The Markowski and Richardson toy simulations revisited: what is the effect of surface drag?

In a recent simulation study of the 8 May 2003 tornadic supercell, Schenkman et al. (2014) concluded that surface drag played an important role in the development of the tornado. It has long been known that surface drag can intensify vortices by preventing cyclostrophic balance and promoting radial inflow, thereby promoting the convergence of angular momentum toward the axis of rotation. However, Schenkman et al.'s analysis suggests that surface drag might have been a crucial source of vorticity for tornadogenesis. In this paper, we resort to the highly idealized ("toy") numerical simulations of Markowski and Richardson (2014) in an attempt to further investigate the role of surface drag on the development of near-surface vertical vorticity.