High-resolution simulations of convective cold pools over the northwestern Sahara
High-resolution simulations of convective cold pools over the northwestern Sahara
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DOI：
10.1029/2008jd011271
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发表时间：
2009-04-24
影响因子：
4.4
通讯作者：
Seifert, A.
中科院分区：
文献类型：
--
作者：
Knippertz, P.;Trentmann, J.;Seifert, A.
沙漠地区深层干燥云下层对流降水蒸发冷却可产生强烈的下沉气流和长期存在的大范围大气密度流。在其前缘的强烈阵风可以引起所谓的哈布沙尘暴。尽管它们的重要性，沙尘循环，国家的最先进的数值天气预报模式，以现实地模拟相关的对流冷池的能力已经调查了很少的日期。2006年5月/6月在摩洛哥南部进行的撒哈拉矿尘实验第一次实地活动期间，观测到了几次密度流。它们是由下午阿特拉斯山脉上空的深层潮湿对流触发的，并在晚上传播到山麓。在这里，我们提出了三个这些密度流的数值模拟使用非流体静力学的小尺度模拟模型与2.8公里的水平网格间距，它允许一个明确的深对流治疗。该模型能够模拟阿特拉斯山脉上空对流单体的及时启动，以及随后形成的具有逼真三维结构的长寿命、大范围冷池。从现有的地面和卫星观测的偏差是密切相关的模式在模拟阿尔及利亚撒哈拉地区的降水对流的缺陷。与修改后的微物理的敏感性研究揭示了很大的影响雨滴大小分布的蒸发和地面降雨，但相当温和的影响冷池演变。减小边界层中湍流垂直混合的长度尺度会导致更广泛但更弱的降水，更多的蒸发，以及更快和更长的冷池。
Cooling by evaporation of convective precipitation in the deep and dry subcloud layer over desert regions can generate intense downdrafts and long-lived and extensive atmospheric density currents. The strong gusts at their leading edges can cause so-called haboob dust storms. Despite their importance for the dust cycle, the ability of state-of-the-art numerical weather prediction models to realistically simulate the associated convective cold pools has been investigated very little to date. During the first field campaign of the Saharan Mineral Dust Experiment in southern Morocco in May/June 2006, several density currents were observed. They were triggered by deep moist convection over the Atlas Mountains during the afternoon and propagated into the foothills in the course of the evening. Here we present numerical simulations of three of these density currents using the nonhydrostatic Consortium for Small-Scale Modelling model with 2.8-km horizontal grid spacing, which allows an explicit treatment of deep convection. The model is capable of simulating the timely initiation of convective cells over the Atlas Mountains and the subsequent formation of long-lived, extensive cold pools with a realistic three-dimensional structure. Deviations from available surface and satellite observations are closely related to model deficiencies in simulating precipitating convection over the Algerian Sahara. Sensitivity studies with modified microphysics reveal a large influence of raindrop size distributions on evaporation and surface rainfall but a rather moderate influence on the cold pool evolution. Decreasing the length scale for turbulent vertical mixing in the boundary layer leads to more widespread but weaker precipitation, more evaporation, and a faster and more extended cold pool.