The United States Great Plains Diurnal ABL Variation and the Nocturnal Low-Level Jet

Abstract

Observations made from an Oklahoma City TV tower are first used to test the one- and two-dimensional versions of a numerical mesoscale model. Both model versions produce a fairly good reproduction of the observed typical diurnal variation in the wind, temperature, and turbulent mixing coefficient profiles. Next, the models are used to study slope winds and the nocturnal low-level jet (NLLJ) formation, starting with the Prandtl slope wind theory conditions and eventually adding more complex physics (e.g., relating to slope angle, mixing, radiation, background flow, and heat input conditions). The models are able to reproduce the Prandtl analytical solution in the appropriate simple conditions. The time scale to reach this solution from an initial rest state is, however, very long for gently sloping plains. Results with the full model indicate that the main mechanism for the Great Plains ?average? NLLJ is the inertial oscillation due to frictional decoupling after sunset. Under normal conditions the sloping of the plains has only a small guiding effect. Direct radiative solar heating and longwave cooling of the atmosphere also have only a small effect, even at nighttime, as mechanical turbulent mixing is efficient during typical NLLJ flow.