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...