A Boundary Layer Interpretation of the Low-level Jet

The inertial boundary layer theory developed by Charney and Morgan to explain the intensification of currents near the western boundary of an ocean is applied to the northward flowing atmospheric jet stream observed in the lowest two kilometers over the central United States; here the mountainous spine of Central America serves as a barrier to the westward flowing trade-wind current and deflects it to the north. Because of diurnal frictional changes caused by surface heating and cooling, the jet undergoes a marked diurnal variation with a noctural maximum so strong that the current becomes super-geostrophic. The dynamic consequences of the unbalanced current are examined in the light of theories by Tepper and Veronis.