A new comparison of bulk ice water content measurements in cirrus cloud

Independent and reliable measurements of atmospheric Ice Water Content (IWC) from platforms such as research aircraft are difficult to obtain. Measurement of IWC in cirrus clouds is important in order to determine their radiative impact. Methods of determining the size-distributed IWC using optical imaging probes such as the 2D-C suffer from the limitation that particle masses must be specified using an empirical relationship with particle size. Verification of such a relationship requires the accurate measurement of the bulk IWC for a range of different particle size spectra. Constant temperature hot wire probes such as the Nevzorov Total Water Content (TWC) probe offer one method of determining the bulk IWC. A Counterflow Virtual Impactor inlet (CVI) in conjunction with a Lyman-Alpha absorption hygrometer to measure the total water content of evaporated crystals is another method of measuring IWC in cirrus.

As a result of wind tunnel tests, shortcomings with the original Nevzorov TWC collector have recently been identified. This featured an 8mm diameter collector cone with a 120 degree angle. Ice fragments have been observed to bounce out, and melt-water from previous particles may also splash out when impacted by newly-arriving particles. Both processes result in the under-estimation of the IWC by a factor of between 2 and 3. The present measurements feature a cone with a 60 degree angle for which wind tunnel tests have shown a significant improvement in the capture efficiency of ice particles. Data were obtained from the FAAM BAe146 research aircraft during studies of ice- and mixed-phase clouds to the north of Scotland during January 2010. We will show comparisons between Nevzorov and CVI bulk IWC measurements and those obtained by size spectrum integration from a number of different imaging probes. The bulk measurements show good agreement with the principle discrepancies appearing at altitudes above about 8km. We investigate potential sources of error in both measurements including the estimation of the CVI enhancement factor at high altitudes and the estimation of the Nevzorov baseline drift with altitude during periods in which the aircraft remains totally in cloud. We also consider the impact of the CVI cut-size and the size-dependent collection efficiency of the Nevzorov TWC collector.