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Master's Defense

Department of Atmospheric Science

Tues., Apr. 17, 3:10 pm, EN6085

Linking Convective Outflow Boundaries in the Nocturnal Boundary Layer to MCS Maintenance

Coltin Grasmick

University of Wyoming

Abstract

Nocturnal Mesoscale Convective Systems (MCSs) are a frequent occurrence during the warm season in the Great Plains. Not only are they responsible for a nighttime maximum in precipitation in this region but they occasionally produce high impact weather such as flooding, severe wind, hail, and tornadoes. The forecast accuracy for nocturnal MCSs is extremely low, even at short timescales (e.g., 1 day), and even for convection-permitting models, due to complexities in predicting convection initiation and MCS maintenance. Different from the well-mixed and turbulent daytime boundary layer, the nocturnal environment experiences the development of a stable boundary layer (SBL) and (at least in the Great Plains) a low level jet (LLJ). These two features interact with MCS outflow, influencing new cell growth a process that is essential for MCS forward propagation and maintenance.

In this project, observations of the July 15, 2015 MCS in Kansas during the Plains Elevated Convection At Night (PECAN) campaign are used to investigate how the vertical structure of the low-level environment and that of outflow boundaries affect new cell growth and thus MCS evolution. The MCS formed in the evening and its outflow produced atmospheric density currents that evolved into bores in some regions of the MCS, especially later in its life, after midnight. Airborne lidar & AERI data reveal different parcel vertical displacements for both types of boundaries. It is shown that, on account of the SBL and the LLJ, the environment in which the MCS cold pool spread became suitable for the formation and maintenance of bores, and that bore lifting was essential for convection initiation. In other words, without bores the MCS would have been smaller and shorter-lived.


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