Hurricane Heat

1) The methodology is trained on the Atlantic. It has no parameters to allow for different structures or size of storms, and there is no good reason why it should work well on storms in other basins. Given the different land-sea configurations and the different role of ENSO in the different basins, and the fact that disturbances in other basins do not form from easterly waves from off of Africa, there is every reason to expect that storms in other basins have different characteristics. For instance, there is greater activity in the Pacific Northwest, and the tropopause is higher in the western Pacific, and this affects brightness temperatures at tops of clouds. If the size of storms differs then the fixed form of EOFs will not be able to capture that form. The analysis must be able to account for differences among basins in order to have confidence in variability or trends. It would be easy enough to test whether the storms in other basins had different characteristics by also performing an EOF analysis for each region. This basic test was not done. It should be.

2) The results are suggestive of these problems. In the SIO where the method gives 0, 1 or 2 storms vs up to 6 in the best track data, there is a serious bias. Similar large biases exist in the SPAC (up to 2 vs 5 in best track). Obviously the threshold is effectively different and it is a comparison of apples and oranges.

3) In addition, this version of the paper deals with PDI. The earlier version of the paper dealt with intensity of storms and that was abandoned because the results were not very good. In particular, the presumption is that the older results were the problem because operational methods have improved. But the Kossin et al results showed bigger and greatest discrepancies with those from best track in recent years: there is no convergence over time. This is harder to see with PDI, because the biggest storms are emphasized, but the question of why is there not good agreement in recent years is not answered.

3. Dr. Judith Curry:

The most vexing thing about the tropical cyclone data sets is the uncertainty that analyst subjectivity contributes to this. The Dvorak scheme for determining tropical cyclone intensity is notoriously subjective, see the recent BAMS article on this. The importance of what Jim Kossin has done is to take this subjectivity out of the analysis.

Kossin’s method matches well the historical data in the North Atlantic (NATL) and East Pacific (EPAC). The method was trained using North Atlantic data, and the East Pacific regime in terms of dynamical and thermodynamical conditions is very close to the North Atlantic conditions. However, Kossin’s method diverges substantially from the established data sets in the Western Pacific, South Pacific, and Indian Oceans. Does this mean that the established data sets or in error, or that Kossin’s method (trained in the Atlantic) does not translate well to the other ocean regions?

Owing to problems with dealing with historical satellite data, Kossin’s study was extended only back to 1983 (the period for which the satellite data are well calibrated), and it is almost certain that this data set cannot be extended back prior to 1977. By itself, this data set is too short to say anything about a trend in intensity. But it can in principle be used to assess uncertainties in the established data sets.

My own analysis of the discrepancies has focused on the Western Pacific (WPAC) data, where 40% of the global tropical cyclones form. During the period 1983-1987, Kossin’s data overlaps with aircraft reconnaissance data; I would expect the WPAC TC data during this period to be of comparable quality to that in the NATL and WPAC, but large discrepancies are seen. I would also expect the agreement to be better in more recent years with the advent of more sophisticated satellite systems, but the discrepancies are largest during the most recent period. Its would not be surprising for this method trained in the NATL not to work well in the other regions. The method does not allow for different structures or sizes of storms in different basins. The NATL cyclones form primarily from easterly waves, while those in other regions do not. The role of ENSO is different in the different basins. The tropopause height is higher in the WPAC. etc.

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