Coherent structures in the atmospheric surface layer under stable and unstable conditions

Simultaneous measurements of the instantaneous values of absolute temperature at seven heights within the lower 36 m of the atmospheric boundary layer under different stability conditions were carried out, accompanied by measurements of the wind velocity components at two levels and of solar radiation flux at the surface. The data obtained allow one to investigate individual convective cells known as coherent structures (CS). Outside the CS, i.e., during quiet periods, an instant temperature profile is in close agreement with the dry-adiabatic lapse rate, but within CS the temperature changes much faster with height, and the shape of the profile varies significantly. A method was developed to transform temperature records from sensors at several heights into an other form, namely, into temporal variations of the heights of isothermal surfaces. Since coherent structures were found to advect with the mean wind velocity, these temporal height variations may be transformed into the spatial ones, i.e., into the xoz-plane section of the temperature field. In such a pictorial presentation coherent structures look like asymmetric columns of heat, penetrating the whole atmospheric surface layer. Coherent structures also exist in the stable stratified surface layer, but they have an inverse asymmetry and occupy only the lower several metres. Wavelike activity dominates in the upper part of the stable surface layer. The characteristic time of surface-layer adjustment to the rapid changes of solar radiation (due to cloud shadows or cloud gaps) was found to be on the order of one minute. Such a time interval is required for coherent structure to reach the top of surface layer.