Cloud-Radiation Interaction in the West African Monsoon: An Idealized Study

The West African monsoon is characterized by large seasonal and intraseasonal variability which is governed by numerous mechanisms et processes that are not yet well understood. One of the them is the cloud-radiation interaction, whose impact on the mean monsoon climate is still not clear. Indeed modification of the boundary layer moist static energy content is associated with modifications of the monsoon circulation, which in turn changes the cloud distribution. The study is thus dedicated to a better understanding of the feedback loop between the monsoon mean climate and the cloud-radiation interaction.

A meridional-vertical two dimensional idealized numerical model developed by Peyrillé and Lafore (2007) is used here. It allows to represent the main features of the West African monsoon. A particular care has been given to compare the broad feature of the idealized monsoon to observations collected during the AMMA field campaign and to satellite products.

Experiments are conducted under a perpetual summer regime to simplify the system. The two dimensional model is able to reproduce the main characteristics of the monsoon (rain, circulation, cloud fraction) using NCEP2 or ERA40 forcing, but with different features depending on the forcing used. A sensitivity study is then perform where clouds do not interact with radiation. Results show that cloud-radiation interaction has a significant impact on the distribution of rainfall and tends to smooth the differences between the simulations obtained with NCEP2 or ERA40 forcing. The processes at play in the feedback loop are analyzed in detail in the study.