Air–sea interaction processes in warm and cold sectors of extratropical cyclonic storms observed during FASTEX

Abstract

Mid-Atlantic measurements from the research vessel (RV) Knorr during the Fronts and Atlantic Storm-Tracks Experiment (FASTEX) are presented, and then used to examine the near-surface environment and air–sea interaction processes during the passage of ten frontal systems. This dataset includes measurements of the surface momentum, sensible-heat and moisture fluxes obtained from three different methods. The inertial dissipation (ID) drag coefficients from the RV Knorr are consistent with the ID data from other measurements in open-ocean storm environments. The covariance drag coefficients are generally larger than the ID values, indicating either the presence of flow distortion problems in the covariance data or a failure of the assumptions inherent to the ID technique at these higher wind speeds. Estimates of the wind speed dependence of the momentum, sensible-heat and latent-heat transfer coefficients are based on averaged values from the two methods. These measurements: (i) contribute significantly to the limited set of surface flux measurements for 10 m neutral winds in the 15–21 m s−1 range; (ii) contain the only ship-based covariance flux measurements successfully obtained in an open-ocean, high wind speed, storm environment; and (iii) include coincident wave-height measurements. The relationships between the surface layer and the synoptic atmospheric environment is examined using composites of atmospheric and oceanic surface-layer characteristics computed in ten storms for which the RV Knorr passed through the open-wave warm sector and the cold front. These composites show minima in the sensible- and latent-heat fluxes, and a maximum in the momentum flux, just before the frontal passage during the warm-sector peak in wind speed. A second momentum flux maximum of comparable magnitude occurs in the middle of the post-frontal regime. Though the warm-sector sensible-heat flux minimum is slightly negative, the sum of the two heat fluxes is positive, suggesting a positive impact on the synoptic development of these storms. Wave heights increase steadily from the eastern half of the warm sector to the frontal passage, remaining high through most of the post-frontal regime before decreasing. Differences between covariance and ID stresses are largest during the times bracketing the cold front when the wave heights and stresses are large. Differences between covariance and bulk stresses are greatest in the pre-frontal low-level jet, when the frequency of waves with periods of 6–9 s maximizes, and in the post-frontal regime where the wind direction veers. Systematic differences between stress direction and wind direction are observed.