Scale Dependence of the Thermodynamic Forcing of Tropical Monsoon Clouds: Results from TRMM Observations

Clouds exert a thermodynamic forcing on the ocean–atmosphere column through latent heating, owing to the production of rain, and through cloud radiative forcing, owing to the absorption of terrestrial infrared energy and the reflection of solar energy. The Tropical Rainfall Measuring Mission (TRMM) satellite provides, for the first time, simultaneous measurements of each of these processes on the spatial scales of individual clouds. Data from TRMM are used to examine the scale dependence of the cloud thermodynamic forcing and to understand the dominant spatial scales of forcing in monsoonal cloud systems. The tropical Indian Ocean is chosen, because the major monsoonal cloud systems are located over this region. Using threshold criteria, the satellite data are segmented into rain cells (consisting of only precipitating pixels) and clouds (consisting of precipitating as well as nonprecipitating pixels), ranging in scales from 10³ km² to 10⁶ km². For each rain cell and cloud, latent heating is estimated from the microwave imager and radiative forcing is estimated from the Cloud and the Earth’s Radiant Energy System radiation budget instrument. The sizes of clouds and rain cells over the tropical Indian Ocean are distributed lognormally. Thermodynamic forcing of clouds increases with rain cell and cloud area. For example, latent heating increases from about 100 W m−2for a rain cell of 10³ km² to as high as 1500 W m−2for a rain cell of 10⁶ km². Correspondingly, the liquid water path increases tenfold from 0.3 to nearly 3 kg m−2, the longwave cloud forcing from 30 to 100 W m−2, and the diurnal mean shortwave cloud forcing from −50 to −150 W m−2. Previous studies have shown that in regions of deep convection, large clouds and rain cells express greater organization into structures composed of convective core regions attached to stratiform anvil cloud and precipitation. Entrainment of moist, cloudy air from the stratiform anvil into the convective core helps to sustain convection against the entrainment of unsaturated air. Thus large clouds produce more rain, trap more terrestrial radiation, and reflect more solar energy than do smaller clouds. The combined effect of increased forcing and increased spatial coverage means that larger clouds contribute most of the total forcing. Rain cells larger than 10⁵ km² make up less than 2% of the rain cell population, yet contribute greater than 70% of the latent heating. Similarly, the clouds larger than 10⁵ km², in which t