Coupling between Sea Surface Temperature and Low-Level Winds in Mesoscale Numerical Models

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.

Storm track sensitivity to sea surface temperature resolution in a regional atmosphere model

“…In addition, Chelton et al (2006) found that fine-scale wind structure in the atmospheric boundary layer is underestimated even in high resolution models driven by high resolution SSTs, and Song et al (2009) have shown that this arises due to an underestimate of vertical diffusion in the boundary layer. Although these studies used different models to that used here, it is possible that limitations in the boundary layer scheme could also be leading to an underestimate of the sensitivity of the storm track to SST resolution in our experiments.…”

Sea Level Pressure Minimum along the Kuroshio and Its Extension*

“…The intense heat flux over the K-KE is adequately represented in JRA-25 regarding its distribution and amplitude. While we will not pursue the exact cause of JRA-25 differences from ICOADS, we found, by analyzing other reanalysis datasets, that this problem is general, indicating problems of current assimilating AGCMs in representation of physics, especially those for MABL (Song et al 2009). …”

“…More recently, Minobe et al (2008Minobe et al ( , 2010 suggest that the hydrostatic adjustment along the Gulf Stream leads to convective cloud formation and enhanced precipitation. Song et al(2006) show that, based on the RAM experiments, pressure gradient across the Gulf Stream front is main force modifying the surface winds. Xu et al (2010) also detected a hydrostatic effect over the cool water pool formed between the Pacific coast of Japan and the large meander of the Kuroshio.…”

Remotely Sensed Winds and Wind Stresses for Marine Forecasting and Ocean Modeling

“…This ocean-atmosphere interaction is clearly evident in the surface winds in operational weather forecast models [73], but is underestimated by about a factor of two because of deficiencies in the parameterization of atmospheric vertical mixing [102]. Observations [68] and mesoscale models with sufficient sensitivity of vertical mixing to stability reveal that the SST influence extends throughout the troposphere in all of the dynamic fields, e.g., the vertical velocity (Fig.…”