Water vapor variations in echo plumes in the convective boundary layer
Bart Geerts, University of Wyoming, Laramie, WY; and Q. Miao
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.
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.
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.
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.
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.
Extended Abstract (2.0M)
Supplementary URL: http://www-das.uwyo.edu/wcr/projects/ihop02/
Session 1, International H2O Project (IHOP)
Monday, 10 February 2003, 10:45 AM-2:30 PM
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