Climatology of drizzle in marine boundary layer clouds based on 1 year of data from CloudSat and Cloud-Aerosol Lidar and Infrared Pathfinder Satellite Observations (CALIPSO)

A survey of the frequency and characteristics of precipitation from low clouds over the oceans based on data from CloudSat and CALIPSO from July 2006 through June 2007 is presented. The low-cloud fraction, drizzle occurrence, and estimated cloud base precipitation rate are examined globally and for eight subtropical and midlatitude stratocumulus (Sc) regions. This analysis is restricted clouds below 4 km. Drizzle detection and characterization is further restricted to clouds with tops above 1 km altitude. The maximum radar reflectivity within an individual CloudSat profile (Z(max)) is used to classify the profile as precipitating or nonprecipitating. The distribution of Z(max) for all profiles is bimodal with peaks around -23 and -12 dBZ interpreted as originating from populations of nondrizzling and drizzling clouds, respectively. Profiles where Z(max) exceeds -18 dBZ are classified as drizzling. Drizzle is detected for 19-34% of cloudy profiles in the subtropical Sc regions and 37-44% of profiles from the midlatitude Sc regions. The cloud base precipitation rate is estimated using the relation: R-cb = 2 . Z(max)(0.7). The lower quartile/median/upper quartile precipitation rates are 0.25/0.6/2.0 mm d(-1) for the subtropical Sc regions and 0.28/0.7/2.3 mm d(-1) for midlatitude regions. A consistent nighttime increase in low-cloud fraction and drizzle occurrence is observed for the subtropical Sc regions. For clouds with r(eff) > 17 mu m the drizzle occurrence can be treated as a function of LWP alone and exceeds 50% (75%) for a LWP of 50 ( 110) g m(-2). For r(eff) < 17 the drizzle occurrence is strongly dependent on both LWP and r(eff).