Observations of wakes from individual wind turbines and a multi-megawatt wind energy installation in the Midwestern US indicate that directly downstream of a turbine (at a distance of 190 m, or 2.4 rotor diameters (D)), there is a clear impact on wind speed and turbulence intensity (TI) throughout the rotor swept area. However, at a downwind distance of 2.1 km (26 D downstream of the closest wind turbine) the wake of the whole wind farm is not evident. There is no significant reduction of hub-height wind speed or increase in TI especially during daytime. Thus, in high turbulence regimes even very large wind installations may have only a modest impact on downstream flow fields. No impact is observable in daytime vertical potential temperature gradients at downwind distances of >2 km, but at night the presence of the wind farm does significantly decrease the vertical gradients of potential temperature (though the profile remains stably stratified), largely by increasing the temperature at 2 m.

A large eddy simulation (LES) model and the Advanced Regional Prediction System (ARPS) model, which does not resolve turbulent eddies, are used to study the effect of a slope angle decrease on the structure of katabatic slope flows. For a simple, uniform angle slope, simulations from both models produce turbulence kinetic energy and momentum budgets that are in good overall agreement. Simulations of a compound angle slope are compared to a uniform angle slope to demonstrate how a changing slope angle can strongly affect the strength of katabatic flows. Both ARPS and the LES model show that slopes with a steep upper slope followed by a shallower lower slope (concave shape) generate a rapid acceleration on the upper slope followed by a transition to a slower evolving structure characterized by an elevated jet over the lower slope. In contrast, the case with uniform slope (having the same total height change) yields a more uniform flow profile with stronger winds at the slope bottom. Higher average slope in the uniform slope angle case generates greater gravitational potential energy, which is converted to kinetic energy at the bottom of the slope. Analysis of the total energy budget of slope flows indicates a consistent structure where potential energy generated at the top of the slope is transported downslope and converted into kinetic energy near the slope base.

Sundowners are downslope windstorms that occur over the southern slopes of the east–west-trending Santa Ynez range in Santa Barbara County, California. In the past, many extreme fires in the area, including the Painted Cave, Montecito Tea, Jesusita, and Sherpa fires, have occurred during sundowner events. A high-resolution 11-yr dynamically downscaled climatology was produced using a numerical weather prediction model in order to elucidate the general dynamical characteristics of sundowners. The downscaled climatology is validated with observations during the 2016 Sherpa fire. A sundowner index (SI) is computed from the climatology that quantifies the magnitude of adiabatic warming and northerly (downslope) wind component during sundowner events. The SI allows for the classification of historical events into categories of various strengths. The primary characteristics of strong sundowners from this classification include 1) internal gravity wave breaking over the Santa Ynez range, 2) initiation in the western Santa Ynez range with eastward progression over the course of a day, 3) a maximum likelihood of occurrence in April and May near 2000 Pacific standard time, and 4) a limited downstream extent for most events, such that the long-term historical weather station, Santa Barbara airport, often does not experience moderate events. Analysis of an operational forecast rubric composed of the surface pressure difference from Bakersfield to Santa Barbara indicates that this rubric is not skillful. However, offshore pressure gradients are skillful and are related to the strong northwesterly alongshore jet. The findings presented herein can be used to provide guidance for fire weather forecasts and support resource allocation during fire suppression efforts.

Diablo winds are dry and gusty north-northeasterly downslope windstorms that affect the San Francisco Bay Area in Northern California. On the evening of 8 October 2017, Diablo winds contributed to the ignitions and rapid spread of the "Northern California firestorm", including the Tubbs Fire, which burned 2800 homes in Santa Rosa, resulting in 22 fatalities and $1.2 B USD in damages. We analyzed 18 years of data from a network of surface meteorological stations and showed that Diablo winds tend to occur overnight through early morning in fall, winter and spring. In addition to the area north of the San Francisco Bay Area, conditions similar to Diablo winds commonly occur in the western Sierra Nevada. Both of these areas are characterized by high wind speeds and low relative humidity, but they neither tend to be warmer than climatology nor have a higher gust factor, or ratio of wind gusts to mean wind speeds, than climatology.

Abstract. Downslope Sundowner winds in southern California's Santa Ynez Mountains favor wildfire growth. To explore differences between Sundowners and Santa Ana winds (SAWs), we use surface observations from 1979 to 2014 to develop a climatology of extreme Sundowner days. The climatology was compared to an existing SAW index from 1979 to 2012. Sundowner (SAW) occurrence peaks in late spring (winter). SAWs demonstrate amplified 500 hPa geopotential heights over western North America and anomalous positive inland mean sea-level pressures. Sundowner-only conditions display zonal 500 hPa flow and negative inland sea-level pressure anomalies. A low-level northerly coastal jet is present during Sundowners but not SAWs.