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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.

Abstract. Since May 1992 a small, 915-MHz profiler has been operated continuously in downtown Montreal. It is a five-beam system employing a microstrip array antenna, located atop a 14-story office building that houses several academic departments of McGill University. The data are used for research on precipitation physics and the clear-air reflectivity in addition to wind profiling. We are especially interested in situations in which the reflectivities of the clear air and the precipitation are comparable. This permits the study of interactions between the precipitation and the clear air, a new area of research made possible by wind profilers. On clear days in the summer, 30-min consensus winds can often be measured to an altitude of 3 km, but ground clutter in the antenna sidelobes interferes with measurements below 600 m. Rain when present often permits wind profiling down to 100 m and up to 6 km or higher. On cold winter days there are some periods when the reflectivity is too weak at all levels to permit wind estimation. Falling snow, however, provides readily detectable echoes and serves as a good tracer of the wind and so allows profiling over its full altitude extent. The best conditions for observing interactions between precipitation and the clear air are when light rain falls through a reflective layer associated with a frontal surface or inversion. Unexpectedly, flocks of migrating birds sometimes completely dominate the signal at night in the spring and fall seasons.

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