Aircraft icing potential and ice- and mixed-phase cloud particle size distributions

Description

Ice clouds play an important role in Earth's climate by influencing the radiation balance and hydrological cycle. Improved parameterizations of cold clouds in the climate models require good understanding of the cloud properties, and especially of the role of the ice particles’ size distribution (PSD). Ice and mixed-phase clouds have an important impact on aviation. In-flight icing is a significant threat to aircraft, resulting in loss of lift, reduced airspeed, and, in some cases, loss of control (Bernstein et al. 2005). Based on recent data, freezing precipitation often forms through nonclassical formation mechanisms, without requiring the formation of a melting layer. However, these relationships are still not thoroughly studied. The focus of this research is to help improve our understanding of winter aircraft icing occurrence through better parameterizations of the ice microphysical cloud properties. The study explores possible relationships between different ice crystals’ size distributions, and airplane icing. The study utilizes data for different ice crystal size spectra in winter cold clouds and data for the corresponding airplane icing occurrences. The microphysical cloud properties are primarily a function of the ice water content (IWC) and the size distributions (SDs), while the radiative properties also depend on temperature (T) and the size distributions (Mitchell 2001). Hence, it is important to know not only the IWC, but how the ice particle size distribution itself is likely to vary as a function of environmental parameters, because they are the main input for the ice cloud parameterizations and may relate to the possible aircraft icing. Cold-cloud interactions with aircrafts that fly through them require knowledge of cloud microphysics. Aircrafts must be designed to fly into supercooled clouds, or they must avoid those clouds in order to prevent problems associated with airframe and engine icing. De-icing or anti-icing systems must be engineered to withstand reasonable extremes in terms of ice water content (IWC), supercooled liquid water content (LWC), ice particle size distributions (SDs), and temperature. The aircraft design or certification envelopes (FAR 25, Appendix C; Federal Aviation Administration, 1999) were developed before the advent of modern cloud physics instrumentation. In the case of ice and mixed-phase clouds, data from the aircraft measurements during recent field campaign suggest that cloud temperature is one of the main parameters governing cloud microstructure, the size distributions, and the current icing potential (CIP). This study may help improve airplane icing prediction through better understanding of the ice microphysical properties.