Cumulus convection and climatic temperature perturbations

Abstract

The influence of cumulus convection on the equilibrium temperature perturbations due to the doubling of CO2 is investigated by using a one‐dimensional radiative‐turbulent model. The model includes parameterizations of solar and infrared radiative flux transfer in clear and cloudy conditions, vertical eddy sensible and latent heat fluxes, surface albedo feedback, and an interactive cumulus convection. The cumulus parameterization employed is basically a Kuo‐type scheme described and modified by Anthes. In addition the criteria for cumulus convection to take place is derived from values associated with the surface relative humidity and the vertical profile of moist static energy. In the numerical experiments it is found that the extent of condensational heating due to cumulus convection is determined by the depth of the conditionally unstable layer and the total amount of buoyancy force generated by the excess cloud temperature over the surrounding temperature. With the incorporation of cumulus convection, an atmosphere with a high relative humidity will generate a higher temperature near the surface. Also, a higher surface relative humidity will generate a larger upward shift in equilibrium condensational heating rates. Perturbation experiments reveal that the sensitivity of cumulus convection to the surface temperature increase due to doubling of CO2 is not apparent under mean annual global conditions with a realistic surface relative humidity of 85%. However, with the incorporation of cumulus convection and fixing the horizontal transport of sensible and latent heat fluxes, it is shown that the surface temperature increase in a tropical atmosphere due to doubling of CO2 significantly reduces from 4.3 to 2.5°K with the upward shift of temperature increases especially evident in upper tropospheric levels. Finally, it is also noted that the tropospheric temperature profile in the tropics generated by the present one‐dimensional model with cumulus convection agrees closely with that provided by Oort and Rasmusson for mean annual climatological conditions.