2022年4月云南罕见寒潮天气成因及预报偏差分析

The analysis of the cause and forecast bias of a rare cold wave in Yunnan in April,2022

2022年3月31日—4月2日,云南省出现历史同期罕见的寒潮天气过程.通过地面观测资料、高空观测资料和NCEP FNL资料,对此次寒潮过程的成因进行分析,并对欧洲中期天气预报中心(European Centre for Medium-Range Weather Forecasts,ECMWF)模式的形势场及最低气温进行检验.结果表明:此次寒潮过程发生在北极涛动负位相期间,西伯利亚高压偏强,欧亚中高纬形成两槽一脊的环流形势.东亚大槽后侧强劲的偏北风引导低层冷空气南移,南支槽向东移动为云南省输送暖湿气流,700 hPa切变线和地面冷锋的南侵,共同造成了此次寒潮天气过程的发生;此次寒潮过程的强降温是由过程中近地层强冷平流的作用,冷锋后部较强的垂直上升运动,引起绝热膨胀冷却作用,加剧了局地气温的下降;通过定量分析导致昆明局地气温变化的各项因子发现,昆明的降温主要受温度平流项的影响,其次是非绝热项的影响. ECMWF模式能较好地预报此次寒潮过程的高低空环流形势及降温的范围,但对降温强度的预报效果相对较差.

A rarely seen cold wave hit Yunnan Province from March 31st to April 2nd,2022.This study analyzes the formative factors of the cold wave based on the data of in situ observations,upper air observations and NCEP FNL.Meanwhile,the study also examines the situation fields and the lowest temperature by using the European Centre for Medium-Range Weather Forecasts(ECMWF)model.The result shows that this cold wave occurred in a negative phase of Arctic Oscillation,when there happened an abnormally strong Siberian High,and a large-scale two-trough-and-one-ridge circulation pattern in the Eurasian mid-and-high latitudes.Northerly wind behind the Eastern Asian trough piloted the low level cold air towards the south;meanwhile,the southern branch trough moved eastward and transported warm wet air towards Yunnan.The 700 hPa wind shear and near surface cold front jointly caused this cold wave when they invaded southward.The severe temperature drop was caused by the strong near surface cold air flux during the event.Furthermore,the strong vertical upward motion of the cold front rear and its corresponding adiabatic expansion cooling quickened the local temperature drop.A quantitative analysis of the factors causing the local temperature change in Kunming finds out that the horizontal temperature flux dominates the temperature drop in Kunming,followed by the non-adiabatic item.As for this cold wave,the ECMWF model was fairly good at forecasting the high-and-low-level circulation pattern and the geographical range of temperature drop,while less good at forecasting the strength of temperature decrease.