Radiative cooling at the top of a stratus cloud deck can lead to a dynamic formation of cell structures within the entire stratus layer. To study this interaction between radiation, cloud microphysics and dynamic processes, the experiment REFLEX III (Radiation and Eddy Flux Experiment) was conducted in summer 1995 in the European Arctic north of Svalbard. Airborne measured were taken of the three wind components, temperature and humidity solar and terrestrial irradiance and particle spectra on flights through the stratus cloud cover over sea ice.
In this paper results from measurement on 27 July 1995 are presented, when a stratus cloud was at first shielded by a higher altocumulus cloud. The lower cloud then drifted into a region without higher clouds leading to the onset of radiation cooling at the top. In the stratus layer complex cell structures formed that can be visualized by a tomography method.
Shortly after the onset of the radiative cooling small particles formed at the cloud top. About 30 minutes later the number of large droplets was significantly increased in the entire cloud, and the small drops at the top had dissapeared. This observation is interpreted as the result of a convective mixing process within the cloud layer.
Data from horizontal flight sections through the cloud layer at various height levels show regular and correlated variations of the liquid water content, temperature, radiation and wind components. With a conditional sampling technique an averaged cell structure is computed. Since the cloud has been traversed in different directions, a 2-dimensional picture of an averaged cell can be constructed.
In the cell centre maxima show in the vertical wind speed and the long wave irradiance. The short wave irradiance shows a minimum in the far end with respect to the direction of the sun.