forecasting the maintenance of mesoscale convective systems crossing the appalachian mountains
DESCRIPTION
Forecasting the Maintenance of Mesoscale Convective Systems Crossing the Appalachian Mountains. Casey Letkewicz CSTAR Workshop October 28, 2010. 9 August 2000. 20 April 2000. Observational Study. 20 crossing and 20 noncrossing cases from Keighton et al. 2007 database - PowerPoint PPT PresentationTRANSCRIPT
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Forecasting the Maintenance of Mesoscale Convective Systems
Crossing the Appalachian Mountains
Casey LetkewiczCSTAR WorkshopOctober 28, 2010
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9 August 2000
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20 April 2000
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Observational Study
• 20 crossing and 20 noncrossing cases from Keighton et al. 2007 database
• Two observed soundings chosen for each case– One to represent upstream environment, one to
represent downstream environment– Soundings modified with surface conditions within
1 hour of MCS passage• Downstream environment discriminated
between crossing and noncrossing cases
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Observational Study• Key discriminatory parameters:– MUCAPE, combined with MUCIN
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Observational Study
• Key discriminatory parameters:– 0-3 and 0-6 km shear; 3-12 km mean wind speed– Mountain-perpendicular 0-3 km shear and 3-12
km wind speed• Crossing cases on average had weaker shear and mean
wind…why?
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Conceptual Model
Frame and Markowski (2006)
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Influence of Mean Wind
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Influence of Low-level Shear
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Questions
• Do changes to the wind profile alone result in a crosser or noncrosser?
• Is the influence of the wind profile greater in smaller CAPE (i.e. noncrossing) environments?
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Idealized Modeling
• CM1 model, version 1.14• ∆x, ∆y = 500 m; ∆z stretched from 150 m at
model surface to 500 m aloft • Gaussian-bell shaped barrier, 100 km wide and
1 km tall• Squall lines allowed to evolve and mature for
3 hours before reaching the barrier
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Experimental Design
SBCAPE = 1790 J/kgSBCIN = -20 J/kg
MUCAPE = 2290 J/kgMUCIN = 0 J/kg
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Experimental Design
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ControlW
ithou
t ter
rain
With
terr
ain
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Control--dry
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Mean Wind ExperimentsM
ean
win
d +5
m/s
Mea
n w
ind
-5 m
/s
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Shear Experiments
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Shear Experiments
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Wind Profile Experiments
• Conceptual model of Frame and Markowski (2006) upheld– The environmental hydraulic jump in the lee also
contributed to system redevelopment• Changes to the wind profile alone do not
discriminate crossing vs. noncrossing systems– What about a less favorable thermodynamic
environment?
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Thermodynamic Experiments
MUCAPE = 2290 J/kgMUCIN = 0 J/kg
SBCAPE = 825 J/kgSBCIN = -150 J/kg
Cool 6K
Cool 12KSBCAPE = 0 J/kgSBCIN = 0 J/kg
MUCAPE = 1370 J/kgMUCIN = -5 J/kg
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Lee Cooling
-Increasing the mean wind did not prevent system redevelopment in the lee
Still have ample MUCIN and small MUCIN!
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Thermodynamic Experiments
SBCAPE = 600 J/kgSBCIN = -20 J/kg
MUCAPE = 600 J/kgMUCIN = -20 J/kg
Drying to Observed RH
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Lee Drying
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Thermodynamic ExperimentsCooling, drying, midlevel warming
SBCAPE = 110 J/kgSBCIN = -720 J/kg
MUCAPE = 575 J/kgMUCIN = -100 J/kg
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Lee Cooling, Drying, Midlevel Warming
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Thermodynamic Experiments
• MUCAPE upheld as most important forecasting parameter, especially when combined with MUCIN
• Changes to wind profile have greater influence in low CAPE, high CIN environments
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Conclusions
• Greatest influence on MCS maintenance is the downstream thermodynamic environment – Especially MUCAPE and MUCIN
• Wind profile does not play a primary role in determining MCS maintenance over a barrier
• Wind profile exerts a stronger influence in low CAPE, high CIN environments
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Publications
• Letkewicz and Parker, 2010: Forecasting the maintenance of mesoscale convective systems crossing the Appalachian mountains. Wea. Forecasting, 25, 1179-1195.
• Modeling study submitted for publication in Monthly Weather Review
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Shear Experiments