This study develops parameterizations of cloud microphysical properties in terms of bulk variables, provides new calculations of cloud susceptibility, and explores applications for climate studies. On the basis of cloud microphysical properties acquired during the Indian Ocean Experiment (INDOEX), the cloud effective radius re is observed to be proportional to the cube root of the ratio of cloud liquid water content (LWC) to total cloud droplet concentration Nt. A shape parameter k, depending on the skewness and dispersion of the cloud droplet size distribution, determines the proportionality constant and is highly dependent on the number of aerosols Na observed; k values range from 0.83¿0.07 in pristine clouds (Na1500 cm-3). Further, Nt is parameterized as a function of Na and cloud vertical velocity or LWC. Although the parameterization coefficients vary between pristine and polluted clouds, the parameterization equations are similar to those developed for other geographic locations and other cloud types, suggesting that they might be applied in climate models with varying coefficients. Using an empirically derived relation between Nt and the projected area of the cloud droplet distributions ac, the cloud susceptibilities are 2.16 times larger than those calculated using the original definition of susceptibility which does not take into account changes in cloud spectral width. Because susceptibility is shown to not vary significantly with wavelength in the solar regime, the average visible susceptibilities of 1.4¿10-3 cm3 and 0.4¿10-3 cm3 for pristine and polluted clouds are representative of the albedo effect. Observed susceptibilities and re agree with those calculated from the parameterization mostly within 30%, suggesting the parameterization can be applied in models to predict indirect radiative forcing. ¿ 2001 American Geophysical Union