Thursday, December 16, 2010
Over the months, I get lots of questions about the various models local forecasters use...and considering that the weather is relatively benign right now....perhaps this is a good time to talk about them.
Carpenters have hammers and power saws, plumbers have wrenches, and meteorologists have computer models. Now these are not physical models of course, but rather simulation models.
The equations that describe the atmosphere are programmed on digital computers--that is the model. We start with a 3D description of the atmosphere based on observations, known at the initialization, and then the model can integrate the equations forward in time to provide the future state of the atmosphere in 3D. The equations are generally solved on a 3D array of grid points and the distance between them is generally termed resolution. A 12-km resolution model has the horizontal grid points separated by 12 km. These models are some of the most complicated constructs of our species...encompassing hundreds of thousands to millions of lines of code...and represent physical processes from the microscopic to the planetary. The most powerful computers of the planet are used to run the models. Be impressed, very impressed.
Now at a typical NWS weather service office, the forecasters have access to a dozen or more models. Models with different resolution, different physical descriptions, different ways of solving the equations, different approaches to initialization and run at different modeling centers. So a meteorologist is like a carpenter with a dozen or more hammers, each different, each better in some situations than others.
The National Weather Service runs two major models: The Global Forecast System Model (GFS), which runs over the entire globe at a resolution of roughly 35km, and NAM model (also called NMM-WRF) which runs at 12-km resolution over North America and adjacent oceanic areas. The best global model is NOT American--a terrible and unnecessary embarrassment for our nation--- and is run by the European Center for Medium Range Weather Forecasting. The UK Met Office has an excellent global model as well. The Navy runs a global model (NOGAPS) that is second tier, but useful, and has a very good regional model called COAMPS.
The research community mainly uses a model called WRF (Weather Research and Forecasting Model) developed at the National Center for Atmospheric Research. This is a state-of-the-art high resolution weather prediction system with all the bells and whistles. At the UW we run WRF operationally at 36, 12, 4, and 1.3 km resolution and our colleagues at the National Weather Service use it heavily. This is the model output you see most frequently on this blog
One of the most important tasks of an operational forecaster is to look at this palette of model forecasts and make a decision of what they think will be the most probable evolution of the atmosphere. Which models have been good lately? Which has done best in the past for similar situations? And the forecast can use the variability of the model forecasts to reach conclusions of the possible future evolutions and which are most likely to occur.
And there is MUCH, MUCH more. The model output can often be improved by statistical post-processing. For example, if a model is typically too cool at certain hours, you could alter the output with the average error. And I haven't even talked about the newest prediction technology....ensemble prediction....in which we run the model tens or hundreds of times with slightly different initializations and physics to get probabilities of what will happen in the future.
This is a highly sophisticated technology--and I am highly optimistic for future improvements. And forecast HAS gotten much better over the past decades.
So next time you are thinking about making fun of meteorologists or laugh at the classic jokes (you guys get paid for forecasting 50%!, meteorologists have a pair of dice in the backroom, I wish I could get paid for being wrong all the time, etc), bite your lip!
Posted by Cliff Mass at 6:05 PM