R. H. SIMPSON AND ROGER A. PIELKE* Introduction whereas the extratropical wave cyclone has a broader more homogeneous swath of damaging winds. A small Economic losses due to hurricanes in the United States change in the hurricane track may cause a major shift currently averagesmore than $800 million a year! and in the coastal threat (see e.g., Brand and Blelloch, 1975). the trend over the last three decades (see Fig. 1 next Thus, one of the greatest challenges in hurricane fore- page) is exponentially upward? This reflects the en9r- casting is the prediction of landfall, and most of the mous increases in construction along the hurricane- forecast improvement resources have been devoted to vulnerable coastlines as Americans migrate to the sea- the problems of movement and landfall. shore. Over the same period the annual loss of life in Hurricane forecasting comprises three closely related hurricanes has steadily diminished. If this is a measure but (at present) independent prediction tasks, the failure of increasing effectiveness of the hurricane warning of anyone of which may vitiate the success of the service, it is also a frail statistic. There are at least three others. These tasks are: (1) the prediction of movement reasons: first is the increasing numbers of people direct- and landfall; (2) the simulation of the storm surge3 in- ly exposed to hurricanes, many of whom have never cluding inland and riverine flooding; and (3) the predic- experienced one (N. L. Frank, 1974); second, the near- tion of extreme winds and areal extent of damaging plateau reached some years ago in prediction skill winds. Of these three, the greatest progress has been (R. H. Simpson, 1973) which persists despite the prog- made in the simulation of storm surge profiles 0 eles- ress which continues to be made in understanding hur- nianski, 1968, 1972). The least progress has been made ricane structure and the energy transformations which in predicting development or change in circulation drive it (J. Malkus and H. Riehl, 1960; LaSeur and strength. In fact, the modeling of development has ~een Hawkins, 1963). Finally, the great killer hurricanes are so ponderous and the initial value problem so intract- rare and comprise not only a critical challenge to the able that nearly all prediction models designed for experience and wits of the forecaster (R. Simpson, operational use seek only to forecast movement. 1971), but also to the formulation of the dynamic In the sections which follow we shall trace the prediction models upon which he is ever more de- progress in developing prediction techniques primarily pendent each year; models which, because of a num- for movement and landfall. We will examine the reasons ber of limiting factors described in this paper, are better why forecast skills have plateaued and what roadblocks able to predict the run-of-the-mill hurricane than the must be removed before significant new improvements extreme event. can be expected. However, the related oceanographic The hurricane poses a more difficult problem of problems relating to the sea-air interface, and the hy- modeling and of warning than do storms of temperate drologic problems of flash and riverine flooding due to latitudes because most of the hurricane's destructive excessive rains are beyond the scope of this paper and forces are concentrated within 20-50 miles of its center, are treated only inferentially. *See end of article for authors' affiliations and addresses. 3 An abnormal rise in sea level due to the influence of hurricane 1 National Weather Service, 1975: Hurricane preparednessbtief- pressuregradients and wind stresseson the sea. ing documents (slides). Similar terms which are common to the vQcabulariesof fluid 2Por a summar) of the 1975 hurricane seasonwith an illustra- dynamicists and are essential to the discussionin this text are tion of storm tracks see Mon. Wea. Rev., April 1976, Vol. 104 defined in "Glossary of Meteorology," 1959: Amer. Met. Soc., (Dollar amounts are adjusted to 1957-59 values). Boston, Mass. 601 APPLIED MECHANICS REVIEWS (/) z 2000 ~ 2- 0 <( (/) ..J II: ..J W 0 a- 0 u. u. 0 0 U) II: Z w 0 ~aJ ..J :: 1 ~ 1000 aJ 1910 1930 1950 1970 1910 1930 1950 1970 FIG.1 TRENDS IN HURRICANE RELATED DAMAGE AND DEATHS IN THE U.S. (5-YEAR AVERAGES) The Conceptual Problem in Predicting Movement performance of the model with hurricanes which are Before the advent of dynamic prediction models, hur- rapidly intensifying as they approach the coast, as in ricane movement was regarded conceptually as a re- Celia (1970), Carmen (1974), Eloise (1975), and the sponse to a "steering current" in the environment, and Florida hurricane of September, 1947. In such cases the development of prediction methods sought to iden- the influence of the developing vortex on its own move- tify and relate the properties of a steering level or layer ment may turn out to be significant. If so, another to the direction and speed of movement of the hurricane means must be found to deal with the initial value center. For example, Norton (1948) used the prevailing problem if such sophisticated primitive equation models wind direction and speed "at the top of the hurricane" are to cope with anomalous movement of the "bad as an index to movement of the vortex. Dynamically, actor," high-risk hurricanes more effectively than the however, the hurricane moves as a complex response simpler barotropic or statistically founded models which to the net forces imposed by environmental circulations can, at best, provide results of uncertain value in such interacting with the entire vortex (Beebe and Simpson, cases. 