Transcript On the dynamics of drylines Fine

On the dynamics of drylines
P2R.3
Fine-scale vertical structure of drylines during the International H2O Project (IHOP) as seen by an Airborne Doppler Radar
Qun Miao and Bart Geerts
University of Wyoming
Summary
Dryline structure and dynamical characteristics are examined by means of
aircraft and airborne Doppler radar observations collected in the central Great
Plains in late spring. Drylines in the southern and central Great Plains have
received considerable attention in the literature. One key reason is that
sometimes severe thunderstorms tend to break out along the dryline. The focus
here is on late morning to early evening, when the convective boundary layer
(CBL) is well-developed.
Of particular use is the Wyoming Cloud
Radar (WCR) aboard the University of
Wyoming King Air aircraft (UWKA). The
reason is that the radar, in profiling
mode, gives vertical structure
information, which can be interpreted by
means of in situ data.

positive means that
sustained updrafts
(and thus stronger
echoes) occur in the
less dense air
QM: true??
Variations with height
solenoidal tilt
positive means that the le
(dry) air rises relative to th
(QM verify)
Differences of variables across dryline are defined as:
[mean of 3 km east of dryline] – [mean 3 km west of dryline]
• Lighter colors correspond to later times.
June 19 has Clear skies,
no deep convection in any
direction apparent, at least
before 20:30 Z. The dryline
progresses from west to
east.
solenoidal circulation
May 22
confluent
difluent
dry side is slightly denser!
Small θv gradient
The UWKA conducted several
traverses perpendicular to drylines as
they became more defined, sometimes
prior to convective initiation (CI).

June 19
Dual-Doppler
The WCR operated in profiling mode,
with beams both below and above the
aircraft, and in vertical-plane dualDoppler (VPDD) mode.

Flight level
Note sloping boundary
and solenoidal circulation
SPOL May 22
• θv gradient reverses sign in time, consistent with a change vertical echo tilt, a
flip in sign of the solenoidal vorticity, and a change in boundary propagation speed.
ground level
EAST
WEST
Comparison between May 22 and June 19
May 22
June 19
22:00 to 00:00
19:30 to 21:30
westward
eastward
SSE to NNW
SSE to NNW
Mean MR difference (g/kg)
2.0
1.5
Mean θv difference (K)
-0.8
-0.1
opposite to MR difference
reverses from early time
-1.4
-6.7
SSE to S
SSE to S
towards the denser air
(lower θv)
towards the denser air
Time (UTC)
Composite variations across dryline
Direction of movement
Different variables are averaged in 200-m bins across the dryline for 9 cases on May 22 and 7 cases on June 19.
Orientation of dryline
May 22
21:50 UTC
00:12 UTC
sign of θv difference
On May 22, UWKA flew
across a dryline in the OK
Panhandle. Flight levels
are from 150 m to 1500
m AGL from late
afternoon to early
evening. The dryline
retrogresses from east
to west.
Obvious mixing ratio (MR)
gradient
The vertical tilt of the
dryline fine-line, the
vertical velocity couplet,
and the density
temperature gradient are
all consistent with a
solenoidal circulation, i.e.
the basis of a density
current.
Note: qv’ is proportional
to buoyancy
solenoidal tilt
solenoidal circulation
Dotted lines are individual
cases and bold lines are
averages of all cases. Lighter
colors correspond later times.
Zm: close-flight-level mean
reflectivity (14 gates)
Wm: close-flight-level mean
vertical velocity (14 gates)
dry side is less dense (warmer)
June 19
• Gentle θv gradient.
• MR change is less than May 22
as well.
• Confluence is much stronger
than May 22.
Mean confluence (m/s)
Change in mean wind
direction across
dryline (W to E)
Tilting of the dryline
Conclusions
• The echo plume and updraft plume at the dryline tilt towards the denser air (lower
θv). The tilted updraft/downdraft couplet and the convergent flow are part of a
solenoidal circulation. The θv gradient shows that this circulation is thermally
directed.
• On May 22, MR difference decreases with height and so does θv difference as well.
This decrease is consistent with a density current.
• On June 19, strongly confluent flow (synoptically driven) happens to
concentrate the regional moisture gradient and produce a dryline. Later in the day,
surface sensible heating makes the cooler western side hotter and reverse the θv
gradient.
• Although large-scale convergence and the regional slope of the terrain is important,
on much smaller scales, the dryline definition appears to be driven by density current
dynamics.