Forecasting the Maintenance of Mesoscale Convective Systems

Crossing the Appalachian Mountains

Casey LetkewiczCSTAR WorkshopOctober 28, 2010

9 August 2000

20 April 2000

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)

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/kgSBCIN = -20 J/kg

MUCAPE = 2290 J/kgMUCIN = 0 J/kg

Experimental Design

ControlW

ithou

t ter

rain

With

terr

ain

Control--dry

Mean Wind ExperimentsM

ean

win

d +5

m/s

Mea

n w

ind

-5 m

/s

Shear Experiments

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/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

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/kgSBCIN = -20 J/kg

MUCAPE = 600 J/kgMUCIN = -20 J/kg

Drying to Observed RH

Lee Drying

Thermodynamic ExperimentsCooling, drying, midlevel warming

SBCAPE = 110 J/kgSBCIN = -720 J/kg

MUCAPE = 575 J/kgMUCIN = -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, 1179-1195.

• Modeling study submitted for publication in Monthly Weather Review

Shear Experiments