In recent years, operational regional numerical models designed for tropical cyclone applications (e.g., the Japanese Typhoon Model, and the model developed by the Geophysical Fluid Dynamics Laboratory, Princeton) have become capable of simulating tropical cyclones with realistic structure and large magnitudes of intensity change that approach those which can occur in nature. Intensity predictions from real-time model runs are now routinely distributed to tropical cyclone warning centers. The level of skill of these intensity predictions (and of those made by the warning agencies) is difficult to evaluate since there are few simple baselines against which to compare them.
Many specific problems reside under the umbrella of tropical cyclone intensity forecasting. At present, the forecaster is required to produce intensity forecasts in short (e.g., 12 hour) increments for periods of up to 72 hours. Additional forecast challenges remain largely unexplored such as the determination of the peak intensity and the timing of the peak intensity from a given stage in the development of the tropical cyclone. Contained in the widely used Dvorak techniques is a basic rule that an intensifying tropical cyclone intensifies at an average rate of one T number per day. This corresponds to an increase of wind speed of approximately 20 kt per day. Dvorak also found that westward moving tropical cyclones require a longer time to peak than those which move northwestward, which, in turn, take longer to peak than northward moving tropical cyclones.
A simple technique, developed by Mundell (a former typhoon forecaster at the Joint Typhoon Warning Center, Guam) provides guidance to the questions of tropical cyclone peak intensity and the timing of the peak intensity. Mundell found that the time to peak intensity as measured from certain intensity thresholds such as minimal tropical storm intensity (35 kt) and minimum typhoon intensity (65 kt) was a strong function of the latitude at which these thresholds were reached. Further, the magnitude of the peak intensity was also a function of the latitude of occurrence of certain benchmark intensities (e.g., the lower the latitude of achieving minimal typhoon intensity, the higher the peak and the longer the time delay until peak).
In this paper, the results of a test of Mundell's techniques are presented using the statistics of the typhoons of 1995-97 in the western North Pacific. The techniques appear to work well (e.g., up to 50% of the variance in the time-delay to peak is explained only by the latitude of acquisition of threshold intensities). Being so simple, and with a demonstrated level of skill, it is proposed that Mundell's techniques be used as a benchmark against which to evaluate other intensity forecasting schemes. Further, it is hoped that biases and conditions under which deviations from the intensity forecasts yielded by Mundell's techniques can be discovered so as to be used by forecasters to gain further advantage.