SUMMARYShip-based radar measurements obtained during the East Pacific Investigation of Climate 2001 stratocumulus (Sc) cruise are used to derive characteristics of the rainfall from drizzling Sc. Reflectivity to rain rate (Z-R) relationships are determined from shipboard raindrop-size distribution measurements obtained from observations using filter-paper, and compared to Z-R relationships derived from aircraft probe data from below north-east Atlantic drizzling Sc and stratus. A model for the evaporation and sedimentation of drizzle is combined with reflectivity profiles from a millimetre-wavelength cloud radar to derive information on the mean raindrop radius and drizzle drop concentrations at cloud base, and to show how Z-R relationships change with height below the cloud base. The Z-R relationships are used in conjunction with shipborne C-band radar reflectivity data to estimate areal average precipitation with uncertainties at cloud base and at the surface. In the Sc drizzle Z-R relationship, Z = aR b (where a and b are constants), the estimated exponent b = 1.1 to 1.4 is lower than commonly observed in deep convective rain (b = 1.5). Analyses indicate that variations in Sc rain rates and reflectivities are influenced both by fluctuations in drizzle drop concentration and in mean radius, but that number concentration contributes more to the modulation of rain rate in Sc. Rain rates derived using the scanning C-band radar are found to be spatially variable, with much of the accumulation originating from a small fraction of the drizzling area. The observations also suggest that rain rate in marine Sc is strongly dependent on cloud liquidwater path, and inversely correlated with cloud droplet concentration.

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The relationship between drizzle and cloud morphology, as manifest in transitions to spatially compact regions of open cellular convection, is studied using data collected from recent field studies over the northeast and southeast Pacific.

[1] Geostationary and Sun-synchronous satellite data and in situ observations from ship cruises are used to investigate the formation of open cellular structure in marine stratocumulus clouds over the southeast Pacific (SEP). Open cellular convection either forms spontaneously as pockets of open cells (POCs) within overcast stratocumulus, or is advected into the region from midlatitude regions. POC formation occurs most frequently during the latter part of the night, demonstrating that this transition is not caused by solar absorption-driven decoupling. The transition preferentially occurs in clouds with low 11-3.9 mm nighttime brightness temperature difference (BTD) which is found to be well correlated with both in situ measured accumulation mode aerosol concentration and cloud droplet concentration estimates derived from MODIS. Besides indicating that nighttime BTD is an excellent proxy for stratocumulus cloud droplet concentration N d , this also suggests that low aerosol concentrations favor POC formation. Indeed, extremely low accumulation mode aerosol concentrations are found during the passage of open cell events over the ship. Free-tropospheric moisture is not found to be an important factor in POC formation. Significant subseasonal variability occurs in the fractional coverage of open cellular convection over the broader SEP. This coverage is well correlated with a MODIS-derived drizzle proxy (MDP) proportional to the ratio of liquid water path (LWP) to N d for predominantly overcast regions. Both LWP and N d variability influences the MDP. Periods of low MDP have significant positive large-scale N d anomalies and are preceded by offshore winds at 850 hPa, which suggests a potential continental influence upon open cell formation over the SEP. Together, the results suggest important two-way interactions between aerosols and drizzle in marine stratocumulus and a role for drizzle in modulating the large-scale albedo of these cloud systems.

Observations from the East Pacific Investigation of Climate (EPIC) 2001 field campaign are well suited for exploring the relationships among the diurnal cycle, mesoscale (10-100 km) structure, and precipitation in the stratocumulus region in the southeast Pacific. Meteorological time series and observations from a scanning C-band radar, vertically pointing cloud radar, and ceilometer, as well as satellite data, are used to show that drizzle is associated with increased variability in cloud and boundary layer properties compared to nondrizzling periods. The stratocumulus-topped boundary layer is typically well mixed at night, transitioning to less well mixed in the afternoon, with drizzle most frequently occurring in the early morning. Coherent patches of drizzle, or "cells," can have large areas with radar reflectivities of greater than 5 dBZ of up to about 100 km 2 . Individual cells have long lifetimes, up to 2 h, and appear to be replenished by moisture in the boundary layer.