Spatial and Temporal Mechanisms Controlling Convection Over The Great Plains

Abstract

The central United States exhibits an anomalous summertime diurnal cycle of precipitation, characterized by an afternoon maximum over the Rocky Mountains that transitions to a nocturnal maximum over the Great Plains. This phenomenon, which remains a challenge for many numerical models, is governed by the interaction of processes spanning multiple scales. This thesis investigates the spatial, temporal, and dynamical mechanisms controlling this diurnal cycle through a comprehensive, climatologically-grounded multi-year analysis of satellite-derived precipitation data Integrated Multi-satellitE Retrievals for GPM (IMERG) and hourly meteorological analyses Rapid Refresh and Rapid Update Cycle (RAP/RUC).

The research is presented in three parts. First, the spatial variation in the synoptic structure of convective systems is examined. The analysis reveals a distinct transition in dominant forcing mechanisms with distance from the mountains: convection in the “Near Plains” (west of 100◦W) is significantly influenced by mountain-initiated solenoidal circulations, while convection in the “Far Plains” is more closely associated with the dynamics of the Great Plains Low-Level Jet (GPLLJ).

Second, the thesis investigates the diurnal evolution of vertical profiles of convection. A systematic diurnal shift from surface-based to elevated convection is identified, which consistently occurs as the nocturnal boundary layer stabilizes. This shift is linked to a threshold in the low-level lapse rate of approximately -4 to -5 K/km, providing a quantifiable metric for the influence of boundary layer thermodynamics on the convective mode.

Finally, the thesis examines the climatological eastward propagation of rainfall. The analysis demonstrates that the diurnal, clockwise rotation of the GPLLJ’s wind vector drives a propagating pattern of low-level mass convergence across the plains. This mechanism is modulated by topographically-induced suppression of afternoon convection via enhanced convective inhibition (CIN), enabling the nocturnal, dynamicallydriven rainfall maximum to dominate.