Section 1: INTRODUCTION

Tropical cyclone (TC) intensity is a measure of the maximum wind speed associated with a TC. From an operational point-of-view, the accuracies of intensity and track forecasts are equally important to the users during the entire life cycle of a TC. Intensity can be defined as the near-surface maximum sustained wind speed (1 or 10 minute mean speed) around the TC circulation center, or as the minimum surface pressure at the TC pressure center. In essence, TC intensity is a part of TC structure.

Forecasts of TC intensity are considerably more difficult than forecasts of TC motion. In the relative sense, atmospheric processes responsible for the major portion of TC motion are rather obvious, not overly complex, and of large scale. In contrast, those governing TC intensity changes are not so obvious, very complex, and are of multiple scales. Confounding the issue is that the initial intensity and intensity trend are often not observed with a sufficient degree of precision. In general the nature of intensity forecasting is more probabilistic than that of motion forecasting.

As a consequence, intensity forecasts are highly subjective and not very skillful when compared to forecasts based on simple climatology and persistence. Numerical and statistical models have had reasonable success with motion forecasts, but not with intensity forecasts. Indeed, up until very recently, those working with models dealing with intensity changes have been discouraged with their inability to improve over forecasts based on climatology and persistence. For example, Merrill (1987), attempting to use synoptic predictors in a statistical intensity prediction model concludes, "These results...would seem to indicate that prediction of intensity change using classical statistical treatment of basic climatology and persistence and existing synoptic data or analyses has reached a dead end..."

More recently, some limited success in the prediction of TC intensity change by both statistical and numerical models has been achieved. However, for the most part, skill in the prediction of motion is much greater that that of predicting intensity. As will be shown, intensity prediction errors can be rather high.

Operational forecasters should recall that one of the most significant decisions required by operational commanders is whether or not to sortie ships that are threatened by TCs. If the threat turns out to be a false alarm, a decision to evacuate is very expensive. On the other hand, a decision not to evacuate, should the threat materialize, could be even more expensive.

Tropical cyclone motion and intensity forecasts and associated errors are closely related. From an operational commander's point of-view, a reasonably correct forecast of TC motion could be completely offset by an incorrect intensity forecast or vice versa. In addition to these meteorological factors, much depends on the type of ship, the degree of readiness, the protection afforded by the port, etc. Thus, the question of whether or not to evacuate is a complex issue and is based both on meteorological and non-meteorological factors.

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Chapter 6

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