大涡模拟试验中台风梯度风平衡分析

TYPHOON GRADIENT WIND BALANCE IN LARGE EDDY SIMULATIONS

关键词:  台风  大涡模拟  梯度风平衡

Key words:  typhoon  large eddy simulation  gradient wind balance

摘要:

尽管经典台风强度理论是基于梯度风平衡模型，但是已有的飞机观测资料和数值模拟研究发现，边界层中和眼墙附近存在非梯度风平衡气流，前人数值模拟发现，眼墙附近最大的超梯度风可以达到切向风速的16.7%。大涡模拟可以模拟出滚涡和龙卷尺度涡旋等小尺度系统，这些小尺度系统对梯度风平衡可能产生影响，使用中尺度天气预报模式结合大涡模拟（WRF-LES）对模拟台风内核区域方位角平均的梯度风平衡进行了分析，三个大涡模拟试验的水平分辨率分别为333 m、111 m和37 m，结果表明非梯度风平衡气流并未因为分辨率提高而显著增强或产生结构差异，最大超梯度风出现在边界层顶处最大切向风半径的内侧，达到梯度风速的10.8%~16.1%。进一步分析发现，不同台风中心定位方法会影响非梯度风平衡气流，最小气压方差法和最大切向风法可以得到与前人模拟一致的结构，而气压权重法得到的低层超梯度风中心远离眼墙、靠近台风中心，位涡权重法得到的低层超梯度风的径向范围向内扩大，最小气压方差法和最大切向风法更适合用于梯度风平衡研究中的台风中心定位。

Abstract:

Despite considerable typhoon intensity theories are based on gradient wind balance models, there exist unbalanced flows in the boundary layer and the vicinity of eyewall as shown in aircraft measurements and numerical simulations. Approximately 16.7% of the tangential wind close to the eyewall is unbalanced in previous simulations. The large eddy simulation (LES) with Advanced Weather Research and Forecast (WRF) is capable of simulating the horizontal rolls and tornado-scale vortices. To understand the possible influences of fine-scale coherent structures on unbalanced flows, the structure of azimuthally averaged flows in the simulated typhoon inner-core region is examined through three numerical experiments with respective horizontal grid sizes of 333 m, 111 m, and 37 m in the LES-WRF framework. The analysis yields that grid sizes make no pronounced difference in the structure and intensity of unbalanced flows in that the supergradient wind maxima of three experiments are located inside the radius of maximum wind at the top of the boundary layer and the maxima range from 10.8% to 16.1% of tangential wind velocities. Though the structure of unbalanced flows is insensitive to the mesh resolution, typhoon center-detecting methods have impacts on the structure. The minimum pressure variance center and the maximum tangential wind center show structures similar with that of previous simulations, comparing with which the pressure centroid center displays the supergradient wind maximum away from the eyewall but close to the typhoon center at lower troposphere and the potential vorticity centroid expands the low-level supergradient wind radially. This study suggests that the minimum pressure variance center and the maximum tangential wind center both perform well in the study of gradient wind balance.