Observations and the Parameterization of Air-Sea Fluxes during DIAMET

DIAMET, the DIAbatic influence on the Mesoscale structures in Extra tropical storms, is part of the UK Natural Environment Research Council (NERC) funded Storm Risk Mitigation (SRM) program. Although the storms are well forecast on the synoptic-scale, the precise timing, location and morphology of the mesoscale and convective-scale structures such as strong winds and intense precipitation within these cyclones remain uncertain. So we need to parameterise key processes, such as the turbulence from the wind over the sea surface leading to an upwards flux of momentum, heat and moisture which adds to the storms, to better understand and forecast the mesoscale structure of severe storms over the UK.

In DIAMET we are using aircraft measurements from only 30-40 m above the sea surface to determine the air-sea fluxes. Low level wind, temperature and humidity are measured at 32 Hz (giving a spatial resolution of ~3 m), and from these the turbulent wind stress, friction velocity, sensible heat flux and latent heat flux are calculated by the Eddy Covariance technique. At present in DIAMET low level measurements have been obtained from 4 flights, divided into a total of 118 straight line runs of 2 minute length (~12 km), though this year there will be further flights to obtain more low level measurements.

These flux values can be used to develop and validate bulk flux parameterisation algorithms. The preliminary results from the measurements are over a range of wind speeds 7 – 24 m/s (at 10 m), and have a range of calculated wind stresses 0 – 3 N/m2, sensible heat fluxes -40 – 280 W/m2, and latent heat fluxes -60 – 570 W/m2. Although detailed quality control is required to find the data with pure turbulence and the sea-surface temperature measurements may need to be corrected. The preliminary results have coefficients for the bulk fluxes that are ~50% greater at large wind speeds (>15 m/s) than those developed in previous studies.