[Objective] The aim was to study the reasons of low temperature and characteristics of weather in Yuxi City.[Method] The reasons of low temperature in Yuxi City were expounded.The types of low temperature and their in...[Objective] The aim was to study the reasons of low temperature and characteristics of weather in Yuxi City.[Method] The reasons of low temperature in Yuxi City were expounded.The types of low temperature and their influences on agricultural crops were introduced.The route of cold air and the typical circulation characteristics of low temperature were analyzed.[Result] The low temperature in Yuxi City was formed because of the influence of its topography and climate.The low temperature and cold disaster included frost,low temperature in spring and autumn;the influencing path of cold air in Yuxi City had four paths:northeast,north,northwest and east path.The circulation characteristics of snowing included southerly trough,strong and cold air,and cold front shear;under the long-term control of cold advection of upper air and cold high pressure in the low layer,the radiation,cooling and temperature reducing were the main circulation characteristics of heavy frost;the circulation background of low temperature in August was conductive to the strong cold air intruding the upper air circulation of Yuxi City.The cold trough in the north and low pressure in the south influenced the circulation in Yuxi City.[Conclusion] The study provided reference for the weather report and relevant professionals.展开更多
Based on the NCEP/NCAR reanalysis dataset and in situ meteorological observations of daily precipitation in boreal summer from 1979 to 2008, the features of circulation anomalies have been investigated using the compo...Based on the NCEP/NCAR reanalysis dataset and in situ meteorological observations of daily precipitation in boreal summer from 1979 to 2008, the features of circulation anomalies have been investigated using the composite analysis for the extreme events and non-extreme events of regional mean daily rainfall(RMDR) occurring over the midand lower- Yangtze valley(MLYV). The extreme RMDR(ERMDR) events are the events at and above the percentile99 in the rearranged time-series of the RMDR with ascending order of rainfall amount. The non-extreme RMDR events are those at the percentiles 90-85 and 80-75 separately. Our results suggest that the threshold value is 25 mm/day for the ERMDR at percentile 99. Precipitation at all the percentiles is found to occur more frequently in the Meiyu rainfall season in MLYV, and the ERMDR events have occurred with higher frequency since the 1990 s. For the percentiles-associated events, the MLYV is under the control of an anomalous cyclonic circulation in the mid- and lower- troposphere with vastly different anomalous circulation at higher levels. However, at both low and high levels, the ERMDR events-related anomalous circulation is stronger compared to that linked to the non-ERMDR events. The dominant sources of water vapor differ between the ERMDR and non-ERMDR events. During the ERMDR events plentiful water vapor is transported from the Bay of Bengal into the MLYV directly by divergence while there is distinctly increased water vapor from the South China Sea(SCS) in non-RMERMDR episodes. The diabatic heating rates < Q1>, < Q2> and< Q1>- < Q2> have their anomalous patterns and are consistent with each other for these percentiles but their strength decreases markedly with the drop of rainfall intensity. For the precipitation at percentiles 99 and 90-85, the sea surface temperature anomalies(SSTA) in the Pacific distribute positively(negatively) in the south(north), and are stronger when the ERMDR emerges, with little or no SSTA as the events at percentile 80-75 occur. Besides, these results suggest that the genesis of the ERMDR event is directly related to intense local circulation anomalies and the circulation anomalies over the Pacific and SCS in tropical to mid-latitudes, and probably linked with the Pacific SSTA closely while the non-ERMDR events are mainly associated with the anomalous circulation on a local basis. The findings here help understand and predict the happening of ERMDR events over the MLYV.展开更多
This article examined the meteorological features of the second Meiyu/Baiu episode(hereafter Meiyu Ⅱ)in 1998 and the results are summarized as follows:(1)The Meiyu Ⅱ period in 1998 has been the longest and the most ...This article examined the meteorological features of the second Meiyu/Baiu episode(hereafter Meiyu Ⅱ)in 1998 and the results are summarized as follows:(1)The Meiyu Ⅱ period in 1998 has been the longest and the most delayed ending one since 1885,which caused the great flood around the Yangtze River Basin.The circulation situation in Meiyu Ⅱ is so typical that the average geopotential height and wind fields at 500 hPa and 850 hPa are very similar to monthly mean in June.The abnormal circulation in Meiyu Ⅱ is associated with an adjusting of the Okhotsk high and short period south-north oscillation of the subtropical high.(2)The heavy rainfall around the Yangtze River during Meiyu Ⅱ period is closely related to a persistent shear line,a clear eastward moving vortex structure and a very strong jet flow at lower levels.It is very clear that the strong divergence at higher levels and convergence at lower levels occurred in situ during Meiyu Ⅱ period. (3)It is very clear that the convective activity propagates eastward quickly with a period about 7 days during Meiyu Ⅱ period.展开更多
文摘[Objective] The aim was to study the reasons of low temperature and characteristics of weather in Yuxi City.[Method] The reasons of low temperature in Yuxi City were expounded.The types of low temperature and their influences on agricultural crops were introduced.The route of cold air and the typical circulation characteristics of low temperature were analyzed.[Result] The low temperature in Yuxi City was formed because of the influence of its topography and climate.The low temperature and cold disaster included frost,low temperature in spring and autumn;the influencing path of cold air in Yuxi City had four paths:northeast,north,northwest and east path.The circulation characteristics of snowing included southerly trough,strong and cold air,and cold front shear;under the long-term control of cold advection of upper air and cold high pressure in the low layer,the radiation,cooling and temperature reducing were the main circulation characteristics of heavy frost;the circulation background of low temperature in August was conductive to the strong cold air intruding the upper air circulation of Yuxi City.The cold trough in the north and low pressure in the south influenced the circulation in Yuxi City.[Conclusion] The study provided reference for the weather report and relevant professionals.
