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Three high resolution large eddy simulations (LES) with two bulk air–sea flux algorithms, including the effects of water phase transition, are performed in order to study the influence of sea spray on the marine atmospheric boundary layer (MABL) structure and cloud properties. Because sea spray has a notable impact under severe wind conditions, the CBLAST-Hurricane experiment supplies the initial realistic conditions as well as turbulence measurements for their assessment. However a hurricane boundary layer (HBL) simulation is not in the scope of this study. Although the simulations in the final state depart from the initial conditions, all three momentum flux distributions are found at the low end of the observed range. The spray-mediated sensible heat flux is opposite to the interfacial flux and reaches up to 60% of its magnitude. When the spray-mediated contribution is taken into consideration, the simulated moisture flux increases by up to 45% and gets closer to the observations. Small scale stream-wise velocity streaks are arranged, probably due to spray effects, into large scale structures where the scalars' variations tend to concentrate. However, the vertical velocity structure below mid-MABL is not greatly affected as the buoyancy forces locally within these structures are negligible. Spray effects greatly enhance the magnitude of the quadrant components of the scalar fluxes, but the net effect is less pronounced. Spray-mediated contribution results in more extended cloud decks in the form of marine stratocumulus with increased liquid water content. The visually thicker clouds reduce the total surface radiation by up to 30 Wm - 2 . [ABSTRACT FROM AUTHOR]

A total of 15 fog events from two field campaigns are investigated: the High Energy Laser in Fog (HELFOG) project (central California) and the Toward Improving Coastal Fog Prediction (C-FOG) project (Ferryland Newfoundland). Nearly identical sensors were used in both projects to sample fog droplet-size spectra, wind, turbulence, and thermodynamic properties near the surface. Concurrent measurements of visibility were made by the present weather detector in both experiments, with the addition of a two-ended transmissometer in the HELFOG campaign. The analyses focused first on contrasting the observed fog microphysics and the associated thermodynamics from fog events in the two locations. The optical attenuation by fog was investigated using three methods: (1) derived from Mie theory using the measured droplet-size distribution, (2) parametrized as a function of fog liquid water content, and (3) parametrized in terms of total fog droplet number concentration. The consistency of these methods was investigated. The HELFOG data result in an empirical relationship between the meteorological range and liquid water content. Validation of such relationship is problematic using the C-FOG data due to the presence of rain and other factors. The parametrization with droplet number concentration only does not provide a robust visibility calculation since it cannot represent the effects of droplet size on visibility. Finally, a preliminary analysis of the mixed fog/rain case is presented to illustrate the nature of the problem to promote future research. [ABSTRACT FROM AUTHOR]

The blue dots are from the PWD sensors, red from WATT (HELFOG only) and all green dots are results from the Mie calculation. 10 Variation of visibility (MOR) with LWC (ql) from (a) HELFOG, and (b) C-FOG fog events. [Extracted from the article]

ircraft turbulence data from the Autonomous Ocean Sampling Network project were analyzed and compared to the Coupled Ocean-Atmosphere Response Experiment (COARE) bulk parametrization of turbulent fluxes in an ocean area near the coast of California characterized by complex atmospheric flow. Turbulent fluxes measured at about 35 m above the sea surface using the eddy-correlation method were lower than bulk estimates under unstable and stable atmospheric stratification for all but light winds. Neutral turbulent transfer coefficients were used in this comparison because they remove the effects of mean atmospheric conditions and atmospheric stability. Spectral analysis suggested that kilometre-scale longitudinal rolls affect significantly turbulence measurements even near the sea surface, depending on sampling direction. Cross-wind sampling tended to capture all the available turbulent energy. Vertical soundings showed low boundary-layer depths and high flux divergence near the sea surface in the case of sensible heat flux but minimal flux divergence for the momentum flux. Cross-wind sampling and flux divergence were found to explain most of the observed discrepancies between the measured and bulk flux estimates. At low wind speeds the drag coefficient determined with eddy correlation and an inertial dissipation method after corrections were applied still showed high values compared to bulk estimates. This discrepancy correlated with the dominance of sea swell, which was a usually observed condition under low wind speeds. Under stable atmospheric conditions measured sensible heat fluxes, which usually have low values over the ocean, were possibly affected by measurement errors and deviated significantly from bulk estimates. [ABSTRACT FROM AUTHOR]