Tropical Convective Clouds

Deep convective cloud systems in the tropics provide the primary mechanism whereby solar heating of the ocean is moved upward into the free troposphere where it can be transported poleward and eventually emitted to space. In the process, these great engines of the global climate produce precipitation and drive the global-scale circulation. The combined effect of these convective systems and the large-scale circulations with which they interact determines the cloud, moisture, and temperature structure of the troposphere in the tropics, which plays a central role in determining the climate. The spatial organization and temporal development of these convective systems can be monitored with satellite instruments and related to the large-scale dynamic and thermodynamic conditions. Deep convection in the Pacific and Indian ocean area is strongly modulated by large-scale interannual variability associated with the Southern Oscillation. The following figure shows SST, 850 mb wind vectors, and the frequency of occurrence of clouds with tops colder than 235 K. Each of these fields are averaged over the four-month period from November to February for each of the years 1986-87, 1987-88, and 1988-89. The first two years are influenced by warm events in the central Pacific, while 1988-89 is a colder year with a well-developed cold tongue of SST along the equator in the eastern Pacific. These SST changes are associated with major shifts in the distribution of 850 mb wind and also in the location and intensity of deep convection. It is easy to see that the SST, the convection, and the large-scale wind patterns are closely related.

Deep convection in the tropics also has an important diurnal variation, which can be observed in the longwave radiation measured by geosynchronous satellites. In the following figure, it is demonstrated that the diurnal variation of clouds with tops colder than 208 K is out of phase with that of clouds with top temperatures between 208 and 235 K.

Deep convection is the source of water vapor for the upper troposphere in the tropics. This upper-tropospheric humidity plays an important role in maintaining the natural greenhouse effect in the atmosphere. Instruments in geosynchonous orbit that include a water vapor channel allow high spatial and temporal resolution monitoring of the effect of deep convection in moistening the upper troposphere of the tropics. The following figure shows cloud top temperatures (pink scale) and upper-tropospheric humidities derived from GOES for 00 GMT on 3 September 1992. The clouds are surrounded by a shroud of high humidity air, which has an e-folding distance of about 300 km. At greater than about 500 km from upper-tropospheric convection, on average, the upper-tropospheric humidity is very low. Where the mean winds blow from clear to cloudy regions the shroud of high humidity air is narrower, suggesting that advection by large-scale winds has an important effect on how the humidity introduced at upper levels by convection spreads to other locations.

The role of advection in spreading humidity is perhaps expressed a little better in the following figures, which show the upper-tropospheric cloud amount and humidity averaged over five days in August of 1991. A tongue of high humidity air extends across the cloudless subsidence zone west of South America along what appears to be the mean wind direction there.

Geosynchronous satellite data and data from global analyses and field experiments such as TOGA COARE can be used to test models of tropical convection and validate modeling of the interaction of tropical convection with the large-scale environment there. Such modeling and intercomparison work is also being conducted under this investigation. This work may eventually lead to better parameterizations of the interaction of tropical convection with the large-scale climate.

The basic maintenance of the tropical greenhouse effect, tropical and subtropical cloud optical properties, and their possible sensitivity to climate change are some of the questions being addressed by this study. Satellite and in situ data will be used to initialize and validate detailed simulations of tropical convective clouds and their interaction with the large-scale environment. Critical EOS instruments that will provide data appropriate to these studies include AIRS/AMSU/MHS for humidity and temperature structure, MODIS, CERES, MIMR, and MISR for cloud structure and radiative properties, and assimilated data sets for large-scale circulation. MLS and GLAS will provide unique information on upper-tropospheric water vapor and thin cirrus cloud decks, respectively.

Recent Modeling Work

Recent modeling work with the NCAR MM5 model has been directed toward understanding the role of large-scale motions and self-aggregation in leading to well-developed tropical mesoscale convective complexes. A conference abstract describing some of this work is available here as a .pdf file.