Convection Initiation by Boundaries

1
Convection Initiation by Boundaries
Wendy Schreiber-Abshire, COMET with Thanks to Dr.
Tammy Weckwerth, NCAR
NCAR
NCAR
2
Overview

• Progression of relative science
• Boundary types
• Gust fronts
• Sea breeze fronts
• Drylines
• Horizontal convective rolls
• Geographic specific boundaries
• To convect, or not convect ?

3
Convergence Convection Initiation

• The Thunderstorm. Byers and Braham, 1949
• The role of surface divergence and vorticity in
  the life cycle of convective rainfall. Part I.
  Observations and analysis. Ulanski and Garstang,
  JAS, 1978
• The relationship between total area divergence
  and convective precipitation in south Florida.
  Watson and Blanchard, MWR, 1984

4
Boundaries from Satellite

• Satellite Cloud Arc Lines
• Meso-highs and satellite imagery. Purdom, MWR,
  1973
• Subjective interpretations of geostationary
  satellite data for nowcasting. Nowcasting,
  Purdom, K. Browning, Ed. Academic Press, 1982
• Thunderstorm trigger mechanisms over the
  southeast U.S., Purdom and Marcus, 12th Conf.
  SLS, 1982

5
Wilson and Schreiber Results

• No apparent rhyme or reason to convective
  storm initiation locations along the Front Range
  of Colorado!

6
Wilson and Schreiber Results (cont.)

• Storm initiation NOT random when viewed
  relative to boundary-layer convergence zones!

7
Wilson and Schreiber Results (cont.)

• 79 of storms gt 30 dBZ that were not advected
  into study area formed along a boundary
• 92 of storms gt 60 dBZ that were not advected
  into study area formed along a boundary

8
Known Convergence Lines

• Synoptic Fronts
• Sea Breeze/Land Breeze Circulation (Lake
  Breezes)
• Davis et al., 1889
• Gust Fronts
• Fujita, JM, 1959

9
Density Currents

• Laboratory
• Forced ascent at head
• Mixing at interface

Simpson, 1987
10
Gust Front Conceptual Model
Droegemeier and Wilhelmson, 1987

• Leading edge of low-level thunderstorm cold-air
  outflow
• Outflow depth 1 km
• Forced ascent at head

11
Storm Initiation by Gust Front

• Cells initiated by gust front
• Not continuous convective line
• Organized by Kelvin-Helmholtz instability
  internal gravity waves

NCAR
Weckwerth and Wakimoto, 1991
12
Known Convergence Lines (cont.)

• Urban Heat Island Circulation
• Changnon, Meteor. Monogr. 40, 1981
• Cloud Shading Boundaries
• Purdom and Gurka, 5th Conf. WFA, AMS, 1974
• Segal et al., MWR, 1986
• Soil Moisture/Vegetation Inhomogeneities
• Ookouchi et al., MWR, 1984
• Nice review of all these in Meso. Met. and
  Fcsting, Chap. 23, Ray, 1984

13
Known Convergence Lines (cont.)

• Slope/Valley Circulations
• COMET module on mtn./valley winds available at
  http//meted.ucar.edu/mesoprim/mtnval/index.htm
• Cloud Streets
• (Visual Observations) Kuettner, Tellus, 1971
• Longitudinal/Horizontal Rolls
• (Aircraft) Lemone, JAS, 1973
• (Radar) Kelly, JAS, 1984

14
Horizontal Convective Rolls
The COMET Program

• Counter-rotating horizontal vortices in CBL
• Aligned along mean wind direction
• Due to combination of surface heat flux and wind
• Clouds often above updraft branches

15
Sea Breeze Front and Rolls

• Sea breeze convection initiation impacted by
  coast shape
• Pielke 1974 Laird et al. 1995

16
Storm Initiation by SBF and Rolls

• Rolls lifted at sea breeze front
• Deeper, stronger updrafts at intersection points
  ? increased likelihood of convection initiation

Atkins et al., 1995
17
Animation of Storm Initiation by Rolls
Weckwerth, 2000

• Rolls alone initiate storms some days
• Accurate measure of stability imperative to be
  able to forecast

18
Specific Geographic CZs

• The Dryline
• Beebe, JM, 1958
• The San Fernando Convergence Zone
• Edinger and Helvey, BAMS, 1961
• The Puget Sound Convergence Zone
• Mass, Weatherwise, 1980
• Denver Convergence Vorticity Zone
• Szoke et al., MWR, 1984

19
The Dryline

• Zone between warm, moist maritime air and hot,
  dry continental air
• Occurs 40 of the time over Plains region
• Also occurs in India and Central West Africa
• CI Rhea, 1966

Bluestein,1993

• Series of papers by Hane, MWR, 2001 and 2002

20
Storm Initiation by Dryline

• Intense, deep mesoscale lifting leads to CI
• Requires lifting depth be gt the LFC height
• Need to the know boundary depth and moisture to
  accurately forecast CI

Ziegler and Rasmussen, 1998
21
Clear-air Radar Echo Example

• The Denver Convergence Vorticity Zone
• We can see why it is called that!

