CFSR/GFS based growing degree days (GDD)
Introduction
The Climate Forecast System Reanalysis (CFSR) and the Global Forecast System (GFS)
creates analyses of many fields including the 2 meter temperature. The CFSR/GFS analyses
were used to create the growing degree days. The growing degree days is a factor
in the plant and pest development.
Celcius and Fahrenheit GDD
For corn, a commonly used GDD uses a minimum temperature (Tlower or Tbase)
of 50F and a maximum temperature (Tupper or Tmax) of 86F. Now
50F and 86F corresponds to 10C and 30C. If you use the 10_30 model
and multiply the results by 9/5, you'll get the GDD from 50F-86F model
in Fahrenheit.
Calculation
- For the CFSR, find the average daily 2m temperature (6 hour forecast)
- For the GFS, find the average daily 2m temperature by averging the daily Tmax and Tmin
- Convert the data from Kelvin to Celsius.
- Calculate the growing degree (crop dependent): GD = min(Tmax,max(Tbase,T)) - Tbase
where Tbase and Tmax depend on the crop model. The formula is the same as:
- if (T < Tbase) GD = 0
- if (Tbase < T and T < Tmax) GD = T - Tbase
- if (Tmax < T) GD = Tmax - Tbase
- For the grib files, the GDD is accumulated from January 1. To get the GDD for June 15 starting
from March 15, you would take the June 15th GDD and subtract the GDD for March 14.
- The CFSR climatology GDD is the average GDD from 1979-2008.
Models
The models are in Celcius and the first number is Tbase and the second number is Tmax.
- 5.5_30: (5.5C/30C) wheat, barley, rye, oats, flaxseed, lettuce, asparagus
- 10_30: (10C/30C) same as the 50F-86F model, corn, sorghum, rice, soybeans, tomato, Black cutworm, European Corn Borer
Details and Caveats
The CFSR uses a system very similar to those run by the National Weather Service to
produce the 1-6 day weather forecasts that you see on TV. (Other countries run
similar systems for their 1-6 day forecasts.) As part of producing a weather forecast,
an analysis of the current atmospheric state is created. This analysis is made
using data from satellites, aircraft, weather ballons and other sources but surprising no
2-meter temperature observations. The 2-m temperature is model derived product which
depends on the land-surface model, the model physics, the model parameterizations and the
analyses of the winds, temperatures and humidity at the model levels. There
are many reasons why the analyses will differ from observations. For example,
you make your temperature observation on a patch of grass and the model is using
a forest. (The land surface is suppose to be representative of the average conditions
in a 35x35 km grid box.) Yes, grass vs. forest can be a big difference. Another
problem is model uses the average height of the grid box. Your measurement will
be at a different elevation. An other problem is the model doesn't know the
local orography. You could be on an exposed peak or in a sheltered valley.
These features are not resolved within a 35-km grid used by the CFSR model.
One way to view the CFSR temperature is they are a model estimate of average
temperature of a 35km x 35km grid box. It will not give you the same
number as an observation.
Links
wikipedia
plots and data
Comments
Calculations with different Tbase/Tmax can be accomodated.
However, you will need to provide references for selecting the
particular Tbase/Tmax values.
Please send comments to: Wesley.Ebisuzaki@noaa.gov
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