2014 Vol. 20, No. 3
column
2014, 20(3): 193-201.
Abstract:
Based on the satellite data from the National Oceanic and Atmospheric Administration and the NCEP/NCAR reanalysis data, the variation of the intensity of convection over the Intertropical Convergence Zone (ITCZ) in summer and its impacts on tropical cyclones are studied. In this paper, an intensity index of the ITCZ is proposed according to Outgoing Longwave Radiation (OLR) in the region of (5°–C20°N, 120°–C150°E) in the western North Pacific (WNP). Then strong and weak ITCZ years are classified and different variables during the strong/weak ITCZ years are analyzed. The composite results show that the ITCZ anomaly is connected to the general atmospheric circulation and SST distribution. In the strong ITCZ years, the subtropical anticyclone weakens and shifts northward. Besides, there is salient cyclonic anomaly at the low level and anticyclonic anomaly at the high level. SST patterns in the preceding winter resemble to those of La Niña. It could persist into the succeeding summer. However, it is opposite in the weak ITCZ years. The impact of the ITCZ anomaly on the tropical cyclone (TC) formation and track is also discussed. There are more TCs over the WNP (5°–C20°N, 120°–C150°E) in the strong ITCZ years and there is a significant increase in the northward recurving TCs. In the weak ITCZ years, fewer TCs occur and the frequency of the northwestward track is higher.
Based on the satellite data from the National Oceanic and Atmospheric Administration and the NCEP/NCAR reanalysis data, the variation of the intensity of convection over the Intertropical Convergence Zone (ITCZ) in summer and its impacts on tropical cyclones are studied. In this paper, an intensity index of the ITCZ is proposed according to Outgoing Longwave Radiation (OLR) in the region of (5°–C20°N, 120°–C150°E) in the western North Pacific (WNP). Then strong and weak ITCZ years are classified and different variables during the strong/weak ITCZ years are analyzed. The composite results show that the ITCZ anomaly is connected to the general atmospheric circulation and SST distribution. In the strong ITCZ years, the subtropical anticyclone weakens and shifts northward. Besides, there is salient cyclonic anomaly at the low level and anticyclonic anomaly at the high level. SST patterns in the preceding winter resemble to those of La Niña. It could persist into the succeeding summer. However, it is opposite in the weak ITCZ years. The impact of the ITCZ anomaly on the tropical cyclone (TC) formation and track is also discussed. There are more TCs over the WNP (5°–C20°N, 120°–C150°E) in the strong ITCZ years and there is a significant increase in the northward recurving TCs. In the weak ITCZ years, fewer TCs occur and the frequency of the northwestward track is higher.
2014, 20(3): 202-207.
Abstract:
The results of this study prove that there is significant troposphere biennial oscillation (TBO) in the South Asian climate, especially with the Indian summer monsoon rainfall. In order to explore the mechanism of TBO in the South Asian region, we defined a unified South Asian monsoon index to depict South Asian summer monsoon (SASM) and South Asian winter monsoon (SAWM) and the transition features between SASM and SAWM. Through further analysis, the connection between the abnormity of SASM and SAWM was discovered. Normally, a strong SAWM is beneficial for a weak SASM later, while a weak SAWM favors a strong SASM. Meanwhile, a strong SASM is favorable for a weak SAWM and a weak SAWM always happens after a weak SASM. Such results suggest the evolution of the South Asian monsoon, which may be an important mechanism to excite TBO in South Asia.
The results of this study prove that there is significant troposphere biennial oscillation (TBO) in the South Asian climate, especially with the Indian summer monsoon rainfall. In order to explore the mechanism of TBO in the South Asian region, we defined a unified South Asian monsoon index to depict South Asian summer monsoon (SASM) and South Asian winter monsoon (SAWM) and the transition features between SASM and SAWM. Through further analysis, the connection between the abnormity of SASM and SAWM was discovered. Normally, a strong SAWM is beneficial for a weak SASM later, while a weak SAWM favors a strong SASM. Meanwhile, a strong SASM is favorable for a weak SAWM and a weak SAWM always happens after a weak SASM. Such results suggest the evolution of the South Asian monsoon, which may be an important mechanism to excite TBO in South Asia.
