Mesoscale variability in marine winds at mid‐latitude

Abstract

Wind data were collected by the National Oceanic and Atmospheric Administration WP‐3D aircraft on low‐level (50 and 90 m) crosswind and along‐mean‐wind tracks of approximately 350 km during the Storm Transfer and Response Experiment in November and December 1980. Observed mesoscale variations in the marine wind fields are characterized by the velocity correlation tensor for three atmospheric regimes: cloud streets, open and closed cellular convection, and prefrontal warm air advection. The dominant scale of mesoscale variation in the offshore wind field normal to the mean wind direction in the case of old continental air flowing over a warmer ocean, producing cloud streets, was 27 km. For this case, the standard deviation in momentum transfer, which was calculated from 2‐km subsets of the flight track by the bulk aerodynamic method assuming a constant drag coefficient, was 13% of the synoptic scale (330km) mean. The dominant scale of mesoscale variation for open cellular convection was 62km, and the dominant scale for closed cellular convection was 90 km. The standard deviation of mesoscale momentum transfer (scales greater than 2 km; constant drag coefficient) for a 345‐km flight track containing both cell types was 26% of the synoptic scale mean. The warm air advection case had no measurable mesoscale variability. For each regime a model of the horizontal velocity correlation tensor, which can be used to estimate a mesoscale variability, is fitted to the observed velocity correlation tensor with velocity component and weather regime dependent coefficients. This general model is consistent with an interpretation of the mesoscale wind field as an ensemble of coherent structures, associated with cloud type, in which the spatial variability of the wind field in each weather regime is associated with physically determined dominant length scales (i.e., cells or rolls), as contrasted with a continuum interpretation of two‐dimensional turbulence. To accurately describe regional winds and fluxes at the sea surface, wind speed and temperature data should be averaged over the dominant mesoscale length scale with either a suitable time average or a spatial average, such as can be obtained by scatterometry, or an estimate of the mesoscale variability should be explicitly stated. It is also suggested that enhanced vertical flux in the oceanic mixed layer occurs at length scales of atmospheric boundary layer structures.

Examination of the Mesoscale Features of the GALE Coastal Front of 24-25 January 1986

“…An extensive area of banded cloud cover can be seen extending roughly southward across offshore waters. The presence of banded features is as expected, since such features are typical of coldair outbreaks (Atlas et al 1986;Kuettner 1971;Le Mone 1973;Overland and Wilson 1984;Grossman and Betts 1990). The band wavelength is larger and cloud lines are somewhat more convective over the Gulf Stream waters south and east of Cape Hatteras.…”

Inertial oscillations in the upper ocean during the Mixed Layer Dynamics Experiment (MILDEX)

“…Disparate forcing over the 55-km separation between the two locations during the variable-wind period preceding the front accounts for the different inertial response. Horizontal correlation scales of 30 km to 60 km have been observed in aircraft measurements in the atmospheric boundary layer [Overland and Wilson, 1984]. The results of this study show that small horizontal length scales for mixed layer inertial currents can be significantly forced by similar scales in the wind.…”

“…The complete description of near-inertial wave dispersion therefore depends on an accurate specification of the wave number spectrum of the wind field. This is a rather poorly known quantity over most of the ocean, but satellite scatterometry [Freilich and Chelton, 1986] and aircraft measurements [Overland and Wilson, 1984] are beginning to provide estimates of the large (>100 km) and mesoscale (<100 km) variability, respectively. For translating wind patterns, the ocean responds predominantly to forcing whose wavelength is the product of the translation speed and the local inertial period [Kundu and Thomson, 1985;Thomson and Huggett, 1981;Price, 1983;D'Asaro, 1987].…”

Tropical Cyclone Boundary Layer Rolls in Synthetic Aperture Radar Imagery

“…Therefore, rolls are believed to be closely related to storm development and intensification (Zhang et al, ). Overland and Wilson () estimated the mesoscale variability in marine winds at midlatitude and also suggested that enhanced vertical flux in the oceanic mixed layer occurs at length scales of atmospheric boundary layer structures. Since dynamically rolls impact the vertical turbulent fluxes that are crucial to hurricane forecast model physics, the MABL roll characteristics and their locations relative to the maximum wind will be invaluable information to help evaluate the hurricane models.…”