Meteorology 5540
Mini-Lecture 11: Surface Wind Forecasting

I. Factors to consider when making a surface wind forecast

  • Climatology: Understand predominant winds and typical diurnal effects on wind direction and speed
  • Pressure gradients on synoptic, meso, and local scales
  • Static stability (influences vertical momentum mixing)
  • Interaction of synoptic scale flow with local terrain and orography (influences mesoscale pressure gradient, wind speed, and wind direction)
  • Surface roughness
  • Convection (e.g., outflow, microbursts, etc...)
  • Critical levels (important for downslope winds)

    II. Forecasting wind speed and direction over uniform orography

  • Best approach is to infer winds from model forecast analyses and knowledge of local orographic and diurnal effects
  • Wind speed is "somewhat" proportional to the pressure gradient, with modifications due to static stability and other effects.
  • Low static stability generally means more momentum mixing, stronger sustained winds, and stronger gusts. High static stability means less momentum mixing; in some cases the surface layer can be decoupled from the flow aloft.
  • Surface wind direction is generally cross isobar, toward lower pressure, and 25 degrees from geostrophic over land and 10 degrees from geostrophic over water.

    III. Forecasting wind speed and direction over complex orography

  • Diurnal circulations dominate during periods of weak synoptic forcing
  • Think in terms of the superposition of the synoptic flow with orographically driven mesoscale and diurnal circulations.
  • Flow tends to parallel large terrain features and mesoscale gaps and often appears to be highly ageostrophic when compared to a synoptic scale analysis. Exceptions include downslope winds.
  • Add mesoscale information to coarser winds from model output.
    Updated May 27, 1997