Perhaps the key issue in Emanuel 2005 (as observed in Landsea’s 2005 comment) was the adjustment to the ATL data set performed by Emanuel (citing an old publication, Landsea 1993). We’ve snickered a bit recently at Martin Juckes using older data rather than newer data – only in climate science would this seem possible. And we see another such incident in Holland and Webster – this time with wind speed methodology – where they use a method of Landsea 1993 rather than the recommendations of Landsea 2005.
Holland and Webster stated:
We use the “best-track” tropical cyclone data base from the National Hurricane Center (Jarvinan et al. 1984). The only changes to the data set data have been to include the intensity corrections recommended by Landsea (1993).
Sounds innocent enough. But underneath this is the raging debate between Emanuel and Landsea about adjustment of wind speeds in the Hurdat database. I reviewed Landsea 1993 and Landsea 2005 here which, upon re-reading, stands up as a pretty reasonable representation of the debate. As noted in my earlier post, Landsea 2005 stated:
It is now understood to be physically reasonable that the intensity of hurricanes in the 1970s through to the early 1990s was underestimated, rather than the 1940s and 1960s being overestimated. To examine changes in intensity over time, it is therefore better to use the original hurricane database than to apply a general adjustment to the data in an attempt to make it homogeneous.
Despite this explicit statement from Landsea, Holland and Webster apply the method of Landsea 1993 without even citing Landsea 2005. Whether Landea 2005 is right or wrong, this adjustment is a topic that any competent data analyst needs to deal with before analyzing trends; however, Holland and Webster simply ignore the issue.
I noticed this matter when I experimented with a plot of the average length of a ATL cyclone and the average length of an ATL hurricane (which I examined in the course of examining the supposed constancy of proportion of hurricane-days to cyclone-days.) The average length of an ATL hurricane declines in the late 1960s, coinciding with the issue raised in Landsea 1993. Holland Webster presumably reduce wind speeds prior to 1970, but don’t provide any details on the adjustment other than what I’ve quoted. Landsea 1993 is specific to the aircraft reconnaissance period – but one would need to check whether H and W adjusted wind speeds for earlier periods as well.
The early years with high hurricane-days mentioned previously – 1886, 1887, 1933 – were not done with aircraft reconnaissance and would not, on the face of it, be subject to Landsea 1993 adjustments in any event. But H and W would need to be checked somehow to see if they adjusted data prior to the aircraft reconnaissance period as well. The results that I’ve presented so far have been based on the methodology recommended in Landsea 2005 – using the original Hurdat data – and will therefore not reconcile precisely to HW results (but are obviously a valid sensitivity study that the original data analysts should have done as well). I urge people interested in the topic to read or re-read my earlier post on Landsea 1993 versus Landsea 2005, as this affects H and W as well.