ACP Atmospheric Chemistry and Physics ACP 1680-7324 Copernicus GmbH Göttingen, Germany 10.5194/acp-12-355-2012 Aircraft millimeter-wave passive sensing of cloud liquid water and water vapor during VOCALS-REx Zuidema P. 1 Leon D. 2 Pazmany A. 3 Cadeddu M. 4 Rosenstiel School of Marine and Atmospheric Sciences, University of Miami, Miami, Florida, USA Department of Atmospheric Sciences, University of Wyoming, Laramie, Wyoming, USA Prosensing Inc., Amherst, Massachusetts, USA Argonne National Laboratory, Argonne, Illinois, USA 05 01 2012 12 1 355 369 This is an open-access article ditributed under the terms of the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited. This article is available from http://www.atmos-chem-phys.net/12/355/2012/acp-12-355-2012.html The full text article is available as a PDF file from http://www.atmos-chem-phys.net/12/355/2012/acp-12-355-2012.pdf

Routine liquid water path measurements and water vapor path are valuable for process studies of the cloudy marine boundary layer and for the assessment of large-scale models. The VOCALS Regional Experiment respected this goal by including a small, inexpensive, upward-pointing millimeter-wavelength passive radiometer on the fourteen research flights of the NCAR C-130 plane, the G-band (183 GHz) Vapor Radiometer (GVR). The radiometer permitted above-cloud retrievals of the free-tropospheric water vapor path (WVP). Retrieved free-tropospheric (above-cloud) water vapor paths possessed a strong longitudinal gradient, with off-shore values of one to two mm and near-coastal values reaching ten mm. The VOCALS-REx free troposphere was drier than that of previous years. Cloud liquid water paths (LWPs) were retrieved from the sub-cloud and cloudbase aircraft legs through a combination of the GVR, remotely-sensed cloud boundary information, and in-situ thermodynamic data. The absolute (between-leg) and relative (within-leg) accuracy of the LWP retrievals at 1 Hz (~100 m) resolution was estimated at 20 g m<sup>−2</sup> and 3 g m<sup>−2</sup> respectively for well-mixed conditions, and 25 g m<sup>−2</sup> absolute uncertainty for decoupled conditions where the input WVP specification was more uncertain. Retrieved liquid water paths matched adiabatic values derived from coincident cloud thickness measurements exceedingly well. A significant contribution of the GVR dataset was the extended information on the thin clouds, with 62 % (28 %) of the retrieved LWPs <100 (40) g m<sup>−2</sup>. Coastal LWPs values were lower than those offshore. For the four dedicated 20° S flights, the mean (median) coastal LWP was 67 (61) g m<sup>−2</sup>, increasing to 166 (120) g m<sup>−2</sup> 1500 km offshore. The overall LWP cloud fraction from thirteen research flights was 63 %, higher than that of adiabatic LWPs at 40 %, but lower than the lidar-determined cloud cover of 85 %, further testifying to the frequent occurrence of thin clouds.

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