青藏高原东北边坡复杂地形重力波的数值模拟

Numerical simulation of gravity waves in complex terrain on the northeast slope of the Qinghai-Tibet Plateau

基于2007年7月青海祁连站的野外加密探空资料,结合高分辨率的三维边界层模式,模拟研究了青藏高原东北边坡复杂地形条件下,边界层对流引起的干动力过程对该地区地形重力波产生及传播的影响机理。结果表明:在不同的背景场强迫下,高原东北边坡复杂地形上空对流和重力波的空间结构存在较大差异。当背景风向与山体垂直时,随着风速增加,山脊背风坡混合层顶附近大气不稳定能量加强,激发了下游区域较强的重力波信号,此时对流线组织性增强、重力波波列较长,高水汽含量的空气被波峰传输到较高的高度,为对流云发展提供了有利条件;当背景风向与山脊走向平行时,山顶上空对流发展旺盛,山脊背风坡混合层顶大气状态较稳定,激发的地形重力波信号较弱且波列较短,整个混合层顶附近水汽较少,对流云形成条件减弱;当背景大气浮力频率减小时,整个区域上空对流发展更加旺盛但组织性减弱,背风坡下游重力波向上传输的距离减小,信号不显著,混合层顶附近水汽分布均匀且变化幅度较小,有利于层状云发展。

In this study,we conduct a numerical simulation of the dry dynamical processes induced by boundary layer convection on the generation and propagation of gravity waves on the northeastern slope of the Qinghai-Tibet Plateau under complex terrain conditions.This simulation is based on field-observed sounding data from the Qilian station in July 2007,coupled with a high-resolution 3D boundary layer model.The results reveal significant variations in the spatial structure of convection and gravity waves over the complex terrain of the northeastern plateau slope,contingent on different background forcing scenarios.Specifically,when the background wind direction is perpendicular to the mountain,an increase in wind speed enhances atmospheric instability energy near the top of the mixed layer on the leeward slope of the ridge,thereby exciting a more pronounced gravity wave signal in the downstream region.In contrast,when the background wind direction aligns parallel to the ridge,vigorous convection develops over the summit,resulting in a more stable atmospheric state at the top of the mixed layer on the leeward ridge slope.This,in turn,weakens the topographic gravity wave signal,yielding a shorter wave train.Additionally,there is less water vapor near the top of the entire mixed layer,diminishing the conditions conducive to convective cloud formation.Also,decreased background atmospheric buoyancy frequency leads to more vigorous yet less organized convection across the region,reducing the upward transmission distance of gravity waves downstream of the leeward slope and yielding in less significant signals.Furthermore,under these conditions,there is a uniform distribution of water vapor near the top of the mixed layer with minimal amplitude changes,which mitigates conditions conducive to convective cloud formation.