Marine Boundary Layer and Turbulent Fluxes over Aegean Sea during an Etesian outbreak

We investigate the boundary layer structure over the Aegean Sea during an Etesian outbreak, based upon airborne measurements (Aircraft_BAe146–FAAM), during the Aegean-GAME field campaign (29 August to 8 September 2011). The sorties involved horizontal tracks mainly at 150m and above the mixing layer depth, and vertical profiles up to 4.5 km over Crete, Lemnos and the Central Aegean Sea. The marine boundary layer (MABL) over the eastern Aegean Sea was suppressed at very low levels due to the very cold SST current in the east coming from the Black Sea. Below 1 km altitude, an intense low-level jet was observed with the maximum wind speeds (up to 20 m s-1) having a northern direction, below 500 m asl. The MABL over the western Aegean Sea was deeper than in the eastern part. Enhanced northeastern wind speeds extended from the surface and up to 2 km with their peak values (ca. 20 m s-1) observed at 500 m asl. Atmospheric turbulence measurements (32Hz) 150 m a.s.l of wind components, temperature and humidity measurements are used to calculate turbulent quantities, using the eddy-correlation method with the horizontal averaging length set to 3km (Kalogiros and Wang, 2011). Mean statistics as well as fluxes of momentum, sensible and latent heat are investigated against several factors (e.g. geographical region, time of the day, wind speed, SST). The values of the stability parameter z/L (at 150 m a.s.l.) ranged from -5 to 5. Both friction velocity and sensible heat flux were well correlated with wind speed and dT (temperature difference between sea and air). The friction velocity had a strong variation throughout the whole region of the Aegean Sea, but its larger values (0.4-0.5 m s-1) were not associated with the higher wind speeds. The larger values of friction velocities are calculated for wind speeds in the range of 12 and 17 m s-1 and obtained at the southeast-ern Aegean under local unstable conditions. Most of the other parts of the Aegean Sea exhibited neutral to stable atmospheric conditions with reduced friction velocities and absolute fluxes (momentum and heat). The friction velocities over the northern Aegean were about 0.1 m s-1, while friction velocities of the order of 0.2 m s-1 have also been calculated in the maritime area, upwind of Crete. The largest positive values of sensible heat flux (up to 25 W m-2) were measured under neutral to unstable conditions over the southeastern Aegean Sea (with the largest SST) and wind speeds 15 ± 2 m s-1. The larger negative values (up to -50 W m-2) are mainly calculated over the central and southern Aegean Sea, covering the whole range of wind intensity. The sensible heat fluxes over the northern Aegean Sea were negligible. Preliminary spectra analysis from near sea-surface horizontal legs, normalized by similarity scaling, gives satisfactory similarity predictions, at the southeastern Aegean Sea, where the largest positive values of sensible heat flux were measured. An effort is made to combine the observed turbulence structure of the complex Aegean Marine Boundary Layer with high-resolution simulations. Large eddy simulations are performed with NCAR MMM LES code (Moeng 1984, Sullivan et al. 1994), providing fluxes and turbulent features affected by the sea velocity roughness length z0u and sea surface temperature (SST) heterogeneity, for representative atmospheric conditions over the northern and southern Aegean Sea. For the sea velocity roughness length z0u, different parameterizations are tested (e.g the classic Charnock formula (Charnock, 1955; Fairall et al., 1996), the empirical formula of Drennan et al., (2003) that is based on empirical wind sea wave data).