Why are severe weather and anomalous climate events often associated with the orthogonal convergence of airflows? Weather and Climate Extremes

Severe weather often occurs in the areas where different cold-warm and dry-wet air masses converge orthogonally. The orthogonal convergence of two-adjacent air parcels can be observed from planetary-scale to synoptic-scale and meso-scale circulations. This study pointed out that an orthogonal convergence of two-adjacent horizontal air parcels can produce a significant non-zero shear stress with its modulus as anomalous energy density and form vertical motions of new airflows, while tailgating and head-on horizontal convergences cannot. Therefore, the orthogonal convergence can induce strong vertical motions and result in severe weather. Continuous convergence of airflows can lead to unusual climatic events. Storm cases of single tornado, tornado swarms, a strong tropical cyclone, and extratropical cyclones associated with heavy rainfall or heavy airborne dust show that the shear stress modulus and total kinetic energy of anomalous winds can better explain extreme weather events than other commonly used dynamical parameters such as divergence and vorticity. Therefore, the shear stress modulus can be used in the forecasting of extreme weather events in operation.