小尺度地形引起的切变重力波

GRAVITY WAVES INDUCED BY SMALL SCALE TERRAIN

在大气边界层中,特别是山区的边界层中,经常可以观测到重力波的活动。例如Bull和Neisser分析了1974年4月至7月的2300时的微气压计记录。他们发现约38％的观测时间里都存在振幅至少为10—20 dPa的重力波活动,其中大多数重力波的振幅为25—70 dPa。由于风速切变和温度层结的不同,重力波引起的垂直位移的大小可能为百米,也可能达到整个夜间边界层厚度的量级。 很多大气活动都可能引起重力波,例如锋面、低空急流、局地风切变、逆温及强对流等等。地形特别是在夜间也是产生重力波的重要来源。当稳定度随高度急剧减少或风速随高度急剧增加时,在山脊的背风坡经常可以发现一系列的波动活动。

Wave equations in a two layer model are solved analytically to obtain the vertical displacement of streamlines as airflow passes over a small scale terrain in nighttime boundary layer. The solutions of the perturbation of the streamlines are obtained as the forms of mountain waves and lee waves. The influences of the atmospheric conditions such as wind speed and temperature structure as well as the topographic features upon the characteristics of the gravity waves are discussed. It is shown that the wind shear plays an important role in the forming of large amplitude waves.