Discussion 4/17

Small-scale windsForecasting & Models

Small scale winds

• Land & sea breezes– Lake breeze

• Mountain & valley breezes– Katabatic wind /

Boulder windstorm• Downdraft/Heatburst

• Chinook (“snow eater”)• The Santa Ana Winds• Derecho– Bow echo

Fig. 12-9, p. 360

Number of derecho storms occurring from 1994 to 2003

WEATHER FORECASTING

• FOLKLORE• PERSISTENCE– The weather tomorrow will be the same as the weather

today (two times out of three)

• CLIMATOLOGY– This takes persistence one step further– The average weather say for a particular month is the same

each year

• MODELING

TREND AND ANALOG

• We know that persistence forecasts will eventually be wrong because weather does change.

• A trend forecast assumes that the weather-causing patterns are themselves unchanging in speed, size, intensity, and direction of movement (this is called steady-state).– For instance: we know that an approaching cyclone will bring rain

(weather does change) but assume that the amount of rain or its speed will not change along the path the cyclone will travel.

• The analog forecast also acknowledges that weather changes, but unlike the trend method, it assumes that weather patterns can evolve with time.– The main assumption is that weather repeats itself.– Therefore, this method “searches” for past weather patterns that are

similar (analog) to the current situation. – In this sense, the future weather patterns “should” be similar to those

that happened in the past.

NUMERICAL WEATHER PREDICTION

• Step One: Weather Observations• Step Two: Data Assimilation• Step Three: Forecast Model Integration• Step Four: Tweaking and Broadcasting

Data Assimilation

Fig. 13.9

Numerical Weather Prediction Models

• North America Model (NAM)• Global Forecast System (GFS)• Computational power / nested grids• Errors• Ensemble forecasts