Forecasting the Maintenance of Mesoscale Convective Systems
Crossing the Appalachian Mountains Casey Letkewicz CSTAR Workshop
October 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 windwhy?
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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
Mean Wind Experiments Mean wind +5 m/s Mean wind -5 m/s
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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?
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