Transcript Slide 1
Fine-Scale Observations of a Pre-Convective Convergence Line in the
Central Great Plains on 19 June 2002
JP3J.1
JP3J.1
Benjamin Daniel Sipprell, [email protected], and Bart Geerts , University of Wyoming, Laramie, USA
The Problem
Questions:
1. How do mesoscale atmospheric processes and
surface fluxes alter the convective boundary
layer (CBL) to generate a dryline boundary?
2. How is dryline convergence maintained, at very
small scales (Ziegler and Rasmussen 1998)?
Vertical Structure and Evolution
Density Current Dynamics
Sustained Convergence
Early soundings through the CBL
demonstrate the presence of a strong
capping inversion at 850 mb and
increasing values of CAPE.
A first stepped traverse shows that the
dry air is cooler (θv lower), consistent
with the westward tilt of the dryline &
the negative solenoidal circulation.
DOW3 along with mobile mesonet
data clearly show strong confluence
near the dryline and along the UWKA
flight track.
First series of stepped traverses shows
θe and ‘r’ differences 3 K and 1.5
-1
g kg , resp. across the dryline, later to
increase to 10 K and 6 g kg-1
indicating dryline strengthening.
Dual-Doppler analysis confirms the
dryline tilt to the W and the negative
solenoidal circulation.
The first stepped traverse observes θv
0.5K cooler on the dry side than
moist side, anda peak of 0.5K at the
dryline.
3. How does deep convection initiate along a
dryline at those scales?
Objectives:
1. To describe the kinematic and thermodynamic properties of a preconvective dryline at very high resolutions and in vertical cross sections.
2. To demonstrate via a case study that fine-scale convergence is
driven by the buoyancy gradient, sustained by density current dynamics.
Discussion
There is a weak θv (virtual pot. temp.) gradient
across the dryline. This gradient is consistent
with the vertical tilt of the echo plume, and the
vertical velocity couplet, indicating a thermally
direct solenoidal circulation. The circulation and
tilt reverse when the θv (virtual pot. temp.)
gradient reverses.
At the fine-line convergence zone (the dryline),
anomalously high θv occurred, deepening in time
till CI.
During the 3rd stepped traverse
the dryline becomes quasi-stationary
and better-defined, according to
DOW3 data.
By 21 UTC cross dryline confluence
increases with values of 10 ms-1 over
distances of hundreds of meters.
Murphey et al. (2005) find that
horizontal shearing along the dryline
due to confluence yields high vertical
vorticity along a contorted dryline on
this day.
Early (unusual eastward tilt)
Late (classic westward tilt)
Conclusions
• Updrafts greater than 5 m/s
are observed, collocated with
anomalously high values of θv
• This leads to local deepening
of the CBL along the fine-line,
leading to CI.
• Dual-Doppler wind field
demonstrates the existence of
solenoidal circulations
consistent with horizontal
density differences, and
changes in dryline propagation
speed.
LearJet dropsondes and UWKA
stepped traverses observe a deep core
of positive buoyancy near the dryline.
By the last series of transects the CBL
depth above the dryline exceeds 3200
m AGL. Advection of high θe air into
the CBL ‘dome’ results in the erosion
of CIN.
A remarkable transformation occurs between
the 2nd and 3rd dryline stepped traverses: the
dryline shifts from a westward to an eastward
tilt with a consistent θv gradient reversal.
Denser air flips from the W side to the E side
of the dryline, possibly because of larger
surface sensible heat flux to the W. The
vertical velocity dipole consistently shifts, the
solenoidal circulation becomes positive, and
the eastward propagating fine-line becomes
stationary. All this is consistent with density
current theory.
DOW3 data, local mesonets and
mobile mesonets show an increasingly
intense southerly jet and thus
increasing confluent flow into a
evolving stationary dryline.
REFERENCES:
Murphey, Hanne V. and Wakimoto, Roger M., 2005: Dryline on 19 June 2002 during IHOP. Part I: Airborne Doppler and LEANDRE II Analysis of the Thin Line Structure and Convection Initiation. Mon. Wea. Rev.: in press.
Ziegler, Conrad L. and Rasmussen, Erik N., 1998: The Initiation of Moist Convection at the Dryline: Forecasting Issues from a Case Study Perspective. Wea. and Forecasting: Vol. 13, No. 4, pp. 1106–1131.