12th Conference on Interactions of the Sea and Atmosphere


Air-Sea Interaction Processes in Warm and Cold Sectors of Extratropical Cyclonic Storms Observed During FASTEX

Ola P. G. Persson, CIRES/Univ. of Colorado and NOAA/ETL, Boulder, CO; and J. E. Hare, C. W. Fairall, and W. D. Otto

During the Fronts and Atlantic Storm Tracks Experiment (FASTEX), extensive measurements of the ocean surface, air-sea interaction processes, and boundary layer structure were obtained from four ships strategically placed in the central North Atlantic Ocean during January and February 1997. The data includes measurements of basic near-surface meteorological parameters, including sea-surface temperature; surface flux measurements, including measurements using the inertial dissipation and covariance techniques from the R/V Knorr; wave height and period spectra from a TSK recorder; manual observations of wind wave and swell directions; and high-resolution serial rawinsonde ascents. During January and February, between 10-20 storm systems passed each ship, with surface winds of 15-30 ms-1. This data set allows us to examine how air-sea interaction processes are modulated by the storms and how these processes in turn impact the structures important for the development of these storm systems, especially the dynamically important warm sector region. In addition, the data also suggest how well satellite-based measurements, which rely on some of these air-sea interaction processes, can determine basic near-surface atmospheric parameters in specific regions of the storms.

In order to place the observations in a storm-relative framework, the start of the warm sector, the surface cold-frontal passage, and the end of the post-frontal sector were defined from basic meteorological parameters. The storms' movements produce a northeast-to-southwest section through each storm. With these definitions, statistical composites of storm-relative atmospheric parameters, surface fluxes, and wave characteristics were computed for each ship. The results include: a) the momentum flux maximizes just before the frontal passage during the peak in wind speed associated with the warm-sector low-level jet. A second stress maximum of comparable magnitude occurs in the middle of the post-frontal regime for the data from the R/V Knorr but not from the Suroit. b) The latent and sensible heat fluxes are a minimum just before the frontal passage. Despite the strong surface winds at this time, the moistening and warming associated with synoptic-scale advective patterns and surface fluxes minimize the vertical gradients in specific humidity and temperature. This pattern should affect the surface potential vorticity generation, which has dynamical implications for frontal stability. c) Wave heights increase steadily from near the eastern edge of the warm sector to just before frontal passage, remaining high through most of the post-frontal regime before decreasing. d) Differences between covariance and inertial dissipation fluxes are largest during the times bracketing the cold front when the wave heights are large (the covariance fluxes are larger), and e) the stress direction is consistently 5-10 to the right of the wind direction in the warm sector and 2-15 degrees to the left of the wind direction in the post-frontal regime. This last result implies that satellite-based scatterometer wind directions, which rely on the surface stress field, will underestimate the surface directional wind shift across the front. One case study with simultaneous observations from NOAA P-3 aircraft and a QuikScat satellite overpass during the PACJET 2001 field experiment illustrates this bias in the satellite-determined wind directions.

extended abstract  Extended Abstract (384K)

Poster Session 1, Extratropical Atmosphere-Ocean Interaction
Monday, 10 February 2003, 2:30 PM-4:00 PM

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