UC Santa Cruz

The most expansive and persistent clouds, reducing the net radiation balance on a annually averaged global basis by 15 W/m^2, are the low–lying marine stratocumulus (MSc) which hover over the eastern subtropical oceans. Despite their climatic importance, key processes and feedbacks within the MSc regime have yet to be fully quantified or understood. The goal of this research is to improve our understanding of MSc processes and their impact on the marine boundary layer (MBL). Specifically, using in situ aircraft cloud microphysical measurements, this research pays particular attention to the process of drizzle, the sedimentation of liquid water. Data utilized in this study primarily come from the Artium Flight Phase Doppler Interferometer (F/PDI) and the Cloud Imaging Probe (CIP) during four days of the Marine Stratus Experiment (MASE) in July 2005 in the northeastern Pacific near Monterey, California. Results presented in this dissertation are especially unique because of the broad and continuous range in which the F/PDI samples (∼2–100 μm diameter), a size range measured by no other instrument alone with the same resolution and accuracy. The upper portion of this size range, ∼30 to 100 μm, is of particular importance to the initiation and evolution of drizzle but traditionally, has been difficult to measure well.

The first goal of this research was to characterize in–cloud drizzle during the four MASE days by exploring the horizontal and vertical structure of drizzle. Drizzle statistics indicate two microphysical regimes exist, a high drizzle regime, associated with patches of heavy drizzle occurring in clusters, and a low drizzle regime, associated with more uniform, light-to-no drizzle. Heavy drizzle regimes exhibit significant drop growth by collision-coalescence while low drizzle regimes exhibit drop growth primarily from condensation. The second goal of this research is to quantify the effects of drizzle on the MBL via the process of evaporation. The observations indicate a large range of BL cooling exists among the four study days. Sub-cloud profiles of evaporative cooling show variability in the location of peak and total depth of cooling. Variability is also found to exist in the horizontal. Lastly, cloud top (CT) processes which may be responsible for the initiation of drizzle during the heavy drizzle days are investigated. We utilize drop size distribution (DSD) measured from CIP/PDI and derived from box model simulations to calculate CT collision rates. We found the observational collision rates follow a power law with the observed slope, m , varying at CT between well-developed and less-developed drizzling MSc. No correlations between m and other observed cloud properties were found. Drop size distributions simulated from a box model of collision-coalescence suggest that while collision rates can be impacted by properties such as turbulence and cloud drop residence time, realistic values were insufficient to reproduce observed CT collision rates.