The lodging of crops by tornadoes

It is well known that tornadoes passing over fields can cause significant damage to crops, and tornado tracks of fallen, or lodged, crops can extend for many hundreds of metres. An examination of photographic evidence of such events suggests that, at least for low speed EF0/EF1 events, lodging occurs beneath tornadoes primarily due to a strong radial flow (rather than circumferential flow) at the canopy surface. In order to investigate this effect further, a simple model of a tornado has been developed which, whilst fully satisfying the three dimensional Euler equations, models a circumferential flow at the edge of the tornado boundary layer near the ground, which becomes a radial flow as the ground is approached. This model is then used in a generalised model of lodging to predict lodging track widths and crop fall directions. It is shown that, when expressed in a suitably normalised form, both lodging width and crop fall direction are functions of a normalised translational velocity and a normalised crop lodging velocity. The lodging patterns are of two forms - a forward convergence (FC) where the cropfall converges on the tornado track in a forward direction, and a backward convergence (BC) where the convergence is in the opposite direction to tornado translations. Regions of FC and BC in the normalised parameter plane are calculated. These patterns are very similar to those observed in the field, which gives some confidence in the nature of the model. The model is then used to investigate the sensitivity of lodging width to crop and tornado parameters, and also to carry out a risk analysis to determine the probability distributions of lodging width for specified distributions of crop and tornado parameters.