.
2011, 17(3): 285-292.
Abstract:
The evaporation rate over South China is estimated based on the Climate Prediction Center Merged Analysis of Precipitation (CMAP) data and the NCEP/DOE reanalysis II data from 1979 to 2007. The temporal variation of evaporation over South China and its relationship to precipitation are discussed. Climatologically, the evaporation rate over South China is the largest in July and smallest in March. In spring and summer, the evaporation rate is approximately one half of the precipitation rate. However, the evaporation rate is approximately equal to the precipitation rate in fall and winter. The year-to-year variation of the evaporation rate over South China is quite in phase with that of the precipitation rate in the period from February to May but out of phase with that of the precipitation rate in early winter. Over South China there is a pronounced decreasing trend in the evaporation in colder seasons and a positive correlation between the evaporation variation and the rainfall variation in spring. In summer, the abnormality of rainfall over South China is closely related to the anomalous evaporation over the northeastern part of the South China Sea and its eastern vicinity. In winter, the rainfall variation in South China has a close linkage with the evaporation variation in a belt area covering the eastern Arabian Sea, the Bay of Bengal, the southeastern periphery of the Plateau, the southern part of South China Sea and the central part of Indonesia.
The evaporation rate over South China is estimated based on the Climate Prediction Center Merged Analysis of Precipitation (CMAP) data and the NCEP/DOE reanalysis II data from 1979 to 2007. The temporal variation of evaporation over South China and its relationship to precipitation are discussed. Climatologically, the evaporation rate over South China is the largest in July and smallest in March. In spring and summer, the evaporation rate is approximately one half of the precipitation rate. However, the evaporation rate is approximately equal to the precipitation rate in fall and winter. The year-to-year variation of the evaporation rate over South China is quite in phase with that of the precipitation rate in the period from February to May but out of phase with that of the precipitation rate in early winter. Over South China there is a pronounced decreasing trend in the evaporation in colder seasons and a positive correlation between the evaporation variation and the rainfall variation in spring. In summer, the abnormality of rainfall over South China is closely related to the anomalous evaporation over the northeastern part of the South China Sea and its eastern vicinity. In winter, the rainfall variation in South China has a close linkage with the evaporation variation in a belt area covering the eastern Arabian Sea, the Bay of Bengal, the southeastern periphery of the Plateau, the southern part of South China Sea and the central part of Indonesia.
2011, 17(3): 293-301.
Abstract:
The structure of the South Asia High (SAH) in the stratosphere and the influence of ENSO on the SAH are systematically investigated with the long-term ECMWF reanalysis data. The results show that the SAH only exists in low levels of the stratosphere. The maximum intensity of the High is located at around 150 hPa and there is no obvious anti-cyclonic structure above 50 hPa. The axis of the SAH center tends to be northwest slanting from lower levels to higher levels. Further analyses show that the geopotential height and temperature fields of the SAH have dramatic anomalies during El Niño years and La Niña years. Corresponding to the ENSO, the SAH is weaker (stronger) at the warm (cold) phase.
The structure of the South Asia High (SAH) in the stratosphere and the influence of ENSO on the SAH are systematically investigated with the long-term ECMWF reanalysis data. The results show that the SAH only exists in low levels of the stratosphere. The maximum intensity of the High is located at around 150 hPa and there is no obvious anti-cyclonic structure above 50 hPa. The axis of the SAH center tends to be northwest slanting from lower levels to higher levels. Further analyses show that the geopotential height and temperature fields of the SAH have dramatic anomalies during El Niño years and La Niña years. Corresponding to the ENSO, the SAH is weaker (stronger) at the warm (cold) phase.
2011, 17(3): 302-309.
Abstract:
By analyzing the change of an index for the characteristics of South Asia High and variations of upper-air troughs in 2002–C2005, we studied the impact of South Asia high on the beginning and ending of the Mei-yu in Jiangsu province. Statistic verification is conducted on the relationships between the index and the Mei-yu season in 1991–C2005 to examine the impacts of the SAH characteristics index on a rain intensity index of Mei-yu and regional distribution of a characteristics index. Historical composite is performed of the 100-hPa circulation field using the 100-hPa geopotential height of Northern Hemisphere from 2002 to 2005 and 45-year NCEP reanalysis to study the difference in the circulation. Diagnostic and statistic conclusions have been obtained as follows. (1) The characteristics preceding to and the advancement/retreat of SAH and the movement of westerly troughs can influence the onset time of the Mei-yu season; after the Mei-yu onset, the progression/withdrawal of SAH and how farther east it extends determine how long the Mei-yu lasts and when it ends. (2) During the Mei-yu, the general 100-hPa circulation situation and average characteristics of the SAH are well corresponding to the characteristics of the season and annual patterns of Mei-yu. And the averages of the SAH ridgeline and east-extending index for June, July and the Mei-yu season have some implications to the forecast of the index of Mei-yu intensity. These conclusions can be served as powerful means in determining the starting/ending dates, duration and annual pattern of the Mei-yu season.
