Conceptual model of diurnal cycle of low-level stratiform clouds over southern West Africa

Abstract

The DACCIWA (Dynamics Aerosol Chemistry Cloud Interactions in West Africa) project and the associated ground-based field experiment, which took place during summer 2016, provided a comprehensive dataset on the low-level stratiform clouds (LLSCs), which develop almost every night over southern West Africa. The LLSCs, inaccurately represented in climate and weather forecasts, form in the monsoon flow during the night and break up during the following morning or afternoon, affecting considerably the radiation budget. Several published studies give an overview of the measurements during the campaign, analyse the dynamical features in which the LLSCs develop, and quantify the processes involved in the LLSC formation. Based on the main results of these studies and new analyses, we propose in this paper a conceptual model of the diurnal cycle of the LLSCs over southern West Africa. Four main phases compose the diurnal cycle of the LLSC. The stable and the jet phases are the two steps during which the relative humidity increases, due to cooling of the air, until the air is saturated and the LLSCs form. Horizontal advection of cold air from the Guinean coast by the maritime inflow and the nocturnal low-level jet (NLLJ) represents 50 % of the local total cooling. The remaining half is mainly due to divergence of net radiation and turbulence flux. The third step of the LLSC diurnal cycle is the stratus phase, which starts during the night and lasts until the onset of surface-buoyancy-driven turbulence on the following day. During the stratus phase, interactions between the LLSCs and the NLLJ lead to a modification of the wind speed vertical profile in the cloud layer, and a mixing of the sub-cloud layer by shear-driven turbulence below the NLLJ core. The breakup of the LLSC occurs during the convective phase and follows three different scenarios which depend on the intensity of the turbulence observed during the night in the sub-cloud layer. The breakup time has a considerable impact on the energy balance of the Earth's surface and, consequently, on the depth of the convective boundary layer, which could vary by a factor of 2 from day-to-day.