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Estimation of Microwave Surface Emissivity at SHEBA for use in Cloud Retrievals |
University of Colorado
haggertj@cloud.colorado.edu
Passive microwave radiometers with frequencies ranging from 37 GHz to 220 GHz were flown over the SHEBA experimental site in May and July, 1998. These measurements were motivated by the possibility of determining cloud liquid water path, ice water path, and precipitation over sea ice from data at these frequencies.The comprehensive cloud data set collected in this experiment offers a unique opportunity for improving and adapting passive microwave retrieval methods for application to arctic clouds. However,retrieval of cloud properties from a downward looking radiometer requires an estimate of surface emissivity and an understanding of its spectral, spatial and temporal variability.
In this study, brightness temperature measurements are used to calculate sea ice emissivity at each frequency and polarization using ancillary aircraft data to characterize the atmosphere and obtain surface temperature. Surface emissivities on clear sky days during the FIRE-ACE aircraft campaign have been calculated and compared with previous estimates cited in the literature. Average emissivity at nadir for snow-covered multiyear ice in the experimental region during late May is estimated as 0.89 at 37 GHz, 0.74 at 89 GHz, 0.72 at 90 GHz, 0.73 at 150 GHz and 0.84 at 220 GHz. In early July, the average nadir emissivity for melting multiyear ice is 0.86 at 37 GHz and 0.84 at 90 GHz. Estimates at 50 degrees off nadir are compared with previous satellite and surface-based measurements of dual polarized emissivities at 37 and 90 GHz. Significant variability exists in published emissivity values due to variations in dielectric and physical properties of snow and ice, but our results fall within previously observed ranges.
