The research project seeks to gain a better understanding of the mechanisms that cause Sundowner wind events in Southern California. Sundowner winds are typically associated with extreme heating in the absence of very strong surface wind velocities. Sundowner winds cause conditions favorable for devastating Southern California wildfires making forecasting them critically important and extraordinarily difficult due to their fast and very fine scale onset. The basic hypothesis to be tested spans a multiplicity of scales of motion: At larger scales (meso-alpha/beta) Sundowner wind events are associated with: 1) a low-level coastal jet driven by an upstream coastal pressure gradient accompanying a strong low-level inversion as well as 2) a shallow critical layer and shallow Scorer Parameter discontinuity along the coastal mountains (e.g., Santa Ynez). This low-level coastal jet results from: 1) an inland extension of the thermal gradient between a cold near shore marine boundary layer and an elevated inland, heated convective boundary layer and 2) a reinforcement of coastal high surface pressure (to the NNW of this thermal gradient) by the right exit region of an upstream polar jet streak. At the smaller scales (meso-gamma), wave development involving an internal gravity wave (IGW) breaking develops along the westward-facing slopes of the coastal (Santa Ynez) Mountains, triggering a short-lived rotor above the marine layer. The rotor amplifies and breaks, mixing warmer and drier air rapidly into the marine layer. This mixing represents the first component of rapid Sundowner warming as the cold marine layer is dissipated and replaced by relatively warm air from the residual continental boundary layer. The second component occurs quickly thereafter as the trailing warm pool accompanying the IGW descends the coastal mountains forming a meso-gamma scale wake low resulting in 10-25C° total warming.
The methods employed in this research activity primarily involve numerical modeling, observational data analyses and theoretical analyses of several real and idealized data case studies. The Weather Research and Forecasting (WRF) model will be employed to simulate numerous Sundowner wind case studies ranging from extreme to marginal to null case studies. The events will be classified based on observed surface wind gust and heating intensities and the numerical modeling will involve a complete spectrum of cases. The horizontal grids will range in resolution from ~10 km regionally to ~666 m in the specific locations of Sundowner wind genesis depending on terrain gradient shape and magnitude thus allowing the scale contraction of simulated fields.
The intellectual merit of this research activity is to understand the multi-scale mechanisms causing Sundowner wind events and their sensitivity to synoptic and multiple mesoscale dynamics involving both the marine layer and very fine scale terrain structure. In particular, more detailed understanding will be derived of how larger scale jet adjustments establish the mesoscale wind and temperature gradients that can be perturbed by terrain to generate meso-gamma scale mountain waves, bores and wake lows. Furthermore, how the marine layer, mountain wave, and larger scale jets interact in the Sundowner wind that occurs in the fire-prone regions near Santa Barbara, California.
The broader impacts of this research activity will involve weather forecasting advancements in this region, most notably predicting conditions that result in extreme wildfire danger. The inability of operational computer models to simulate the finer scale characteristics of Sundowner winds makes it much more difficult for the National Weather Service (NWS), private forecasters and local emergency responders to warn/prepare for fire events. The public is extremely vulnerable in this region and improved forecasts derived from the knowledge gained by this research may prevent the loss of life and property by educating forecasters and emergency responders concerning key precursor conditions. In addition, student interns from underrepresented communities will collaborate on this project at the NWS Forecast Office in Oxnard, California.
