On the Search for Dynamical Commonalities Among Southern California ‘Sundowner' Events: Simulation Studies of Several Events

The frequency of destructive wildfires in Southern California has led, over the past two decades, to a body of research on strong downslope windstorm events that has provided a great deal of useful information and improved forecasts of such phenomena and their role in extreme fire behavior. In particular, much has been learned about the now well-known Santa Ana wind environment, from its synoptic forcings down to individual meso-beta and meso-gamma scale internal gravity wave amplifications, hydraulic jumps and rotors. A less well understood wind event is the so-called ‘Sundowner' which is most commonly observed in the vicinity of Santa Barbara and the Santa Ynez Mountains. While a modest body of research exists on Sundowners, it is not yet sufficient to provide for robust forecasting of such events, which can include strong downslope winds and/or strong low-level warmings and dramatically increase fire danger in Santa Barbara and western Ventura counties. Our recent research analyzing surface observational data, presented in a companion paper, suggests the possibility that the current thinking on what constitutes a Sundowner may need to be revisited. However, the surface data and available upper air observations (mostly from profilers and RASS systems) are not sufficient to fully resolve this question, especially as regards whether all the events possess enough dynamical commonalities to be classified as the same type of event (Sundowner) or whether differing dynamics warrants a different classification.

To address this issue, as well as better understand the synoptic/mesoscale interactions involved in Sundowner events, in this paper we present an overview of results from high-resolution nested WRF simulations of several events, with somewhat different characteristics, that span the space of Sundowner events identified via the climatology presented in the companion paper. The goals of the simulations are severalfold: (1) to better understand how the larger synoptic and mesoscale flow dynamics interact with smaller scale processes driven, at least in part, by the presence and orientation of the Pacific Ocean and Santa Ynez Mountains; (2) to determine if there are sufficient dynamical commonalities to allow the construction of an operationally applicable forecast algorithm that could be use by the NWS Oxnard Forecast Office to forecast Sundowner wind events; (3) if the dynamic commonalities are weak, to determine if another way to classify/distinguish between the resulting event types exists that could also be useful for forecasters, and; (4) to determine if there are limits to our ability to simulate such events with current model/data assimilation systems, including how much value is added through data assimilation and more detailed treatment of the upper ocean than is usually done with short-range operational forecasts.

We conclude the presentation with our plans for future modeling work regarding the Sundowner wind phenomena.