Evaluation of passive microwave snow water equivalent retrievals across the boreal forest/tundra transition of western Canada

Composite patterns of monthly averaged (1978–2002) satellite passive microwave derived snow water equivalent (SWE) data produced using the Meteorological Service of Canada land cover sensitive algorithm suite contain an interannually consistent and well-defined zone of high SWE retrievals (> 100 mm) across the northern boreal forest of western Canada. Consistently lower SWE retrievals are present across both the comparatively dense boreal forest to the south, and open tundra to the north. Because of the sparse conventional observing network across this region, a dedicated field sampling campaign was conducted during the 2003/2004 winter season to measure snow properties across these landscape zones in northern Manitoba, Canada. Using road and helicopter access, snow cover measurements were made along an approximately 500 km transect between the communities of Thompson (boreal forest), Gillam (sparse northern boreal forest), and Churchill (open tundra) during late November 2003, and early March 2004. The November 2003 snow survey revealed good agreement between Special Sensor Microwave/Imager (SSM/I) derived SWE retrievals and in situ measurements across the gradient of increasing SWE between Thompson and Gillam, and the well defined zone of high SWE values to the north and east of Gillam. Further to the north, SSM/I derived SWE retrievals over the boreal ecotone and open tundra were consistently lower than the in situ measurements. Results of the March survey (at which point mean snow depths had increased from approximately 30–60 cm) showed an increase in absolute SWE differences between the in situ measurements and passive microwave estimates, but relative agreement between the datasets remained strong. The same SWE gradient across the boreal forest was again captured by both datasets, although tundra retrievals remained consistently low. These findings, along with consideration of snowpack stratigraphy, density, and the potential influence of tundra lakes and ponds on microwave emission and scatter, suggest that development of a tundra-specific SWE retrieval algorithm is necessary. From a scaling perspective, a large range of SWE values were measured on the ground within each satellite footprint, but a comparison of frequency histograms showed that a high percentage of satellite retrievals (November: 93%; March: 63%) fell within ± 20 mm of the median ground measurement.