22
Clear-air Radar Boundaries and Convection
Initiation

• Nowcasting with Doppler radar The
  forecaster-computer relationship.
• Wilson and Carbone, Nowcasting II, K. Browning,
  Ed., 1984
• Initiation of convective storms at radar-observed
  boundary-layer convergence lines.
• Wilson and Schreiber, MWR, 1986

23
Cumulus Cloud Echoes

• First radar echoes from cumulus clouds. Knight
  and Miller, BAMS, 1993 and JAS 1998
• radar used to study entrainment and mixing (via
  Bragg scattering) as well as precipitation
  formation (via backscatter from hydrometeors)
• Mantle Echoes from Bragg scattering
  rediscovered
• Developing thunderstorm forecast rules utilizing
  first detectable cloud radar-echoes. Henry and
  Wilson, 5th Conf. Av. Wx. Sys., AMS, 1993

24
Research by Community 1

• Mesoanalysis of summertime convergence zones in
  central and eastern North Carolina. Koch and Ray,
  WF, 1997

• A boundary in NC is much more likely to initiate
  convection and is twice as likely to interact
  with another boundary than a boundary in Colorado
  is!

25
Research by Community 2

• A numerical investigation of the Las Vegas
  convergence zone. Runk and Schreiber-Abshire, 8th
  Conf. on Meso. Proc., 1999
• Key role in the 9 July 1999 major flooding event
  in Las Vegas

26
Factors Influencing Convection Initiation and
Evolution

• Updraft Source
• Boundary-layer forcing
• Stability
• Changes in moisture, CIN
• Shear (Updraft Tilt)
• Lifting depth at boundary and boundary-relative
  motions

27
Shear Balance
From Weisman, after Rotunno et al., 1988

• No shear gust front moves away from storm
• Shear balances gust front optimal for long-lived
  systems

28
Steering Level Winds

• Deep, vertical updrafts
• Boundary motion storm motion
• Observations supported by
• Moncrieff and Miller,1976 and Weisman and
  Klemp,1986

Wilson and Megenhardt, 1997
29
Automating this Knowledge to Produce a Forecast

• Various Regional, Mesoscale and especially
  Storm-scale Modeling Efforts
• The Auto-nowcaster ? SCAN
• Local Mesonet Data (stability)
• High Resolution Satellite (Cu boundary
  information)
• Radar (Cu, boundary, and wind field information)
• High Resolution Model
• Numerous Algorithms

30
Storm Initiation Logic

• Prior to Storms
• No Convergence Line
• Fair Weather Cu
• Initiation
• Convergence Line Present
• LI lt 0
• Cu in Lifting Zone
• Deep Erect Updrafts
• (Depending on convergence strength, LL Shear,
  Boundary relative cell speed)

31
Storm Initiation Logic (cont.)

• Growth/Merger
• Boundary Motion Storm Motion
• Convergence Strong and Deep
• LI lt 0
• Dissipation
• Convergence Line Moving Away from Storms
• LI gt 0

32
Summary

• Convection Initiation is favored by
• 1) Strong, deep, low-level convergence
• 2) Convergence line moving into existing cumulus
  clouds
• 3) Deep vertical updrafts (moderate low-level
  shear)
• Storm Merger, Organization, and Longevity is
  favored by
• 1) The above for initiation and
• 2) Boundary relative steering flow near zero

33
Summary (cont.)

• Dissipation is favored by
• 1) Convergence line moving away from cell
• 2) Convergence line moving into stable region

34
The Bottom Line

• Low-level boundaries may and often do initiate
  convection, but
• Not all boundaries initiate storms
• Storms not uniform and continuous along
  boundaries
• Sensitivity of moist convection forced by
  boundary layer processes to low-level
  thermodynamic fields. Crook, MWR, 1996
• 1oC, 1 g/kg can make the difference!
• Thermodynamic variability within the convective
  boundary layer due to horizontal convective
  rolls. Weckwerth, Wilson and Wakimoto, MWR, 1996
• Natural variability and limits of observing
  systems are at least this magnitude!

35
This Science is Now Commonplace ?

• NORTHEAST AND NORTH CENTRAL CO AREA FORECAST
  DISCUSSION
• 258 PM MDT THU AUG 5 1999
• ltsnipgt
• CURRENTLY RADAR AND SATELLITE PICTURES ARE
  INDICATING A FEW THINGS. THERES A CONVERGENCE
  AREA GOING ACROSS WASHINGTON AND ELBERT
  COUNTIESWITH A WEAK BROAD CIRCULATION CENTERED
  IN MORGAN COUNTY. SOUTH-SOUTHEASTERLY FLOW ON THE
  INCREASE SOUTH OF THE LINE. THE CURRENT RADAR IS
  SHOWING A DECENT LINE OF STORMS DEVELOPING ALONG
  THE CONVERGENCE ZONE AND WEAKER SCATTERED RETURNS
  COVERING THE MOUNTAINS AND FOOTHILLS. THE LOWER
  CLOUD DECK IS ALSO DISSIPATING OVER A GOOD
  PORTION OF THE PLAINS WITH SUNSHINE ON THE
  INCREASE. EXPECT BOUNDARIES TO GET GOING WITH
  LIKELY TO WIDESPREAD PRECIPITATION A GOOD BET IN
  MOST AREAS.
• ltsnipgt

36
Boundaries and Convection Initiation Reference
List

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•  

37
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39
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40
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42
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44
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