Improving the accuracy of the AFWA-NASA (ANSA) blended snow-cover product over the Lower Great Lakes region

Abstract. The Air Force Weather Agency (AFWA) – NASA blended snow-cover product, called ANSA, utilizes Earth Observing System standard snow products from the Moderate-Resolution Imaging Spectroradiometer (MODIS) and the Advanced Microwave Scanning Radiometer for EOS (AMSR-E) to map daily snow cover and snow-water equivalent (SWE) globally. We have compared ANSA-derived SWE with SWE values calculated from snow depths reported at ~1500 National Climatic Data Center (NCDC) co-op stations in the Lower Great Lakes Basin. Compared to station data, the ANSA significantly underestimates SWE in densely-forested areas. We use two methods to remove some of the bias observed in forested areas to reduce the root-mean-square error (RMSE) between the ANSA- and station-derived SWE. First, we calculated a 5-yr mean ANSA-derived SWE for the winters of 2005–2006 through 2009–2010, and developed a 5-yr mean bias-corrected SWE map for each month. For most of the months studied during the 5-yr period, the 5-yr bias correction improved the agreement between the ANSA-derived and station-derived SWE. However, anomalous months such as when there was very little snow on the ground compared to the 5-yr mean, or months in which the snow was much greater than the 5-yr mean, showed poorer results (as expected). We also used a 7-day running mean (7DRM) bias correction method using days just prior to the day in question to correct the ANSA data. This method was more effective in reducing the RMSE between the ANSA- and co-op-derived SWE values, and in capturing the effects of anomalous snow conditions.