12a.4 Vertical Velocity and Buoyancy Characteristics of Echo Plumes Detected by an Airborne Mm-wave Radar in the Convective Boundary Layer

One outstanding puzzle in wind profiler data is the tendency for vertical motions to be negative over flat terrain, especially in the convective boundarylayer (CBL)(Angevine 1997). Erroneous velocities on the order of 20-40 cm s are reported during the daytime and they virtually disappear at night. After eliminating various sources of possible measurement error, Angevine (1997) concludes with the speculation that the errors are due to small targets (particulate scatterers detected by 915 MHz radar) that have a systematic downward velocity. In this study vertical velocities in the CBL are studied at much higher resolution, by means of an airborne 95 GHz (3 mm) Doppler radar, with as objectives to confirm this downward bias and to interpret it in terms of insect behavior and CBL dynamics. In comparison with 915 MHz radar data, 95 GHz radar profiles can be sampled at higher frequency, and the 95 GHz signal is not affected by Bragg scattering. The benefit of airborne measurements, compared to ground-based radar profiles, is the availability of in situ thermodynamic and kinematic observations, and the direct observation of horizontal structure. Fixed profiling radars can do this indirectly, by converting time to distance, assuming some advection speed. Such procedure is questionable because the evolution time scale of thermals may be smaller than its advective time scale.