| Drought and flood disasters are the most frequent and serious meteorological disasters.Droughts and floods coexist in Yunnan,and droughts and floods turn rapidly,and cause secondary disasters such as landslides and mudslides.These disasters not only threaten the development of agriculture in Yunnan,but also threaten the safety of people's lives and property.In recent years,the frequency of sudden droughts and floods has increased,the scope of impact has expanded,and the distribution between regions is extremely uneven.In order to gain a deeper understanding of the law of rapid transition of drought and flood in low-latitude plateau cities and regional differences,this paper uses Mann-Kendall trend test,mutation test,wavelet analysis and other methods to compare and analyze based on the summer drought and flood rapid transition index and precipitation Z index.The temporal and spatial distribution characteristics of the sudden drought and flood in Qujing City and Chuxiong Prefecture in summer,and the NCEP/NCAR reanalysis data are used to preliminarily analyze the relationship between the drought and flood in summer in the two places and the atmospheric circulation and other possible influence factors.The main conclusions reached are:(1)In the past 59 years,Qujing City and Chuxiong Prefecture's Long Cycle Drought and Flood Rapid Transition Index(LDFAI)has shown a significant decreasing trend,indicating that there will be more rain-waterlogging and drought-to-drought events in the two places in the future,and drought to rain-flooding events will decrease.The frequency of the mayor's cycle of drought and flood in Qujing is higher than that in Chuxiong Prefecture.Among them,there are 14 times in Qujing City and 10 times in Chuxiong Prefecture,and 14 times in Qujing City and Chuxiong Prefecture.7 times.The years of the maximum and minimum LDFAI are not the same.The most severe year of drought in Qujing City was 1999,and that of Chuxiong was 1986;the most severe year of conversion from rainfall to drought in Qujing City was 2003 in Chuxiong Prefecture in 1972.There was no mutation in LDFAI in Qujing City,and there were two mutation points in LDFAI in Chuxiong Prefecture in 1996 and 1974.The sudden change point of the LDFAI intensity in Qujing City was in 1981,but there was no abrupt change in Chuxiong Prefecture.The LDFAI oscillation cycles in Qujing City and Chuxiong Prefecture are not the same,9 years in Qujing City and 20 years in Chuxiong Prefecture.Qujing City's oscillation cycles are more frequent,and droughts and floods have changed sharply.The frequency of incidents is also more frequent.Qujing City and Chuxiong Prefecture LDFAI distribution and the frequency distribution of drought and flood events showed different characteristics.In Qujing City,the drought-to-rainfall year showed a trend of decreasing from the east to the surroundings,while Chuxiong Prefecture showed a trend of decreasing from the west to the surroundings;Qujing City's rain-waterlogging turned to a drought year showed a trend of increasing from the northeast to the southwest,and Chuxiong Prefecture's rain and waterlogging trends.The turning drought year shows a decreasing trend from southeast to northwest.The frequency distribution of sudden drought and flood events is also different.The frequency of drought-to-rainfall events in Qujing City shows an increasing trend from southeast to northwest,and Chuxiong Prefecture shows a decreasing trend from the middle to the surrounding areas;Qujing City's rain-flood-todrought events occur The frequency of Chuxiong Prefecture is high in the central region and low in the southern and northern regions;Chuxiong Prefecture is high in the eastern and southern regions,and low in the central and western regions.(2)In the past 59 years,Qujing City and Chuxiong Prefecture have similarities and differences in the short-period drought-flood transition events.The short-period drought-flood transition index(SDFAI)in Qujing City from May to June showed a significant upward trend,indicating that May-June Drought-to-rain-waterlogging events tend to increase;while SDFAI in Chuxiong Prefecture from May to June showed a significant downward trend,indicating that rain-waterlogging-to-drought events will increase in the future.The SDFAI in June-July and July-August showed a downward trend.Qujing City and Chuxiong Prefecture's distribution and trend analysis of shortperiod drought and flooding events are also different.Generally speaking,the spatial distribution of SDFAI in Qujing City and Chuxiong Prefecture does not have a particularly obvious pattern,but in the comparison of the spatial distribution characteristics of the change trend.Zhanyi and Malong in Qujing City from July to August passed the 90% significance test,and Fuyuan passed the 95% significance test.There was a significant increase in drought-to-rainfall events in these three regions.Other periods and regions None of the trend of changes passed the significance test.In Chuxiong Prefecture,from May to June,Yuanmou and Dayao passed the 90%significance test,and SDFAI showed a significant decrease;from June to July,Lufeng passed the 95% significance test,and SDFAI also showed a significant decrease;From July to August,Yongren passed the 95% significance test,and SDFAI showed a significant decrease trend;these areas all showed a significant increase in rainwaterlogging to drought.Except for the above time and region,none of them passed the significance test.