Passive microwave measurements of tundra and taiga snow covers in Alaska, U.S.A.

Abstract

The microwave emissivity of two snow covers was measured in Alaska in March, 1990. Observations were made on taiga snow near Fairbanks that was 0.83 m thick with a 0.55 m thick basal layer of depth hoar. Other measurements were made on the tundra snow cover at Imnaviat Creek north of the Brooks Range which was 0.27 to 0.64 m thick and consisted of two or more wind slabs overlying a depth hoar layer 0.14 to 0.26 m thick. Density, crystal structure, and grain size were similar in tundra and taiga depth hoar layers. Emissivity was measured at 18.7 and 37 GHz using radiometers mounted on a 1.5 m tall bipod. Measurements were made on undisturbed snow, and then several snow layers were removed and additional measurements were made. This sequence was repeated until all snow had been removed. Effective emissivity values for the full snow depth ranged from 0.6 (37 GHz, H-pol) to 0.95 (18.7 GHz, V-pol) and were similar for both taiga and tundra snow covers. For both snow covers, there was a marked reduction in the effective emissivity (eeff) from that of the underlying ground with a maximum reduction of about 30%. All of the reduction was found to occur within the depth hoar layer. Maximum reduction in eeff could be caused by a depth hoar layer 0.3 m thick. Overlying wind slab or new snow were nearly “invisible”, increasing the effective emissivity only by a small amount due to self-emittance. Thus, it was difficult to distinguish the two different snow covers on the basis of their emissivity, since both contained 0.3 m of depth hoar or more.