Airborne radar observations of convective plumes in the boundary layer

The Wyoming Cloud Radar (WCR), mounted onboard the ARAT remote sensing aircraft, participated in the TRAC project in June and early July 1998. The ARAT made three flights (June 19, 22, and 28 1998) in or just above the boundary layer. The boundary layer in all three cases was well mixed and capped by a strong inversion at roughly 1km altitude. Numerous plumes emanating from the surface are evident in the WCR data. The plumes are visible to the WCR due to small insects carried aloft in air ascending more rapidly than the insects are able to descend. The insects serve as tracers of air motion - they are assumed to follow horizontal air motions while adding a bias of roughly 0.5 to 1 m/s downward to the vertical velocities. These observations provide some unique insights into convection in the boundary layer.

The WCR is a 95 GHz (W-band) Doppler radar with a 0.9 degree beamwidth using a PRF of 10KHz. The ARAT installation of the WCR utilized two beams: one oriented at nadir and the other looking 42 degrees forward of nadir. Due to the motion of the aircraft the nadir beam data overlaps the forward beam data after a time lag of 10 seconds per km below the aircraft. The sample volume is observed from two different angles, thus when there is sufficient signal the two Doppler velocity measurements can be inverted to determine the two-dimensional velocity in a vertical plane. The sensitivity of the WCR/ARAT installation is generally sufficient to observe the plumes from an altitude of 700m or less, however it is not adequate to observe the plumes from above the top of the boundary layer.

When there is sufficient signal the dual-Doppler analysis yields a vertical cross section through the plume. The dual-Doppler analysis has a resolution of roughly 30m in the vertical and 10 m along-track. The vertical-plane dual-Doppler velocities are presented for a few individual plumes. A comparison of the in-situ wind measurements from the ARAT and the vertical plane dual-Doppler results is presented. In addition to the specific cases, this paper includes a discussion of the average size, shape, and spacing of the observed plumes