Anomalous microwave spectra of snow cover observed from Special Sensor Microwave/Imager measurements

Brightness temperature spectra measured by the Spacial Sensor Microwave/Ima ger (SSM/I) flown onboard F8 and F14 satellites of the U.S. Defense Meteoro logical Satellite Program (DMSP) during the 1987-1988 and 1997-1998 winter periods are analyzed concurrently with the data from snow monitoring statio ns over the former Soviet Union. Extensive analysis reveals the existence o f two anomalies in the microwave thermal radiation spectra of snow cover. I t is shown that in the beginning of winter the SSM/I measurements at 19, 37 , and 85 GHz generally follow a classical pattern; that is, the brightness temperatures decrease for both increasing snow depth and increasing frequen cy. Dramatic departures fi um this behavior is observed around the middle o f winter: The brightness temperatures roach a minimum and then begin to inc rease despite the fact that the snow depth remains constant or even continu es to grow. Statistical analysis of the snow pack characteristics and SSM/I measurements is presented around the time when the brightness temperatures reach a minimum. The anomalous spectral characteristics are analyzed using a two-stream radiative transfer model and dense media theory. It is shown how metamorphic changes in the snow crystalline structure are responsible f or the brightness temperature minimum. The second departure from the normal snow signature is the inversion of brightness temperature spectra; that is , the higher-frequency brightness temperature is greater than the low-frequ ency measurements. It is shown that tills phenomenon, observed previously o ver Greenland and Antarctica, is much more extensive. Radiative transfer si mulations were used to show that a dense layer of surface crust on top of o ld coarse-grained snow can produce the invented brightness temperature spec trum.