2014, 20(3): 208-217.
Abstract:
By using the NCEP/NCAR pentad reanalysis data from 1968 to 2009, the variation characteristics of Middle East jet stream (MEJS) and its thermal mechanism during seasonal transition are studied. Results show that the intensity and south-north location of MEJS center exhibit obvious seasonal variation characteristics. When MEJS is strong, it is at 27.5°N from the 67th pentad to the 24th pentad the following year; when MEJS is weak, it is at 45°N from the 38th pentad to the 44th pentad. The first Empirical Orthogonal Function (EOF) mode of 200-hPa zonal wind field shows that MEJS is mainly over Egypt and Saudi Arabia in winter and over the eastern Black Sea and the eastern Aral Sea in summer. MEJS intensity markedly weakens in summer in comparison with that in winter. The 26th-31st pentad is the spring-summer transition of MEJS, and the 54th-61st pentad the autumn-winter transition. During the two seasonal transitions, the temporal variations of the 500-200 hPa south-north temperature difference (SNTD) well match with 200-hPa zonal wind velocity, indicating that the former leads to the latter following the principle of thermal wind. A case analysis shows that there is a close relation between the onset date of Indian summer monsoon and the transition date of MEJS seasonal transition. When the outbreak date of Indian summer monsoon is earlier than normal, MEJS moves northward earlier because the larger SNTD between 500-200 hPa moves northward earlier, with the westerly jet in the lower troposphere over 40°-90°E appearing earlier than normal, and vice versa.
By using the NCEP/NCAR pentad reanalysis data from 1968 to 2009, the variation characteristics of Middle East jet stream (MEJS) and its thermal mechanism during seasonal transition are studied. Results show that the intensity and south-north location of MEJS center exhibit obvious seasonal variation characteristics. When MEJS is strong, it is at 27.5°N from the 67th pentad to the 24th pentad the following year; when MEJS is weak, it is at 45°N from the 38th pentad to the 44th pentad. The first Empirical Orthogonal Function (EOF) mode of 200-hPa zonal wind field shows that MEJS is mainly over Egypt and Saudi Arabia in winter and over the eastern Black Sea and the eastern Aral Sea in summer. MEJS intensity markedly weakens in summer in comparison with that in winter. The 26th-31st pentad is the spring-summer transition of MEJS, and the 54th-61st pentad the autumn-winter transition. During the two seasonal transitions, the temporal variations of the 500-200 hPa south-north temperature difference (SNTD) well match with 200-hPa zonal wind velocity, indicating that the former leads to the latter following the principle of thermal wind. A case analysis shows that there is a close relation between the onset date of Indian summer monsoon and the transition date of MEJS seasonal transition. When the outbreak date of Indian summer monsoon is earlier than normal, MEJS moves northward earlier because the larger SNTD between 500-200 hPa moves northward earlier, with the westerly jet in the lower troposphere over 40°-90°E appearing earlier than normal, and vice versa.
2014, 20(3): 218-227.
Abstract:
The daily patterns and activity of Intertropical Convergence Zone (ITCZ) in the Western-Central Pacific Ocean are analyzed using NOAA interpolated Outgoing Longwave Radiation dataset during the period from 1979 to 2008, and the relationships between ITCZ patterns and Arctic Oscillation (AO) is investigated in this paper. In accordance with the central activity region the daily ITCZ can be divided into six patterns―north, south, equator, double, full and weak pattern, respectively. The statistic result shows that the north (accounting for 30.98% of the total observations), south (31.11%) and weak (24.05%) ITCZ patterns are the most active daily patterns within a 30-year period, while the other three ITCZ patterns occur infrequently. Results show that the February-April AO index has a significant positive (negative) correlation with the frequency of the north (weak) ITCZ pattern from March-May to August-October, with the strongest correlation in April-June (March-May). At the same time, the lower tropospheric atmosphere circulation (850-hPa wind field) and SST anomalies corresponding to the AO change significantly in the tropical Pacific. When AO is in the positive phase, there is an anomalous westerly from the equator to 15°N and warmer SST in the critical north ITCZ active region, while there is an anomalous easterly and insignificant change of SST from the equator to 15°S. The wind and SST anomalies share the same characteristics of the equatorial asymmetry and thus enlarge the gradient between the south and north of equator, which would help reinforce convection in the north of equator and result in more frequent occurrence of the northern type of ITCZ.
