1.National Meteorological Center, Beijing 100081;2.International Center for Climate and Environment Science, Institute of Atmospheric Physics, Chinese Academy of Sciences, Beijing 100029
Forecaster Special Project of National Meteorological Center Grant Y201708;Forecaster Special Project of China Meteorological Administration Grant CMAYBY2019-141Forecaster Special Project of National Meteorological Center (Grant Y201708), Forecaster Special Project of China Meteorological Administration (Grant CMAYBY2019-141)
基于实况观测资料、欧洲中期天气预报中心(European Centre for Medium-Range Forecast, ECMWF)0.5°（纬度）×0.5°（经度）水平分辨率的再分析数据和集合预报数据，对2018年2月一次入海爆发性气旋在黄海南部的爆发性增强时期的动力和热力因子进行了对比分析。根据气旋路径、强度和海面风的检验结果挑选出两组集合成员——好成员组和坏成员组。通过组间对比分析得到如下主要结论 1）在气旋入海之后爆发性增强时，500 hPa高空槽和850 hPa中低层低涡迅速加强，同时低层和高层的西南急流均明显加大，中高层系统快速增强，上述因子均为气旋出现爆发性发展提供有利条件。2）气旋入海之后上升运动快速增强，这加剧了低层辐合与高层辐散，有利于地面降压，促使地面气旋的爆发性发展。水汽在中低层辐合后随气流上升发生凝结并释放潜热，这加强了高层辐散、低层辐合以及上升运动，促使气旋进一步爆发性发展。与此同时，对流层顶的高值位涡下传增强，低层大气斜压性受气旋上空冷暖平流的增强而增大，导致垂直稳定度减小，地面气旋性涡度增强，也有利于气旋爆发性发展。最终此次气旋快速增强并达到中等爆发性气旋的强度。3）虽然集合预报两组成员的平均场均比分析场弱，但是好成员组抓住了气旋上空中高层天气系统的快速增强过程，以及垂直运动、温度平流、水汽条件、位涡等预报因子和物理量的快速增强过程，其预报效果在气旋强度和路径等方面均显著优于坏成员组。
Observations and European Centre for Medium-Range Forecast (ECMWF) reanalysis data were used to compare the ability of ensemble forecast members [with a horizontal resolution of 0.5º(latitude)×0.5º(longitude)] to provide dynamic and thermodynamic factors relating to a cyclone that exploded over the southern Yellow Sea in February 2018. According to validation results for the track and strength of the cyclone and surface wind, two sets of ensemble forecast members were selected as good and bad forecast members, respectively, and the main conclusions were subsequently obtained from a comparative investigation between them as follows: 1) When the cyclone had explosively developed, the trough and vortex at 500 hPa and 850 hPa, respectively, strengthened rapidly, and the southwest jet streams at lower and upper levels simultaneously increased rapidly in association with the rapid strengthening of synoptic systems; this provided a favorable condition for explosive cyclone development. 2) With the rapid strengthening of ascending motion after the cyclone had entered the Yellow Sea, convergence and divergence at lower and upper levels were respectively intensified; this promoted a pressure reduction and the cyclone then explosively developed. Following convergence of water vapor at a mid-low level, latent heat was released under the ascending motion, which promoted divergence and convergence at upper and lower levels, respectively, and then enhanced ascending motion. Therefore, the rapid strengthening of water vapor flux convergence caused the cyclone to explosively develop. The downward transmission of the high potential vorticity (PV) and baroclinicity at a lower level were strengthened, the degree of stability weakened, and cyclonic vorticity was enhanced. These factors were beneficial for explosive cyclone development, and it finally increased to a medium-strength explosive cyclone. 3) Although the forecast fields from the two ensemble sets were both weaker than those of analysis data, the good ensemble members caught the rapid strengthening of the synoptic system at a mid-upper level over the cyclone and the rapid developing processes of these key factors and physical quantities (such as vertical motion, PV, temperature advection, and water vapor). Therefore, the ability of the good members to track the strengthening cyclone was found to be superior to that of bad members.