Measurement and Modeling of Microwave Emission from Forested Snowfields in Michigan

Forest cover was found to confuse the normally inverse relationship between microwave brightness temperature (T_B) and snow depth in dry snowpacks in the lower peninsula of Michigan during February of 1979 and 1980. However, even in the presence of forest cover, consisting of a mixture of hardwoods and softwoods, the average Nimbus-7 SMMR 37 GHz vertically and horizontally polarized T_Bs were 11° and 19° K lower during the study period in February of 1979 versus February of 1980. This is attributed to deeper average snowcover for the study period in 1979 (43.8 cm) as compared to 1980 (9.9 cm) since lower T_Bs accompany deeper snow under dry snowpack conditions. 37 GHz brightness temperature was correlated with snow depth for the study periods in 1979 and 1980, resulting in positive relationships instead of the expected inverse relationships. Because the snow depth increased with increasing forest cover in the lower peninsula of Michigan, it was assumed that the emissivity of the forest cover overwhelmed that of the snow - both in 1979 and 1980 causing higher T_Bs in more heavily forested areas. Using a simple model, we removed the effects of the trees from the T_B/snow depth relationship. A new value, T_BR, residual brightness temperature, derived from the 37 GHz data, was correlated with snow depth for selected time periods in February 1979 and 1980 resulting in inverse relationships of R ≡ .82 and .77 respectively. In addition, SMMR data of August 1979 were correlated with percent forest cover to determine the T_B of the study area in the absence of snow. The lower SMMR frequencies (10.69 and 6.6 GHz) showed statistically significant inverse relationships with forest cover during the August study period. These inverse relationships were probably due to increased soil moisture in the more heavily forested areas in the northern half of lower peninsula of Michigan thus causing lower T_Bs.