A one-year study of the diurnal cycle of clouds and radiation in the West African Sahel region

 

Authors


Mark A. Miller — Rutgers University
Virendra Prakash Ghate — Argonne National Laboratory

Category

Radiation

Description

The Sahel region of West Africa is thought to be particularly sensitive to global climate change due to its location on the tropical margin and its heavy dependence on yearly rainfall delivered by the West African Monsoon. This rainfall nurtures a precariously small and essential agricultural output in this region. To gain a better understanding of the role of clouds and other radiatively active forcing mechanisms, which contribute to the monsoon circulation in the Sahel, radiative fluxes and cloud properties, among other atmospheric quantities, were measured during the RADAGAST Campaign (Radiative Atmospheric Divergence using ARM Mobile Facility, GERB, and AMMA Stations) in Niamey, Niger, during 2006. Shortwave and longwave fluxes were measured at the top of the atmosphere every fifteen minutes using the Geostationary Earth Radiation Budget (GERB) instrument aboard the Meteosat-8 satellite. Coincidently, the ARM Mobile Facility (AMF) measured the radiative fluxes at the surface on a time scale of less than one minute, which enabled the calculation of the cross-atmosphere radiative flux divergence on a time scale of fifteen minutes. This divergence quantifies the net warming or cooling experienced by the atmospheric column above the Sahel. These high-temporal-resolution measurements are combined with high-resolution active remote sensor measurements of clouds and moisture, which are thought to partially control the radiative flux divergence. We use these unique measurements to define and quantify the diurnal cycle of clouds and radiation in the Sahel region and to determine how varying cloud conditions impact this cycle. Our data also demonstrate the power and utility of this measurement approach and its ability to evaluate global climate model simulations of the Sahel region.