Convective Initiation along a Dryline: A High-Resolution Modeling Study and the Role of Horizontal Convective Rolls

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Convective Initiation along a Dryline A
High-Resolution Modeling Study and the Role of
Horizontal Convective Rolls

• 11 October, 2005
• School of Meteorology Seminar

Ming Xue School of Meteorology and Center for
Analysis and Prediction of Storms University of
Oklahoma
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Introduction The dryline

• Definition A narrow zone of strong horizontal
  moisture gradient at and near the surface.
• Most observed in the Western Great Plains of the
  U.S. (also in India, China, Australia, Central
  Africa,....)
• Over the U.S., the dry line is a boundary between
  warm, moist air from the Gulf of Mexico, and hot,
  dry continental air from the southwestern states
  or the Mexican plateau
• The dry line is not a front there is little
  temperature gradient across dryline

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An Example of Dryline
0.6C
19C
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E-W Vertical Cross-section
qv
q
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E-W Cross-section of winds and qv
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The dryline as a focus of convection

• Surface convergence between winds with easterly
  component east of the dry line and westerly
  component west of the dry line.
• Dryline is the westernmost boundary of moist,
  convectively unstable air.
• The capping inversion layer to the east of
  dryline helps with the accumulation of CAPE, in
  the moist but relatively cool low-level boundary
  flow

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Initiation of Convection along Dryline

• Exact WHEN, WHERE, and WHY convection is
  initiated, if at all, remains a forecasting
  challenge
• Goal of this study to answer the above
  questions, for one case at least.

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Dryline CI and IHOP_2002

• Despite the general understandings on the role of
  dryline in CI, the exact processes by which
  convection is triggered or initiated, and the
  specific location of initiation along the dryline
  is not well understood
• Exactly WHEN and WHERE convection is initiated,
  if at all, remains a forecasting challenge
• Understanding convective initiation (CI)
  processes was one of the goals of IHOP_2002
  (International H2O) field experiment

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References to appear in MWR IHOP CI Special
Issue

• Xue, M. and W. Martin, 2005a,b A high-resolution
  modeling study of the 24 May 2002 case during
  IHOP.
• Part I Numerical simulation and general
  evolution of the dryline and convection. Mon.
  Wea. Rev., In press.
• Part II Horizontal convective rolls and
  convective initiation. Mon. Wea. Rev., In press.

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The International H2O Project(IHOP_2002)

• This project took place across the Southern Great
  Plains from 13 May to 25 June 2002
  (http//www.atd.ucar.edu/dir_off/projects/2002/IHO
  P.html)
• Several goals were set for this experiment,
  including
• Study ways to improve Quantitative Precipitation
  Forecasts
• Improve forecasts of timing and location of
  convective initiation
• Supporting both of these were studies in boundary
  layer processes and new instrumentation

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The International H2O Project

• Over the Southern Great Plains from 13 May to 25
  June 2002
• Improving QPF
• Improving understanding and forecasting of CI
• Boundary layer (BL) process studies
• Instrumentation intercomparisons
• Weckwerth et al. (2004 BAMS)

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Objectives of This Study

• Simulate the CI processes at high resolution and
  by assimilating high-resolution observations
• Understand the structure, evaluation and dynamics
  of dryline
• Understand convective initiation along a dryline

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The May 24, 2002 Dryline CI Case
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Methodology

• Make use of special data sets collected during
  IHOP
• Assimilate observations into a high-resolution
  mesoscale model to, the ARPS.
• Verify model simulations against available data
• Analyze realistic high-resolution model
  simulations to understand dryline evolution and
  convective initiation (CI) process

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Synopsis of the Event

• Convection started between 2000 and 2030 UTC in
  Texas panhandle area along a dryline. An
  intensive observation period of IHOP_2002.
• Rapidly developed into a squall line and advanced
  across Oklahoma and northern Texas
• SPC reported almost 100 incidents of large hail,
  15 wind reports, and two tornadoes in central
  Texas

