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

Abstract

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?2 for a rain cell of 103 km2 to as high as 1500 W m?2 for 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 the largest rain cells are embedded, make up less than 3% of clouds, yet are the source of greater than 90% of the total thermodynamic forcing. Significant differences are apparent between the scales of latent heating and radiative forcing, as only about 25% of cloud area is observed to precipitate. The fraction of clouds that contain some rain increases dramatically from about 5% for the smaller scale (103 km2) to as high as 90% for the largest scale considered here (106 km2). The fractional area of the precipitating cloud ranges from 0.2 to 0.4 with a hybrid-scale dependence. Greater than one-half of radiative forcing is provided by nonprecipitating anvil portions of deep convective cloud systems. The results presented here have significant implications for the parameterization of clouds and rain in GCMs and washout of solute trace gases and aerosols in chemistry and transport models.