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Simulating Supercell Thunderstorms in a Horizontally-Heterogeneous
Convective Boundary Layer
Christopher Nowotarski, Paul Markowski, Yvette Richardson
Pennsylvania State University
George BryanNational Center for Atmospheric Research
25th Severe Local Storms ConferenceDenver, CO
September 14, 2010
Motivation
• Previous 3D numerical simulations of supercells and tornadoes generally use horizontally homogeneous environments and exclude surface fluxes.
• Observations and simulations suggest that supercells are favored in areas with significant vertical wind shear and instability (CAPE).
• Convective Boundary Layers (CBLs) are characterized by local variations in these quantities.
• Consequently, supercells simulated in such environments may behave differently than in a homogeneous environment.
Background- Mesoscale variations in low-level vertical
wind shear and moisture “profoundly influence the morphology of deep convective storms” (Richardson et al. 2007, Richardson 1999).
- Storms became more organized and stronger when moving to areas of increased shear.
- Storms propagated towards areas of increased low-level moisture in weak shear regimes
- Isolated supercells in areas of increased moisture showed both higher updraft speeds and stronger low-level rotation.
- Tornadoes tend to be more likely in environments with higher 0-1 km shear and lower lifting condensation levels (Markowski and Richardson 2009)
Background - HCRs- Boundary layer convection is a source of heterogeneity
in the atmosphere.
- Because of low-level shear requirements, supercell environments are likely characterized by rolls or disorganized convection.
- Variations in thermodynamic quantities result from increased convergence in updraft branches. (Weckwerth 1996)
-Potential temperature can be 0.5 K higher in updraft
-Mixing ratio is 1.5 – 2.5 g kg-1 higher in updraft
- Increased instability, lower LCLs, and favored regions for cloud formation in updraft branches.
- Roll axes tends to be aligned with mean CBL wind (Weckwerth 1999)
- Organized boundary layer convection results in local, periodic variations in low-level vertical wind shear.
- Maxima tend to be in areas without strong magnitudes of vertical velocity.
(from: Weckwerth 1996)
(from: Markowski and Richardson 2007)
Background
24 May 2008(from: NOAA comprehensive Large Array-Data Stewardship System)
Experiment Design
• Two high resolution simulations of supercells
• One simulation allows a convective boundary layer to develop before initializing deep convection.
• The other is a “typical” horizontally-homogeneous simulation.
• Compare simulations, focusing on behavior and structure of mature storms (rather than initiation).
Model Configuration• Model
– CM1, version 1, release 14 (with modifications)– dt = 0.75 s, 0.125 s for acoustic calculations– periodic lateral boundary conditions
• Grid – dimensions: 200 km x 150 km x 18 km– dx, dy = 200 m– dz = stretched from 50 m (below 3 km) to 500 m (above 9.5 km)
• Parameterizations– Ice microphysics (Lin et al. 1983)– land surface scheme using two-layer soil model (Noilhan and Planton 1989) CBL run only– Radiation (REFERENCE!!!!) CBL run only
What about mixing out shear?• No large-scale horizontal temperature
gradient or Coriolis force, so vertical wind shear is mixed out in CBL.
• To maintain requisite shear for HCRs and supercells, we need to artificially relax the horizontal winds at low-levels towards the initial state.
• Add constant velocity tendency to each gridpoint on a vertical level that nudges the average wind at that height towards the initial average.
• Some reduction in shear is still allowed!
• Modification of technique applied by Robe and Emmanuel (2001)
+
Base States
• CBL simulation– 35 m s-1 0-6 km shear– SBCAPE: 2819 J kg-1
– Small capping inversion to prevent widespread convection
• Homogeneous simulation– Average of CBL simulation after one hour
of simulation time– Low level moisture and temperature
have increased (CAPE too)– Some reduction in low-level shear from
mixing
Results• Simulated CBL (before storm initiation)
Results
Results
Results
Results
Summary
Future Work