Diurnal Oscillations of Low-Level Jets, Vertical Motion, and Precipitation: A Model Case Study

Abstract

Heavy convection that occurred over Tulsa, Oklahoma, during the evening and night of 26/27 May 1984 is studied through a series of numerical experiments with a regional forecast model. The emphasis is on diurnal oscillations of the boundary-layer flow and the sensitivity of model predictions of nocturnal low-level jets to the presence or absence of cloud radiative interaction and latent heating. The nocturnal low-level jet appears to be established mainly by boundary-layer processes and to be significantly amplified by the latent heat of condensation, which increases during the night over the lower Great Plains. The presence of cloud radiative interactions appears to have less impact on the prediction of the low-level jet and nocturnal convection for the present case, but it is important for the timing of the nocturnal precipitation. The regional model underestimates the extreme precipitation rates observed over Tulsa. The possibility that this underestimation is a consequence of insufficient resolution is investigated by running a high-resolution, convective-scale cloud model initialized with and driven by the regional model forecast state. In the nested mode, the regional model with a grid size of 50 km and the convection model with a grid size of 0.6 km produce local rainfall totals on the order of 10 cm over time periods on the order of 2 h, in agreement with the heavy observed precipitation.