A comparison of ice water content measurement techniques on the FAAM BAe-146 aircraft

This paper presents a comparison of ice water content (q_i) data from a variety of measurement techniques on the Facility for Airborne Atmospheric Measurements (FAAM) BAe-146 research aircraft. Data are presented from a range of cloud types measured during the PIKNMIX field experiment that include mixed phase stratocumulus, cumulus congestus and cirrus clouds. These measurements cover a broad range of conditions in which atmospheric ice particles are found in nature, such as the low ice water content environments typically found in mid-latitude cirrus and the much higher ice water content environments often observed in cold convective clouds. The techniques include bulk measurements from i) a Nevzorov hot-wire probe ii) the difference between the measured total water content (condensed plus vapour) and the water vapour content of the atmosphere and iii) a Counterflow Virtual Impactor (CVI) (only for cirrus measurements). We also estimate the q_i from integration of the measured particle size distribution (PSD) with assumptions on how the density of ice particles varies as a function of size.

The results show that the only bulk ice water content technique capable of measuring high q_i values (several g kg^-1) was the total water content minus water vapour method. For low ice water contents we develop a new parametrization of the Nevzorov base-line drift that enables the probe to be sensitive to q_i ± 0.002 g m^-3. In cirrus clouds the agreement between the Nevzorov and other bulk measurements was typically better than a factor of two for the CVI (q_i > 0.01 g kg^-1) and the total water content minus water vapour method (q_i > 0.03 g kg^-1). Good agreement with the bulk measurements for all cases could be obtained with the estimate from the PSD provided that appropriate a-priori assumptions on the mass-dimension relationship were made. This is problematic in the convective clouds sampled because pristine ice particles, heavily rimed particles and supercooled liquid drops were all present. In a cirrus case we show that using a temperature dependent mass-dimension relation was required to match the bulk measurement of q_i.