Understanding the role of drizzle in stratocumulus microphysics and radiative properties is important since these clouds are ubiquitous and have a large effect on the net radiation balance. However, because of its small rain droplets, this kind of precipitation cannot be observed by radar on satellite yet. To solve this problem we formulate a drizzle detection index, scaled Re, which is defined by the product of LWP(MW)/LWP(SR) and Re(MW) and expected to be able to discriminate drizzling clouds from non-precipitating clouds, here LWP(MW) and LWP(SR) are liquid water path retrieved at the microwave and solar frequencies respectively, Re(MW) donates the effective radius derived from LWP(MW). Unlike effective radius retrieved at near-IR frequency that is biased toward the value near the cloud top, this scaled Re represents droplet radius rather at the lower cloud layer, because Re(MW) is a measurement of an average droplet radius over the whole cloud layer and the ratio of LWP(MW) and LWP(SR) reflects the vertical inhomogeneity of the effective radius in a cloud layer. When the effective radii are vertically homogeneous, the ratio equals unity. When the effective radii decrease with height, the ratio exceeds unity. When the effective radii increase with height, the ratio is less than unity. Generally droplet size increases with height and reaches a maximum near the top in a nonprecipitating cloud layer. In contrast, a precipitating cloud contains the relatively larger raindrops below or near the cloud base. Therefore once precipitation occurs the scaled Re will become larger.