Observations of Size-resolved Drizzle Rates and Radar Reflectivity in Marine Stratocumulus

While it is widely acknowledged that drizzle is a key process in the stratocumulus-topped marine boundary layer (MBL), there remain many outstanding questions regarding its quantitative impacts on the system. Here, we report in situ aircraft measurements of the size-resolved drizzle rate (R) and calculated total radar reflectivity (Z) values in marine stratocumulus using the Artium Flight Phase Doppler Interferometer (F/PDI) during the Marine Stratus Experiment in July 2005. One of the advantages of the F/PDI relative to previous instruments is accurate cloud drop size distribution measurements across a wide range of sizes, 4 to 150 μm diameter. The lower bound of drop size that is often considered drizzle varies substantially, with typical values ~50 μm diameter. However, Nicholls (QJRMS 1984) reports observations that suggest that the contribution of drops smaller than 50 μm to the drizzle rate can be very substantial, particularly at cloud top, although the resolution of the instrumentation allowed for only a coarse analysis.

The size distribution of drizzle is relevant to a number of MBL processes. For example, the rate of drop evaporation after it falls below cloud base into the sub-cloud layer depends on drop size. In turn, such evaporation can be an important for the dynamics within the boundary layer and, via feedbacks, impact the cloud layer itself. Also, drop-drop interactions (such as collision-coalescence) within the cloud are strongly dependent on drop size, and therefore the development and evolution of drizzle itself is size-dependent. Another example is that sedimentation of cloud drops from near cloud top has been proposed to reduce entrainment by decreasing the potential for evaporative cooling in this region.

Preliminary comparisons of Z and R values calculated from measured size distributions illustrates that a single drizzle rate may lead to numerous values of reflectivity. Several studies have attempted to better understand the Z-R relationship in order to accurately predict R from radar-measured Z. Our preliminary studies have found that a wide distribution of data points can be seen at cloud base where significant drizzle occurs and the Z-R relationship can vary greatly within the same cloud layer. Our goal is to understand both the distribution of drizzle and the cloud factors which control the Z-R relationship.