The daily patterns and activity of Intertropical Convergence Zone (ITCZ) in the Western-Central Pacific Ocean are analyzed using NOAA interpolated Outgoing Longwave Radiation dataset during the period from 1979 to 2008, and the relationships between ITCZ patterns and Arctic Oscillation (AO) is investigated in this paper. In accordance with the central activity region the daily ITCZ can be divided into six patterns―north, south, equator, double, full and weak pattern, respectively. The statistic result shows that the north (accounting for 30.98% of the total observations), south (31.11%) and weak (24.05%) ITCZ patterns are the most active daily patterns within a 30-year period, while the other three ITCZ patterns occur infrequently. Results show that the February-April AO index has a significant positive (negative) correlation with the frequency of the north (weak) ITCZ pattern from March-May to August-October, with the strongest correlation in April-June (March-May). At the same time, the lower tropospheric atmosphere circulation (850-hPa wind field) and SST anomalies corresponding to the AO change significantly in the tropical Pacific. When AO is in the positive phase, there is an anomalous westerly from the equator to 15°N and warmer SST in the critical north ITCZ active region, while there is an anomalous easterly and insignificant change of SST from the equator to 15°S. The wind and SST anomalies share the same characteristics of the equatorial asymmetry and thus enlarge the gradient between the south and north of equator, which would help reinforce convection in the north of equator and result in more frequent occurrence of the northern type of ITCZ.
2014, 20(3): 228-235.
Abstract:
By using 1958-2001 NOAA extended reconstructed sea surface temperature (SST) data, ERA40 reanalysis soil moisture data and precipitation data of 444 stations in China (east of 100°E), the possible relationships among South China Sea (SCS) SST anomaly (SSTA), soil moisture anomalies (SMA) and summer precipitation in eastern China as well as their possible physical processes are investigated. Results show that the SSTA of SCS bears an evidently negative correlation with spring soil moisture in the east part of Southwest China. More (less) precipitation happens in the Yangtze River basin and less (more) in the Southeast China in summer when the SSTA of SCS is higher (lower) than normal and the soil in the east part of Southwest China is dry (wet) in spring. Further analysis shows that when the SSTA of SCS is high (low), the southwesterly wind at low level is weak (strong), decreasing (increasing) the water vapor transport in South China, resulting in reduced (increased) spring precipitation in the east part of Southwest China and more (less) soil moisture in spring. Through the evaporation feedback mechanism, the dry (wet) soil makes the surface temperature higher (lower) in summer, causing the westward extension (eastward retreat) of the West Pacific Subtropical High, eventually leading to the summer precipitation anomalies.
By using 1958-2001 NOAA extended reconstructed sea surface temperature (SST) data, ERA40 reanalysis soil moisture data and precipitation data of 444 stations in China (east of 100°E), the possible relationships among South China Sea (SCS) SST anomaly (SSTA), soil moisture anomalies (SMA) and summer precipitation in eastern China as well as their possible physical processes are investigated. Results show that the SSTA of SCS bears an evidently negative correlation with spring soil moisture in the east part of Southwest China. More (less) precipitation happens in the Yangtze River basin and less (more) in the Southeast China in summer when the SSTA of SCS is higher (lower) than normal and the soil in the east part of Southwest China is dry (wet) in spring. Further analysis shows that when the SSTA of SCS is high (low), the southwesterly wind at low level is weak (strong), decreasing (increasing) the water vapor transport in South China, resulting in reduced (increased) spring precipitation in the east part of Southwest China and more (less) soil moisture in spring. Through the evaporation feedback mechanism, the dry (wet) soil makes the surface temperature higher (lower) in summer, causing the westward extension (eastward retreat) of the West Pacific Subtropical High, eventually leading to the summer precipitation anomalies.
2014, 20(3): 236-241.
