The impact of variations of low-level structure associated with surface drag on intensification of simulated tornadoes

The low-level structure is critical to transformation of low-level vorticity, which is known to be the fundamental source of rotation in updraft, and hence impact the intensification of tornadoes. Multiple convergence boundaries have been observed and simulated within surface drag by recent studies, suggesting that tornado development can occur away from the main convergence boundary separating the outflow and ambient inflow. How these variations of low-level structure under effect of surface drag contribute to long lived tornadoes are not well understood. In addition, differences in tornado intensity across different environments may yield different sensitivities to the underlying surface roughness. In this study, two environmental soundings representative of high CAPE, shallow CIN and low cape, deep CIN, respectively, are used to initialize the idealized simulation with the Bryan Cloud Model (CM1), and simulation with different surface drag strength with semi-sip boundary conditions are performed. Results of the simulations show that the inclusion of surface drag substantially alters the low-level structure in several minutes, resulting in stronger vorticity near surface and greater curvature of the convergence boundary. With the increase of surface drag, the tornado develops away from the forward convergence boundary due to the stronger angular momentum of the vortex and weaker ambient inflow. The rotating updraft tends to tilt to the area with greater convergence. The increasing distance beyond certain threshold between the relative location of tornado and convergence boundary indicates enhanced separation of maximum vorticity and maximum positive vertical pressure gradient, resulting in stronger vorticity near surface but slightly transport upward. The trajectory analysis demonstrates that, in drag conditions, most parcels travel through the rear flank downdraft area and merge to the rotating updraft, which intensifies the tilting of horizontal vorticity in that area and form the secondary convergence boundary. The low CAPE and deeper level of CIN require more dynamical vertical pressure gradient force associated with the magnitude of convergence from the low-level boundary and hence are more sensitive to the increase of surface drag.