The important role dispersive Rossby waves play in the propagation of large scale forecast errors has been well understood since the forties of the last century. The first quantitative estimate of the speed, at which the effects of initial condition errors propagate in a numerical weather prediction (Charney 1949), was based on estimating the largest possible group velocity of Rossby waves. While Charney’s estimate of the propagation speed was quite accurate, his theory, based on the assumption of a barotropic atmosphere, could not be used to estimate the growth rate of the propagating errors in a region of enhanced baroclinicity. Norm Philips (1976) was the first to study the effect of baroclinic growth on the eastward propagating forecast errors. He used a two-layer quasi-geostrophic model to show that an adequate observational network over the Pacific could reduce forecast errors over North America by as much as 30%.

Norm Philips, truely understanding the importance of Rossby wave trains in large scale prediction, devoted an entire WMO monograph to the topic of “Dispersion Processes in Large-Scale Weather Prediction” (Phillips 1990). With the notable exception of Anders Persson (2000), this monograph was seemingly forgotten by the numerical weather prediction community. This situation has changed at the turn of the millenium, when the forecast results of the Winter Storm Reconnaissance field programs (Szunyogh et al. 2000 and 2002) led to the rediscovery of the importance of dispersion processes. Based on our experience, accumulated during the targeted observations programs, we believe that improving our understanding of the dispersion processes in complex atmospheric flows, is our best hope to improvef orecasts in the 3-10 days forecast range. Norm Philips’ monograph is a must-read for anybody, who considers conducting predictability research or planning to designe new observing techniques in such future field programs as THORPEX.

References:

Charney, J. G., 1949: On a physical basis for numerical prediction of large-scale motions in the atmosphere. J. Meteor., 6, 371-385.

Persson, A., 2000: Synoptic-dynamic diagnosis of medium range weather forecast systems. Proc. Seminar on Diagnosis of Models and Data Assimilation Systems, Reading, United Kingdom, ECMWF, 123-137.

Phillips, N. A., 1976: The impact of synoptic observing and analysis systems on flow pattern forecasts. Bull. Amer. Meteor. Soc., 57, 1225-1240.

Phillips, N. A., 1990: Dispersion Processes in Large-Scale Weather Prediction. WMO-No. 700, 126 pp.

Szunyogh, I., Z. Toth, R. E. Morss, S. J. Majumdar, B. J. Etherton, C. H. Bishop, 2000: The effect of targeted dropsonde observations during the 1999 Winter Storm Reconnaissance program. Mon. Wea. Rev., 130, 1144-1165.

Szunyogh, I., Z. Toth, A. V. Zimin, S. J. Majumdar, A. Persson, 2002: Propagation pf the effect of targeted observations: The 2000 WinterStorm Reconnaissance program. Mon. Wea. Rev., 130, 1144-1165.