On the Impact and Future Benefits of AMDAR Observations in Operational Forecasting: Part II: Water Vapor Observations

Abstract

lthough wind and temperature observations from commercial aircraft have been shown to improve operational numerical weather prediction (NWP) on global and regional scales, the quality and potential importance of newly available moisture observations are less well recognized. Because moisture changes often occur at much smaller scales than wind and temperature variations, these temporally and spatially frequent moisture observations can have exceptionally large impacts on forecasts of disruptive weather events and could help offset the dwindling number of global moisture observations. Currently, more than 148 aircraft-based Water Vapor Sensing Systems (WVSS; 139 operating in the US and 9 in Europe) provide specific humidity observations en route and in 1200 profiles made daily during takeoff/landing. Results of a series of assessments comparing data from initial WVSS sensors installed on 25 United Parcel Service (UPS) Boeing 757 aircraft with collocated raobs show agreement to within 0.5 g kg?1, with minimal biases. Intercomparisons of observations made among nearby aircraft agree to better than 0.2 g kg?1. The combined results suggest that the WVSS measurements are at least as accurate as water vapor observations from high-quality raobs. Information regarding observed spatial and temporal moisture variability could be important in optimizing the use of these observations in future mesoscale assimilation systems. Forecasts of disruptive weather events made by NWS and airline forecasters demonstrate the benefits obtained from combined temperature/moisture/wind profiles acquired during aircraft ascents and descents. Finally, initial NWP impact studies show that WVSS reports that include moisture obtained throughout the day have greater influence than twice-daily raob humidity data on contiguous U.S. (CONUS) forecasts for 24 h and beyond.