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 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)

Influence of Mean Wind

Influence of Low-level Shear

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

Experimental Design

Control Without terrain With terrain

Control--dry

Mean Wind Experiments Mean wind +5 m/s Mean wind -5 m/s

Shear Experiments

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 = 2290 J/kg MUCIN = 0 J/kg SBCAPE = 825 J/kg SBCIN = -150 J/kg Cool 6K 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 SBCAPE = 600 J/kg SBCIN = -20 J/kg MUCAPE = 600 J/kg MUCIN = -20 J/kg Drying to Observed RH

Lee Drying

Thermodynamic Experiments Cooling, drying, midlevel warming SBCAPE = 110 J/kg SBCIN = -720 J/kg MUCAPE = 575 J/kg MUCIN = -100 J/kg

Lee Cooling, Drying, Midlevel Warming

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, Modeling study submitted for publication in Monthly Weather Review

Shear Experiments