Ground-based retrieval of seasonal cloud and precipitation properties in the Arctic

We have developed a retrieval method for the study of low-level cloud and precipitation in the Arctic. The retrieval method is based on ARM NSA-AAO ground-based remote sensing measurements, and Global Ozone Monitoring (GOME) stratospheric ozone profiles from the ERS-2 satellite. We have modified a retrieval algorithm developed by Mahesh et al. (2001) to include ice crystal precipitation below cloud base, cloud phase, and cloud microphysics. The algorithm is well-suited to the Arctic because clouds there are thin, the atmosphere dry, and there is prolonged darkness. The retrievals show precipitation ice crystals with diameters between 20 and 1000 μm and number concentrations less than 1 cm^-3, with typical values of 226 μm and 52 m^-3, respectively. Precipitation increases during spring and decreases during fall. Most low-level thin clouds are liquid clouds during all seasons, but especially in summer; ice clouds and mixed-phase clouds form mainly in winter, when their incidence is about 40% and 4%, respectively. Retrieved values of the cloud droplet number concentration are maximum and droplet effective radius a minimum in winter, and their seasonal trend correlates strongly with aerosol light scattering measured nearby at the NSA-AAO Global Monitoring Division (GMD) site, operated by NOAA.