Diablo–North wind events are associated with below-normal temperatures east of the Sierra Nevada, with rapid warming of the air as it subsides into coastal California. The coincident development of a coastal sea level pressure trough leads to a large pressure gradient over the Sierra Nevada and Northern California. Before the onset of these downslope wind events, there is substantial amplification of upper-level ridging over the eastern Pacific, with sea level pressure increasing first over the Pacific Northwest and then over the Intermountain West. For the Bay Area sites, there is a maximum frequency during the early morning hours and a large decline midday, while the Sierra Nevada locations have a maximum frequency approximately three hours earlier. There is little trend in event frequency during the past two decades over either region. The Sierra Nevada locations have their minimum frequency from February to August, and a maximum from October to January. For the Bay Area, there is a frequency minimum during midsummer, a maximum in October, and a declining trend from November to June.
These events, sometimes called Diablo or North winds, are more frequent over the Bay Area and nearby coastal terrain than along the western slopes of the Sierra Nevada, where the highest frequency occurs over the midslopes of the barrier. Strong, dry downslope winds over Northern and central California have played a critical role in regional wildfires. The 925-hPa relative humidity anomaly from climatology (% color shading), geopotential heights (m solid lines) contoured every 15 m, and 925-hPa wind anomalies (arrows). 10, but for mean sea level pressure, contoured every 2 hPa.Įvolution of 850-hPa geopotential heights (m) contoured every 15 m, 850-hPa wind anomalies from climatology (arrows), and 850-hPa temperature anomalies (☌ shading). Geopotential height anomalies from climatology (m) are color shaded.Īs in Fig. Black contours are geopotential height in meters, contoured every 60 m.
The 500-hPa geopotential height (m) composite evolution for (top) Bay Area and (bottom) Sierra Nevada events. Graphics based on NCEP–NCAR reanalysis grids and produced using the online facilities of NOAA/ESRL.ĭiurnal variability of Diablo–North wind event frequency (solid lines) and maximum wind speeds (dotted lines) for the (top) Bay Area and (bottom) Sierra Nevada regions. Sea level pressure anomalies (hPa) from climatology (1980–2010) for (top) 2011 and (bottom) 2016 over September–December. The annual totals are the sum of the number of events for all stations in the relevant regions. (top) Annual frequency of Diablo–North wind events and (bottom) maximum and mean wind speeds during such events for the Bay Area and Sierra Nevada regions. The maximum wind speeds for events in both regions are also shown. Seasonal variability of the frequency of Diablo–North wind events for the (top) Bay Area and (bottom) Sierra Nevada regions. The number of Diablo–North wind events by elevation (m) for the (top) Bay Area and (bottom) Sierra Nevada locations shown in Fig. Maximum wind gusts (kt) over the central coastal California for the period from 0000 to 1200 PDT. The total number of Diablo–North wind events for the ASOS (squares) and RAWS (circles) locations for the Bay Area and Sierra Nevada regions for – are shown using color shading. Surface stations used in this study as well as terrain elevation. Vertical black lines indicate the wind directions that define Diablo–North winds in this study for each region. (bottom) The histograms, binned by 10° intervals, of the frequency of Diablo–North wind events as a function of wind direction. (top) Scatter diagrams of the number of 3-h periods with maximum hourly winds greater than 25 kt and hourly relative humidity less than 20% for the (left) Bay Area and (right) Sierra Nevada locations. The observations are color coded by relative humidity.
The black line indicates the wind speed threshold of 12.9 m s −1 (25 kt) used in this analysis. Observed hourly maximum wind speed (m s −1) and wind direction (°) for the (top) Bay Area and (bottom) western Sierra Nevada observing sites shown in Fig.