1976). An unresolved problem is the degree to which The earliest conceptual model to provide objective the vortex circulation may influence its own movement. predictions of hurricane movement was developed by The evidence suggests that when a hurricane circu- H. Riehl, et al. (1956). Riehl considered that the best lation approaches steady state, its movement is almost index to steering of the hurricane was the geostrophic wholly determined by the dynamics of environmental flow of the environment at the level of nondivergence. circulations.4 Hovermale (1975), in 'his development of Zonal and meridional components of geostrophic mo- a new hurricane prediction model for use at the National tion were calculated for the 500 mb surface (-5.5 km) Meteorological Center draws upon this evidence to solve using a large rectangular grid which enclosed the vortex. the very difficult problem of initializing his model. He This was used as input to a regression relation based fIrst removes the vortex and replaces it with one which upon historic data to obtain the westward and north- ward displacements of the vortex for the succeeding is artificially spun up. While further details of this procedure will be dis- 24-hour period. The method worked surprisingly well cussed in a later section, we remark here that the jus- in a research environment but, operationally, suffered tification of this procedure will be severelytested in the from the subjectivity of hurried hand analyses of the 500 mb geopotential fields. During the late 1950's and early 1960's the search "NHEML Field Program Plans-1975 4e.g., NOAA, 1975: for methods less sensitive to subjective circulation {Hurricane)," p. 50. 602 3000' APPLIED MECHANICS REVIEWS analyses led to the use of statistical screening to iden- clons (Garstang and Betts, 1974; Garstang, 1975), which tify predictors from surface charts. Veigas, et al. (1959) imply that dynamic interactions between cumulus-scale and R. G. Miller (1958) used a large storm-centered circulations and those of much larger scale in the en- grid to screen for predictors of 24-hour displacements. vironment supply a brake to the growth of disturbances. This research produced an operational model known as The explosive development of disturbances, and the T-60 (Veigas, 1961, 1962) which, despite the fact that rapid growth of hurricanes into extreme events-present- it took no cognizance of upper-level circulations, dis- ly beyond the reach of operational models-comprise played good skill with westward-moving hurricanes, and serious problems in hurricane prediction and warning: blazed the trail for development of a heirarchy of the first, because the skills of most models are at lowest ebb more powerful models that followed. These included in cases where there are development extremes, and one by B. I. Miller and Paul Moore (1960); and the second, because significant changes in strength and size NHC-64 model by B. I. Miller and P. chase (1966). The of the hurricane strongly influence the distribution of latter attempted to incorporate the better features of storm surge and tidal flooding at landfall, and hence, the work of Riehl and of Veigas, and is the foundation the need for evacuating coastal residents. upon which the subsequent statistically founded meth- ods, described in the next section, have evolved. The ap- Progress with Statistically Founded Prediction Models plication of these models was primarily responsible for a significant increase in forecast skills at the National Statistically founded methods for predicting move- Hurricane Center in the early 1960's. (Dunn, et al., ment are of two types, one employing historical in- 1968). formation whose application depends upon current ob- The problem of predicting hurricane development, servations only as regards the location of storm center from tropical cyclogenesis to the extreme event with and of its instantaneous movement. The second uses winds up to 100 mps, remains one of the most difficult statistical analogs or screening techniques to obtain pre- and intractable problems in meteorology. As such, it is dictors related to circulation characteristics, usually usually treated operationally as a separate problem, over a scale much larger than the hurricane environment.