School of Meteorology (Defense)

The Structure, Evolution, and Dynamics of a Nocturnal Convective System Simulated Using the WRF-ARW Model

Benjamin Blake

School of Meteorology

03 December 2015, 10:00 AM

National Weather Center, Room 4140
120 David L. Boren Blvd.
University of Oklahoma
Norman, OK

Previous studies have documented a nocturnal maximum in thunderstorm frequency across the central United States. Forecast skill for nocturnal convection remains relatively low, and one reason is the difficulty in accurately simulating storm systems associated with the greater occurrence of elevated convection at night. This study utilizes the WRF-ARW model Version 3.6.1 to simulate a nocturnal mesoscale convective system event that occurred over the Southern Great Plains on 3-4 June 2013. The purpose of this study is to advance the knowledge of the dynamics, structure, and evolution of nocturnal convection through examining the structure of the simulated storm from the perspective of several different dynamical frameworks.

The structure and evolution of the storm was influenced by a strong horizontal gradient in CAPE and CIN corresponding to a narrow corridor of high mixing ratios associated with the low-level jet. These CAPE values in the jet exceeded the magnitude of CAPE observed in the daytime boundary layer. During the night, the source of convectively unstable parcels was almost entirely above 1 km, and positive buoyancy was released within the storm above ~4 km. This positive buoyancy could only be realized through the interaction of the inflow with a cold pool that became stronger aloft than at the surface. Significant variation in the depth and structure of the ascent was found around the cold pool. The reasons for this variation are examined via three frameworks: i) The RKW framework for shear/cold pool interactions; ii) The Weisman framework for the generation of internal circulations within the system resulting from horizontal buoyancy gradients; iii) The dynamical framework of wave theory (e.g. Froude number, Scorer parameter). The application of these theories allowed insight into the three-dimensional structure of the convective system and provided a possible explanation for how convection south of a front is maintained at night in the presence of a low-level jet. Convective feedbacks associated with bores/waves with leaky ducts, weak surface cold pools, and advection by the low-level jet are likely responsible for nocturnal convection south of the front.

School of Meteorology (Defense) Seminar Series website