Do rocks, hills, or trees increase or decrease the wind speeds in a tornado?

Unfortunately, there is no clear answer. Both observations (of real tornadoes), computer simulations, and laboratory studies (in tornado vortex chambers) have shown that the "surface roughness", i.e., the measure of how disrupted the wind near the ground is by objects such as dirt, rocks, hills, trees, and even houses, can either increase or decrease the wind speeds in a tornado. How can trees increase the wind speeds? Well, the strongest winds in a tornado occur when air from outside the tornado can flow closest to the center of the vortex. The conservation of angular momentum, e.g., the rotation in the air, requires that as the air flows toward the center of the tornado (as it spirals in) its rotation must increase. Depending on the configuration of the airflow outside of the tornado, sometimes there is not ENOUGH "inflow" toward the center, and so blobs of air outside the tornado do not get very close to the center of rotation before they are lifted upward off the ground. In this ...  more