钟形地形动力抬升和重力波传播与地形云和降水形成关系研究

摘要

Orographic clouds and precipitation have a critical role in regional water cycles,fresh water resources,ecosystems,the environment,and climate change.The relationship between terrain dynamic forcing,mountain waves,orographic clouds and precipitation,and the formation mechanism were investigated in a conditionally unstable moist atmosphere on May 1,2009,by using the Weather Research and Forecasting (WRF) Model and the wet Froude number (Fw).The results show that at a topographic height of 2 km and half-width of 10 km,when unstable airflow gradually increases from 2.5 to 25 m/s,the wet Froude number increases from 0.19 to 1.81.The terrain block has a major role when Fw≤ 1; orographic clouds induced by terrain dynamic lifting form on the windward side,and mountain waves form mainly over the windward side and propagate toward the upstream region,which forms stratiform clouds at first and then evolves as quasi-stable shallow convective wave clouds.The maximum precipitation occurs at the terrain crest near the windward side.When Fw is too small,no precipitation is produced.When Fw＞ 1,the orographic clouds induced by terrain dynamic lifting form primarily at the terrain crest,and mountain waves form mainly over the leeward side and propagate toward the region downstream of the crest,producing quasi-stable shallow convective wave clouds in the downstream region.In addition,in a weak and conditionally unstable moist atmosphere,orographic precipitation is produced mainly by warm microphysical processes induced by terrain lifting,and mountain wave clouds cannot produce precipitation.%地形云和降水过程在区域水循环、水资源、生态环境及气候变化中具有十分重要的作用.本文利用中尺度数值模式WRF数值模拟试验,以及通过引入表示大气层流速度、层结稳定度和地形特征的关系参数——湿Froude数(Fw),研究了北京2009年5月1日湿条件不稳定大气层结下,地形云和降水形成过程与地形动力抬升和地形重力波传播之间的关系及形成机理.研究表明,在地形最大高度2km、半宽10km的条件下,层流速度从2.5 m/s逐步增加到25 m/s时,对应的湿Fw数从0.19增加到1.81.当Fw≤1时,地形的阻挡起主要作用,由地形抬升形成的地形云主要产生在迎风坡一侧.地形重力波主要产生在迎风坡,并向上游传播,先形成层状云,最后演变为准稳定浅对流波状云.最大降水主要发生在紧靠山顶的迎风坡一侧,但当Fw很小时,地形云不产生降水.当Fw＞1时,地形抬升形成的云主要发生在山顶附近,而地形重力波主要形成在背风坡,并向下游方向传播,形成准稳定波状云.最大降水主要产生在紧靠山顶的背风坡一侧.另外,在弱湿条件不稳定大气层流下,地形降水主要由地形动力抬升造成的暖云微物理过程产生,地形重力波形成的波状云几乎不产生降水.