Turbulence Structure of Marine Stratocumulus

Abstract: Aircraft measurements are analysed from the “First Lagrangian” of the Atlantic Stratocumulus Transition Experiment (ASTEX) from south east of the Azores Islands. In this experiment, Lagrangian strategy was used and the marine air mass, that advected southward, was followed during 12 to 14 June 1992. During the experiment, the stratocumulus clouds transitioned into thin and broken stratocumulus with cumulus cloud penetrating from below. To characterise the vertical structure in the marine boundary layer the buoyancy fluxes, the variances, the turbulent kinetic energy, the momentum fluxes and humidity fluxes were examined. The buoyancy flux profiles were used to discover the decoupling of the stratocumulus and the sub-cloud layer. Turbulence analysis for all five flights shows that the cloud layer were decoupled from the underlying layer. In the cloud layer the buoyancy production due to longwave radiative cooling at cloud top, was the main source for driving the turbulence. In the sub-cloud layer, the variances indicate wind shear to be the main generator of turbulence for the first two days. Then, as sea surface temperature increases, buoyancy produced turbulence was more pronounced. The u-, v- and w-spectra and cospectra of wθ and uw give insight into the typical eddy sizes, and how the peak wavelengths vary with height. The peak wavelengths in sub-cloud and cloud layer were larger than layer depth and u- and v-spectral peak wavelengths often larger than the peak wavelength from w-spectra. While peak wavelengths in the sub-cloud layer vary with the height above the surface, they are approximately invariant with height in the cloud layer.