Abstract:
Based on composite analysis and numerical simulations using a regional climate model (RegCM3), this paper analyzed the impact of the LHF anomaly in the tropical western Pacific on the precipitation over the south of China in June. The results are as follows. (1) Correlation analysis shows that the SC precipitation in June is negatively correlated with the LHF of the tropical western Pacific in May and June, especially in May. The SC precipitation in June appears to negatively correlate with low-level relative vorticity in the abnormal area of LHF in the tropical western Pacific. (2) The LHF anomaly in the tropical western Pacific is a vital factor affecting the flood and drought of SC in June. A conceptual model goes like this: When the LHF in the tropical western Pacific is abnormally increased (decreased), an anomalous cyclone (anticyclone) circulation is formed at the low-level troposphere to its northwest. As a result, an anomalous northeast (southwest) air flow affects the south of China, being disadvantageous (advantageous) to the transportation of water vapor to the region. Meanwhile, there is an anomalous anticyclone (cyclone) at the low-level troposphere and an anomalous cyclone (anticyclone) circulation at the high-level troposphere in the region, which is advantageous for downdraft (updraft) there. Therefore a virtual circulation forms updraft (downdraft) in the anomalous area of LHF and downdraft (updraft) in the south of China, which finally leads to the drought (flood) in the region.
Based on composite analysis and numerical simulations using a regional climate model (RegCM3), this paper analyzed the impact of the LHF anomaly in the tropical western Pacific on the precipitation over the south of China in June. The results are as follows. (1) Correlation analysis shows that the SC precipitation in June is negatively correlated with the LHF of the tropical western Pacific in May and June, especially in May. The SC precipitation in June appears to negatively correlate with low-level relative vorticity in the abnormal area of LHF in the tropical western Pacific. (2) The LHF anomaly in the tropical western Pacific is a vital factor affecting the flood and drought of SC in June. A conceptual model goes like this: When the LHF in the tropical western Pacific is abnormally increased (decreased), an anomalous cyclone (anticyclone) circulation is formed at the low-level troposphere to its northwest. As a result, an anomalous northeast (southwest) air flow affects the south of China, being disadvantageous (advantageous) to the transportation of water vapor to the region. Meanwhile, there is an anomalous anticyclone (cyclone) at the low-level troposphere and an anomalous cyclone (anticyclone) circulation at the high-level troposphere in the region, which is advantageous for downdraft (updraft) there. Therefore a virtual circulation forms updraft (downdraft) in the anomalous area of LHF and downdraft (updraft) in the south of China, which finally leads to the drought (flood) in the region.
2014, 20(3): 251-266.
Abstract:
In this paper, based on heavy rain numerical forecast model AREM (Advanced Regional Eta Model), two different initialization schemes, LAPS and GRAPES-3DVAR, are used to run assimilation experiments of AREM-LAPS and AREM-3DVAR with the same data source (NCEP forecast field, surface data and radio-soundings) during the period from 21 May to 30 July 2008 to investigate the effect of the two initialization schemes on the rainfall simulation. The result suggests that: (1) the forecast TS score by the AREM-LAPS is higher than that by the AREM-3DVAR for rainfall in different areas, at different valid time and with different intensity, especially for the heavy rain, rainstorm and extremely heavy rain; (2) the AREM-3DVAR can generally simulate the average rainfall distribution, but the forecast area is smaller and rainfall intensity is weaker than the observation, while the AREM-LAPS significantly improves the forecast; (3) the AREM-LAPS gives a better forecast for the south-north shift of rainfall bands and the rainfall intensity variation than the AREM-3DVAR; (4) the AREM-LAPS can give a better reproduction for the daily change in the mean-rainfall-rate of the main rain band, and rainfall intensity changes in the eastern part of Southwest China, the coastal area in South China, the middle-lower valleys of Yangtze river, the Valleys of Huaihe river, and Shandong peninsula, with the rainfall intensity roughly close to the observation, while the rainfall intensity simulated by the AREM-3DVAR is clearly weaker than the observation, especially in the eastern part of Southwest China; and (5) the comparison verification between the AREM-LAPS and AREM-3DVAR for more than 10 typical rainfall processes in the summer of 2008 indicates that the AREM-LAPS gives a much better forecast than AREM-3DVAR in rain-band area, rainfall location and intensity, and in particular, the rainfall intensity forecast is improved obviously.
