Southern California ‘Sundowner' Events: Simulation Studies and Conceptual Models

The frequency of destructive wildfires in Southern California has led, over the past two decades, to a body of research on Santa Ana windstorm events that has provided a conceptual model and improved forecasts of Santa Ana event impacts on fire behavior.  In particular, much has been learned about the  synoptic forcings and mesoscale internal gravity wave amplifications, hydraulic jumps and rotors common to strong Santa Ana events.

Less well understood are so-called ‘Sundowner’ events in the vicinity of Santa Barbara and the Santa Ynez Mountains.  The modest body of research exists on Sundowners has not yet been sufficient for robust forecasting of such events, which can include strong downslope winds and/or strong low-level warmings that dramatically increase fire danger in Santa Barbara and Ventura counties.   Our recent research incorporating limited observational data and modeling studies suggested that, like Santa Ana  events, some degree of internal gravity wave (IGW) dynamics need to be considered as intrinsic elements in stronger Sundowner events.  However, open questions remain, including: 1) the degree to which regional, as opposed to local, transports at low- to mid-tropospheric levels condition the upstream stability profile such that strong and nonlinear IGW dynamics are in play: 2) the dynamics and propagation of offshore lower tropospheric eddies during certain Sundowner events, and the degree to which these eddies modulate the behavior of the marine layer and thus the evolution of Sundowners along the Santa Barbara county coast; (3) whether differences in the environment leading to trapped versus  propagating IGWs are significant in Sundowner evolution; and (4) what is the nature of a tendency of some Sundowners to propagate in a general west-east direction along the Santa Ynez range, versus others that do not propagate in this fashion.

To address this issue, as well as better understand the synoptic/mesoscale interactions involved in Sundowner events, in this paper we present an extension of previous WRF simulations of several Sundowner events, focusing on the aspects above as well as sensitivity of the simulations to varying the initial conditions, physical parameterization schemes,  and other aspects of the model configuration (including upper ocean initialization and treatment).   We will endeavor to perform simulations of recent events that can be better validated with the advent of a new NOAA wind profiler installation at Santa Barbara airport.  Finally, as a means of evaluating the predictability of these Sundowner events using current (and future) model/data-assimilation systems, sensitivity of the simulations to data assimilation approach will also be briefly discussed. 

We conclude the presentation with preliminary conceptual models of Sundowners based on our results, as well as a brief discussion of how these conceptual models might be utilized in a forecast algorithm for use by NWS Oxnard forecasters.