By far, the most important issue in Emanuel 2005 is Emanuel’s adjustments to West Pacific hurricane wind speeds. I’ve reached this conclusion after expenditure of a considerable amount of time and effort, including looking at every Annual Report of the Joint Typhoon Warning Council from 1959-2003.
After doing this, I stumbled on a Comment by William Gray, which was rejected by Nature – which states the problems with the Pacific adjustments in very clear terms. While Gray’s Comment could be and should be pruned, Nature has distorted the record once again by failing to publish a comment that was actually more pertinent than the comments by Pielke and Landsea, both of which were also meritorious.
Although Emanuel’s Pacific adjustments were new, they were not mentioned in the body of Emanuel 2005, although they were described in the Supplementary Information – which might not even have been considered by reviewers. Emanuel himself has added a link at his website to Cleartheair.org, which has developed a sensationalistic graphic of his results.
It took me longer to find the pea under the thimble than it should have. The debate over Emanuel 2005 has focused on Atlantic hurricanes – in part because these are discussed in the accepted Pielke and Landsea comments. But because the number of Atlantic hurricanes is much smaller than the number of Pacific hurricanes, the totals are dominated by Pacific hurricanes and adjustment issues for Atlantic hurricanes, while interesting, are a much less important issue in the calculation of the total.
Without analyzing the article quite carefully, a reader could obtain the impression that the Atlantic adjustments, discussed in the Landsea article, were the adjustments applicable to Pacific typhoons as well – Judith Curry, for example, missed the distinction as noted in one of the threads. But the Pacific adjustments are quite different. In the Atlantic Basin, Emanuel drew on Landsea 1993 for authority, but for the Pacific made his own adjustments. So the following graphic is really the one that carries the water in Emanuel 2005:
From Emanuel 2005.
The Underlying Data
Jordan and Ho, 1962 online here have an interesting discussion of Pacific cyclones prior to 1962.
The Joint Typhoon Warning Center maintains a large archive with annual reports from 1959-2003 including information on every typhoon and tropical storm. Annual Cyclone Reports Best Tracks . Here is a figure from their 2004 report showing the "trend" of cyclone numbers in the West Pacific – a result obviously at odds with Emanuel. This is not a graph by "stooges", but by the organization that has developed all of the relevant data.
JWTC 2004 Annural Report Figure 1-2. Tropical cyclones of tropical storm or greater intensity in the western North Pacific (1959-2004).
The Pacific wind-speed measurement methods are not homogeneous. I’ll summarize the three main stages in W Pacific hurricane speed measurement, as I presently understand them.
(1) up to 1974, wind speeds were estimated from airplane reconnaissance primarily with an early pressure-wind relationship;
(2) from 1974-1986, wind speeds were estimated from airplane reconnaissance using the pressure-wind relationship of Atkinson and Holliday (1977), resulting in lower wind speeds;
(3) after 1986, when airplane reconnaissance was discontinued in the W Pacific, pressures and winds were estimated from Dvorak-method interpretation of satellite imagery.
Emanuel stated that wind-speeds in the early period were estimated according to the following equation:
Emanuel does not provide any authority for this. A variation of the above equation is discussed in Fletcher and Johannesen 1965, citing Fletcher 1955 and Jordan 1958 for the earlier equation.
The impression that I obtained from the data and literature is that the Fletcher-Jordan equation was an attempt to summarize the data, but was not itself used to generate the wind estimates from pressure information. Fletcher and Johannesen 1965 was written at the time of introducing a new type of airplane reconnaissance using WB-47 instead of WB-50 aircraft, with the new aircraft no longer stressed to penetrate hurricanes at 700 mb.
Previously the reports all included estimates of 700 mb height, which appear to be linked directly to estimates of pressure. Shortly thereafter, the 700 mb heights ceased to be published. I presume that 700 mb heights ceased to measured by WB-47 aircraft – introducing another inhomogeneity into the record, which has not been discussed elsewhere.
