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Radiobrightness at 6.7GHz, 19GHz and 37GHz from Mature Evergreen Trees Observed During the Cold Lands Processes Experiment in Colorado

The University of Michigan Microwave Geophysics Group participated in the Cold Lands Processes Experiment (CLPX) during Feb-Apr 2003 by deploying its Truck Mounted Radiometer System-3 (TMRS-3) to perform temporal monitoring of the snowpack at the Local Scale Observation Site (LSOS). The LSOS was located at the Fraser Experimental Forest headquarters in the mountains near Fraser, Colorado, USA. The clearing in which the TMRS-3 was deployed was only about 20m x 20m, and adjacent to a tree line composed of ~10m tall evergreen trees. To quantify the amount of the downwelling brightness from these trees onto the snow pack, the TMRS-3 was periodically lowered and pointed toward the tree line. Microwave brightness data was collected from the trees in 15 degree increments from horizontal incidence to 45 degrees from zenith. Both polarizations were observed for 6.7, 19 and 37GHz. A rapid decrease in brightness is evident as the radiometers were pointed progressively upwards. The next May an upward looking hemispherical ("fish-eye") photograph was taken for the purpose of estimating the longwave radiation sources to the snow pack. This photograph was taken from the point on the ground where the majority of snow pack microwave brightness observations was taken, and reveals a significant sky background through the incomplete canopy of these trees. By superimposing Gaussian approximations to the microwave antenna gain pattern of the individual TMRS-3 radiometers onto the photograph, we estimated the amount the main beams were filled with canopy and with sky. Comparison of the measured data to that expected for a partially filled main beam indicates that, except for the very top of the evergreen canopy, it emits radiobrightness in a roughly isotropic fashion. Investigations into possible contributions of scattering are ongoing. An analysis of the upwelling brightness from the same trees will also be reported.