The CloudSat satellite's Cloud Profiling Radar (CPR) is a highly sensitive 94 GHz (W-band) nadir viewing radar system flown in retrograde sun synchronous orbit useful for determining the vertical structure of cloud hydrometeors down to sensitivity of ~ -30 dBZ reflectivity factor. Given this sensitivity and a recent correction to various near-surface range bins to suppress surface clutter, it is possible to unambiguously measure precipitation rates in marine stratocumulus clouds over a spectrum extending from ~0.08 mm hr-1 to ~3.0 mm hr-1 down to altitudes of 0.5 km with 0.25 km vertical resolution. This enables an effective means to measure much of the drizzle and light rain spectrum in marine stratocumulus (MSC) clouds. However, because of its sun synchronous orbit, CloudSat cannot sample the diurnal cycle of precipitation in MSC clouds -- which is the focus of our investigation. Our objective is to understand the diurnal properties of MSC precipitation, to determine if there are in fact two modes of precipitation in MSC clouds, one involving intermittent shallow afternoon convective showers, the other involving steady drizzle discharge from the saturated nighttime cloud layer. Although there is ample published evidence of the latter mode, only a few reports of the afternoon mode have appeared in the literature. By the same token, Yang and Smith have reported in two recent papers the appearance of mid-afternoon diurnal precipitation maxima in regions frequented by MSC cloudiness based on observations from Tropical Rainfall Measuring Mission (TRMM) Precipitation Radar (PR) measurements.

The TRMM satellite enables diurnal monitoring of precipitation because of its 35º moderately inclined prograde orbit which creates precession through the diurnal cycle. Furthermore, the TRMM PR scans over a 220 km wide horizontal swath producing a much greater duty cycle in cloud sampling. On the downside, the TRMM PR is a Ku-band radar (13.8 GHz) with only 17 dBZ sensitivity, thus its effective lower end for measuring the rain rate spectrum is only ~ 0.3 mm hr-1 -- which is not sufficient to detect a substantial portion of the drizzle spectrum. Therefore, as will be shown in this study, we have acquired numerous coincident CloudSat CPR and TRMM PR reflectivity cross-sections in the world's MCS regions and are now developing transform schemes that estimate the missing CPR rain rate spectrum given the nature of the measured PR spectrum, based on the relationships found between the coincident data. Various case studies will be discussed which illustrate the strengths and weaknesses in the methodology as it pertains to MSC cloud environments.