湛江极端低温事件特征及个例对比分析

ANALYSIS AND CASE STUDY OF CHARACTERISTICS OF EXTREME MINIMUM TEMPERATURE IN ZHANJIANG

关键词:  极端低温  阻塞高压  西伯利亚高压  温度平流  北极涛动  ENSO

Key words:  extreme minimum temperature  blocking high  Siberian High  temperature advection  Arctic Oscillation  El Niño-Southern Oscillation

摘要:

利用广东湛江1951—2018年逐日气温资料、NCEP再分析资料和NOAA重建海温资料，分析近67年湛江冬季极端低温事件的特征，对比分析1967年1月、1975年12月和2016年1月三次极端低温事件过程的天气要素和环流特征，从ENSO和北极涛动（AO）方面探讨形成极端低温的原因。研究表明：1951—2017年平均最低气温和极端最低气温均呈增暖趋势，且极端最低气温比平均最低气温增暖更快。湛江冬季极端最低气温受东亚冬季风南北一致型模态影响，东亚冬季风一致偏强时湛江极端最低气温偏低，其关键环流系统为乌拉尔山及其以东地区的高压脊、贝加尔湖以南地区的低压槽和西伯利亚高压。进一步分析表明，三次冷空气过程受ENSO和AO的影响不同，其中1967年冷空气受东部型拉尼娜和AO负位相共同影响，1975年冷空气主要受中部型拉尼娜影响，2016年冷空气则主要受AO负位相影响。三次过程均以偏西路冷空气活动为主，都表现出对流层中层东亚地区位势高度偏低、对流层低层异常北风和西伯利亚高压偏强的特征。但高空环流形势不同，1967年冷空气过程为高压脊发展型，预报关键是槽后脊的发展时间；1975和2016年冷空气过程环流形势均为横槽转竖型，预报关键是阻塞高压崩溃、横槽转竖时间。地面受冷空气影响时间与925 hPa温度平流有较好的对应关系，但由平流降温造成的过程最低气温与850 hPa冷平流强度对应，过程最低气温往往发生在850 hPa冷平流最强的次日，另外还要关注辐射降温作用对最低气温的影响。

Abstract:

Based on the daily temperature data of Zhanjiang, the reanalysis data from the NCEP/NCAR and the Extended Reconstructed Sea Surface Temperature (SST) V5 from NOAA during 1951—2018, this study discussed the characteristics of winter extreme minimum temperature (EMT) in Zhanjiang during recent 67 years, compared the weather elements and circulation characteristics of three cases (January 1967, December 1975 and January 2016), and discussed the reasons of EMT from the perspective of El Ni?o-Southern Oscillation (ENSO) and Arctic Oscillation (AO). The results showed that the average minimum temperature and the EMT had a warming trend during 1951—2017, and the latter increased faster than the former. The winter EMT in Zhanjiang was affected by the mode of East Asian winter monsoon (EAWM) with the same sign of anomalies throughout eastern China. It was in favor of the EMT reducing in Zhanjiang when the EAWM was uniformly stronger. The key circulation systems were the ridge over the Ural Mountains and its east, the trough on the south of Baikal Lake and Siberian High. Further analysis showed that the influence of ENSO and AO on the three cases was different. The 1967 cold air was jointly affected by the negative phase of AO and the Eastern-Pacific La Ni?a, the 1975 cold air was mainly affected by the Central-Pacific La Ni?a, and the 2016 cold air was mainly influenced by the negative phase of AO. The three processes were dominated by the cold air activities in the west side, and the circulation showed the characteristics such as low geopotential height in the middle troposphere in East Asia, abnormal northerly wind in the lower troposphere and strong Siberian High. The three cases had different high-altitude circulation patterns. The 1967 cold air high-altitude situation was high pressure ridge development, and the key to forecast was the development time of the ridge behind trough. The 1975 and the 2016 cold air high-altitude situations were the transverse trough turning anticlockwise to meridional with blocking high-pressure activity, and the forecast key was the time of blocking high collapse and the zonal trough turning to a meridional one. The influence time of cold air on the ground corresponded well with the 925 hPa temperature advection, but the lowest process temperature caused by advection cooling corresponded to the intensity of 850 hPa cold temperature advection. The lowest process temperature usually lagged behind the strongest 850hPa cold temperature advection for one day. In addition, we should pay attention to the impact of radiation cooling on the lowest temperature.