2b. Spectral Models
Spectral Models: Alternative Computing
For the record, the GFS is not a grid-point model. Rather, it's a spectral model, meaning that the GFS describes the present and future states of the atmosphere using mathematical equations whose graphical solutions look like undulating ocean waves. Such an approach may seem odd, but remember that the atmosphere is indeed an ocean of air that ripples with waves of pressure, temperature, wind, and humidity. To support this viewpoint, you need only to glance below at the GOES-11 water-vapor image of the eastern Pacific basin (full size) to observe a set of atmospheric waves.
Thus, one advantage that spectral models have is that they are more attuned with the wave-like motions in the real atmosphere. As a result of their more natural mathematical fit, spectral models save computational time. Given these savings, spectral models better lend themselves to longer-range forecasts than grid-point models like the WRF, which push supercomputers to their limits just to churn out a respectable three-day forecast. In fact, the GFS is routinely run out to 384 hours (16 days) four times a day (initialized at 0Z, 6Z, 12Z and 18Z), though the number of “waves” that the model can resolve is reduced beyond the 180-hour forecast period to save computational time.
If you ever visit the Penn State Weather Station, you’ll find copies of computer forecasts from several computer models on the new electronic “map wall" (here's a photograph taken in 2009). Meteorologists refer to these weather charts as progs, which is short for prognostications (see image below for an example). At the map wall, forecasters engage in spirited discussions, debating the merits and deficiencies of the various computer guidance. They know all too well that computer forecasts can be, at times, far from reality. The consensus of forecasters, especially those with the most experience, consistently provides the mold for the most accurate forecast.
So how do meteorologists defend against the inherent errors associated with computer guidance? Let’s start with the specifics of the offensive game plan of computer models.