Cumulus Momentum Fluxes in the Tropics and Subtropics

积云对流引起的垂直动量通量是大气环流的一个重要因素，但人们对此知之甚少，特别是在热带和亚热带地区。对积云动量通量的可靠观测很少。目前在不同的全球天气和气候模式中使用的积云动量通量的参数化法的准确性是很难判断的。热带对流也表现出许多类型的中尺度组织。参数化的一个重要问题是，在不知道对流的中尺度组织的情况下，能否充分预报积云动量通量。该项目将使用三个广泛和互补的新数据集来加强对积云动量通量的观测知识。具体地说，动量通量将被计算为(1)TOGA-COARE地区、其他对流区域的全球再分析的预算残差，以及纬向平均，(Ii)来自TOGA-COARE的热带中尺度对流系统的广泛的双多普勒机载雷达分析，以及(III)来自热带对流的云系模式模拟的输出。TOGA-COARE实验特别值得研究，因为持续的深对流与垂直风切变的大时间变化相耦合，有望在积云动量通量中产生大且容易检测到的变化。该项目的目标是量化积云动量通量对热带大尺度瞬变和平均环流的重要性，特别是季节内振荡，以及(Ii)在各种风切变条件下，确定中尺度对流系统对流与层状区的环流在产生动量通量方面的相对重要性，以及(Iii)将结果与已提出的几种不同的参数化进行比较。这些分析应该有助于改善热带地区天气和气候模式的表现。

Vertical fluxes of momentum due to cumulus convection are a significant but poorly understood contributor to atmospheric circulations, particularly in the tropics and subtropics. Reliable observations of cumulus momentum flux are scarce. It is very difficult to judge the accuracy of the widely varying parameterizations of cumulus momentum fluxes currently used in different global weather and climate models. Tropical convection also exhibits many types of mesoscale organization. An important issue for parameterization is whether the cumulus momentum fluxes can be predicted adequately without knowing the mesoscale organization of the convection. This project will bolster observational knowledge of cumulus momentum fluxes using three extensive and complementary new data sets. Specifically, momentum fluxes will be computed (1) as budget residuals in global reanalyses for the TOGA-COARE region, other convective regimes, and as zonal averages, (ii) from an extensive set of dual-Doppler airborne radar analyses of tropical mesoscale convective systems from TOGA-COARE, and (iii) from output from cloud ensemble model simulations of tropical convection. The TOGA-COARE experiment is particularly promising to study because the persistent deep convection coupled to the large temporal variations in vertical wind shear can be expected to produce large and easily detectable changes in the cumulus momentum fluxes.The goals of the project are to quantify the significance of cumulus momentum fluxes on large scale transient and mean circulations in the tropics, particularly the intraseasonal oscillation, and (ii) to determine the relative importance of the circulations in the convective versus stratiform region of mesoscale convective systems in producing the momentum fluxes, in a variety of wind shear regimes, and (iii) to compare the results to several different parameterizations that have been proposed. These analyses should help to improve the performance of weather and climate models in the tropics.