复杂山区上空垂直速度场和热力对流活动的理想数值模拟

利用英国气象局高分辨率的边界层数值模式BLASIUS,针对中国西北一个复杂山区进行了一系列的理想数值模拟,分析了在不同天气条件下山区上空的垂直速度场分布和对流特征以及地形对热力对流活动的影响,同时讨论了与地形有关的对流触发机制。模式结果表明,复杂山区的垂直运动在稳定层结和风速较大的情况下较易预测,而在中性层结下,山区上空的垂直运动分布随机性强。在Froude数小于0.5的条件下,气流往往被山峰阻塞而在迎风坡造成地形强迫和辐合性抬升,从而易在迎风坡触发深对流活动;在背风坡则由于迎风坡的绕流重新辐合而造成垂直运动。绕流的辐合是触发深对流活动的另一重要因子。在大风或Froude数较大的条件下,地形重力波容易在山地下游被激发。地形重力波与对流活动的相互影响在模式中清楚可见。在适当的条件下,重力波除了可以与对流活动相耦合从而使气团上升到更高的高度外,重力波的走向很可能会影响到深对流系统的传播路径。研究还发现稳定度对相邻两条对流线之间的距离长短也有影响。稳定度较小时,相邻两条对流线之间的平均宽度趋向变大而单个对流线的强度也相应变大。定量化的结论和理论升华值得进一步的数值模拟研究。

IDEALIZED SIMULATIONS OF VERTICAL VELOCITY FIELDS AND THERMAL CONVECTION OVER A COMPLEX HILLY TERRAIN

A series of idealised model simulations have been performed over a real hilly terrain using a high resolution boundary layer model.The model used is the Met Office boundary layer model called Boundary Layer Above Stationary,Inhomogeneous Uneven Surface(BLASIUS).Based on those simulations,the characteristics of vertical velocity fields and thermal convection over the real/complex terrain are investigated and effects of orography are diagnosed.The model results indicate that the vertical velocity field is more predictable under higher wind speeds in a stable atmosphere,while it is more randomly distributed in a neutral atmosphere.When Froude number is less than 0.5,air flow tends to be blocked and uplifted over the windward slope by orography,and under such circumstances deep convection is likely to be triggered.Due to the division of airflow over the windward slope the vertical motion can also be induced on the lee side.The confluence of air flow on the lee side of mountains is another important factor which may induce deep convective systems.When the wind speed is high or Froude number is larger,gravity waves can be triggered on the lee side of mountains,and interaction between terrain-induced gravity waves and thermal convection can be clearly seen.When there is no energy input at the model's surface,the vertical velocity field is dominated by the forced ascents and gravity waves,and when the model's surface is heated gradually,thermal convection begin appear.In the lee side of the mountain,there are convective lines tending to converge due to the confluence of the flow from both sides of the mountain.The convergence of convective lines has an important impact on convection triggering.The top of the convective mixed layer is the interface for the interaction between thermal convection and gravity waves.When the upward motion of convection is overlapped by the ascent due to gravity waves,the upwards vertical motion tends to be strengthened,otherwise,the convection is likely to be depressed. Because terrain-induced gravity waves are able to affect the strength of convective activity,the propagation of convective system may also be modulated by gravity waves.The alignments of convective lines and gravity waves seem to be related to each other,and.more model simulations are worthy to be performed to obtain quantitative conclusions and theoretical mechanisms.