In this paper, based on heavy rain numerical forecast model AREM (Advanced Regional Eta Model), two different initialization schemes, LAPS and GRAPES-3DVAR, are used to run assimilation experiments of AREM-LAPS and AREM-3DVAR with the same data source (NCEP forecast field, surface data and radio-soundings) during the period from 21 May to 30 July 2008 to investigate the effect of the two initialization schemes on the rainfall simulation. The result suggests that: (1) the forecast TS score by the AREM-LAPS is higher than that by the AREM-3DVAR for rainfall in different areas, at different valid time and with different intensity, especially for the heavy rain, rainstorm and extremely heavy rain; (2) the AREM-3DVAR can generally simulate the average rainfall distribution, but the forecast area is smaller and rainfall intensity is weaker than the observation, while the AREM-LAPS significantly improves the forecast; (3) the AREM-LAPS gives a better forecast for the south-north shift of rainfall bands and the rainfall intensity variation than the AREM-3DVAR; (4) the AREM-LAPS can give a better reproduction for the daily change in the mean-rainfall-rate of the main rain band, and rainfall intensity changes in the eastern part of Southwest China, the coastal area in South China, the middle-lower valleys of Yangtze river, the Valleys of Huaihe river, and Shandong peninsula, with the rainfall intensity roughly close to the observation, while the rainfall intensity simulated by the AREM-3DVAR is clearly weaker than the observation, especially in the eastern part of Southwest China; and (5) the comparison verification between the AREM-LAPS and AREM-3DVAR for more than 10 typical rainfall processes in the summer of 2008 indicates that the AREM-LAPS gives a much better forecast than AREM-3DVAR in rain-band area, rainfall location and intensity, and in particular, the rainfall intensity forecast is improved obviously.
2014, 20(3): 267-278.
Abstract:
The sea-land breeze circulation (SLBC) occurs regularly at coastal locations and influences the local weather and climate significantly. In this study, based on the observed surface wind in 9 conventional meteorological stations of Hainan Island, the frequency of sea-land breeze (SLB) is studied to depict the diurnal and seasonal variations. The statistics indicated that there is a monthly average of 12.2 SLB days and an occurrence frequency of about 40%, with the maximum frequency (49%) in summer and the minimum frequency (29%) in autumn. SLB frequencies (41%) are comparable in winter and spring. A higher frequency of SLB is present in the southern and central mountains due to the enhancement effect of the mountain-valley breeze. Due to the synoptic wind the number of SLB days in the northern hilly area is less than in other areas. Moreover, the WRF model, adopted to simulate the SLBC over the island for all seasons, performs reasonably well reproducing the phenomenon, evolution and mechanism of SLBC. Chiefly affected by the difference of temperature between sea and land, the SLBC varies in coverage and intensity with the seasons and reaches the greatest intensity in summer. The typical depth is about 2.5 km for sea breeze circulation and about 1.5 km for land breeze circulation. A strong convergence zone with severe ascending motion appears on the line parallel to the major axis of the island, penetrating 60 to 100 km inland. This type of weak sea breeze convergence zone in winter is north-south oriented. The features of SLBC in spring are similar both to that in summer with southerly wind and to that in winter with easterly wind. The coverage and intensity of SLBC in autumn is the weakest and confined to the southwest edge of the central mountainous area. The land breeze is inherently very weak and easily affected by the topography and weather. The coverage and intensity of the land breeze convergence line is significantly less than those of the sea breeze. The orographic forcing of the central mountain exhibits significant impacts on low-level airflow. A windward land breeze front usually occurs along the coastline between the wee hours and the morning in summer, with an arc-shaped convergence zone about 10 to 30 km off shore. In winter the arc-shaped convergence zone is weak and appears only in the southeast coastal area. Landing on the flat regions of northern to western parts of the island and going inland from there, the sea breeze front at the leeward side meets with that at the windward side in the centre of the island when sea breeze fully develops, causing an intense convergence zone throughout the whole island. Consistent with prevailing winds in direction, the windward sea breeze and leeward land breeze develop quickly but are not distinguishable from background winds.
