Scales of aircraft icing: a comparison of icing PIREPs to liquid water measurements from research aircraft

A pilot report (PIREP) of icing severity is the result of a complex combination of the icing conditions present, the way in which the aircraft encountered those conditions, the aircraft type, its ice protection system, and the pilots experience and perception. The intensity of the atmospheric icing condition itself is a function of the temperature, liquid water content, and drop sizes present within the clouds and/or precipitation. By combining information on these quantities for a given volume of airspace at a given time (e.g. at 10,000 ft. over Chicago at 21Z today), one can assess an instantaneous estimate of the icing intensity expected at that point in time and space. This intensity must further be related to how an aircraft will interact with the conditions present to estimate icing severity, which is an inferred or observed measure of the impact that the icing has on the aircrafts ability to fly. Given the complexity of the interaction between the meteorological conditions, the aircraft, and the pilot, which leads to a voiced PIREP of icing severity, it is instructive to compare these reports with objective measurements of the conditions. Aircraft measurements of liquid water content and temperature were compiled from numerous flight programs made in the U.S. and Canada between 1997 and 2004, using a variety of aircraft platforms (e.g. Twin Otter, Convair 580, and C-130). Icing PIREPs were matched to these data if they occurred nearby in time and space, both horizontally and vertically. Time and location differences were varied to determine how the correlations were affected. Ideally, high (low) values of measured SLW would correspond to more (less) intense icing reports in the vicinity. In general, this correlation was lower than expected, though the results did improve somewhat with decreasing differences in location and time. The reasons for the lack of correlation vary from case to case, but most can be traced back to the different scales at which icing occurs. It is possible to have PIREPs of varying intensities or high and low SLW values within a small volume. A closer examination of a few cases will also be presented. The scale of icing events can have implications for products that diagnose and forecast aircraft icing likelihood and severity. Icing conditions can change very quickly over a small volume so the conditions in one grid box may be completely different from those in an adjacent one. This also adds a degree of difficulty to the verification of such products.