Kenneth L. Davidson

A Model of Marine Aerosol Generation Via Whitecaps and Wave Disruption

We have, over the past several years, as one element in the development of a time-dependent model of the aerosol population of the marine atmospheric boundary layer, attempted to define, in terms of aerosol droplet radius (r) and 10m-elevation wind speed (U), a model of open-ocean sea-surface aerosol generation. This source function is represented by the expression dF(r, U)/dr, which states the rate of production of marine aerosol droplets, per unit area of the sea surface, per increment of droplet radius. In the initial modeling efforts only the indirect aerosol production mechanisms associated with the bursting of whitecap bubbles (see Figure 1) were considered. The model for sea surface aerosol generation by the indirect mechanisms, first introduced in our Canberra SSAG-1 (Monahan, et al, 1979) and Manchester SSAG-2 (Monahan, 1980) papers, is given by Equation 1, where W is the

Wind vector retrieval using ERS-1 synthetic aperture radar imagery

{\rm{d}}{{\rm{F}}_0}/{\rm{dr = W}}{\tau ^{ - 1}}{\rm{dE/dr}}

An analysis of the surface production of sea-salt aerosols

instantaneous fraction of the sea surface covered by whitecaps, τ is the time constant characterizing the exponential whitecap decay (measured in seconds), and dE/dr is the differential whitecap aerosol productivity, i.e. the number of droplets per increment droplet radius produced during the decay of a unit area of whitecap (expressed in m−2 μm−1 ). The necessary expression for W(U) was obtained from shipboard photographic observations of white- caps (Monahan, 1971; Toba and Chaen, 1973), while values for τ and dE/dr were derived from measurements made using the University College, Galway, whitecap simulation tank.

Estimating the Refractive Index Structure Parameter (Cn2) over the Ocean Using Bulk Methods

Turbulent fluxes have been measured in the atmospheric surface layer from a boom extending upwind from the Dutch offshore research platform Meetpost Noordwijk (MPN) during HEXMAX (Humidity Exchange over the Sea Main Experiment) in October–November, 1986. We started out to study eddy flux of water vapour, but discrepancies among simultaneous measurements made with three different anemometers led us to develop methods to correct eddy correlation measurements of wind stress for flow distortion by nearby objects. We then found excellent agreement among the corrected wind stress data sets from the three anemometers on the MPN boom and with eddy correlation measurements from a mast on a tripod. Inertial-dissipation techniques gave reliable estimates of wind stress from turbulence spectra, both at MPN and at a nearby ship. The data cover a range of wave ages and the results yield new insights into the variation of sea surface wind stress with sea state; two alternative formulas are given for the nondimensional surface roughness as a function of wave age.

Synthetic aperture radar imaging of upper ocean circulation features and wind fronts

An automated algorithm intended for operational use is developed and tested for estimating wind speed and direction using ERS-1 SAR imagery. The wind direction comes from the orientation of low frequency, linear signatures in the SAR imagery that the authors believe are manifestations of roll vortices within the planetary boundary layer. The wind direction thus has inherently a 180/spl deg/ ambiguity since only a single SAR image is used. Wind speed is estimated by using a new algorithm that utilizes both the estimated wind direction and /spl sigma//sub 0/ values to invert radar cross section models. The authors show that: 1) on average the direction of the roll vortices signatures is approximately 11/spl deg/ to the right of the surface wind direction and can be used to estimate the surface wind direction to within /spl plusmn/19/spl deg/ and 2) utilizing these estimated wind directions from the SAR imagery subsequently improves wind speed estimation, generating errors of approximately /spl plusmn/1.2 m/s, for ERS-1 SAR data collected during the Norwegian Continental Shelf Experiment in 1991.

The relationship between the microwave radar cross section and both wind speed and stress: Model function studies using Frontal Air-Sea Interaction Experiment data

The aerodynamic roughness for all major types of sea ice is specified on the basis of surface layer measurements from ship and ice floe platforms. Median neutral surface drag coefficients, Cdn×1000, for grease and nilas ice are 0.7 and 1.6. Small, medium, large, and fused pancake ice have Cdn equal to 0.9, 1.6, 2.4, and 1.9. Young ice Cdn ranges from 2.3 to 3.1. Very smooth first-year or multiyear ice with no pressure ridges has Cdn equal to 1.5. Pack ice has median values of 2.0 and 2.2 for first-year and multiyear ice. Marginal ice zone ice averages 3.1 and 3.4 (first-year, multiyear) if unaffected by wave action. Wave-affected ice has Cdn values of 4.2 and 4.6 (first-year, multiyear). Extremely rough multiyear ice in intense shear or compaction regions has Cdn equal to 8.0. Open ocean areas within 100 km of an ice edge averaged 1.8, while ice-free water downwind of ice, with small or no waves is 1.4. These values can be adjusted if heat fluxes are present. The accuracy of the Cdn averages is estimated to be 20% or better.

OBSERVATIONAL RESULTS ON THE INFLUENCE OF STABILITY AND WIND-WAVE COUPLING ON MOMENTUM TRANSFER AND TURBULENT FLUCTUATIONS OVER OCEAN WAVES

The rate of production of sea water droplets by bubble bursting at the ocean surface is very difficult to measure directly. Laboratory simulations are also difficult. However, the surface flux can be inferred from measuremerits of the rate of change of aerosol densities and changes in the height of the marine boundary layer. Data from CEWCOM-78 have been analyzed to produce the aerosol surface flux volume spectrum from 0.8 to 15 pm radius at a wind speed of 9 m s-’. Using this flux spectrum and equilibrium aerosol spectra from JASIN. flux spectra are calculated for wind speeds from 6 to 18 m s-’.

Measurements of the rate of dissipation of turbulent kinetic energy, ɛ, over the ocean

Abstract Infrared scintillation measurements were obtained along a 7-km path over San Diego Bay concurrently with meteorological measurements obtained from a buoy at the midpoint of the path. Bulk estimates of the refractive index structure parameter were computed from the buoy data and compared with scintillation-derived values. The bulk estimates agreed well with the scintillation measurements in unstable conditions. In stable conditions the bulk estimates became increasingly higher than the scintillation values as the air–sea temperature difference increased. This disagreement may be due to enhanced wave-induced mixing of the lower atmosphere that decreases the vertical temperature and humidity gradients in stable conditions from the assumed Monin–Obukhov similarity (MOS) theory forms, resulting in bulk values that are too high. The bulk estimates decrease rapidly when the absolute air–sea temperature difference approaches small positive values. These predicted decreases in were not observed in either ...