There’s a very interesting re-statement of the early data in a recent article by Lander et al, in which they attempt to place the 2002 Pongsona typhoon in the context of wind-pressure measurements prior to the introduction of the Atkinson-Holliday method as shown in the graphic below. The red line shows the Atkinson-Holliday relationship. So presumably Lander et al didn’t think that the early airplane measurements needed to be re-stated a la Emanuel.
From Lander et al. Figure 4. Figure 4. Relationship between maximum sustained wind and minimum sea-level pressure, where “b” values indicate theoretical limits of the relationship. Dots are actual aircraft reports for 1960’s and 1970’s prior to JTWC’s use of the Atkinson-Holliday (1974) wind-pressure relationship (dashed line). Dotted line is the best-fit solution that provides the value used for Pongsona in this assessment.
The Atkinson-Holliday Relationship
In 1974, the following relationship between wind speed and pressure was introduced, being mentioned in the 1974 JWTC Annual Cyclone Report.
Emanuel doesn’t mention either Atkinson-Holliday or the 1974 Annual Cyclone Report, but reports in his SI that a new relationship was introduced – supposedly in 1973. Here is an extended excerpt:
For the first decade or so of airborne reconnaissance, surface winds were estimated mostly by visual inspection of the sea surface. Beginning in the early 1950s, radar altimeters aboard the aircraft made possible an accurate determination of the aircraft’s absolute altitude. When combined with direct pressure measurements, this gives a good estimate of geopotential height at flight level. Surface pressure can then be estimated using empirical relationships between surface and flight level pressure. This technique, developed during the 1950s, was used without significant modification through the end of aircraft reconnaissance in the western North Pacific and until the advent of accurate dropwindsondes in the North Atlantic. Minimum surface pressure estimates were converted to maximum sustained surface wind using semi-empirical wind-pressure relations which, however, have evolved with time.
For the western North Pacific, the wind-pressure relationship used until 1973 was
where in this case is given in knots and is the central surface pressure in hPa. In 1973, JTWC adopted a revised wind-pressure relationship given by:
where is again given in knots. Eliminating [tex[ p_c $ between (4) and (5) gives a formulafor correcting the pre-1973 JTWC wind speeds:
This yields a substantial reduction in pre-1973 wind speeds, amounting to an average of around 10 ms^à⣃ ’ ”¬’¢1 over the observed range of hurricane-force winds.Although the JTWC best track data does not record estimates of central pressure, the latter was tabulated as part of the Annual Tropical Cyclone Reports available from JTWC. To test whether the wind-pressure relations given by (4) and (5) [SM – should be (2) and (3) ] are indeed valid in the best track data, we tabulated the maximum sustained surface wind and minimum surface pressure for each storm between 1959 and 2003, covering the whole period of time that the Annual Tropical Cyclone Reports are available. This shows that (5) [SM – should be (3) ] is very closely followed in the interval 1973-1987, and maintains exactly thereafter, when the Dvorak method was used to estimate surface winds, while minimum pressure was calculated directly from (5). [SM – should be (3)]
In principle, applying correction (6) [SM – should be (4) ] to the pre-1973 JTWC best track winds should result in their satisfying (5) [SM – (3)] closely, but this does not turn out to be the case.Supplementary Figure 1 shows that while, as expected, the unadjusted winds are much too strong relative to the recorded pressures, the correction introduced by (6) [ SM – (4)] goes too far, and the adjusted winds are somewhat too weak. A better fit is obtained by compromising between the raw winds and the adjusted winds given by (6) [SM- 5?]:
where is given by (6) and w is a weight. For the period 1959-1966, a good fit is obtained by choosing w = 0.3 (see Supplementary Figure 1), while for 1967-1972, a better fit is obtained by taking w = 0.8. (The winds during and after 1973 are unadjusted, equivalent to taking w =1.) Having no pressure data before 1959, we use the same adjustment as for the period 1959-1966.