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Surface analysis satellite images
From Wakimoto et al. (2005, MWR).
1900
2000
2200
2100
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1900 UTC
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2000 UTC
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2045 UTC
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Time and Location of Initiation(Loop time 17UTC
22 UTC)
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1932UTC
22
2002UTC
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2032UTC
24
2102UTC
25
2132UTC
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2202UTC
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2258UTC
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2358UTC
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Animation 20UTC-00UTC, KLBB radar
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Animation 20UTC-00UTC, KFRD radar
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Surface Charts at 18 Z
L
gt3200 J/kg
Pmsl, qv, T and V
CAPE and CIN
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Upper airChartsat 18 Z
500 hPa
250 hPa
700 hPa
850 hPa
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18 UTC May 24, 2002 I.C. 3 km / 1km grid
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Model Configurations

• 1 km grid nested inside a 3 km grid
• ADAS analyses for ICs and 3 km BCs
• ARPS model with full physics, including ice
  microphysics soil model PBL and TKE-SGS
  turbulence
• 12 hour forecast, starting at 18 UTC

CI 2000UTC
1800 UTC
1200 UTC
0006 UTC
0000 UTC
3km
1km
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Obsevations Used by ADAS

• ARM
• Colorado Agriculture network
• IHOP Composite Upper Air - rawinsondes
• KS Ground Water District 5
• OK Mesonet
• SAO
• SW Kansas Mesonet
• Western TX Mesonet
• Profiler data absent

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Surface analysis plus obs at 18 Z
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3km model forecast
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Animation of 3km model forecasthttp//twister.ou
.edu/may24ci/gmeta_3km_rf_cyc_anim.mov
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1 km model forecast
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t3h, 2100 UTC
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t4h, 2200 UTC
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t5h, 2300 UTC
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t6h, 0000 UTC
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Animation of 1 km forecasthttp//twister.ou.edu/
may24ci/A_gmeta_rf_1kmd_sml_anim.mov
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t3h, 2100 UTC
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t2h t2h 15min t2h 30min t2h
45min
C
C
B
B
B
A
A
A
C
B
A
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Animation of Vertical Cross-section and Extracted
soundings
W, C and E indicate locations of extracted
soundings
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Animation of Vertical Cross-section
http//twister.ou.edu/may24ci/B_gmeta_1kmd_vpte_a
nim.movExtracted soundingswest, east and at
the dryline but averaged along the dryline
direction.
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From Etling and Brown (1993 BLM)
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from Wakimoto et al. (2005)
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Animation of surface moisture convergence
fieldsaround cell group A http//twister.ou.edu/
may24ci/D1_gmeta_divq_1kmd_1bA_color_anim.mov
http//twister.ou.edu/may24ci/D1_gmeta_divq_1kmd_
1bA_sml_anim.mov
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Animation of surface moisture convergence
fieldsaround cell group Chttp//twister.ou.edu/
may24ci/D1_gmeta_divq_1kmd_1bC_color_anim.mov
http//twister.ou.edu/may24ci/D1_gmeta_divq_1kmd_1
bC_sml_anim.mov
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30x30km
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4min intervals
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x
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5min intervals
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Conceptual model
(Xue and Martin 2005b)
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Conceptual Model (from Xue and Martin 2005b)

• A conceptual model of dryline convective
  initiation as related to the interaction of
  horizontal convective rolls (HCRs) with the
  primary dryline convergence zone (PDCZ) is
  proposed.
• HCRs develop on both sides of the dryline in the
  afternoon due to surface heating over elevated
  terrain. Close to the PDCZ, the HCRs are aligned
  at an acute angle, a, with the dryline. The HCRs
  on the west side are more intense and deeper and
  their updraft speed can reach several meters per
  second.
• The low-level convergence bands associated with
  these updrafts are enhanced at the dryline
  location.
• The PDCZ is the would-be location of the
  convergence zone between the moist and dry air
  masses if the HCRs were absent. The actual
  convergence zone drawn as a thick solid line is
  distorted into a wavy pattern by the intersecting
  HCRs.
• Often, HCRs cease to exist or significantly
  weaken immediately east of the PDCZ, due to
  suppression by a broad branch of descending
  motion that is part of the broader mesoscale
  dryline circulation.
• Convective initiation is preferred close to the
  central portion of the leading HCR convergence
  bands at the PDCZ, where surface convergence is
  maximized due to opposing winds on each side of
  the bands as well as along-band flow convergence
  found at these locations. These preferred
  locations are also the intercepting points of the
  leading HCRs with the hypothetically unperturbed
  (straight) PDCZ.
• As a result, the spacing between initial
  convective cells along the dryline tends to be
  equal to the distance between successive HCR
  updraft or near-surface convergence bands
  multiplied by sec(a).
• The low-level environment has been preconditioned
  for easy triggering of convection along this zone
  because of sustained mesoscale lifting at the
  PDCZ.

