祁连山冬季降雪个例模拟分析(Ⅰ):降雪过程和地形影响

Numerical Simulation of Snowfall in Winter of Qilian Mountains. Part(Ⅰ):Snowfall Process and Orographic Influence

利用部分改进了的中尺度数值模式MM5V3对2006年2月7~8日祁连山一次降雪过程进行了三重双向影响嵌套模拟研究,模式对雪带分布的模拟与实测基本吻合。重点分析了此次降雪过程中的热力动力特征和云的微物理结构,并通过地形敏感性试验,研究了祁连山地形对降雪的作用。结果表明：降雪过程中有低层西北湿冷气流向祁连山区输送水汽在山前形成大值区,气流除在祁连山周围绕流外同时沿祁连山北坡爬升,降雪前期空气饱和层和上升气流区比较深厚,为祁连山北坡降雪中心的形成提供了有利的动力热力条件,降雪后期有高空干冷下沉气流侵入使降雪减弱。这次过程为冷性稳定层云降雪过程,水成物含水量大值区也主要分布于祁连山北坡和山顶附近,冰晶和雪分布在6 km以下,在冷云顶存在0.06 g.kg-1的过冷云水。祁连山高大地形对大范围降雪落区无明显影响,但对祁连山北坡降雪中心形成有直接影响。降低地形高度后,山顶无法形成上升运动和云粒子,迎风坡云体发展减弱。地形对降雪增幅中心主要位于祁连山北坡,24 h最大增幅达3 mm。

A case of snowfall in Qilian Mountains during 7-8 February 2006 is simulated using the mesoscale model MMSV3. The simulated snow band is basically consistent with observation. The dynamical and thermal conditions and microphysical characteristics of the snowfall are mainly discussed. The orographic influence of Qilian Mountains is also investigated through sensitivity tests. Results show that there is a south-west cold moist flow in front of the mountain in low levels. The flow is separated into two parts, one is around the mountain, the other flow up along the northern slope of the mountain. During the early stage of the snowfall, the thick saturated layer and strong updraft provide beneficial conditions to snowfall. Afterwards the dry and cold downdraft invades from the high levels which weakened the snow- fall. This snowfall process is of cold and stable stratiform characteristics. The maximum hydrometers distributed on the northern slope and mountain top. Ice and snow is below 6 km. There is 0.06 g · kg^-1 supercooled water at the cloud top. Qilian Mountains influent directly on formation of the snowfall center at the northern slope of the mountain. If the height of the mountain is lowered down, there are no updraft and cloud particles at mountain top, and cloud at the northern slope is weakened. High mountains increase the snowfall at the northern slope with 3 mm in 24 h.