Response of Deep and Shallow Tropical Maritime Cumuli to Large-scale Processes

A spectral diagnostic method of cumulus ensemble (Nitta, 1975) is compared with a bulk diagnostic method (Yanai et al., 1973), using the same data set taken in the Marshal Islands area for a 100-day period in 1956. For the total vertical cloud mass flux and mass detrainment, both methods give nearly identical results. Using the spectral diagnostic method, daily spectra of cloud base mass flux as functions of the detrainment height are obtained. The effects of radiative cooling upon the background cloud mass spectrum is examined. Typical radiative cooling rates in the tropics tend to produce a bi-modal distribution of mass spectrum showing deep and shallow clouds. Then the dependence of daily cloud mass spectra on the large-scale vertical motion and the evaporation from the sea is examined by data stratification, by correlation analysis, and by time-spectrum analysis. Under disturbed conditions, the bi-modal distribution is further enhanced. Under suppressed conditions, a uni-modal distribution consisting of only shallow clouds prevails. Deeper clouds are more highly correlated with the large-scale vertical motion at higher levels than that at lower levels, showing the controlling influence of the upper tropospheric vertical motion upon the deep clouds. Shallow clouds persist nearly all the time due to radiative cooling and surface evaporation, but they are enhanced by the large-scale ascent. Time-spectrum analysis applied to the computed mass flux and the large-scale parameters shows that both deep and shallow clouds are modulated by large-scale disturbances.