On the dynamics of drylines 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

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On the dynamics of drylinesFine-scale vertical
structure of drylines during the International
H2O Project (IHOP) as seen by an Airborne Doppler
RadarQun Miao and Bart GeertsUniversity of
Wyoming
P2R.3
Summary
Variations with height
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.
solenoidal tilt
Differences of variables across dryline are
defined as mean of 3 km east of dryline
mean 3 km west of dryline
June 19 has Clear skies, no deep convection in
any direction apparent, at least before 2030 Z.
The dryline progresses from west to east. Small
?v gradient

• Lighter colors correspond to later times.

solenoidal circulation
May 22

• 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.
• The UWKA conducted several traverses
  perpendicular to drylines as they became more
  defined, sometimes prior to convective initiation
  (CI).
• The WCR operated in profiling mode, with beams
  both below and above the aircraft, and in
  vertical-plane dual-Doppler (VPDD) mode.

June 19
Dual-Doppler Note sloping boundary and
solenoidal circulation
SPOL May 22

• ?v 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
Comparison between May 22 and June 19
May 22 June 19
Time (UTC) 2200 to 0000 1930 to 2130
Direction of movement westward eastward
Orientation of dryline SSE to NNW SSE to NNW
Mean MR difference (g/kg) 2.0 1.5
Mean ?v difference (K) -0.8 -0.1
sign of ?v difference opposite to MR difference reverses from early time
Mean confluence (m/s) -1.4 -6.7
Change in mean wind direction across dryline (W to E) SSE to S SSE to S
Tilting of the dryline towards the denser air (lower ?v) towards the denser air
Composite variations across dryline

• Different variables are averaged in 200-m bins
  across the dryline for 9 cases on May 22 and 7
  cases on June 19.

May 22
2150 UTC
0012 UTC
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
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)
solenoidal circulation
Conclusions

• The echo plume and updraft plume at the dryline
  tilt towards the denser air (lower ?v). The
  tilted updraft/downdraft couplet and the
  convergent flow are part of a solenoidal
  circulation. The ?v gradient shows that this
  circulation is thermally directed.
• On May 22, MR difference decreases with height
  and so does ?v 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 ?v 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.

June 19

• Gentle ?v gradient.
• MR change is less than May 22 as well.
• Confluence is much stronger than May 22.