Abstract. Jet engine power loss due to ice particle accumulation is a recognized
aviation hazard occurring in cloud conditions difficult to forecast or
visually recognize. High-altitude cirrus clouds can have ice particle
concentrations high enough to be dangerous; therefore, pilots must be
informed when aircraft enter such environments. One approach to determining
ice particle concentration is an onboard lidar system. Concurrent lidar
measurements are compared to backscatter coefficients derived from particle
size distributions obtained from wing-mounted, in situ probes during four
case studies consisting of sixty-second flight segments at different
temperatures: +7 and +4 ∘C for water droplet
analysis, and −33 and −46 ∘C for ice particle analysis.
Backscatter coefficients derived from external cloud probes (ECP) are
correlated (0.91) with measurements by an airborne lidar system known as the
Optical Ice Detector (OID). Differences between OID and ECP backscatter
coefficients range from less than 1 to over 3 standard deviations in terms of uncertainties. The backscatter coefficients are mostly in agreement for
liquid clouds and are in disagreement for the −33 and −46 ∘C cases, with ECP-derived backscatter coefficients lower than
the OID for three out of the four cases. Measurements over four 60 s research flight segments show that measured total water content is
correlated (0.74) with the OID backscatter coefficient, which indicates that
the OID is a useful instrument for determining ice particle concentrations
over a broad range of environments, including at ice water contents as low
as 0.02 g m−3. Additionally, concurrent measurements from cloud imaging
probes and the OID provide improved knowledge of cloud conditions, which may
help in understanding cloud processes.