Aircraft-based observations of air-sea turbulent fluxes around the British Isles

Observations of turbulent fluxes of momentum, heat and moisture from low-level aircraft data are presented. The fluxes are calculated using the eddy covariance technique from flight legs typically ~40 m above the sea surface. Over 400 runs of 2 minutes (~12 km) from twenty-six flights are evaluated. The flight legs are mainly from around the British Isles although a small number are from the seas around Iceland and Norway. Sea surface temperature observations from two on-board sensors (the ARIES inteferometer and a Heimann radiometer) and a satellite-based analysis (OSTIA) are used to determine an improved SST estimate. Most of the observations are from moderate to strong wind speed conditions: the latter being a regime short of validation data for the bulk flux algorithms that are necessary for numerical weather prediction and climate models. Observations from both statically stable and unstable atmospheric boundary-layer conditions are presented. There is a particular focus on several flights made as part of the DIAMET (Diabatic influence on mesoscale structures in extratropical storms) project. Observed neutral exchange coefficients are broadly consistent with previous studies, although higher for the momentum coefficient. Commonly used bulk flux algorithms are generally in line with the observations. Examining the results as a function of aircraft heading shows higher fluxes and exchange coefficients in the across-wind direction, compared to the along-wind direction (although this comparison is limited by the relatively small number of along-wind legs). A multi-resolution spectral decomposition technique demonstrates a lengthening of spatial scales in along-wind variances in along-wind legs, implying the boundary-layer eddies are elongated in the along-wind direction. It appears that the along-wind runs may not be able to adequately capture the full range of turbulent exchange that is occurring.