Airborne passive microwave measurements of arctic clouds and sea ice

Characterization of arctic cloud processes, although important for many applications, remains a difficult challenge. Few in situ observations of arctic clouds are available, and satellite retrieval of polar cloud properties has not been as successful as in other parts of the world. The purpose of this work is to investigate the feasibility of relating liquid water path in arctic clouds to microwave radiances over open water and sea ice surfaces. Data collected from the Beaufort Arctic Storms Experiment, the FIRE Arctic Clouds Experiment, and the Surface Heat Budget of the Arctic experiment provide a comprehensive set of cloud and surface measurements in various seasons. Airborne passive microwave radiometer data from these experiments are used to examine the spectral properties of liquid water clouds and the underlying surfaces.; The Airborne Imaging Microwave Radiometer (AIMR), originally used for operational sea ice measurements, has been refurbished and deployed for cloud observations. An algorithm for estimating liquid water path over oceans from satellite sensors is adapted for use with AIMR data and applied with measurements over both water and sea ice. Aircraft in situ measurements of cloud microphysical properties are used for validation.; Over sea ice, the surface emission term is a significant part of the upwelling radiance, so the physical and dielectric properties of the surface must be well-characterized. Surface emissivity at a range of microwave frequencies is estimated from measurements on clear sky days. Surface temperature is derived from infrared radiometer measurements with corrections for atmospheric emission and absorption applied. The spatial and temporal variability of the surface temperature and emissivity are examined.; Sea ice surface emissivity and temperature data are used in radiative transfer model simulations of upwelling brightness temperature at microwave frequencies. It is demonstrated that liquid water clouds produce detectable increases in brightness temperature, and that the magnitude of the increase is proportional to liquid water path. Brightness temperatures at 90 GHz show the largest response to liquid water when compared to responses at 37, 150, and 220 GHz. Sensitivity analyses show that uncertainties in surface emissivity have the most significant influence on brightness temperature, especially at low values of liquid water path.; Liquid water path retrievals over sea ice are performed for a series of cases during the FIRE-SHEBA experimental period. Comparisons of retrieved liquid water path with in situ measurements show a correlation of 0.989 for the cases considered. Agreement between the two data sets is best for liquid water path values of 70 g m−2 and higher. Accurate specification of surface emissivity is most important for low liquid water path since thick liquid clouds tend to mask the effects of surface variations. The feasibility of qualitatively estimating liquid water path using polarization differences is analyzed.; Assuming that reasonable estimates of liquid water path over sea ice can be obtained by this method when the value exceeds 70 g m−2 , independent measurements of liquid water path throughout the year suggest that useful measurements could be obtained from a downward looking microwave radiometer 10% of the time in winter, 28% of the time in spring, and 50% of the time in summer.