Interactions Between Clear-Air Reflective Layers and Rain Observed with A Boundary-Layer Wind Profiler

We present observations with a 915-MHz boundary layer profiler of reflective layers in the clear air that persist as light rain falls through them. In the three cases presented a clear-air echo is initially present over several hours and confined to a horizontal layer a few hundred meters thick. Subsequently, light rain falls through the layer but does not eliminate the spatial pattern of refractive index irregularities which presumably accounts for the clear-air echo. When the reflectivity of the rain is comparable with that of the clear air, Doppler spectra in the vertical profiler beam have separate modes arising from Rayleigh scattering by the rain and Bragg scattering by the clear air. The modes are distinguishable because of the fall velocity of the drops relative to the air. Using an algorithm to separate the clear air and precipitation components of these bimodal spectra, we can isolate the power and velocity of each mode. This allows us to see interactions of the two backscattering mechanisms. In one puzzling case the clear-air reflectivity in a layer increases by an order of magnitude as rain falls through it, but in the other cases no such strong effect occurs. We discuss possible causes of the increase in reflectivity but find no convincing explanation.