Tornadogenesis in a Simulated Mesovortex: The Role of Surface

FrictionAlex Schenkman

Co-authors: Ming Xue and Alan Shapiro

Experiment Design• A 100-m simulation is nested within two larger, lower

resolution grids. • On the two outer grids, radar and conventional

observations are assimilated via the Advanced Regional Prediction System (ARPS) 3DVAR and cloud analysis.

• The outermost grid (2-km horiz. spacing) is used to forecast overall MCS and line-end vortex development.

• A 400-m grid is nested within the 2-km grid and simulates mesovortices associated with the MCS.

• The 100-m grid forecasts detailed evolution of a long-lived mesovortex and associated tornado-like vortex.

• For more details on the experiment design see Schenkman et al. (2011a,b) and Schenkman et al. (JAS, in press).

Vorticity generated by friction

Stretching in rotor

Vorticity decreases as parcel enters updraft

Simulation without drag

• Experiment is run without drag to further verify frictionally generated vorticity is cause of rotor.

• Note: BCs and ICs still come from same 400-m simulation

• Should have a limited impact as most of the frictionally-generated vorticity developed during the 100-m simulation.

Conclusion

Surface drag in the model is playing a key role in the generation of the rotor, associated low-level updraft, and thus tornadogenesis in this case.

But how?

Mountain rotor comparison

• Doyle and Durran (2002) showed that rotors formed in the lee of mountains in simulations with surface drag turned on.

• They attribute rotor formation to boundary layer separation that occurs when the flow encounters the adverse PGF associated with the first lee-wave.

• To determine the dynamics behind the rotor formation we examine rotors that form in the lee of mountains.

Figure from Doyle and Durran (2007)

Mountain rotor comparison (cont’d)

• To better compare our studies we note the following similarities:– In both studies there is a strong jet, beneath

which there is a large horizontal vorticity maximum

– An adverse PGF causes boundary layer separation in both studies. Lee-wave in Doyle and Durran. Gust front in our case.

– Both studies are stably stratified.

Conceptual Model

Generality of Results?

Figure adapted from Dowell and Bluestein (1997)

Adapted from Frame and Markowski (2010)