28 November 2016
Why do global climate models struggle to represent low-level clouds in the West African summer monsoon?
Publication: J. Climate (in press)
DOI Number: doi:10.1175/JCLI-D-16-0451.1
Author: Hannak, L.; Knippertz, P.; Fink, A. H.; Kniffka, A.; Pante, G.
Climate models struggle to realistically represent the West African monsoon (WAM), which hinders reliable future projections and the development of adequate adaption measures. Low-level clouds over southern West Africa (5–10°N, 8°W–8°E) during July–September are an integral part of the WAM through their effect on the surface energy balance and precipitation, but their representation in climate models has received little attention. Here we use 30 (20) years of output from 18 (8) models participating in the Coupled Model Intercomparison Project Phase 5 (Year of Tropical Convection) to identify cloud biases and their causes. Compared to ERA-Interim re-analyses, many models show large biases in low-level cloudiness of both signs and a tendency to too high elevation and too weak diurnal cycles. At the same time, these models tend to have too strong low-level jets, the impact of which is unclear due to concomitant effects on temperature and moisture advection as well as turbulent mixing. Part of the differences between the models and ERA-Interim appear to be related to the different subgrid cloud schemes used. While nighttime tendencies in temperature and humidity are broadly realistic in most models, daytime tendencies show large problems with the vertical transport of heat and moisture. Many models simulate too low near-surface relative humidities, leading to insufficient low cloud cover, abundant solar radiation, and thus a too large diurnal cycle in temperature and relative humidity. In the future, targeted model sensitivity experiments will be needed to test possible feedback mechanisms between low clouds, radiation, boundary-layer dynamics, precipitation, and the WAM circulation.