Clouds cover vast areas of Earths surface and significantly impact the hydrology and ecology cycles. Marine stratocumulus clouds persist year-round off the coast of California, at times during the summer months covering more than 1000 square miles. These clouds lie about 2000 feet above the ocean surface and are nominally only few hundred feet thick. With a droplet size of about 10 micrometers (less than a millionth of an inch), these clouds are extremely reflective of solar radiation, and thus they cool Earths surface. They must be represented accurately in global climate models aimed at predicting future climate and energy needs.
Key processes that can change the coverage of stratocumulus clouds are (1) mixing of the dry, warm layer above the cloud into the cloud layer, called entrainment, and (2) conversion of cloud droplets into drizzle-sized drops, which then fall as precipitation.
The field campaign Stratocumulus Entrainment and Precipitation Studies (SEPS), conducted from mid July to mid August of 2014, was a study of the controls on entrainment and precipitation processes in stratocumulus clouds. SEPS was led jointly by Bruce A. Albrecht from the University of Miami, Haflidi H. Jonsson from the Naval Postgraduate School, and Virendra P. Ghate from EVS.
An instrumented Twin Otter research aircraft was operated for SEPS by the Center for Interdisciplinary Remotely Piloted Aircraft Studies (CIRPAS). Flight operations were based at the Marina municipal airport in Marina, California, about 10 miles north of Monterey. The campaign and the research were supported by the Marine Meteorology Program of the Office of Naval Research (ONR).
The Twin Otter was equipped with multiple instruments designed for measuring aerosols, cloud and precipitation drop sizes, and thermodynamic variables like temperature, pressure, and humidity. Also aboard the Twin Otter was a vertically pointing, frequency-modulated, continuous-wave (FMCW) Doppler cloud radar. The FMCW radar operated at a frequency of 95 GHz and collected key sets of cloud observations (specifically, droplet backscatter and velocity) at very high temporal (0.2 sec) and spatial (5 m) resolution.
Also participating in SEPS was a mobile phased-array weather radar (MWR), operating from a truck at a frequency of 10 GHz. The MWR was originally designed for identifying aircraft from the ground, for military purposes; key modifications made it suitable for atmospheric research. The MWR had scanning abilities and collected observations of precipitation at 150-m resolution, with an observable range of about 5 miles from the truck.
To understand the controls on precipitation in stratocumulus clouds, hygroscopic aerosols (salt) were artificially introduced into non-precipitating or lightly precipitating stratocumulus clouds. The salt aerosols set up conditions for assessing the impact of giant (3-micrometer-diameter) aerosols on the processes by which cloud drops grow to drizzle size. Preliminary data analysis suggests that adding giant aerosols successfully produced drizzle-sized (precipitation) drops in a non-precipitating cloud, but further analysis is required for validation.
To gain insights into the entrainment process, radar chaff (pre-cut, metallic-coated fiber needles) was obtained, sized at half the wavelengths of the two radars. For the FMCW radar the chaff was 1.5 mm long, while for the MWR the chaff was 1.5 cm long. Because of their nature and sizes, the chaff needles were visible only to their respective radars, generating echoes much larger than those of the clouds. The chaff sizes and composition were selected to give the needles negligible fall velocity, so that the researchers could track air motion by monitoring the chaff.
The limited range of the MWR confined the field of study for entrainment processes to 5 miles off the coast. Chaff needles of both sizes were introduced above the cloud by using rare military technology, and the motions of the needles were tracked by both radars as the chaff descended from above the cloud top into the cloud layer. Analyses are continuing to extract additional information from the observations.
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