Simple Multicloud Models for the Diurnal Cycle of Tropical Precipitation. Part I: Formulation and the Case of the Tropical Oceans
Simple Multicloud Models for the Diurnal Cycle of Tropical Precipitation. Part I: Formulation and the Case of the Tropical Oceans
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DOI：
10.1175/2011jas3568.1
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发表时间：
2011-10-01
影响因子：
3.1
通讯作者：
Majda, Andrew J.
中科院分区：
文献类型：
--
作者：
Frenkel, Yevgeniy;Khouider, Boualem;Majda, Andrew J.
本文用两个简单的热带对流模式研究了太阳加热日循环对热带降水的影响。该模型利用三种云类型congestus，深，层状被认为是有组织的热带对流的特点，是基于两个第一斜压模式的垂直结构加上边界层模式。这两种模型的不同之处主要在于它们处理边界层动力学的方式。第一个是纯物理的，并减少到一个单一的方程的等效位温θ（e）连接边界层对流层上部通过下降气流和地面通过蒸发，而第二个使用完整的整体边界层（FBBL）动力学与感热和潜热通量之间的仔细分离和非降水浅积云的参数化。事实证明，在波文比小的海洋降水的情况下，这两个模型产生了定性相似的解决方案，其特征在于一夜之间的启动和清晨的降水峰值与观测一致。模拟的海洋降水日循环分为四个循环阶段：1）CAPE（再）生成阶段，其特征是中午和下午早些时候边界层theta（e）和水分通量的增强，随后是2）下午晚些时候以浓热加热和下降气流湿润为主的（再）湿润阶段（由于浅积云的消散，特别是在FBBL模式中）和持续到午夜的辐射冷却。3)当对流层中部足够潮湿和凉爽时，深对流在午夜左右开始，并（重新）建立了接近其辐射对流平衡的降水水平（1 K日（-1）），然后4）在0600 LST的日出时达到峰值，在0900 LST左右产生约2 K日（-1）的降水最大值，干燥对流层，消耗CAPE并关闭循环。
The variation of tropical precipitation due to the diurnal cycle of solar heating is examined here in the context of two simple models for tropical convection. The models utilize three cloud types-congestus, deep, and stratiform-that are believed to characterize organized tropical convection and are based on the two first baroclinic modes of vertical structure plus a boundary layer mode. The two models differ mainly in the way they treat the boundary layer dynamics. The first one is purely thermodynamical and is reduced to a single equation for the equivalent potential temperature theta(e) connecting the boundary layer to the upper troposphere through downdrafts and to the surface through evaporation while the second uses full bulk boundary layer (FBBL) dynamics with a careful separation between sensible and latent heat fluxes and parameterization of nonprecipitating shallow cumulus. It turns out that in the case of the precipitation over the ocean where the Bowen ratio is small, both models yield a qualitatively similar solution, characterized by an overnight initiation and early morning peak in precipitation consistent with observations. The modeled diurnal cycle of precipitation over the ocean is divided into four cyclic phases: 1) a CAPE (re)generation phase characterized by the enhancement of the boundary layer theta(e) and moisture fluxes during midday and early afternoon that is followed by 2) a (re)moistening phase dominated by congestus heating during the late afternoon and moistening from downdrafts (due to detrainment of shallow cumulus, specifically in the FBBL model) and radiative cooling that lasts until midnight. 3) Deep convection is initiated around midnight when the midtroposphere is sufficiently moist and cool and (re)establishes the precipitation level near its radiative convective equilibrium (1 K day(-1)) and then 4) peaks with sunrise at 0600 LST to yield a precipitation maximum of roughly 2 K day(-1) at around 0900 LST that dries the troposphere and consumes CAPE and closes the cycle.