Regional icing algorithm performance analysis

Verification of in-flight icing forecasts previously has involved evaluations on either the national scale or in the areas around large cities. However, forecast performance within specific regions also is an important aspect of the evaluation of icing forecasts and diagnoses. In particular, regional differences in performance may indicate changes or improvements that are needed to accommodate the varying characteristics of the forecasts and icing situations. Moreover, these evaluations may identify particular strengths and weaknesses in the forecasts.

Performance of the Integrated Icing Diagnostic Algorithm (IIDA) and icing AIRMETs, the operational icing forecasts issued by the National Weather Service's Aviation Weather Center, are evaluated over 15 regions across the continental United States. These regions were defined to cover the AIRMET region while creating areas that are relatively homogeneous in terms of both climate and topography. Naturally, the occurrence of icing varies among these regions. Moreover, the frequency of pilot reports (PIREPs) of icing is dependent both on the climate and on aircraft flight patterns. The regional measures of overforecasting, the percent of total area and volume covered by the forecast, are normalized using the maximum expected icing extent, based on numerical-model-based temperature and humidity analyses. Finally, a variety of IIDA thresholds are evaluated to determine the extent to which the calibration of IIDA varies from region to region. Results indicate that the performance of both IIDA and the AIRMETs varies a great deal among the regions.