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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
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Forecasting the Maintenance of Mesoscale Convective Systems Crossing the Appalachian Mountains Casey Letkewicz CSTAR Workshop October 28, 2010
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
Observational Study • Key discriminatory parameters: • MUCAPE, combined with MUCIN
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?
Conceptual Model Frame and Markowski (2006)
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?
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
Experimental Design SBCAPE = 1790 J/kg SBCIN = -20 J/kg MUCAPE = 2290 J/kg MUCIN = 0 J/kg
Control Without terrain With terrain
Mean Wind Experiments Mean wind +5 m/s Mean wind -5 m/s
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 Cool 6K SBCAPE = 825 J/kg SBCIN = -150 J/kg MUCAPE = 2290 J/kg MUCIN = 0 J/kg Cool 12K SBCAPE = 0 J/kg SBCIN = 0 J/kg MUCAPE = 1370 J/kg MUCIN = -5 J/kg
Lee Cooling -Increasing the mean wind did not prevent system redevelopment in the lee Still have ample MUCIN and small MUCIN!
Thermodynamic Experiments Drying to Observed RH SBCAPE = 600 J/kg SBCIN = -20 J/kg MUCAPE = 600 J/kg MUCIN = -20 J/kg
Thermodynamic Experiments Cooling, drying, midlevel warming SBCAPE = 110 J/kg SBCIN = -720 J/kg MUCAPE = 575 J/kg MUCIN = -100 J/kg
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
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
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