An Improved Retrieval for Cloud Water Contents from the Second Generation Closed Path Laser Hygrometer

A second generation University of Colorado closed-path laser hygrometer (CLH-2) is an airborne in situ instrument that fits in a wing canister on the NSF/NCAR Gulfstream-V aircraft and is designed to measure cloud water content (CWC) over a wide range of altitudes and total water values. We report on the results of recent work to improve the retrieval for total water (and, hence, CWC) from the CLH-2 by more reliably estimating the background laser signal profile and by more precisely fitting the line shape of the water absorption feature at 1368.6 nm. Using three different line profiles to model the absorption feature, including Voigt, Lorentz, and Gaussian, we find the Voigt and Lorentz profiles to produce the more versatile and reproducible results. Intercomparisons in clouds with CWC measured with the NCAR counterflow virtual impactor (CVI) and in clear air to ambient water measured with a Picarro cavity ringdown spectrometer demonstrate good agreement down to 500 ppm and below, with reliable identification of the water vapor absorption line in individual 1-second spectra. This method appears to consistently resolve meaningful water vapor variations of several hundred ppm at absolute values of less than 1000 ppm, which is at least a factor-of-three improvement over previous retrievals for this instrument. Even lower detection thresholds are achievable with additional averaging. After accounting for the enrichment of droplets through the forward facing sub-isokinetic sampling inlet, this represents a minimum resolvable CWC of approximately 0.01 g/kg. With additional minor modifications, including second harmonic detection the instrument should be capable of resolving variations of 10 ppm at low ambient water mixing ratios. This work will help to improve the performance of the CLH-2 in future field campaigns using the Gulfstream V or other suitable aircraft and allows us to refine CWC results from the Deep Convective Clouds and Chemistry field campaign which took place over the continental United States in the summer of 2012.