Marchand, RogerTansey, Emily2024-09-092024-09-092024-09-092024Tansey_washington_0250E_26807.pdfhttps://hdl.handle.net/1773/51788Thesis (Ph.D.)--University of Washington, 2024Low cloud decks with tops below 3 km are ubiquitous year-round over the Southern Ocean (SO). Low clouds are crucial to the albedo of the SO, reflecting a large portion of incident shortwave radiation back to space. Climate models struggle to capture cloud properties, which drives biases in SO radiative fluxes. Long-term satellite observations are commonly relied upon for assessment of model performance, but are limited to properties at cloud top, highlighting the need for observational data collected at the surface.This dissertation presents an analysis of SO low cloud and precipitation microphysical and thermodynamic phase properties from the Macquarie Island Cloud & Radiation Experiment (MICRE), which took place from April 2016 to March 2018, providing the lengthiest set of continuous surface-based observations to date. Cloud properties retrieved from MICRE instrumentation are compared with retrievals from the Moderate Resolution Imaging Spectroradiometers (MODIS) aboard the Terra and Aqua satellites, which are relied upon for a spatial analysis of a larger region surrounding Macquarie Island. Low cloud optical depth feedbacks are also studied using the surface microphysics observations. In Chapter 2, the surface radar and depolarization lidar are used to study cloud macrophysical characteristics and the phase of cloud and below-cloud precipitation. Low clouds are present 65% of the time, with cloud-top temperatures below freezing 2/3 of the time. About 85% of low clouds with tops below 0°C are supercooled liquid phase with minimal seasonal variability; about 35%-40% of the time, these clouds produce mixed or ice phase precipitation, indicating the importance of ice formation to SO low cloud precipitation processes. Ice is found to be the dominant phase of below-cloud precipitation when radar reflectivity factors surpass about -5 dBZ to 0 dBZ. In Chapter 3, MODIS data are used to study diurnal & seasonal cloud properties in the environment surrounding Macquarie Island. Winter has significantly greater cloud and ice phase fractions than summer, and optically thicker clouds. Liquid phase occurs more often in the morning than the afternoon (even though clouds are cooler in the morning), implying that updrafts may be required to loft ice particles near cloud-top for detection by MODIS. A significant increase in cloud droplet number concentration in the island’s wake to the east reveals that lee wave patterns are forming. MODIS aerosol retrievals do not show significant differences inside the cloud droplet “enhancement zone” vs. outside. MODIS aerosol properties have moderate correlations and low bias relative to ground-based aerosol retrievals. Lee-wave clouds are found to be associated with higher wind speeds, lower boundary layer inversion altitudes, greater lower tropospheric stability, and decay of the Scorer parameter with altitude. Cloud microphysics retrievals from MODIS and MICRE are compared in Chapter 4, with correlation coefficients ranging from 0.5 to 0.7. The lowest biases in cloud droplet effective radius occur at the MODIS 3.7 μm band, and MODIS is observed to underestimate (overestimate) effective radius in the case of larger (smaller) particle sizes. Lastly, the MICRE data are used to assess low cloud optical depth feedbacks. Optical depths retrieved during MICRE are weakly sensitive to warming cloud-top temperatures, and this sensitivity can be explained by changes in liquid water path. A weak positive feedback emerges for clouds with top temperatures ≥ 0°C, with decreases in warm-cloud liquid water paths driven primarily by reductions in cloud geometric thickness.application/pdfen-USnoneCloud physicsLidarPrecipitationRadarAtmospheric sciencesRemote sensingClimate changeAtmospheric sciencesThesis