Cloudiness and Marine Boundary Layer Dynamics in the ASTEX Lagrangian Experiments. Part II: Cloudiness, Drizzle, Surface Fluxes, and Entrainment

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  • 1 Atmospheric Science Department, University of Washington, Seattle, Washington
  • | 2 Atmospheric Sciences Programme, University of British Columbia, Vancouver, British Columbia, Canada
  • | 3 Monash University, Clayton, Victorta, Australia
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Abstract

The Analysis of the Atlantic Stratocumulus Transition Experiment (ASTEX) Lagrangians started in Part I is continued, presenting measurements of sea surface temperature, surface latent and sensible heat fluxes from bulk aerodynamic formulas, cloud fraction, and drizzle rate for the two Lagrangians, mainly using data from horizontal legs flown by the Electra and C130. Substantial drizzle, averaging 1 mm day−1 at the surface, was measured during the first Lagrangian. The surface fluxes increased rapidly as the air mass advected over rapidly increasing SST. Cloud fraction remained high throughout. During the second Lagrangian, drizzle formed in the stratocumulus layer but mainly evaporated in the deep, dry cumulus layer and the subcloud layer before reaching the surface. Stratocumulus cloud cover was thickest when moist air lay above the inversion and then it dissipated to leave only cumuli once dry air advected over the inversion.

Three methods are compared for determining entrainment rate (European Centre for Medium-Range Weather Forecasts analyses of mean vertical motion, calculation of a water budget, and the ozone flux–jump method). While all three methods have significant uncertainties, their predictions are all consistent with an entrainment rate of 0.7 ± 0.3 cm s−1 for the first Lagrangian and 0.6 ± 0.3 cm s−1 for the second Lagrangian. Corresponding estimates of the time-dependent horizontal divergence are also presented.

Estimates of the cumulus mass flux, internal mixing time, and entrainment dilution time for the boundary layers observed during the two Lagrangians are also presented.

Abstract

The Analysis of the Atlantic Stratocumulus Transition Experiment (ASTEX) Lagrangians started in Part I is continued, presenting measurements of sea surface temperature, surface latent and sensible heat fluxes from bulk aerodynamic formulas, cloud fraction, and drizzle rate for the two Lagrangians, mainly using data from horizontal legs flown by the Electra and C130. Substantial drizzle, averaging 1 mm day−1 at the surface, was measured during the first Lagrangian. The surface fluxes increased rapidly as the air mass advected over rapidly increasing SST. Cloud fraction remained high throughout. During the second Lagrangian, drizzle formed in the stratocumulus layer but mainly evaporated in the deep, dry cumulus layer and the subcloud layer before reaching the surface. Stratocumulus cloud cover was thickest when moist air lay above the inversion and then it dissipated to leave only cumuli once dry air advected over the inversion.

Three methods are compared for determining entrainment rate (European Centre for Medium-Range Weather Forecasts analyses of mean vertical motion, calculation of a water budget, and the ozone flux–jump method). While all three methods have significant uncertainties, their predictions are all consistent with an entrainment rate of 0.7 ± 0.3 cm s−1 for the first Lagrangian and 0.6 ± 0.3 cm s−1 for the second Lagrangian. Corresponding estimates of the time-dependent horizontal divergence are also presented.

Estimates of the cumulus mass flux, internal mixing time, and entrainment dilution time for the boundary layers observed during the two Lagrangians are also presented.

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