The sea-land breeze circulation (SLBC) occurs regularly at coastal locations and influences the local weather and climate significantly. In this study, based on the observed surface wind in 9 conventional meteorological stations of Hainan Island, the frequency of sea-land breeze (SLB) is studied to depict the diurnal and seasonal variations. The statistics indicated that there is a monthly average of 12.2 SLB days and an occurrence frequency of about 40%, with the maximum frequency (49%) in summer and the minimum frequency (29%) in autumn. SLB frequencies (41%) are comparable in winter and spring. A higher frequency of SLB is present in the southern and central mountains due to the enhancement effect of the mountain-valley breeze. Due to the synoptic wind the number of SLB days in the northern hilly area is less than in other areas. Moreover, the WRF model, adopted to simulate the SLBC over the island for all seasons, performs reasonably well reproducing the phenomenon, evolution and mechanism of SLBC. Chiefly affected by the difference of temperature between sea and land, the SLBC varies in coverage and intensity with the seasons and reaches the greatest intensity in summer. The typical depth is about 2.5 km for sea breeze circulation and about 1.5 km for land breeze circulation. A strong convergence zone with severe ascending motion appears on the line parallel to the major axis of the island, penetrating 60 to 100 km inland. This type of weak sea breeze convergence zone in winter is north-south oriented. The features of SLBC in spring are similar both to that in summer with southerly wind and to that in winter with easterly wind. The coverage and intensity of SLBC in autumn is the weakest and confined to the southwest edge of the central mountainous area. The land breeze is inherently very weak and easily affected by the topography and weather. The coverage and intensity of the land breeze convergence line is significantly less than those of the sea breeze. The orographic forcing of the central mountain exhibits significant impacts on low-level airflow. A windward land breeze front usually occurs along the coastline between the wee hours and the morning in summer, with an arc-shaped convergence zone about 10 to 30 km off shore. In winter the arc-shaped convergence zone is weak and appears only in the southeast coastal area. Landing on the flat regions of northern to western parts of the island and going inland from there, the sea breeze front at the leeward side meets with that at the windward side in the centre of the island when sea breeze fully develops, causing an intense convergence zone throughout the whole island. Consistent with prevailing winds in direction, the windward sea breeze and leeward land breeze develop quickly but are not distinguishable from background winds.
2014, 20(3): 279-288.
Abstract:
This paper comprehensively studies the spatio-temporal characteristics of the frequency of extremely heavy precipitation events over South China by using the daily precipitation data of 110 stations during 1961 to 2008 and the extremely heavy precipitation thresholds determined for different stations by REOF, trend coefficients, linear trend, Mann-Kendall test and variance analysis. The results are shown as follows. The frequency distribution of extremely heavy precipitation is high in the middle of South China and low in the Guangdong coast and western Guangxi. There are three spatial distribution types of extremely heavy precipitation in South China. The consistent anomaly distribution is the main type. Distribution reversed between the east and the west and between the south and the north is also an important type. Extremely heavy precipitation events in South China mainly occurred in the summer-half of the year. Their frequency during this time accounts for 83.7% of the total frequency. In the 1960s and 1980s, extremely heavy precipitation events were less frequent while having an increasing trend from the late 1980s. Their climatological tendency rates decrease in the central and rise in the other areas of South China, and on average the mean series also shows an upward but insignificant trend at all of the stations. South China’s frequency of extremely heavy precipitation events can be divided into six major areas and each of them shows a different inter-annual trend and three of the representative stations experience abrupt changes by showing remarkable increases in terms of Mann-Kendall tests.
This paper comprehensively studies the spatio-temporal characteristics of the frequency of extremely heavy precipitation events over South China by using the daily precipitation data of 110 stations during 1961 to 2008 and the extremely heavy precipitation thresholds determined for different stations by REOF, trend coefficients, linear trend, Mann-Kendall test and variance analysis. The results are shown as follows. The frequency distribution of extremely heavy precipitation is high in the middle of South China and low in the Guangdong coast and western Guangxi. There are three spatial distribution types of extremely heavy precipitation in South China. The consistent anomaly distribution is the main type. Distribution reversed between the east and the west and between the south and the north is also an important type. Extremely heavy precipitation events in South China mainly occurred in the summer-half of the year. Their frequency during this time accounts for 83.7% of the total frequency. In the 1960s and 1980s, extremely heavy precipitation events were less frequent while having an increasing trend from the late 1980s. Their climatological tendency rates decrease in the central and rise in the other areas of South China, and on average the mean series also shows an upward but insignificant trend at all of the stations. South China’s frequency of extremely heavy precipitation events can be divided into six major areas and each of them shows a different inter-annual trend and three of the representative stations experience abrupt changes by showing remarkable increases in terms of Mann-Kendall tests.
