Development and assessment of combined Terra and Aqua snow cover products in Colorado Plateau, USA and northern Xinjiang, China

This study presents our methods to produce multi-day Terra, Aqua, and Terra-Aqua moderate resolution imaging spectroradiometer (MODIS) snow cover composite images respectively from daily Terra MODIS, Aqua MODIS, and both Terra and Aqua MODIS snow cover products, with flexible starting and ending dates and a user-defined cloud cover threshold. The methods are applied to Colorado Plateau, USA and northern Xinjiang, China. The statistical comparison gives the following results. For the 2003-2004 hydrologic year, the daily Terra-Aqua composite images exhibited ~10-15% less on annual mean cloud cover and ~1-4% more on annual mean snow cover, compared with their daily Terra or Aqua counterparts. Using 10% cloud cover as a user-defined threshold, we produced 152 (northern Xinjiang) and 162 (Colorado Plateau) multi-day Terra-Aqua composite images for the 20032004 hydrological year, respectively. On average, it is 2.4 and 2.2 days per composite image for northern Xinjiang and Colorado Plateau, respectively. The multi-day Terra, Aqua, and Terra-Aqua composite products result in similar annual mean snow covers (~15% for the Colorado Plateau and ~30% for the northern Xinjiang), as those from the corresponding standard NASA 8-day products, ~3 times as those from the standard NASA daily products. The lower snow cover percentage retrieved from the daily standard products is mainly due to larger cloud cover in the daily products. The Aqua products always have lower annual mean snow cover and higher annual mean cloud cover than those of the Terra products. The daily Terra-Aqua composite products have higher agreement with ground measurements than either of the standard NASA daily Terra or Aqua products. The multi-day Terra-Aqua composite products have much higher agreement with ground measurements than that of the standard daily products and have similar agreement as that of the standard 8-day products. Therefore, those new composite products generated from our methods are a significant contribution to the current MODIS snow cover product series.