Maritime differences between wind direction and stress: Relationships to atmospheric fronts and implications

Ship-based data from the Fronts and Atlantic Storm Tracks Experiment (FASTEX) in the wintertime North Atlantic Ocean and aircraft data from the Pacific Landfalling Jets Experiment (PACJET) over the northeast Pacific Ocean show that systematic differences between the wind direction and stress direction occur in the vicinity of atmospheric fronts. Furthermore, these differences change sign from the warm sector to the post-frontal regions.

According to theory (Grachev et al 2003), if the stress is due entirely to wind waves, then the stress direction should be the same as the wind direction. If other factors, such as the swell, are influencing the stress, then the stress direction should be between the swell and wind directions. Composites of time-series through 10 frontal passages during FASTEX show that the stress direction is often greater than the wind direction by 5-12° (that is, the stress direction is to the right of the wind direction) in the central and western portion of the warm sector east of the surface cold front, while in the post-frontal regime the opposite appears to be true. Manual observations of the swell direction show that the warm sector stress direction is frequently between the swell direction and the wind direction, in qualitative agreement with theory. During PACJET, low-level flights with the NOAA P-3 aircraft on both sides of surface cold fronts showed similar differences. Stress directions 150-200 m above the surface were oriented 6-26 degrees to the right of the wind direction east of the cold fronts while they were up to 26 degrees to the left of the wind direction west of the cold fronts. These results suggest that in the vicinity of fronts, the stress vector may not be an accurate indicator of the wind direction, either in the warm sector, post-frontal regime, or both.

One possible explanation is that stress effects from swells are present and that swell orientations change much less or more slowly across a cold front than does wind direction. Physically, this may result either because the swells move faster than the cold front so post-frontal swell orientations are found ahead of the cold front, or because a cold front moves fast enough to not influence the swells over a long enough time period to change their orientation to that of the winds. Strong along-wind thermal gradients present within the warm sector and in the post-frontal regime also make it plausible that thermal wind effects reorient the turbulent eddies, as suggested by Geernaert (1996). However, angular differences between the stress and wind direction are 3-4 times smaller than that predicted by his theory.

The presence of these systematic stress-wind direction differences implies that satellite-based scatterometer wind directions, which rely on the surface stress field, will be in error and will underestimate the surface directional wind shift across the front and thus the derivative fields, such as convergence and vorticity. However, if such directional biases are seen in the scatterometer comparisons with other wind measurements, their magnitude is often such that they may be ascribed to "uncertainties" in the observations. This talk will present the observations showing these stress-wind direction differences, the available evidence for the cause of the differences, and the consequence of these differences for scatterometer-derived surface winds and fields derived from these winds.