Multi‐level measurements of turbulence over the sea during the passage of a frontal zone

Abstract

AbstractA Doppler radar technique originally proposed by Lhermitte (1968) has been used to measure the horizontal components of turbulence simultaneously at closely spaced height intervals from an altitude of 80m to 2500m. The observable range of turbulence scales was between about 200m and 5000m. Observations were made over the sea, with the radar situated on the Isles of Scilly, to minimize the effects of in‐homogeneities in terrain.The measurements reported in this paper were made during the passage of a warm frontal zone with strong surface winds; they show the change with height in the structure of turbulence simultaneously in both the planetary boundary layer and a free shear zone situated some way above it. Below 600m, in the planetary boundary layer, the two horizontal components of turbulence were rather similar in intensity, the total variance of horizontal velocity in the observed spectral range decreasing from 1·7m2s−2 at 80m to less than 0·1m2s−2 at 700m. For much of the observed spectral range the turbulence spectra were of the form kS(k) α k−n, with n decreasing from 1·2 below 300m to 0·7 at 700m. The lapse rate was slightly less than dry adiabatic in the planetary boundary layer with broken cloud above 300m, consistent with slight stability below 300m and some instability above. The decrease in the value of n is thought to have been due to the decrease of stability with height. The spectral scale λm increased with height reaching a value of 1·6km at 900m. Above the boundary layer for most of the time there was a placid layer several hundred metres deep and this was surmounted by a free shear layer in which the variance of velocity increased again to typically 0·5m2s−2. In the free shear layer the horizontal components of turbulence resolved parallel and perpendicular to the local shear vector were rather similar in intensity at scales less than 500m but were markedly unequal at scales between 500m and 2km. This is consistent with the occurrence of Kelvin‐Helmholtz billows with a wavelength of 1 to 2km.The Doppler radar measurements also enabled the vertical flux of horizontal momentum to be calculated. The quantity −u′w′ was found to decrease from about 0·2m2s−2 in the planetary boundary layer to about 0·01m2s−2 in the placid layer, before increasing again to about 0·1m2s−2 in the free shear layer. The energy dissipation rate ϵ was estimated from the turbulence spectra in those cases where the spectral slope n was not greatly different from 2/3. Values of ϵ decreased from 10cm2s−3 at 400m to 2cm2s−3 in the placid layer, before increasing to a second maximum of about 20cm2s−3 in the free shear zone. The rate of mechanical production of turbulence was calculated from the Doppler radar measurements of momentum flux and shear and it was found to vary with height in a rather similar manner to ϵ.