Passive microwave SWE data for the Canadian Prairies
The Climate Research Division of the Science and Technology Branch of Environment and Climate Change Canada has an ongoing program to develop SWE retrieval capabilities from spaceborne passive microwave brightness temperatures for major Canadian landscape regions (Walker and Goodison, 2000). Initial algorithm development focussed on the relatively simple scattering environment of the open prairies (Goodison and Walker, 1995), while subsequent research produced algorithms for general categories of boreal forest cover (Goita et al., 2003). The open prairie algorithm is based on the brightness temperature gradient between the 37 and 19 GHz (18 GHz with SMMR) vertically polarized channels, while the three forest algorithms are based on the brightness temperature difference of these same channels. The resulting suite of land cover sensitive SWE retrieval algorithms can be applied to both SMMR and SSM/I brightness temperatures, with per-grid cell SWE estimates produced as the sum of the SWE values obtained from each land cover algorithm weighted by the percentage land cover type (F) within each grid cell:
SWE = FDSWED + FCSWEC + FSSWES + FOSWEO ,
where D represents deciduous forest, C coniferous forest, S sparse forest, and O open Prairie environments. Evaluation of the algorithm suite for various regions and time periods (Derksen et al., 2002; Walker and Silis, 2002; and Derksen et al., in press) has shown that the algorithms are typically capable of producing SWE retrievals within +/- 15 mm of surface observations, although consistent underestimation of SWE is a problem in heavily forested areas due to the complex impact of dense vegetation on microwave emission and scatter.
The passive microwave SWE database provided for this site are 5-day (pentad) averaged SWE maps from December 1 to March 1 covering the period 1978 to 2002. The data have a nominal resolution of 25 km, and the SMMR brightness temperatures were standardized to an SSM/I F-8 baseline using the frequency, polarization, and overpass time specific coefficients of Derksen and Walker (2003). Real-time SWE maps are also generated with the same algorithm.
- Derksen, C., A. Walker, and B. Goodison. In press. A comparison of 18 winter seasons of in situ and passive microwave derived snow water equivalent estimates in Western Canada. Remote Sensing of Environment.
- Derksen, C., and A. Walker. 2003. Identification of systematic bias in the cross-platform (SMMR and SSM/I) EASE-Grid brightness temperature time series. IEEE Transactions on Geoscience and Remote Sensing. 41(4): 910-915.
- Derksen C., A. Walker, E. LeDrew, and B. Goodison. 2002. Time series analysis of passive microwave derived central North American snow water equivalent imagery. Annals of Glaciology. 34: 1-7.
- Goita, K., A. Walker, and B. Goodison. 2003. Algorithm development for the estimation of snow water equivalent in the boreal forest using passive microwave data. International Journal of Remote Sensing. 24(5): 1097-1102.
- Goodison, B., and A. Walker. 1995. Canadian development and use of snow cover information from passive microwave satellite data. In Choudhury, B., Y. Kerr, E.Njoku, and P. Pampaloni (eds.). Passive Microwave Remote Sensing of Land-Atmosphere Interactions. VSP BV, Utrecht, Netherlands, 245-262.