Multi-scale numerical simulations of the synoptic environment, Diablo windstorm, and wildfire formation mechanisms for the Tubbs Fire (2017)

Abstract

The Advanced Research Weather Research and Forecasting (WRF-ARW) model was used to simulate the downscale evolving atmospheric dynamical processes conducive to the intensification and propagation of the Tubbs Fire (2017). This wildfire impacted Napa and Sonoma Counties, California, spreading quickly and erratically through complex mountainous terrain due in large part to downslope Diablo Winds. The Tubbs Fire spread over 36,000 acres and destroyed 5,636 structures, killing 22. The simulations and supporting observations during the pre-Diablo Wind period indicate a well-defined inverted surface trough in Northern California’s Central Valley, along with a strong amplifying trough in the mid-troposphere and attendant cold frontogenesis over the Sierra Nevada. Mid-upper tropospheric jet streak flow, along with simulated and observed soundings from Reno, Nevada, indicate a mid-upper tropospheric jet indirect, exit-region descending, secondary circulation in conjunction with lower mid-tropospheric cold air advection caused by the southwestward low-level jet under the upper level jet’s entrance region. These adjustments enabled the organization of a deepening and ascending inversion over the Sierra Nevada, as well as a self-induced wave critical layer between 850 and 700 hPa prior to Diablo Wind formation. As the organizing jet streak departed, the discontinuously stratified atmosphere over the Sierra Nevada and coastal mountains in Northern California provided a favorable environment for mountain wave amplification. Intensifying leeside sinking motion coupled with wave steepening resulted in strong downslope winds in Northern California. Upward propagating mountain waves are present coinciding with the steepening of the isentropic surfaces consistent with the resonant interaction of nonlinear gravity waves. The model also simulated the development of a hydraulic jump in the lower troposphere on the lee side of the mountain range during Diablo Wind development. The simulation and observations indicate that the favorable environment for Diablo Winds resulted from the baroclinic jet-front system propagating over the Sierra Nevada when it produced a highly discontinuously stratified atmosphere favorable for nonlinear mountain wave amplification. However, the main surge of momentum down the leeside is only indirectly coupled with the jet streak’s exit region, being the result of cold frontogenesis, which allows for vertically differential cold air advection and its attendant discontinuously stratified vertical atmospheric structure.