Modeling the West African Monsoon using the Superparameterized CAM

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Wednesday, 26 January 2011: 5:00 PM
Modeling the West African Monsoon using the Superparameterized CAM
608 (Washington State Convention Center)
Rachel McCrary, Colorado State University, Fort Collins, CO; and D. A. Randall

The West African Monsoon (WAM) is the term commonly used to describe the seasonal rains that occur in West Africa from May to October. Since rain-fed agriculture is one of the main sources of food and income for the people in West Africa, societies in this region are highly vulnerable to variability in monsoon rains. As our planet warms due to increasing greenhouse gas concentrations, it is probable that we can expect changes in the monsoon system which will impact water resources in West Africa. Unfortunately, conventional global circulation models (GCMs) are unable to represent the complex multi-scale interactions known to be associated with the monsoon and so are unable to represent the timing, spatial patterns and magnitude of the WAM. The inability of these models to capture the observed monsoon system, greatly undermines their ability to represent potential changes to the monsoon in a warmer climate. 
 It is possible however, that GCMs which have implemented the multi-scale modeling framework (MMF) are better equipped to represent the changes in the monsoon circulation over West Africa. The MMF is an innovative computational strategy that investigates the interactions between clouds and the global circulation of the atmosphere. MMFs have been uniquely designed to examine the interactions between small-scale circulations and large-scale dynamics. In an MMF, the cloud parameterizations of a GCM are replaced by embedding a 2D cloud resolving model within each grid box. This modeling approach has been pioneered by CMMAP in the Community Atmosphere Model (CAM), commonly referred to as the Superparameterized-CAM (SP-CAM). When compared to the standard CAM, the SP-CAM has been shown to improve the representation of many aspect of the global climate including the diurnal variability of convection over land and oceans, intraseasonal variability of tropical convection and the MJO. In this study we have performed a 10 year AMIP style run covering the years 1997-2006 using the SP-CAM version 3.5. With this simulation we have investigated the ability of the SP-CAM to represent both the seasonal cycle and the diurnal cycle of convection over West Africa during the monsoon season. To understand the seasonal cycle of convection during the monsoon season we have examined many important dynamical features associated with the monsoon including the African Easterly Jet, the Saharan heat low, the low-level monsoon winds, and the Harmattan winds. To examine the diurnal cycle over the Sahel, we have tried to understand how convection can propagate from East to West across West Africa and how this influences when and where it will rain over the region. Results from this study will help us to determine if the MMF and the SP-CAM are suitable for use in future climate change assessments involving the West African region.