No abstract

This study evaluates the impacts of sea surface temperature (SST) specification and grid resolution on numerical simulations of air-sea coupling near oceanic fronts through analyses of surface winds from the European Centre for Medium-Range Weather Forecasts (ECMWF) model. The 9 May 2001 change of the boundary condition from the Reynolds SST analyses to the NOAA Real-Time Global (RTG) SST in the ECMWF model resulted in an abrupt increase in mesoscale variance of the model surface winds over the ocean. In contrast, the 21 November 2000 change of the grid resolution resulted in an abrupt increase in mesoscale variability of surface winds over mountainous regions on land but had no significant effect on winds over the ocean.To further investigate model sensitivity to the SST boundary condition and grid resolution, a series of simulations were made with the Weather Research and Forecasting (WRF) model over a domain encompassing the Agulhas return current (ARC: also called "retroflection") region in the south Indian Ocean. Results from three WRF simulations with SST measured by the Advanced Microwave Scanning Radiometer on the Earth Observing System Aqua satellite (AMSR-E) and the Reynolds and RTG SST analyses indicate the vital importance of the resolution of the SST boundary condition for accurate simulation of the air-sea coupling between SST and surface wind speed. WRF simulations with grid spacings of 40 and 25 km show that the latter increased energy only on scales shorter than 250 km. In contrast, improved resolution of SST significantly increased the mesoscale variability for scales up to 1000 km.Further sensitivity studies with the WRF model conclude that the weak coupling of surface wind speeds from the ECMWF model to SST is likely attributable primarily to the weak response of vertical turbulent mixing to SST-induced stability in the parameterization of boundary layer turbulence, with an overestimation of vertical diffusion by about 60% on average in stable conditions and an underestimation by about 40% in unstable conditions.

No abstract

[1] The response of surface winds to ocean fronts characterized by sharp gradients in both sea surface temperature (SST) and ocean currents was analyzed using scatterometer (NSCAT and QuikSCAT) wind data and Gulf Stream path positions in conjunction with simulations made with the Pennsylvania State University (PSU)-National Center for Atmospheric Research (NCAR) Mesoscale Model (MM5). All match-ups, between each scatterometer pass and the Gulf Stream path, were visually examined and only those for which the wind field was free of atmospheric fronts or large curvature over a reasonably straight segment of the Gulf Stream were selected. Ten match-ups met these criteria for the period studied from 16 September 1996 to 29 June 1997 for NSCAT and from 24 July 1999 to 31 December 2000 for QuikSCAT. Changes in the modeled surface wind field across the front in each of the ten cases agree well with changes in the observed winds. Our findings suggest that the perturbation pressure gradient resulting from the thermal forcing by the front accounts for the decrease in wind speed when moving from warm to cold water and the increase observed in the converse. In the cases examined, the adjustment of the surface wind to the front occurred as a result of the vertical motion induced by horizontal divergence/convergence and advection in the marine atmospheric boundary layer (MABL). The dynamical forcing associated with strong surface currents is also shown to modify scatterometer-derived winds. Finally the numerical simulations suggest that the dynamical and thermal effects are very nearly additive.