Emanuel illustrated this shift in his Supplementary Figure 1:
Emanuel 2005 Supplementary Figure 1. Scatter plot of storm maximum surface winds (knots) versus minimum surface pressure (hPa) during the period 1959-1966 for the raw JTWC best track data (blue), the winds adjusted using (4) from Supplementary Methods (red), and the winds adjusted using (5) from Supplementary Methods (green). The heavy black curve is the wind-pressure relation given by (3) from Supplementary Methods
The most important aspect of Emanuel’s methodology is the total failure to consider what happens after 1986 when airplane reconnaissance ceased and all information was derived from satellites – introducing inhomogeneity not simply in the wind-pressure equation, but even in the estimation of pressure – which was now done by satellite imagery. Critics of Spencer and Christy would do well to reflect on exactly how hurricane pressures are estimated – since the microwave methodology seems to includes estimates of temperature using satellite imagery in one of its stages.
Gray’s Comments on Emanuel
After I’d spent a lot of time doing this, I stumbled across an online Comment on Emanuel by William Gray, submitted to Nature in 2005 and, needless to say, rejected. Gray seems to be much criticized but his website shows a lengthy list of publications, including relevant publications on estimating hurricane pressure and wind strength using satellite microwave technology – experience which seems to me to be highly relevant to Emanuel 2005, fossilized or otherwise.
Gray states that there is a major inhomogeneity between Dvorak estimation and Atkinson-Holliday estimation, amounting to a estimating difference of approximately 7.5 m/sec (Dvorak higher) and that when this difference is cubed, the bias is multiplied accounting for the entire effect observed by Emanuel. Gray:
The AH [Atkinson-Holliday] scheme was discontinued after 1986 when aircraft reconnaissance in the NW Pacific was terminated, and central pressures could no longer be directly measured. Vmax values have since been obtained solely from satellite. The Dvorak satellite TC intensity scheme (1975, 1984) is known to give systematic higher Vmax estimates than the AH scheme. Knaff and Zehr’s (2005) recent analysis shows that the Dvorak satellite (1975, 1984) scheme for the estimation of Vmax (used in the Pacific since 1987) gives, on average, about 7.5 m/s higher Vmax value than the AH scheme for all wind speed categories. There is no question that the Dvorak scheme is superior to the AH scheme.
The differences between the Dvorak and the AH schemes causes large differences in Vmax. For 7.5 m/s wind differences of 32.5 m/s for Dvorak versus 25 m/s for AH, the cubed ratio of the Dvorak to the AH maximum wind speed is 2.2. For higher Vmax values (say 57.5 vs. 50 m/s) this ratio is 1.5. There are many more TC time periods in Emanuel’s analysis of the lower Vmax values when this ratio of the cubed Vmax of the satellite to the AH is close to 2 to 1.
Most of the large increase in Emanuel’s NW Pacific TC energy dissipation calculations from the early 1970s to the early 2000s can be explained by the cubed differences of the Vmax estimates between the more recent (since 1987) Dvorak pure satellite scheme for Vmax and the earlier (1973-1986) period when the AH aircraft scheme was used. Gray et al. (1991) and Martin and Gray (1993) have discussed the many complications that arise for Vmax determination from utilization of satellite-only versus aircraft-only measurements and when both measurements are available.
Gray is not the only person to comment on inhomogeneity between Dvorak and airplane measurements. Kossin and Yelden, 2004, is entitled "A Pronounced Bias in Tropical Cyclone Minimum Sea Level Pressure Estimation Based on the Dvorak Technique. I haven’t considered it, but it sounds like this should have been specifically considered by Emanuel.
