Experimental studies of convection in the atmospheric boundary layer: Coherent structures under clear skies and cumulus clouds

Variations in the temperature fields of the atmospheric surface (to a height of 46 m) and boundary (to a height of 650 m) layers were studied. In the atmospheric surface layer, temperature was measured with seven fast-response highly stable thermometers mounted on a tower, and, in the atmospheric boundary layer, in addition to the aforementioned thermometers, a TsAO profiler (microwave radiometer) was used. The measuring instrumentation also contained a pyranometer, two sonic anemometers, and two additional resistance thermometers placed near the anemometers. Under clear skies, during daylight hours, the basic mode of variations in the temperature profile of the atmospheric surface layer is characterized by its rapid transitions from a calm state corresponding to a low average temperature and an adiabatic gradient to a state with a higher average temperature and a superadiabatic gradient. These transitions are observed during the passage of convective structures. The time necessary for a steady-state average profile and its corresponding convection regime to be established after a rapid change in insolation was close to 1 min. In central Russia, under broken clouds in summer, a variation in the temperature profile was observed in the atmospheric boundary layer. The main feature of this variation was almost synchronous temperature variations throughout the whole atmospheric layer under study (0-650 m), which repeated cloud-induced variations in the flux of incident solar radiation with a delay on the order of 1 min. It turned out that a sufficiently large cloud produces a specific form of circulation with a subsidence under its central part and with rising airflows at its periphery. The transformation of the upper portion of the profile is mainly due to advection, i.e., transport of the described circulation structure together with the cloud.