Extended Abstract (256K)12A.4
Vertical velocity and buoyancy characteristics of echo plumes detected by an airborne mm-wave radar in the convective boundary layer
Bart Geerts, University of Wyoming, Laramie, WY; and Q. Miao
Reflectivities and Doppler velocities from an airborne 95 GHz radar, the Wyoming Cloud Radar, are used to describe the detailed (~25 m) vertical structure of plumes and in the optically-clear convective boundary-layer (CBL). This study focuses on the of the ?undisturbed? CBL, i.e. away from mesoscale convergence zones. Some 30 hours of combined radar and in situ aircraft data were collected in the undisturbed, mature CBL over the central Great Plains of North America in late spring of 2002, as part of IHOP_02 (Weckwerth et al 2003). Multiple radar configurations are used, including a profiling mode, with fixed antennas looking up and down from the aircraft, and a vertical-plane dual-Doppler mode, employing a nadir and slant-forward antenna.
The detectability of coherent buoyant eddies in the optically clear CBL came as a surprise in IHOP. This allows us to document the vertical echo and vertical velocity structure away from the much stronger radar ?fine-lines?, an example of which is analyzed in another paper for this Conference (Geerts, Leon and Trudel). Echo plumes clearly mark the depth of the quiescent CBL depth. These plumes generally contain updraft cores, yet the WCR data, from both the up and down antennas, indicate that subsidence prevails. This is consistent with vertical velocity averages documented by ground-based vertically-pointing radars, and wind profilers. In IHOP we had the unique capability to compare WCR vertical velocities at the nearest gate above and below the aircraft to those measured by the gust probe. The latter measure the air vertical velocity with an accuracy of ~0.1 m/s at matching frequency (25 Hz). A preliminary comparison indicates that echoes tend to subside, at an average rate of 0.8 m/s. Also, this sedimentation of echoes, which are believed to be mostly insects, is independent of echo strength and height within the CBL, but is larger when the vertical air velocity is larger. That is, insects oppose the updraft in which they are embedded, at an adjustable speed.
This study will describe the vertical structure of CBL echoes and their updrafts/downdraft properties, and quantify the buoyancy and water vapor anomalies in plumes. We will first show some examples of echo plumes and vertical velocities. The typical horizontal and vertical dimensions of plumes, and the vertical velocity characteristics, will be documented by means of pdfs of frequency-by-altitude diagrams. Then evidence will be supplied that that these plumes are equivalent to buoyant thermals. Next, we use all suitable combined WCR profile and aircraft data to assess the accuracy of the radar-derived vertical motion as a measure of vertical air motions, and to quantify the bias. Finally, vertical velocity pdfs will be shown after correction for insect motion.
Reference: Weckwerth, Parsons, Koch, Moore, LeMone, Demoz, Flamant, Geerts, Wang, and Feltz, 2003: An Overview of the International H2O Project (IHOP_2002) and Some Preliminary Highlights. Bull. Amer. Meteor. Soc., in press.
Supplementary URL: http://www-das.uwyo.edu/wcr/projects/ihop02/
Session 12A, IHOP/Boundary Layer
Monday, 11 August 2003, 4:00 PM-6:00 PM
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