A simple analytical model including both diurnal thermal forcing over sloping terrain (the Holton mechanism) and diurnally varying boundary-layer friction (the Blackadar mechanism) is developed to account for the observed amplitude and phase of the LLJ over the Great Plains and to understand better the role of each mechanism. The present model indicates that for the pure Holton mechanism (time-independent friction coefficient) the maximum southerly wind speed occurs (depending on the assumed friction coefficient) between 1800 and 2300 LST (Local Standard Time) which is earlier than the observed maximum (~ 0100 LST). For the pure Blackadar mechanism (time-independent thermal forcing), the present model shows that the maximum southerly wind speed generally occurs later (~ 0300 LST) than observed and has a strong latitudinal dependence. For both mechanisms combined, the present model indicates that the maximum southerly wind speed occurs near the observed time which lies between the time obtained in the pure Holton mechanism and the time obtained in the pure Blackadar mechanism; furthermore the maximum southerly wind speed is larger (and closer to that observed) than in each one considered individually. The amplitude and phase of the LLJ as a function of latitude can be obtained by the combined model by allowing for the observed latitude-dependent mean and diurnally varying thermal forcing.
A Simple Analytical Model of the Nocturnal Low-level Jet over the Great Plains