Abstract:

The weather research and forecasting (WRF) model along with its three-dimensional data assimilation (3D-Var) system was used to improve the initial field by assimilating radar reflectivity and radial velocity from Doppler radars. A rare strong squall line occurring on 4 March 2018 in Jiangxi Province was simulated and studied by using the adjusted initial field. It is found that only assimilation of rainwater, snow, graupel particles retrieved from radar reflectivity and water vapor derived from radar reflectivity could not make a stable improvement in forecasting the composite reflectivity, especially got an opposite effect on forecasting surface gale and precipitation. The assimilation of reflectivity data could significantly improve forecast skill when radar radial velocity was assimilated jointly. The reason is that assimilation of radar reflectivity can efficiently adjust initial hydrometeors and thermal field, but it has little effect on initial dynamic field. As the simulation time went by, the adjustment of dynamic field was unreasonable, a false divergence wind field appeared in the upper troposphere, thus a stratiform cloud area appeared in front of the squall line, which was not captured in reality. So the model could not improve the simulation of vertical wind shear and cold pool as well as rear inflow, and then there was a large gap between forecasted results and observations. Only assimilation of radar radial velocity could improve simulation results, and assimilation of both could efficiently adjust initial hydrometeors and thermal fields as well as dynamic field, and make the physical configuration more compatible with reality.The vertical wind shear and wind field structure were more favorable to occurrence of the strong squall line, and then formed a strong cold pool close to reality, further the forecasting results of radar composite reflectivity, surface wind and precipitation of the squall line agreed much better with observations compared with only assimilation of radar radial velocity.

Key words: rare squall line, thunderstorm gale, Doppler radar data, data assimilation

摘要：

选用WRF（weather research and forecasting）模式及其3D-Var（three-dimensional variation）同化系统,针对2018年3月4日发生在江西的一次罕见强飑线天气,探讨同化多普勒雷达不同观测资料对极端雷暴大风天气模拟预报的影响。结果表明：仅同化由雷达反射率反演的雨水、雪和霰粒子以及由其估算的水汽不能稳定改善模式对飑线雷达反射率的预报效果,尤其对地面大风和降水的预报起反效果;当联合同化雷达反射率与雷达径向风资料后,显著改进了模式对飑线发展演变过程中雷达反射率、地面降水和地面大风的预报效果,雷达反射率的同化呈现显著正效果。原因是仅同化雷达反射率对初始水成物及热力场影响较大,而对动力场调整微弱,随着积分时间增加,热力场对动力场的反馈作用不真实,高层出现虚假辐散风场,飑线前侧模拟出虚假层状云区,且未能改进飑线系统低层垂直风切变、冷池以及对流层中下层后侧入流的模拟,模拟的飑线移动和演变过程与实况有很大差距;当联合同化雷达反射率与雷达径向风资料后明显调整了初始动力、热力和水成物场,物理配置更符合实际,形成更有利于强飑线发生的垂直风切变和风场结构,产生与实况接近的强冷池,模拟结果与实况的吻合度明显高于仅同化雷达径向风资料的试验。

关键词: 罕见飑线, 雷暴大风, 多普勒雷达观测资料, 资料同化