Extending the capabilities of the polarimetric WSR-88D: observations of cirrus clouds and moist layers

A technique to enhance detectability of atmospheric scatterers has been implemented with the polarimetric WSR-88D KOUN, which allows measurements of echoes with reflectivity of -25 dBZ at 10 km (-33 dBZ in the single-polarization mode), a sensitivity on par with that of mm-wavelength cloud radars. Such sensitivity allows radar observations of cirrus clouds. KOUN's measurements in clouds are compared with satellite visible and IR images, and the networking WSR-88D KTLX situated 27 km from KOUN. It is demonstrated that KOUN's data frequently show cirrus cloud layers not captured by satellite. Radar observations of clouds can significantly complement satellite data by supplying a more complete description of cloud structures, altitudes of the upper and lower boundaries, wind velocities, and turbulence parameters. The dependence of radar differential reflectivity on the antenna elevation angle provides information on the shape of cloud particles. It is shown that dominant shapes of particles in observed cirrus clouds are plates. The presence of pristine plates in clouds brings differential reflectivity to values of 8 dB.

Sensitivity of the enhanced KOUN radar permits the detection of atmospheric layers of increased humidity. Two types of layers have been observed with KOUN: 1) nearly uniform layers, and 2) layered structures with bubbles of enhanced reflectivity. For the first time, polarimetric parameters of clear air radar returns are presented. These polarimetric data support our conclusion that these layers are anomalously moist, since the differential reflectivities in the layers are close to zero and the copolar correlation coefficients are nearly unity in contrast to biological scatterers in the atmosphere. Radar data are compared with the moisture profile obtained from rawinsonde soundings. Time evolution of the layers obtained over series of volume scans points to the convective nature of the humid bubbles.