Differentiating Sundowner from Santa Ana Wind Environments

Sundowner events, characterized by extreme warming in the Santa Barbara, California region, are important coastal phenomena as they often trigger local fires. The Sundowner creates a favorable fire environment, not by generating gusty surface wind flow but by producing extraordinary heating rates in excess of 10°C/hour for 2-3 hours thus reducing the relative humidity to dangerously low levels. It is not uncommon for surface temperatures to exceed 40°C during these events while just a few miles away surface temperatures are more than 20°C cooler. The National Weather Service often finds forecasting such features in advance to be a highly challenging task because so little is understood about how the larger scale environment sets up the Sundowner-induced local shallow downslope heating along the Central California coastal regions near Santa Barbara. This is different from Santa Ana winds in which the precursor meso-α/β scale environment is well understood.

Observations and numerical simulations are employed to differentiate the favorable meso-α/β scale environments for Sundowner versus Santa Ana wind environments. The results of the multi-scale analyses employing the North American Regional Reanalysis (NARR), radiosonde and surface observations as well as very high resolution numerical simulations with grid resolutions <<1 km indicate key features of the favorable Sundowner environment. The focal point of the very different meso- α/β scale precursors to Sundowner winds (as opposed to Santa Ana winds) are the following: 1) a massive west-southwesterly jet streak above 500 hPa with strong anticyclonic shear and curvature north of Santa Barbara resulting in widespread mid-tropospheric sinking motions, 2) substantial offshore ridging north-northwest of Santa Barbara at and below 950 hPa in response to the mid-tropospheric mass flux convergence, 3) a very strong jet at and below 950 hPa exhibiting strong ageostrophic backing between the lower and middle troposphere from the north-northwest parallel to the coast but perpendicular to the mountains between Vandenberg AFB and Santa Barbara, 4) an extraordinary inversion below 950 hPa (often exceeding 20°C/500 m) in the Vandenberg AFB sounding with near moist neutrality between the inversion maximum and the mid-troposphere, 5) a nearly moist neutral layer between the inversion and the mid-troposphere, and 6) very short waves in the vertical velocity field ~5-10 km in length below 900 hPa oriented nearly perpendicular to the coast with a descending node near Santa Barbara.

These conditions are shown to be favorable for the evolution of shallow mountain wave phenomena that produce excessive surface heating but only modest wind velocities during Sundowner events in highly isolated regions near Santa Barbara.