飞机结冰研究：云模拟和表征中的挑战

      Recently, FAA requirements for aircraft icing certification have been updated to include characterization of supercooled large drops (SLD). Cloud measurements and icing wind tunnel simulations of existing Federal Aviation Regulations, Appendix C, Part 25 and these new conditions (Appendix O) require advances in instrumentation capable of measuring droplet size and liquid water content (LWC) under a much wider range of conditions. Cloud microphysics investigations and measurements have revealed that some clouds containing large drops can have droplet size distributions that are bi-modal. Such conditions present an additional challenge to the instrumentation. Existing instrumentation based on concepts developed in the 1970’s are known to have difficulties measuring spray drops in the SLD range. Currently, up to five or six different instruments utilizing different measurement principles are needed to characterize icing clouds and icing cloud simulations in icing wind tunnels. The phase Doppler interferometer flight probe, dual range (PDI FPDR) has been developed to address these new and challenging spray measurement conditions. This instrument is capable of simultaneously measuring droplets in the size range of 1 μm to 2000 μm or larger and LWC over a full range of cloud and cloud simulation conditions. To ensure reliable operation under the harshest icing conditions, the probe incorporates deicing and temperature controls. Since the instrument is considered to be relatively new to the meteorological and icing research community, extensive tests and evaluations of the instrument were conducted to demonstrate its measurement capabilities. An extensive program of instrument comparisons between the well-established PMS FSSP and OAP instruments was conducted. Results of these comparisons are reviewed and discussed. Significant measurement differences were found under higher droplet number density spray conditions and these differences are discussed and reconciled. Comparisons of LWC measurements are also provided and compared to the well-established hot wire devices and icing blade data.