The Use of Amdar Observations For Verifying Cloud Ceiling and Icing Forecasts

Abstract

The World Meteorological Organization (WMO 2015) AMDAR observing system provides automatic measurement of atmospheric conditions using aircraft as an observation platform. Variables include latitude, longitude, pressure altitude, time, and temperature. In addition, NOAA/NWS has installed the Water Vapor Sensing System (WVSS-II) in support of the U.S. contribution to the WMO AMDAR observing system. There are 130 WVSS-II instruments equipped on UPS (25) and Southwest Airlines (105) aircraft. These instruments provide reports of water vapor mixing ratio, as well as Relative Humidity (RH) and dew point (both derived from the mixing ratio). These data are of particular interest near airports where aircraft record measurements during takeoff and landing so that these measurements form a vertical profile, also called a sounding. The presence of cloud layers can be inferred from the profiles of these moisture observations, helping to identify cloud bases (i.e., ceilings). Combining the moisture and temperature data identifies, especially, regions where icing is not expected (too warm or too dry) but also regions where icing is possible. This presentation will illustrate the utility of AMDAR observations for evaluating forecasts of weather impacting aviation. For example, verifying icing potential forecasts against AMDAR data produces results consistent with verification against other observing platforms (e.g., PIREPs, satellite, etc.), giving confidence in the use of AMDAR temperature and moisture data as a truth set. Concerning identification of ceilings, transitions from low AMDAR RH values to values >80% are in good agreement with METAR cloud base observations. Individual soundings show the height of the maximum AMDAR RH to align well with the METAR cloud base height (and the height of maximum radiosonde RH values). In addition, it may be possible to use AMDAR to help inform the use of other observing platforms. For example, when multiple co-located observations are available (e.g., AMDAR, radiosonde, METAR, radar, etc.), and two or more of them agree, the behavior of the other observations can be investigated against that truth”.