2014, 20(3): 289-296.
Abstract:
An improved Smith iterative method, which generally applies to microwave bands, is described. The moisture profiles retrieved from the brightness temperature data of microwave humidity sounder onboard FY-3A satellite, pertaining to clear sky, are shown over western North Pacific by using a vector-discrete ordination radiative transfer model. The retrieved profiles are compared with those from MODIS products, and the results of single point show that they are in good agreement with the results for lower layers. The distribution of retrieved humidity at 500 hPa is basically consistent with the actual situation, and the values are lower than that of the MODIS products. Compared with MODIS products, RMS is within 5.76g/kg at every single level separately.
An improved Smith iterative method, which generally applies to microwave bands, is described. The moisture profiles retrieved from the brightness temperature data of microwave humidity sounder onboard FY-3A satellite, pertaining to clear sky, are shown over western North Pacific by using a vector-discrete ordination radiative transfer model. The retrieved profiles are compared with those from MODIS products, and the results of single point show that they are in good agreement with the results for lower layers. The distribution of retrieved humidity at 500 hPa is basically consistent with the actual situation, and the values are lower than that of the MODIS products. Compared with MODIS products, RMS is within 5.76g/kg at every single level separately.
2014, 20(3): 242-250.
doi: 10.16555/j.1006-8775.2014.03.007
Abstract:
Long-term observational data indicated a decreasing trend for the amount of autumn precipitation (i.e. 54.3 mm per decade) over Mid-Eastern China, especially after the 1980s (~ 5.6% per decade). To examine the cause of the decreasing trend, the mechanisms associated with the change of autumn precipitation were investigated from the perspective of water vapor transportation, atmospheric stability and cloud microphysics. Results show that the decrease of convective available potential energy (i.e. 12.81 J kg-1/ decade) and change of cloud microphysics, which were closely related to the increase of aerosol loading during the past twenty years, were the two primary factors responsible for the decrease of autumn precipitation. Our results showed that increased aerosol could enhance the atmospheric stability thus weaken the convection. Meanwhile, more aerosols also led to a significant decline of raindrop concentration and to a delay of raindrop formation because of smaller size of cloud droplets. Thus, increased aerosols produced by air pollution could be one of the major reasons for the decrease of autumn precipitation. Furthermore, we found that the aerosol effects on precipitation in autumn was more significant than in other seasons, partly due to relatively more stable synoptic systems in autumn. The impact of large-scale circulation dominant in autumn and the dynamic influence on precipitation was more important than the thermodynamic activity.
Long-term observational data indicated a decreasing trend for the amount of autumn precipitation (i.e. 54.3 mm per decade) over Mid-Eastern China, especially after the 1980s (~ 5.6% per decade). To examine the cause of the decreasing trend, the mechanisms associated with the change of autumn precipitation were investigated from the perspective of water vapor transportation, atmospheric stability and cloud microphysics. Results show that the decrease of convective available potential energy (i.e. 12.81 J kg-1/ decade) and change of cloud microphysics, which were closely related to the increase of aerosol loading during the past twenty years, were the two primary factors responsible for the decrease of autumn precipitation. Our results showed that increased aerosol could enhance the atmospheric stability thus weaken the convection. Meanwhile, more aerosols also led to a significant decline of raindrop concentration and to a delay of raindrop formation because of smaller size of cloud droplets. Thus, increased aerosols produced by air pollution could be one of the major reasons for the decrease of autumn precipitation. Furthermore, we found that the aerosol effects on precipitation in autumn was more significant than in other seasons, partly due to relatively more stable synoptic systems in autumn. The impact of large-scale circulation dominant in autumn and the dynamic influence on precipitation was more important than the thermodynamic activity.
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