The Role of Low-Level Clouds over Southern West Africa in the Regional Monsoon System – Experiments Using the ICON Model

Climate and weather models still struggle to realistically represent the complex multi-scale West African monsoon (WAM) system. One potential reason, which has received relatively little attention so far, is the extensive ultra-low stratus clouds that form during nighttime in the southern parts of West Africa (8°W–8°E, 5–10°N) in connection with the nocturnal low-level jet. Since the clouds often persist until midday, they have a strong impact on the local radiation budget, which in turn influences the development of the boundary layer, in particular in terms of heating, convection, and mixing. A recent study based on output from global climate models participating in CMIP5 and Years of Tropical Convection (YoTC) indicates that the majority of models are not capable to reproduce the amount, height, and diurnal cycle of low clouds in this area. The objective of the present study is to investigate the impact of the radiative effects of these clouds on the larger WAM system and its diurnal cycle.

In order to address this task, sensitivity simulations with the numerical weather prediction model ICON of the German Weather Service are conducted. For these simulations, the optical thickness of the lowest cloud decks in the model are increased or decreased prior to calling ICON's radiation scheme. This way the radiative effects of the clouds on the further development of the model's state of the atmosphere can be analysed in a fully nonlinear fashion. The experiments cover one monsoon season (July–September 2006; i.e., the special observing period of the African Monsoon Multidisciplinary Analysis (AMMA)) and include standard as well as convection-permitting simulations. The simulations are initialized with European Centre for Medium-Range Weather Forecasts (ECMWF) ERA-Interim re-analysis data and include 5-day runs started (i) daily for a background model climatology and (ii) every 4 days for the sensitivity experiments in standard operational and convection-permitting configurations. We made use of ICON's nesting capability to obtain grid-spacings of 14 km (standard run) and 6.5 km (convection-permitting) over the region of interest. The presented work is part of the project DACCIWA (Dynamics-Aerosol-Chemistry-Cloud Interactions in West Africa), which aims to investigate the impact of the drastic increase in anthropogenic emissions in West Africa on the local weather and climate, for example through cloud-aerosol interactions or impacts on radiation and stability.

Results show a strong response of the model, particularly for the cloud water content, cloud cover, and precipitation. An altered diurnal cycle of cloud development occurs in the region of interest. Reduction of cloud optical thickness (COT) leads to an intensified nocturnal low-level jet and more nocturnal cloudiness, which tends to break up later on the following day and is shifted to a higher altitude. In the afternoon, the increased radiation in experiments with reduced COT leads to higher surface temperatures and earlier as well as more convection, which in turn leads to more rainfall over southern West Africa and increased specific humidity in the lowest layers. Consistently, optically thickened clouds produce less rain. The surface pressure over southern West Africa decreases for more transparent clouds, leading to an increased north-south gradient near the coast. The observed effects on the meteorological variables propagate northwards with the monsoon flow in the course of the night and can lead to decreased precipitation directly north of the influenced region and increased precipitation even farther north. Details of the response depend on the representation of convection in the model. Convection-permitting simulations show overall increased precipitation and an even stronger influence on regions outside of the domain modified in the sensitivity experiments.