The Effect of Doubling the CO2 Concentration on Convective and Non-convective Precipitation in a Gen

Vast differences exist in the treatment of clouds among the various GCMs that have been used for CO2-sensitivity studies (see e.g., Rutledge and Schlesinger, 1985). However, a common feature has been found among several CO2 sensitivity experiments (Hansen et al., 1984; Washington and Meehl, 1984; Wetherald and Manabe, 1986, 1988; Wilson and Mitchell, 1987; also see the review by Schlesinger and Mitchell, 1987). That is, when the CO2 concentration in the model climate is doubled, the cloudiness decreases throughout most of the tropical and middle latitude tropospheres of both hemispheres, even though the tropics remains a moist and convectively active region with precipitation increasing in these regions. Based on this result, it is inferred that in the semi-arid tropical regions the increase in precipitation is expected to be in the form of convective rainfall, giving an increase in the intensity but not necessarily in the frequency of rainfall (Jaeger,1988). However, to date no study as yet has investigated the CO2-induced changes in convective and non-convective types of precipitation and their frequency distributions on a global scale. This topic will be examined in the present note, based on a preliminary CO2 sensitivity experiment, performed with the use of a general circulation model developed at the Meteorological Research Institute (MRI). Since cloudiness and area of precipitation are closely correlated, the CO2-induced cloud feedback process will be discussed with respect to the response in the convective and non-convective areasas of precipitation.