Evaluation of sea surface temperatures and wind fields in WRF coupled simuations

The coupled responses of surface wind dynamics and ocean surface heating are analyzed by comparing simulations of the Weather and Research Forecasting (WRF) model coupled with and without a sea surface temperature diurnal heating model. For the diurnally varying simulation, a prognostic scheme of the sea surface temperature diurnal variability produces a sea surface temperature field that depends on the incoming solar radiation and surface wind speed and responds back to the atmospheric model via surface fluxes. The SST initial boundary conditions are from the Multiscale Ultra-high Resolution (MUR) SST and the atmospheric boundary and initial conditions are from the GFS Final Analysis. The MUR SSTs represent an SST without diurnal variability because it only contains nighttime satellite data. The difference between two seven-day simulations’ for 7-14 July, 2014 is explored.

The prognostic scheme produces diurnal warming that exceeds 3°C on certain occasions. Furthermore, in these particular large warming events, the cycle of heating does not necessarily reset overnight from convective mixing. The comparisons of the modeled SSTs to SEVIRI and GOES SST, however, show that the model is overproducing the magnitude of the diurnal heating. Though the model overestimates the amplitude of heating, the magnitudes themselves are fairly consistent with observations and other modeling studies. The surface wind speeds in areas of diurnal warming over the course of the simulation can differ more than 3 m/s. On average, these same areas also experience shifts in the surface divergence and wind stress curl. Surface wind trajectories computed for each grid point show that, as small winds travel over the largest diurnal warming (or diurnal warming gradient), the wind stress curl becomes smaller. A comparison to ASCAT-A wind data shows that modeled winds are in better agreement with satellite data when diurnal SST variability is included, especially in larger wind magnitudes. This is likely the result of the nonlinear changes to the atmosphere that produce shifts in storm track of regional shortwave systems surrounding areas of diurnal warming. These findings suggest significant changes to the atmospheric boundary layer on not only local, but regional scales due to the diurnal variability of the upper ocean.