Modeling of Cloud/Radiation Processes for Tropical Anvils

Satellite imagery suggests that large portions of the tropics are covered by extensive cirrus cloud systems. Tropical cirrus clouds evolve during the life cycle of the mesoscale convective systems and are modulated by large- scale disturbances. Outflow cirrus clouds from tropical cumulonimbi appear to be maintained in a convectively active state by radiative flux gradients within the clouds, as suggested by Danielson 1982. Extensive anvils are likely to become radiatively distabilized by cooling at tops and warming at bases. This would drive convective fluxes which in turn would provide an upward flux of water vapor within the cloud. The additional moisture at cloud top levels would promote rapid ice crystal growth and fallout. Ackerman et al. 1988 have computed radiative heating rates in typical tropical anvils. The heating rate differences between the cloud bottom and top ranges from 30 to 200 Kday. Lilly 1988 has analyzed the dynamic mechanism of the formation of cirrus anvils using a mixed layer model, and has shown that destabilization of the layer could be produced by strong radiative heating gradients. The importance of radiative processes in the life cycle of tropical anvils and convective systems has also been illustrated by Chen and Cotton 1988 and Dudhia 1989.