黄海海雾 WRF 数值模拟中垂直分辨率的敏感性研究

摘要

基于 WRF 模式探究不同垂直分辨率下模式对黄海海雾的模拟表现。将3种垂直分层方案（35η、44η与63η）和2种边界层方案（YSU、MYNN）组合，对10次海雾做模拟研究。统计分析了水平雾区与雾顶高度对垂直分辨率的敏感性，并利用1次典型个例剖析了雾顶长波辐射与雾体湍流的作用。统计结果显示，提高垂直分辨率能显著改进水平雾区的模拟效果，改善明显的个例在不同垂直分辨率试验下的雾顶高度差异也较大；YSU 方案相比于 MYNN 方案更敏感，从35η层增至44η层其试验个例的击中率（POD）和公正预兆得分（ETS）的平均改进率分别提高了13．29％和10．22％。典型个例研究结果揭示，模式对雾顶长波辐射和雾体湍流作用的模拟程度强烈依赖垂直分辨率：粗垂直分辨率给出的湍流强度很弱，导致模拟失败；雾顶存在“云水含量增多→长波辐射增强→降温加大→云水含量增多”的正反馈过程，细垂直分辨率比粗垂直分辨率更容易维持与增强此反馈；只有细垂直分辨率才能模拟出雾顶长波辐射冷却产生的贯穿雾体直抵海面、强度不弱于近海面机械湍流的浮力湍流，它导致雾体降温而出现符合观测事实的海温高于气温的现象。%The performance and sensitivity of the Weather Research and Forecasting (WRF)model to different vertical resolu-tions for simulating the Yellow Sea fog are studied.Numerical experiments with combinations of three vertical resolutions (35η,44ηand 63η)and two planetary boundary layer schemes (YSU,MYNN)are designed and conducted for the simulation of 10 sea fog cases.The sensitivity of fog-area and fog-top height to vertical resolution is statistically analyzed,and a typical case is investigated in detail to reveal the effects of fog-top long-wave radiation cooling and turbulence inside the fog.The sta-tistical results show that:(1 )the fog-area simulation is significantly improved with increases in vertical resolution,and the model also performs better in fog-area simulations for those cases that have large fog height differences between the experiments with different vertical resolutions;(2)the YSU scheme is more sensitive than the MYNN scheme.Compared to the 35η-exper-iments,the averaged probability of detection (POD)and equitable threat score (ETS)in the 44η-experiments have improved by 13.29% and 10.22%,respectively.Detailed analysis of the typical case indicates that a reliable modeling of fog-top long-wave radiation cooling and turbulence inside the fog strongly depends on vertical resolution.It can be outlined as:(1)a coarse verti-cal resolution with a weak turbulence intensity leads to the failure of simulation;(2)a positive feedback———"increased cloud liquid water → enhancing long-wave radiation → strengthening cooling → cloud liquid water increased"———forms near the fog top,and fine vertical resolution is more helpful to maintain and strengthen this feedback loop than the coarse resolution;(3) only the simulations with fine vertical resolution can capture the downward buoyancy turbulence that is produced by fog-top long-wave radiation cooling.The intensity of the buoyancy turbulence is comparable to that of the shear turbulence near the sea surface,and it results in the phenomenon that sea surface temperature is higher than air temperature,which is often observed during sea fog occurrence.