西北干旱区对流边界层发展的热力机制模拟研究

A simulative study of the thermal mechanism for development of the convective boundary layer in the arid zone of northwest China

观测发现,西北干旱区对流边界层高度在夏、冬两季极大值分别达到4km和800m。利用热力学方法对西北干旱地区深厚对流边界层的原因进行了定量分析。用建立的热力数值模型模拟表明,平均而言,夏季和冬季对流边界层高度的51.5%和61.4%分别由感热产生,对于夏、冬季对流边界层高度可分别达到4km和800m的极大值,感热贡献更大,为63.4%和73.1%,感热对对流边界层的贡献率冬季明显大于夏季;夏、冬季浮力夹卷对对流边界层的贡献是18.1%和9.4%;包括感热和浮力夹卷的热力因素可以解释夏、冬季对流边界层高度的69.6%和70.8%,而对于对流边界层极大值,则其解释可高达81%和84.6%。热力因素是西北干旱区对流边界层产生的主要原因,其中,感热又是最根本原因。

The convective boundary layer （CBL） in the arid zone of northwest China can reach heights of 4 km in summer and 800 m in winter according to the observation. Its cause was quantitatively studied using the thermodynamic method. The re sults show that： （1） the averaged CBL height can be accounted in terms of sensible heat for its 51.5% in summer and 61.4% in winter with the percentages by sensible heat up to 63.4% in summer and 73.1% in winter when the maxima of CBL heights reached, and the degree of contribution of sensible heat to CBL in winter bigger distinctly than that in summer; （2） the contri- butions of entrainment caused by buoyancy are 18.1% in summer and 9.4% in winter, respectively; （3） the coupled effects of sensible heat and buoyancy entrainment are as high as 69.6% in summer and 70.8% in winter as applied to averaged CBL heights, but even as higher as 81% in summer and 84.6~ in winter as to their maxima. In conclusion, thermal effects inclu- ding sensible heat and buoyancy entrainment together are crucial to forming such high CBL height over the arid area, in which sensible heat is more important, in some sense, in consistent with the general knowledge.