Contributions of Swell to Air-Sea Interactions under Weak Wind Conditions

We have analyzed the Pelican aircraft data and compared them with the ASIT tower data. Due to the orientation of the boom on the ASIT, the Pelican instrument performance and flight days, there are six good comparison flights. We found that occurrence of swell was well correlated to spectral peaks of the vertical velocity, w, at the frequency of about 0.2 s-1 and how high the spectra peak can be observed in the marine atmospheric boundary layer. We found that the amplitude of the w spectral peak increases with the wave age except for 15 August when the marine atmosphere was most stable among the comparison days and atmospheric disturbances were strongly suppressed. In addition, the depth of the vertical layer where the w spectral peak was found increases with the wave age as well. We found that the momentum flux from the Pelican aircraft and ASIT tower agree reasonably well. Using the w spectra peak as the indicator of swell, we found that momentum flux from both the Pelican flights and the ASIT data for swell and wind-waves are in two different regimes. For wind-sea cases, the momentum flux decreases with height, therefore, the Pelican momentum flux is generally smaller than the one from the ASIT. For swell cases, the momentum flux does not change with height much, therefore, the Pelican momentum flux agrees with the ASIT one better. By normalizing the momenutm flux with wind speed, we found that the high drag coefficient at low wind is associated with swell cases. The result implies that the most important features that are associated with weak winds are atmospheric stability and swell.