High-Resolution Image

This is a ‘hot’ topic, because smoke is literally hotter than clouds, but also because a number of scientists are looking into this enigmatic issue of smoke (and other aerosol) interaction with clouds. This true-color Aqua-MODIS image acquired on June 14, 2007 at 12:45 UTC, over South-central Africa and the adjoining East Atlantic, portrays a largely cloud-covered scene, with the exception of the lower right quarter, which shows numerous red dots marking the locations of fires. Why is this lower right quarter practically cloud-free, even though it is supposed to have the same land or ocean surface characteristics as the rest of the scene, which is very cloudy? A previous study found that cloud-cover went from about 38% in clean conditions down to 0% in the presence of smoke; thereby demonstrating that smoke inhibits the formation of clouds. This phenomenon may also be linked to the positive impact of smoke on the radiative forcing of climate.
A related question is: why is the sky least cloudy directly above the fires than over other areas further downwind where the smoke is transported? A hypothesis that we are investigating is that the heat from large fires probably plays a significant role, not only in inhibiting cloud formation, but also in causing the evaporation of existing clouds directly above the fire. It is common knowledge that fires generate tremendous heat energy, which is transmitted by conduction, convection, and radiation. Conduction is ensured by the ground, convection by the circulation of the air carrying the smoke, while radiative heat is propagated in space by infrared waves. The heat we feel from a fire, even when we are not close to it, is due to the energy transmitted by radiation, and is also what enables satellite sensors to detect fires from space in the first place. Fortunately, MODIS measures the rates of release of Fire Radiative Energy (FRE), which we plan to use to examine the role of the heat from fires in inhibiting cloud formation or in causing the evaporation of existing clouds.

(Submitted by Charles Ichoku. Image provided by the MODIS Rapid Response team.)