CONDITIONAL SAMPLING AND SCALE ANALYSIS OF THE MARINE ATMOSPHERIC MIXED LAYER -- SOFIA EXPERIMENT

Abstract

During the SOFIA experiment, performed in the Azores region in June 1992, airborne missions were conducted in the atmospheric boundary layer with two aircraft instrumented for turbulence measurements. We show how the conditional sampling technique, applied to the velocity, temperature and moisture fluctuations, is able to describe the various parcels which constitute the turbulent field. Each parcel, so identified, is characterized by its fractional area and by its contribution to the transfers of sensible heat and latent heat. On the other hand, a scale analysis is conducted by filtering the turbulent signals in five non-overlapping frequency bands, defined according to the characteristic turbulent scales. The contribution of each band to the turbulent energy and to the transfers is thus presented. The importance of the lowest frequencies, which are generally removed from the signals by high-pass filtering before computing turbulent fluxes, is shown. In the final section, the conditional sampling technique is applied to the signals filtered in the various bands. Despite a slight deformation of the eddies due to the filtering technique, the contribution of each parcel can be estimated at the various scales analysed.