By analyzing the change of an index for the characteristics of South Asia High and variations of upper-air troughs in 2002–C2005, we studied the impact of South Asia high on the beginning and ending of the Mei-yu in Jiangsu province. Statistic verification is conducted on the relationships between the index and the Mei-yu season in 1991–C2005 to examine the impacts of the SAH characteristics index on a rain intensity index of Mei-yu and regional distribution of a characteristics index. Historical composite is performed of the 100-hPa circulation field using the 100-hPa geopotential height of Northern Hemisphere from 2002 to 2005 and 45-year NCEP reanalysis to study the difference in the circulation. Diagnostic and statistic conclusions have been obtained as follows. (1) The characteristics preceding to and the advancement/retreat of SAH and the movement of westerly troughs can influence the onset time of the Mei-yu season; after the Mei-yu onset, the progression/withdrawal of SAH and how farther east it extends determine how long the Mei-yu lasts and when it ends. (2) During the Mei-yu, the general 100-hPa circulation situation and average characteristics of the SAH are well corresponding to the characteristics of the season and annual patterns of Mei-yu. And the averages of the SAH ridgeline and east-extending index for June, July and the Mei-yu season have some implications to the forecast of the index of Mei-yu intensity. These conclusions can be served as powerful means in determining the starting/ending dates, duration and annual pattern of the Mei-yu season.
2011, 17(3): 310-316.
Abstract:
First, based on routine meteorological data, the synoptic characteristics of a heavy warm-sector rainfall that occurred on June 13, 2008 in the Pearl River Delta were analyzed. Second, a mesoscale numerical model, Weather Research and Forecasting (WRFV2.2), was used to simulate the heavy rainfall. Diagnostic analyses were done of moist potential vorticity (MPV) for its horizontal components (MPV2) and vertical components (MPV1) based on the simulation results of WRFV2.2 to identify the mechanism of the rainfall development. The results showed that the heavy rainfall occurred when there were high MPV1 in the upper levels and low MPV1 and high MPV2 in the lower levels. Disturbances of high MPV1 in the upper levels came from the southwest or northwest, those of low MPV1 in the lower levels came from the southwest, and those of high MPV2 came from the south. Disturbances of low MPV1 at low levels were the direct cause of convective instability. Enhanced vertical shear of meridional wind led to increased MPV2 at lower levels, strengthened baroclinicity, and active warm and wet flows. These distributions of MPV helped to trigger the release of unstable energy and produce warm-sector heavy rainfall. As it integrates the evolution of dynamic and thermal fields, MPV is able to reveal the development of this heavy rainfall effectively.
First, based on routine meteorological data, the synoptic characteristics of a heavy warm-sector rainfall that occurred on June 13, 2008 in the Pearl River Delta were analyzed. Second, a mesoscale numerical model, Weather Research and Forecasting (WRFV2.2), was used to simulate the heavy rainfall. Diagnostic analyses were done of moist potential vorticity (MPV) for its horizontal components (MPV2) and vertical components (MPV1) based on the simulation results of WRFV2.2 to identify the mechanism of the rainfall development. The results showed that the heavy rainfall occurred when there were high MPV1 in the upper levels and low MPV1 and high MPV2 in the lower levels. Disturbances of high MPV1 in the upper levels came from the southwest or northwest, those of low MPV1 in the lower levels came from the southwest, and those of high MPV2 came from the south. Disturbances of low MPV1 at low levels were the direct cause of convective instability. Enhanced vertical shear of meridional wind led to increased MPV2 at lower levels, strengthened baroclinicity, and active warm and wet flows. These distributions of MPV helped to trigger the release of unstable energy and produce warm-sector heavy rainfall. As it integrates the evolution of dynamic and thermal fields, MPV is able to reveal the development of this heavy rainfall effectively.
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