(3)The research on the characteristics of atmospheric circulation shows that from the perspective of the altitude field,the mid-high latitude cold air cannot reach Qujing during the drought period(May-June)during the drought-to-rain-flooding year,and the subtropical high is relatively strong and the location is biased.In the west,during the flood period(July-August),the high pressure in the middle and high latitudes increases,and the intensity of the main subtropical high is reduced in the east.The cold air flows southward,and it merges with the water vapor from the Bay of Bengal and the western Pacific to form precipitation;rain-waterlogging turns into arid waterlogging.During the period,the subtropical high is located to the south and weak,and the warm and humid air flow is transported to the low-latitude plateau areas.The southeast wind prevails in the Indo-China Peninsula,which also brings water vapor to Qujing City.During the drought period(July-August),the cold air at mid-high latitudes weakens.,The impact of the subtropical high extends westward,and the prevailing westerly wind in the Bay of Bengal is not conducive to precipitation in Qujing;Chuxiong Prefecture shows similar characteristics.From the perspective of the anomalous wind field,the drought period(May-June)in Qujing City is not conducive to precipitation,and the anticyclonic circulation occurs during the flood period(July-August).Chuxiong Prefecture experienced drought during the same period.Cyclone circulation occurred during the flood period,and anticyclonic circulation occurred during the flood period;Qujing City experienced anticyclonic circulation during the flood period(May-June)when the rain and flood turned to drought,and the cyclone circulation occurred during the dry period.Chuxiong Prefecture experienced flooding during the same period.Anticyclonic circulation occurred during the period,and cyclonic circulation occurred during the dry period.However,Qujing City is in an area with a larger anomaly.In addition,Qujing City is also affected by the easterly winds of the Pacific and South China Sea in addition to the southwest monsoon in summer.As a result,the frequency of sudden droughts and floods in Qujing City is higher than that of Chuxiong Prefecture,and the intensities of droughts and floods are not synchronized.From the perspective of vertical wind field anomalies,there is a sinking airflow in the dry period(May-June)when the drought turns to rain and flood in Qujing City,which is located in the positive anomaly area,and the updraft is located in the negative anomaly during the flood period(July-August)In the same period,Chuxiong Prefecture was located in the negative anomaly area during the drought period,and updraft occurred in the negative anomaly area during the same period.The updraft occurred in the positive anomaly area during the flood period.Qujing City was in the updraft during the flood period(May-June)when the rain-waterlogged to arid.Located in the negative anomaly area,the downdraft prevails during the dry period(July-August)and is located in the positive anomaly area;the downdraft prevails in the flood period of the same period in Chuxiong Prefecture,which is located in the positive anomaly area,and the updraft prevails in the negative anomaly during the dry period.Anomaly zone.It can be clearly seen that the 0Pa/s isoline is often located near 101°E,which happens to separate Qujing City and Chuxiong Prefecture.The two places are in opposite anomalies,and different air currents prevail,thus causing the intensity of droughts and floods in the two places to turn sharply.Out of sync.In addition to being affected by the atmospheric circulation,factors such as high terrain,geological differences,city size,population and other factors may also cause the frequency of sudden droughts and floods in Qujing City to occur higher than that of Chuxiong Prefecture.The impact of the city,the karst landform of Qujing City has also increased the impact of the sudden turn of droughts and floods. |
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