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The observed diurnal cycle of low-level stratus clouds over southern West Africa: a case study

机译：南部非洲南部低水平层云云观察到的昼夜循环：一个案例研究

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摘要

This study presents the first detailed observational analysis of the complete diurnal cycle of stratiform low-level clouds (LLC) and involved atmospheric processes over southern West Africa (SWA). The data used here were collected during the comprehensive DACCIWA (Dynamics-Aerosol-Chemistry-Cloud-Interactions in West Africa) ground-based campaign, which aimed at monitoring LLC characteristics and capturing the wide range of atmospheric conditions related to the West African monsoon flow. In this study, in situ and remote sensing measurements from the supersite near Save (Benin) collected during a typical day, which is characterized by the onset of a nocturnal low-level jet (NLLJ) and the formation of LLC, are analyzed. The associated dynamic and thermodynamic conditions allow the identification of five different phases related to the LLC diurnal cycle: the stable, jet, stratus I, stratus II, and convective phases. The analysis of relative humidity tendency shows that cooling is a dominant process for LLC formation, which leads to a continuous increase in relative humidity at a maximum rate of 6% h(-1), until finally saturation is reached and LLC form with a cloud-base height near the height of NLLJ maximum. Results of heat budget analysis illustrate that horizontal cold-air advection, related to the maritime inflow, which brings the cool maritime air mass and a prominent NLLJ wind profile, has the dominant role in the observed strong cooling of -1.2 Kh(-1) during the jet phase. The contribution from horizontal cold advection is quantified to be up to 68 %, while radiative cooling and sensible heat flux divergence both contribute 16% to the observed heat budget below the NLLJ maximum. After the LLC form (stratus phases I and II), turbulent mixing is an important factor leading to the cooling below the cloud base, while strong radiative cooling at the cloud top helps to maintain thick stratus.
机译：本研究提出了第一次详细观察层面云（LLC）的完整昼夜循环，以及涉及南部非洲（SWA）的大气过程。这里使用的数据在综合的DACCIWA（西非的动态 - 气溶胶化学 - 云互动）基于地面的运动期间，旨在监测LLC特征​​，并捕获与西非季风流有关的广泛大气条件。在本研究中，分析了在典型日内收集的Saveite附近的Superite和遥感测量，其特征在于夜行低级射流（NILLJ）的发作和LLC的形成。相关的动态和热力学条件允许鉴定与LLC昼夜循环相关的五个不同阶段：稳定，喷射，层状I，层状II和对流阶段。相对湿度倾向的分析表明，冷却是LLC形成的主要方法，这导致相对湿度的最大速率连续增加6％H（-1），直到最后达到饱和度，并具有云的LLC形式-base高度靠近nllj的高度最大。热预算分析结果表明，与海上流入有关的水平冷空气平流，带来凉爽的海上空气质量和突出的NLLJ风型，在观察到的强化冷却中具有主导作用-1.2 kH（-1）在喷射阶段。横向冷平面的贡献量高达68％，而辐射冷却和明智的热通量发散均可贡献16％，以至于最大值低于NLLJ的观察热预算。在LLC形式（STRATUS阶段I和II）之后，湍流混合是导致云底部低于冷却的重要因素，而在云顶上的强烈辐射冷却有助于保持厚层。

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