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

Monday, 7 July 2014
Steven J. Abel, Met Office, Exeter, United Kingdom; and R. J. Cotton, P. A. Barrett, and A. K. Vance

This paper presents a comparison of ice water content (qi) 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 qi 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 qi 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 qi ± 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 (qi > 0.01 g kg-1) and the total water content minus water vapour method (qi > 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 qi.

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