RELATIONSHIP BETWEEN SURFACE CONDITIONS AND SUBSEQUENT RAINFALL IN CONVECTIVE STORMS

This paper describes the relationship between surface conditions (temp erature and humidity) and subsequent rainfall. The focus is on convect ive storms that are forced and maintained locally due to conditional i nstability in the vertical distribution of atmospheric temperature. Th ese storms are described using two probabilistic measures: (1) the pro bability of occurrence of storms given surface conditions and (2) the average storm rainfall. The surface conditions are described by a sing le variable: surface wet-bulb temperature. The proposed theoretical re lationships are tested using an hourly data set on rainfall and wet-bu lb temperature from the Amazon region. These observations confirm that both measures increase linearly with wet-bulb temperature. However, f or the occurrence of anp storm the wet-bulb temperature has to exceed a threshold of about 22 degrees C. The sensitivity of the frequency of storms to changes in the climatology of surface wet-bulb temperature is larger than the corresponding sensitivity of the average storm rain fall. These general concepts are applied in discussing the potential i mpact of changes in land cover on rainfall patterns using two specific examples: deforestation in the Amazon region and development of irrig ation projects in the Columbia River basin.