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Conceptual Model (from Xue and Martin 2005b)

• The general confluent flow pattern between the
  two air masses, and the significant local
  enhancement of westerly or southwesterly winds by
  downward momentum transport due to intense
  eddies, are the primary sources for the
  enhancement and maintenance of the PDCZ.
• Because of PDCZ convergence, the top of the
  well-mixed moist layer is often half a kilometer
  or so higher in the PDCZ region than that to the
  east, making the LFC easier to reach by
  individual parcels. The further lifting by HCR
  convergence pushes the air parcels above the LFC
  and triggers moist convection.
• Behind the leading convergence bands are
  elliptically-shaped asymmetric surface divergence
  patterns with the asymmetry arising from the fact
  that the air feeding the downdrafts already
  possesses westerly or southwesterly momentum.
• The surface divergence patterns give rise to
  convergence maxima near the center of the bands.
  The convergence at the end of the long axis of
  the ellipses is weaker although directional shear
  is largest there.
• Vertical vorticity centers are found at such
  locations (marked by circled Vs in the figure)
  and the centers may split into pairs with each
  located near the end of the HCR convergence
  bands.
• Interestingly, despite the enhanced vertical
  vorticity at these locations and the possible
  enhancement of vertical motion due to Ekman
  pumping, the maximum vorticity centers do not
  appear to be favorable locations of CI. This is
  an equally interesting finding.

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Summary

• Mesoscale convergence associated with the
  confluent flow around the dryline is shown to
  produce an upward moisture bulge, while surface
  heating and boundary layer mixing are responsible
  for the general deepening of the boundary layer.
  These processes produce favorable conditions for
  convection.
• Horizontal convective rolls (HCRs) develop on
  both sides of the dryline. The main HCRs that
  interact with the primary dryline convergence
  boundary (PDCB) are those from the west side and
  they are aligned at an acute angle with the
  dryline.
• Often, HCRs cease to exist or significantly
  weaken immediately east of the PDCZ, due to
  suppression by a broad branch of descending
  motion that is part of the broader mesoscale
  dryline circulation.
• The HCRs intercept the PDCB and create strong
  moisture convergence bands at the surface and
  force the PDCB into a wavy pattern. The
  downdrafts of HCRs and the associated surface
  divergence create localized maxima of surface
  convergence that trigger convection.

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Summary continued.

• Sequences of convective cells develop at the
  locations of persistent maximum surface
  convergence forcing, then move away from the
  source with the mid-level winds. When the initial
  clouds propagate along the convergence bands that
  triggers them, they grow faster and become more
  intense.
• The surface divergence associated with HCRs also
  helps concentrate the background vorticity and
  the vertical vorticity created by tilting of
  environmental horizontal vorticity into vortex
  centers or misocyclones, and such concentration
  is often further helped by cross-boundary shear
  instability. The misocyclones, however, do not in
  general co-locate with the maximum vertical
  forcing or the locations of convective
  initiation, but can help enhance surface
  convergence to their south and north.
• While the mesoscale convergence of dryline
  circulation preconditions the boundary layer by
  deepening the mixed layer and lifting moist air
  parcels to their LCL, it is the localized forcing
  by the HCR circulation that initiates the
  convection.

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References

• Xue, M. and W. Martin, 2005a,b A high-resolution
  modeling study of the 24 May 2002 case during
  IHOP. Mon. Wea. Rev., In press. Also at
  http//twister.ou.edu/visa.htmlpubs.
• Part I Numerical simulation and general
  evolution of the dryline and convection.
• Part II Horizontal convective rolls and
  convective initiation.
• Expanded version of this talk and more movies can
  be found at http//twister.ou.edu/may24ci

Thanks!
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Central At the dryline
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West of dryline
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East of dryline