Radar Investigations of Convective Patterns in the Clear Atmosphere

Abstract

Two types of convective patterns are observed with radar in the clear atmosphere. One pattern consistsof small thermal-like cells (Type I) which are in the order of 1-3 km in diameter and several hundredmeters in height. The cells may persist for 20-30 min. In plan view, the Type I cell echoes are doughnut-shaped; that is, they typically appear circular or elliptical and have echo-free centers. The structure ofthe cell and its time history are consistent with the view that the relative flow within the cell is upward inits center, and outward and possibly downward around its periphery. These clear-air cells are detectedregularly with ultra-sensitive radars at wavelengths >10 cm. The echoes are caused by scattering fromfluctuations in refractive index which are particularly marked at the cell boundary. The other pattern is made up of clear air Be'nard-like convection cells (Type II) which are 5-10 km indiameter and 1-2 km in height. The centers of these cells are also echo-free. The overall pattern may persistfor up to 4 hr and individual Benard-like cells may have lifetimes of at least 30 min. The Benard-like patternis composed of several small cells organized around the circumference of the larger Type II cells. Thesesmall cells are probably established through the same air flow that generates Type I cells. Thus, it is expected that the Type II cell is characterized by cores of updrafts around its periphery and by downwardflow in its center. Echoes from the Type II Benard-like cells were observed on three different clear dayswith a 3.2-cm radar of moderate sensitivity. These echoes were caused by the scattering from an unusuallylarge number of insects. Benard-like clear air patterns have also been observed with sensitive 10-cm radarsby virtue of the scattering from refractive index fluctuations. The 10-cm radar observations confirm thatthe Type II pattern is composed of Type I convection cells organized around the periphery of the largerBenard-like cells.