提高模式分辨率后热带气旋生成迟缓的原因分析

Analysis of the delay of tropical cyclone formation in a higher resolution numerical simulation

利用目前国际上最先进的中尺度WRF模式模拟热带气旋生成,网格分辨率从9 km增加到3 km,3 km网格中积云参数化方案不起作用,依靠微物理方案来模拟对流尺度系统特征,模式中热带气旋的生成过程变得迟缓。当低压扰动发展到一定程度后再加入3 km网格,生成过程有加快趋势。本研究针对该现象进行分析。结果表明:只用微物理方案使低层(950~700 hPa)风速的垂直切变减小,不利于对流发展;切变减小主要是由于动量垂直输送项的差异所致。在加入细网格的6 h内,低层对流尺度(减去区域平均)的动量垂直输送量平均增加了一倍,某些时刻达到了5倍以上;动量混合增加是由于微物理方案模拟的垂直速度增加所致。此外,只用微物理方案导致对流有效位能迅速被消耗。低层垂直切变和对流有效位能的减小都不利于对流发展,从而导致热带气旋生成发展过程迟缓。本研究表明,目前WRF中的微物理方案在模拟热带气旋生成过程中的对流发展时仍然存在问题。

The most advanced mesoscale WRF model in the world is used to simulate the genesis process of tropical cyclones.When the grid resolution is increased from 9 km to 3 km,the cumulus parameterization scheme in 3 km grid does not work,so the microphysical scheme is used to simulate the characteristics of convective-scale system in the 3 km domain.As a result,the formation process of tropical cyclone in the model becomes slow.If the 3 km domain is added after the low pressure disturbance has developed to a certain extent,the formation process tends to accelerate.This study is focused on the analysis of the phenomenon.Results show that the low-level (950—700 hPa) vertical shear of horizontal wind speed decreases by using only the microphysical scheme,which is not conducive to the development of convection.The decrease of low-level vertical shear is mainly due to the difference of vertical momentum advection between the two experiments.During the 6 h after adding the inner nest,the convective-scale (removing the area average component) vertical momentum advection at low-level is double on average and 5 times in maximum compared to control experiment.The increase of momentum mixing in the low-level is due to the increase of vertical velocity simulated by microphysical scheme.In addition,the CAPE (convective available potential energy) is rapidly consumed by using only the microphysical scheme.The decreases of low-level vertical shear and CAPE are not conducive to the development of convection near the disturbance center,resulting in the delay of genesis and development of tropical cyclone.This study demonstrates that there are still some problems in simulating the real development process of convection during the formation of tropical cyclone with the present microphysical schemes in WRF model.