Vertical motions of drops of different sizes and the growth of drizzle in marine stratus

The Wyoming King Air and the Wyoming Cloud Radar made observations in unbroken marine stratus off the coast Oregon in September 1995 and August 1999. In this study data collected 8 days. Cloud tops were below 1 km agl, cloud thicknesses were less than 400 m, and environmental conditions were similar for each day studied.; A comparison of drop concentrations within narrow size ranges in regions of more intense upward versus downward vertical air velocities, reveals a robust pattern. The comparison is quantified with the parameter χw defined as the ratio of the concentration of drops in a given size interval in the upper 20th percentile of vertical velocities, to the concentration in the lower 20th percentile of vertical velocity. As a function of drop size, χw showed two maxima (χw > 1). The first occurs at small cloud drop sizes of 9–18 μm near the mode of the drop size distribution. Values of χw for this size range were generally between 1.5–3. The second maximum occurred for diameters of 75–150 μm with values around 2. The minimum occurred at intermediate drop diameters of 24–30 μm, with an average value of 0.6.; The results of this study call attention to the role of weak vertical motions in marine stratus in shaping the drop size distribution. Upward motions allow for increased in-cloud dwell time and give rise to the maximum in χw, at drizzle sizes. The minimum in χw near 30 μm diameter is most likely due to a depletion of these drops in upward moving air due to collection by drizzle drops. The maximum in χw at small cloud drops is the result of a difference between updrafts and downdrafts owing to the lapse of time since initial condensation at cloud base. Other factors such as recirculation of partially evaporated drops, entrainment at cloud top or the fragmentation of large drops may also contribute to the minimum in χw at small sizes.