The Imprint of Al Chang on Canadian Achievements in Passive Microwave Remote Sensing of Snow Cover

Anne Walker and Barry Goodison
Climate Research Branch, Meteorological Service of Canada
4905 Dufferin Street, Downsview, Ontario, Canada M3H 5T4
Ph: 416-739-4357, Fax: 416-739-5700
E-mail: anne.walker@ec.gc.ca

The Climate Research Branch (CRB) of the Meteorological Service of Canada (MSC) has a long-standing research program focused on the development of methods to retrieve snow cover information from passive microwave satellite data for Canadian regions. Snow cover is a dominant feature of the Canadian landscape for at least 6-8 months of the year, with snow water equivalent (SWE) having a particularly significant influence on the water and energy budgets. Spatial and temporal variability of SWE can be large due to climate, land cover and topographic variations; yet ground based observations are very limited over most of the country. Hence, there is a continuing need to develop and evaluate new methods which will allow determination of SWE and its variability over the different regions of Canada for both climatological and hydrological applications.

The CRB research activity to derive SWE from passive microwave satellite data began in the early 1980s. A collaborative multi-agency experiment involving NASA's airborne MFMR, airborne gamma flights by the US National Weather Service and Canadas Geological Survey of Canada, extensive ground sampling led by CRB and SMMR satellite data was the basis for the development of the CRB SWE airborne algorithms. It was the start of long-term co-operation between scientists and their agencies that has led to new knowledge and products to derive SWE from passive microwave satellite data from regional to global scales. Canadian research continued, with a focus on the development of regional products from satellite data to meet the user community's needs. On-going validation of SWE satellite products to meet real-time uses has been trademark of the CRB approach.

Over subsequent years a suite of algorithms, using SMMR and SSM/I, to derive SWE from satellite data have been developed by, and validated for, several landscape regions including prairie, boreal forest and taiga. Now algorithms are used with SSM/I data to generate regional snow cover products that support a number of research and operational applications. Maps depicting SWE distribution over areas in western Canada are produced on a regular basis each winter (e.g. weekly) using SSM/I data accessed in near real-time. Maps are distributed to a variety of users such as national and provincial water resource agencies, agricultural agencies, hydropower companies, and meteorological forecast offices to support their operational activities. Recently, the application of SWE algorithms to the 25-year time series of satellite passive microwave data - Nimbus-7 SMMR (1978-1987) and SSM/I (1987-present) - has provided CRB with new insights on long term spatial and temporal variability of snow cover and relationships with atmospheric circulation and climate variability. New opportunities always come along, the most recent being evaluation and use of AMSR-E data for improved SWE retrievals.

The Canadian achievements noted above would not have been possible without the collaboration and involvement of Al Chang and his colleagues at NASA, spanning from the very initial research activities to today. Al was instrumental in arranging for NASA microwave radiometers to be flown on aircraft as part of 2 major field experiments in western Canada in February 1982 and during the February 1994 BOREAS winter campaign. Microwave radiometer data sets resulting from these airborne programs were provided to CRB scientists who coupled the data sets with airborne gamma and ground-based measurements of snow cover (SWE) to derive the empirical SWE algorithms that are used today by CRB to generate various SWE map products. Als research and published papers on the theory behind retrieval of snow cover properties using microwave remote sensing had a significant influence on the development of the approach used by CRB to acquire in situ measurements of snow cover physical characteristics. Based on our field data collection, we were able to also "educate" Al and his colleagues on the very complex, layered nature of real world snow packs and challenge him to apply his modeling approach to such heterogeneous cases.

CRB has been fortunate to have enjoyed over 20 years of collaboration with the NASA team of Al Chang, Dorothy Hall and Jim Foster - ranging from shared participation in field experiments, scientific discussions at various meetings and conferences, joint projects, and contributing to the validation of new SWE products from the AMSR-E sensor (one of Al's last responsibilities). Al Chang's impact on the CRB program has been significant and long-lasting and his friendship will be missed. The authors are honored to be able to contribute to this memorial symposium in his honor and describe to his family, friends and colleagues his imprint on Canadian achievements in passive microwave remote sensing of snow cover.

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