Not All Chaos on the Weather Map is Equal

Not all chaos on the weather map is equal, researchers have found, providing insights that are hoped to improve weather forecasting. Researchers usually assume that all spots on a weather map are equally chaotic, meaning that small uncertainties in initial conditions grow to the point at which the conditions become unpredictable. Now, a multidisciplinary University of Maryland team of meteorologists, physicists, and computer scientists (DJ Patil, 301-405-4842, dpatil@ipst.umd.edu) has developed a technique that identifies what can be considered as chaos "hotspots," regions in which small changes in conditions are believed to magnify most quickly into large perturbations in the weather.

Chaos hotspots shift their location on a regular basis, but tend to cover only about 20% of the global map at any given time. Making more meteorological observations in hotspots can help reduce forecasting errors, the researchers believe. Since 1992, the National Weather Service has provided "ensemble forecasts," in which a computer model generates a main forecast and several slightly adjusted forecasts providing a range of possible outcomes for the weather. The Maryland researchers look at global wind predictions from five of these forecasts at a particular level in the atmosphere (where the pressure is 500 millibars).

Placing these five forecasts on the map, the researchers then look at wind vectors, which specify how each forecast deviates from the main forecast in wind strength and direction. Analyzing 1100 km-by-1100 km squares in a global map, they identify regions where the vectors tend to line up with one another (see figure at Physics News Graphics).

The aligned wind vectors have "low dimensionality," transforming the regions in which they reside into chaos hotspots where good initial observations become most crucial for reducing forecasting errors. All other points on the map are less important for forecasting, the authors say. (Patil et al., Phys. Rev. Lett., 25 June 2001; text at Physics News Select.)