中性层结下局地平缓地形起沙的次网格参数化研究

Study of the subgrid scale parameterization of dust emissions caused by gentle local terrain under the neutrally stratified atmosphere

中文关键词:  局地地形  狭管效应  下滑效应  次网格参数化

英文关键词:Focal terrain  Narrow-pipe effect  Downslide effect  Subgrid scale parameterization

中文摘要:

      局地地形起沙对沙尘模式的预报精度具有重要影响.基于狭管与下滑效应以及气流过山的已有研究成果,研究了中性层结与局地平缓地形条件下风速在这两种效应下次网格参数化问题,然后基于平坦地形的起沙公式、引入局地地形的摩擦速度,得到了相应的次网格起沙公式,最后对狭管效应、下滑效应以及两效应的叠加进行了理想试验.试验表明:(1)对于狭管效应的次网格参数化问题,风从狭管的阔口进(出)、窄口出(进)时,风速、跃移通量与起沙量均增大(减小);(2)下滑效应致使风速、跃移通量与起沙量均增大,且山脚增幅大于山坡;(3)对于两种效应的叠加,当气流从狭管的阔口进、窄口出时,风速、跃移通量和起沙量同时增大,山脚风速大于山坡,大粒径沙粒跃移通量增幅大于小粒径沙粒.反之,窄口进、阔口出时,出入口宽度之比存在一临界值,大于此临界值,风速减小;反之增大.而且,除了在山脚的小粒径沙粒在一定风速条件下跃移通量会增大外,其余情况均减少.最后,提出了本参数化方法与数值模式耦合的分算法与整算法.

英文摘要:

      Dust emissions caused by local terrain have an important effect on forecast precision of dust model.Based on the existing research results of narrow-pipe effect and downslide effect as well as the speed-up equation of wind flowing over hills the subgrid-scale parameterizations of surface wind velocities associated with both narrow-pipe effect and downslide effect were firstly studied on conditions of neutral stratification and local gentle terrain, then the subgrid-scale-parameterization of dust emissions were also proposed based on equations for flat terrain in which friction velocity of flat terrain was replaced with that of complex one.At last three sets of ideal experiments were applied to test the narrow-pipe effect, downslide effect and superposition of both effects.The experiments show that as to narrow-pipe effect,the velocity in the narrow pipe saltation flux and the emissions will increase (decrease) when wind flows in from the wide (narrow) entrance and flow out from the narrow(wide) exit of the pipe.While as to the downslide effect,the velocity saltation flux and the emissions increase, too and the increasing amounts at hill foot are bigger than those at hillside.As far as the superposition of both effects is concerned,when wind flows in from the wide end and flow out from the narrow end,the velocity saltation flux and the emissions also increase with the velocity at hill foot bigger than that at hillside and the saltation flux associated with big-sized grains of sand bigger than those associated with small-sized grains.On the contrary, when wind flows in from the narrow end and flow out from the wide end,there exists a threshold value for the ratio of exits width to entrance's.When beyond it,the velocity decreases otherwise the velocity increases.Furthermore,the saltation flux will decrease except for the case in which there arc small-sized grains Locatcd on hill foot.