Marine Atmospheric Boundary Layer Circulations Forced by Gulf Stream Sea Surface Temperature Gradients

Abstract

The Penn State/NCAR mesoscale model is initialized with calm winds, a barotropic temperature pattern, and a uniform surface pressure in studies of the response of the marine atmospheric boundary layer (MABL) to realistic differential fluxes of heal and moisture at the sea surface in the vicinity of the Gulf Stream. A maritime sounding from the GALE data region during Intensive Observation Period 2 (IOP 2) is used to define the initial vertical structure of the temperature and humidity fields. The sensitivity of the MABL to two sea surface temperature (SST) patterns is tested. One is a relatively smooth analysis that is typical of those used by research and operational models applied on the synoptic scale and mesoscale. The other is based on the experimental 14 km high-resolution analysis of NOAA. In addition, other simulations are used to determine the sensitivity of the MABL response to physical factors such as surface moisture fluxes, latent heating, and the sea-surface roughness. These studies have two purposes: one is to provide a better understanding of the three- dimensional MABL response to a realistic SST pattern; the other is to isolate the mesoscale circulations produced by this differential thermal forcing so that their interaction with other processes, such as cyclogenesis, can be inferred in real-data simulations. The results of simulations using the two SST analyses are quite different. For example, the MABL front that develops near the north wall of the Gulf Stream is much stronger with the high-resolution analysis. Horizontal temperature gradients below 950 mb are 2–3 times larger, horizontal velocities near the surface are in excess of 7 m s−1 instead of ∼2 m s−1, and the vertical velocity patterns showed significantly different spatial characteristics and amplitudes. In both simulations, responses to the surface forcing extended upward to about 800 mb. In the experiment with the high-resolution SST analysis, a moderately strong mesoscale circulation was produced in the MABL within 12 h. Additional factors found to be important contributors to the MABL response are latent heat release in the lower atmosphere and sea-surface fluxes of moisture. The enhancement of the heat and moisture fluxes associated with the higher winds in the vicinity of the MABL front also significantly contributes to the amplitude of the circulation.