High-Resolution Multi-Scale Simulations of the 2010 Los Angeles/Southern California Heat Burst Event

An extraordinary mesoscale ‘heat burst' event that occurred during the afternoon of 27 September 2010 set several temperature records within the Los Angeles Basin. The event was unusual not only due to the extreme temperatures, but also for the relatively early point in the diurnal cycle (before maximum solar heating) that the extreme heat was manifested in the Basin as well the fact that the heat extended essentially all the way to the Pacific coastline with minimal modification. In a companion paper, we provided an analysis of the event using available observational datasets and discussed the likelihood that multiscale interactions, leading to downward transport of heat over the San Gabriel Range, played a key role in the event. However, even with the relatively high density of observed data in the Los Angeles area, the available data are not sufficient to fully understand the nature of the interactions and downward transport, not to the mention the roles of sea breeze and mountain-plains solenoidal circulations in the evolution of the event. In this paper we extend the observational analysis with multiscale WRF-ARW simulations that can fully resolve meso-gamma and upper micro-alpha scale processes. The goals of the simulations are severalfold: (1) to better understand how the larger mesoscale flow dynamics interact with smaller scale processes largely driven by the presence and orientation of the Pacific Ocean and San Gabriel, San Bernadino and Sierra Pelona Mountain Ranges; (2) to determine an operationally applicable forecast algorithm that could be use by the NWS Oxnard Forecast Office to discriminate such extreme heating events from lower amplitude heat waves as well as Santa Ana and Sundowner wind events, and; (3) to determine if there are limits to our ability to simulate such events with current model/data assimilation systems, particularly the timing of the event, and how much value is added through data assimilation.

In this presentation we will provide an overview of our simulation methodology, briefly discuss points relevant to goal (3) above (including model validation) but focus primarily on elucidating what the simulations add to the understanding of the heat burst event gained from our observational analysis, especially as pertains to meso-gamma scale phenomena forced in part or in toto by the local terrain. We conclude the presentation with our plans for future work on a forecast algorithm for such extreme events.