基金National Natural Science Foundation of China(41330425)National Key Technology R&D Program(2007BAC29B02)"Qinglan"Project of Jiangsu Province for Cultivating Research Teams
文摘Based on the NCEP/NCAR reanalysis dataset and in situ meteorological observations of daily precipitation in boreal summer from 1979 to 2008, the features of circulation anomalies have been investigated using the composite analysis for the extreme events and non-extreme events of regional mean daily rainfall(RMDR) occurring over the midand lower- Yangtze valley(MLYV). The extreme RMDR(ERMDR) events are the events at and above the percentile99 in the rearranged time-series of the RMDR with ascending order of rainfall amount. The non-extreme RMDR events are those at the percentiles 90-85 and 80-75 separately. Our results suggest that the threshold value is 25 mm/day for the ERMDR at percentile 99. Precipitation at all the percentiles is found to occur more frequently in the Meiyu rainfall season in MLYV, and the ERMDR events have occurred with higher frequency since the 1990 s. For the percentiles-associated events, the MLYV is under the control of an anomalous cyclonic circulation in the mid- and lower- troposphere with vastly different anomalous circulation at higher levels. However, at both low and high levels, the ERMDR events-related anomalous circulation is stronger compared to that linked to the non-ERMDR events. The dominant sources of water vapor differ between the ERMDR and non-ERMDR events. During the ERMDR events plentiful water vapor is transported from the Bay of Bengal into the MLYV directly by divergence while there is distinctly increased water vapor from the South China Sea(SCS) in non-RMERMDR episodes. The diabatic heating rates < Q1>, < Q2> and< Q1>- < Q2> have their anomalous patterns and are consistent with each other for these percentiles but their strength decreases markedly with the drop of rainfall intensity. For the precipitation at percentiles 99 and 90-85, the sea surface temperature anomalies(SSTA) in the Pacific distribute positively(negatively) in the south(north), and are stronger when the ERMDR emerges, with little or no SSTA as the events at percentile 80-75 occur. Besides, these results suggest that the genesis of the ERMDR event is directly related to intense local circulation anomalies and the circulation anomalies over the Pacific and SCS in tropical to mid-latitudes, and probably linked with the Pacific SSTA closely while the non-ERMDR events are mainly associated with the anomalous circulation on a local basis. The findings here help understand and predict the happening of ERMDR events over the MLYV.
基金National Natural Science Foundation of China under Grant No.49794030.
文摘This article examined the meteorological features of the second Meiyu/Baiu episode(hereafter Meiyu Ⅱ)in 1998 and the results are summarized as follows:(1)The Meiyu Ⅱ period in 1998 has been the longest and the most delayed ending one since 1885,which caused the great flood around the Yangtze River Basin.The circulation situation in Meiyu Ⅱ is so typical that the average geopotential height and wind fields at 500 hPa and 850 hPa are very similar to monthly mean in June.The abnormal circulation in Meiyu Ⅱ is associated with an adjusting of the Okhotsk high and short period south-north oscillation of the subtropical high.(2)The heavy rainfall around the Yangtze River during Meiyu Ⅱ period is closely related to a persistent shear line,a clear eastward moving vortex structure and a very strong jet flow at lower levels.It is very clear that the strong divergence at higher levels and convergence at lower levels occurred in situ during Meiyu Ⅱ period. (3)It is very clear that the convective activity propagates eastward quickly with a period about 7 days during Meiyu Ⅱ period.
