83rd Annual

Monday, 10 February 2003: 1:30 PM
Water vapor variations in echo plumes in the convective boundary layer
Bart Geerts, University of Wyoming, Laramie, WY; and Q. Miao
Poster PDF (1.9 MB)

The Wyoming Cloud Radar (WCR), a 95 GHz Doppler radar, extensively sampled the optically-clear, quiescent convective boundary layer (CBL) during IHOP aboard the University of Wyoming King Air (UWKA). The term ‘quiescent’ CBL indicates that the CBL does not contain a mesoscale convergence zone evident as well-defined singular radar fine-line. WCR vertical reflectivity profiles mark the CBL by plumes of stronger echo. Most plumes extend throughout the CBL depth. Their width and spacing is variable. They are absent at night and grow in depth and spacing during the morning hours as the CBL deepens. They can be referred to as bug plumes because the WCR echo is largely due to small insects.

The WCR operated in various antenna modes, including one where the nadir and zenith antennas operated simultaneously. In this mode the flight-level data can be combined with the WCR echo and vertical velocity profiles, which are continuous except near flight level. On many occasions the UWKA flew around 60 m above ground level, with the WCR looking upward only. This too is a useful configuration. The quasi-instantaneous transects of CBL echo and vertical velocity structure have a resolution of about 30 m.

The objective of this poster is to use the extensive IHOP record of combined WCR/UWKA data to demonstrate that echo plumes represent CBL thermals, to document the vertical velocity structure of these thermals, to show that they contain more water vapor than the interstitial CBL air, and that they contribute significantly to the upward moisture flux within the CBL. The WCR vertical velocity profile will allow an estimation of the plume-induced moisture flux divergence in the CBL.

Through comparison between flight-level vertical air motion and close-range echo vertical motion above and below the aircraft, we will first assess whether the vertical echo motion is systematically biased, i.e. whether insects actively oppose updrafts in which they become embedded. If needed the WCR vertical motions will be corrected to represent air motion. Next flight-level data are used to assess whether echo plumes are positively buoyant at various levels within the CBL, and we will compare the plume buoyancy estimates to that of thermals that are simulated numerically or in the lab.

The key question then relates to the moisture anomalies in thermals. Flight-level data will be used to assess such anomaly and to estimate the flight-level water vapor flux. Large differences may be found regionally and on different days. A water vapor flux profile can further be derived, assuming the conservation of mixing ratio in plumes of known vertical motion. Such flux profile will only capture the upward transfer, as echoes are generally too weak between plumes to estimate the rate of subsidence there. 

We may delve into the dynamical interpretation of echo plumes in the quiescent CBL. The plumes may reflect variations in land surface conditions or topography, or else they may reflect inherently atmospheric BL dynamics. To address this we will examine stationarity and use aircraft-based land surface characterizations (albedo, NDVI and skin temperature), and aircraft-based soundings conducted at regular intervals.

Supplementary URL: http://www-das.uwyo.edu/wcr/projects/ihop02/