Abstract
Abstract. 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−2 and 3 g m−2 respectively for well-mixed conditions, and 25 g m−2 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−2. 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−2, increasing to 166 (120) g m−2 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.
Highlights
The VOCALS Regional Experiment, held in October– November 2008 in the southeast Pacific (SEP), had one focus on acquiring field measurements capable of illuminating the processes linking aerosols, clouds, and precipitation (Wood et al, 2011b), field measurements that could help assess modeled cloud-aerosol interactions (e.g., Saide et al, 2011, and references therein)
Radiosonde-derived water vapor paths calculated at 150 m, a representative altitude for the aircraft sub-cloud legs, show VOCALS-REx WVPs lower than for other years, with values between eight to twenty mm (Fig. 1b)
An error in the cloud base temperature or height by 1 K or ∼100 m is estimated to correspond to an error in the boundary-layer WVP of 0.13 mm, assuming a WVP of 20 mm distributed over 1.5 km depth
Summary
The VOCALS Regional Experiment, held in October– November 2008 in the southeast Pacific (SEP), had one focus on acquiring field measurements capable of illuminating the processes linking aerosols, clouds, and precipitation (Wood et al, 2011b), field measurements that could help assess modeled cloud-aerosol interactions (e.g., Saide et al, 2011, and references therein). These considerations motivated the choice of instruments placed upon the NCAR C130 plane These included in-situ aerosol and microphysical probes, and cloud remote sensors that could routinely provide information on stratocumulus precipitation and its vertical profile, the cloud boundaries, and liquid water (Wood et al, 2011b). The emission from the wings of the absorption line retained sensitivity to LWP, and LWP can be retrieved given an input water vapor path specified independently using other aircraft datasets This is a new and different application of the GVR from its previous applications in the Arctic, and the GVR’s usefulness for cloud process studies within the dry subtropics are undocumented. Background: initial retrieval design, further instrument description, and radiative absorption models
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