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Solar Energy Forecasting Using Numerical Weather Prediction (NWP) Models
Patrick Mathiesen, Sanyo Fellow, UCSDJan Kleissl, UCSD
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Solar Radiation Reaching the SurfaceIncoming solar radiation can be reflected, absorbed, or transmitted to the surface
Kiehl and Trenberth, 1997
Incoming Solar Radiation- Date/Time
Geometry Effects- Location- Date/Time
- Incoming Angle
Atmospheric Effects- Trace gases- Aerosols- Water vapor
Cloud Effects- Liquid water content- Composition
GHI = Usable solar energy at surface
Diffi
culty
of P
redi
cting
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Surface GHI is Highly VariableLarge variability in surface GHI is undesirable for utility scale applications
Atmospheric Conditions- Static intra-hour
conditions- Effect on surface GHI is
consistent and well documented
Cloud Conditions- Dynamic weather
conditions change on short time scales
- Difficult to predict- Clouds are the largest
contributors to GHI variability
Cloud Events
Clear Skies
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Solar Forecasting Methods are Designed to Predict Cloud Dynamics
Solar forecasting mitigates high GHI variability – increasing efficiency of PV plants
Clear Sky Models– Function of date, time, and
location only– Accurate for sunny conditions
Persistence Modeling– Assumes conditions remain static
for hours or more
Cloud Detection and Forecasting– Local sky imagery (minutes ahead)– Satellite imagery (hours ahead)
Numerical Weather Prediction– Intra-hour to days ahead
prediction of large scale weather patterns
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Local Sky Imagery:Cloud Motion Detection
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Numerical Solar ForecastingNumerical Weather Prediction (NWP) simulations predict weather patterns essential for
determining surface radiation
• Conservation of mass, energy, and water equations numerically solved – Prognostic Variables: Temp, pressure,
water mixing ratio, etc.
• Radiative models calculate surface GHI– Parameters: Water Vapor, ozone, trace
gases, aerosols, cloud parameterizations
• Models Analyzed: North American Mesoscale Model (NAM), Global Forecasting System (GFS), and European Centre for Medium Range Weather Forecasts (ECMWF)– NAM/GFS are freely distributed
weather forecasts
Wm-2
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NWP GHI Outputs are Biased!GHI forecasts using NWP are expected to be significantly biased
• NWP models are not designed for accurate solar radiation forecasting– Radiation used only to drive
surface energy balance– Temporal variability
unimportant
• Spatial discretizations are too coarse to resolve intermittent cloud dynamics– Only general cloud
properties can be parameterized
Wm-2
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Using MOS to Correct NWP BiasNWP biases are consistent and predictable
Error Processing• Compare to ground truth data
• SURFRAD network• Calculate mean bias error (MBE)• Establish MBE as a function of forecast
parameters• MBE profile may reveal information about
under which conditions is the NWP scheme biased
Model Output Statistics (MOS) Correction• Establish a correction function for MBE in
terms of prognostic forecast variables• Prescribed an expected MBE for future
forecasts• Subtract expected bias from base forecast
More on the method: Lorenz et. al., 2009
parametersFBIASHISTORICAL
parametersGBIASEXPECTED
EXPECTEDFORECASTIMPROVED BIASGHIGHI
ObservedGHIBIAS FORECAST
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Example: Bias as a function of clear sky indexNWP model positively biased for clear skies and negatively biased for cloudy
conditions
NWP Model predicts too much GHI
NWP Model predicts too little GHI
CLEAR CONDITIONSCLOUDY CONDITIONS
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Bias Correction for a Single Forecast (W m-2)Based on clearness index, the basis NWP forecast can be improved
W m-2W m-2
200 Wm-2 - -80 Wm-2 = GHIIMPROVED
ORIGINALFORECAST
EXPECTED BIAS
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Prediction interval applicationMOS correction improves forecast
Base Forecast Corrected Forecast
MOS CORRECTION
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ConclusionsNWP Models as a GHI Forecast
– Inherently biased– Cloud parameterization is a likely
source of error
Simple MOS Correction– MOS correction reduces average
MBE by nearly 40 W m-2 for most conditions
Next Steps– Application of MOS to prognostic
variables (such as liquid water content) to evaluate accuracy of cloud parameterization models
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THANK YOU!
Acknowledgements: Sanyo Corporation
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Where is the source of this error?
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MBE as a Function of Measured Clear Sky Index (ktm)For true clear conditions (ktm > .9) the radiative model is unbiased
NAM GFS
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Two-Dimensional MOS (Wm-2)MBE significantly reduced (~100 W m-2) in target areas
Average MBE = 41.9 Wm-2 Average MBE = -2.9 Wm-2
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RMSE (Wm-2) varies with measured clear sky index (ktm)In general, the ECMWF provides the best forecast
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RMSE Improved Forecasts