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Finescale Structure and Microphysics of Coastal Stratus

Gabor ValiDepartment of Atmospheric Science, University of Wyoming, Laramie, Wyoming

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Robert D. KellyDepartment of Atmospheric Science, University of Wyoming, Laramie, Wyoming

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Jeffrey FrenchDepartment of Atmospheric Science, University of Wyoming, Laramie, Wyoming

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Samuel HaimovDepartment of Atmospheric Science, University of Wyoming, Laramie, Wyoming

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David LeonDepartment of Atmospheric Science, University of Wyoming, Laramie, Wyoming

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Robert E. McIntoshMicrowave Remote Sensing Laboratory, University of Massachusetts, Amherst, Amherst, Massachusetts

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Andrew PazmanyMicrowave Remote Sensing Laboratory, University of Massachusetts, Amherst, Amherst, Massachusetts

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Print Publication:
01 Dec 1998
DOI:
https://doi.org/10.1175/1520-0469(1998)055<3540:FSAMOC>2.0.CO;2
Page(s):
3540–3564
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Abstract

Observations were made of unbroken marine stratus off the coast of Oregon using the combined capabilities of in situ probes and a 95-GHz radar mounted on an aircraft. Reflectivity and Doppler velocity measurements were obtained in vertical and horizontal planes that extend from the flight lines. Data from three consecutive days were used to examine echo structure and microphysics characteristics. The clouds appeared horizontally homogeneous and light drizzle reached the surface in all three cases.

Radar reflectivity is dominated by drizzle drops over the lower two-thirds to four-fifths of the clouds and by cloud droplets above that. Cells with above-average drizzle concentrations exist in all cases and exhibit a large range of sizes. The cells have irregular horizontal cross sections but occur with a dominant spacing that is roughly 1.2–1.5 times the depth of the cloud layer. Doppler velocities in the vertical are downward in all but a very small fraction of the cloud volumes. The cross correlation between reflectivity and vertical Doppler velocity changes sign at or below the midpoint of the cloud, indicating that in the upper parts of the clouds above-average reflectivities are associated with smaller downward velocities. This correlation and related observations are interpreted as the combined results of upward transport of drizzle drops and of downward motion of regions diluted by entrainment. The in situ measurements support these conclusions.

Corresponding author address: Gabor Vali, Department of Atmospheric Science, University of Wyoming, Box 3038, Laramie, WY 82071.

Email: vali@uwyo.edu

Abstract

Observations were made of unbroken marine stratus off the coast of Oregon using the combined capabilities of in situ probes and a 95-GHz radar mounted on an aircraft. Reflectivity and Doppler velocity measurements were obtained in vertical and horizontal planes that extend from the flight lines. Data from three consecutive days were used to examine echo structure and microphysics characteristics. The clouds appeared horizontally homogeneous and light drizzle reached the surface in all three cases.

Radar reflectivity is dominated by drizzle drops over the lower two-thirds to four-fifths of the clouds and by cloud droplets above that. Cells with above-average drizzle concentrations exist in all cases and exhibit a large range of sizes. The cells have irregular horizontal cross sections but occur with a dominant spacing that is roughly 1.2–1.5 times the depth of the cloud layer. Doppler velocities in the vertical are downward in all but a very small fraction of the cloud volumes. The cross correlation between reflectivity and vertical Doppler velocity changes sign at or below the midpoint of the cloud, indicating that in the upper parts of the clouds above-average reflectivities are associated with smaller downward velocities. This correlation and related observations are interpreted as the combined results of upward transport of drizzle drops and of downward motion of regions diluted by entrainment. The in situ measurements support these conclusions.

Corresponding author address: Gabor Vali, Department of Atmospheric Science, University of Wyoming, Box 3038, Laramie, WY 82071.

Email: vali@uwyo.edu

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