The issue is discussed in further detail in in Knaff and Sampson (2006) online here and Knaff and Zehr (2006), submitted which I have not yet seen. However Knaff and Zehr 2006 is discussed here by Jeff Masters as follows:
Knaff and Zehr (2006) make some convincing arguments that typhoon intensities during the 1973-1986 period were too low due to measurement error, and the number of Category 4 and 5 storms in the region have been roughly constant for the past 50 years. This paper has been accepted for publication in Weather and Forecasting, and will likely be published late this year. Dr. Knaff and Charles Sampson of the Naval Research Laboratory have performed a preliminary re-analysis of maximum typhoon intensities for the period 1966-1987 based on the Knaff and Zehr (2006) results, and this re-analysis will be presented at the upcoming 27th Conference on Hurricanes and Tropical Meteorology (April 24-28, 2006). In the extended abstract, they show that after correcting for the AH technique errors, the number of Category 4 and 5 typhoons during the 1966-1987 period increased by 1.5 per year, leaving only a slight upward trend in Category 4 and 5 typhoons during the period 1970 – 2004.
Masters shows the following graphic:
Masters website Figure 6. Number of Category 4 and 5 hurricanes in the Northwest Pacific Ocean since reliable records began in 1945. Data taken from the Joint Typhoon Warning Center "best track" database. Typhoon intensities from the period 1973 – 1986 were estimated using the "Atkinson-Holliday" (AH) technique, which may have underestimated typhoon intensity.
I think that Gray’s arguments should be appraised on their merits – rather than according to whether Nature published them. At a minimum, to an outsider who has happened to have browsed all of the JWTC Annual Reports from 1959-2003, they seem highly plausible and merit a reply. Perhaps the issue will get into discussion as a result of Knaff and Zehr.
While doing this, I revisited Emanuel’s website. He links to ClearTheAir.org, which has publicized his work and displays the following image
linking in turn to an animated graphic illustrating Emanuel’s graphic.
This chart is based on the dataset from my 2005 Nature paper and updated with two additional years of data. To produce the chart, the temperature index was converted to Fahrenheit. The chart was produced by Clear the Air. For high-resolution non-watermarked versions, download from ftp://188.8.131.52. User: ClearTheAir. Password: hurricane. http://www.cleartheair.org/hurricanes.vtml
While Emanuel was quick to update this graphic to incorporate 2005 data, it will be interesting to see if the graphic is equally quickly updated to include low 2006 Atlantic hurricane results.
Atkinson, G.D. and C.R. Holliday, 1977: Tropical cyclone minimum sea level pressure/maximum sustained wind relationship for the western North Pacific. Mon. Wea. Rev., 105, 421-427.
Emanuel webpage http://wind.mit.edu/~emanuel/anthro2.htm
Emaneul 2005 SI http://www.nature.com/nature/journal/v436/n7051/suppinfo/nature03906.html
Fletcher, R.D. 1955, Comparison of maximum surface winds in hurricanes, BAMS 36, 242-250.
Fletcher and Johannesen, 1965. MWR. http://ams.allenpress.com/archive/1520-0450/4/4/pdf/i1520-0450-4-4-457.pdf
Gray, William M., 2005, Comments on: “Increasing destructiveness of tropical cyclones over the past 30 years” by Kerry Emanuel, Nature, 31 July 2005, Vol. 436, pp. 686-688 http://arxiv.org/ftp/physics/papers/0601/0601050.pdf
Gray, William. Website http://typhoon.atmos.colostate.edu/Includes/Documents/Publications.html
Jordan and Ho, 1962. VARIATIONS IN THE ANNUAL FREQUENCY OF TROPICAL CYCLONES, 1886–1958 Monthly Weather Review Volume 90, Issue 4 (April 1962) pp. 157–164
Jordan, CL 1958. Estimation of surface central pressure in tropical cyclones from aircraft observations. BAMS 39, 345-352.
John A. Knaff and Charles R. Sampson, 2006 , REANALYSIS OF WEST PACIFIC TROPICAL CYCLONE MAXIMUM INTENSITY 1966-1987, http://ams.confex.com/ams/pdfpapers/108298.pdf
Knaff and Zehr 2006..
JAMES P. KOSSIN AND CHRISTOPHER S. VELDEN, 2004. A Pronounced Bias in Tropical Cyclone Minimum Sea Level Pressure Estimation Based on the Dvorak Technique Mon We Rev. http://www.ssec.wisc.edu/~kossin/articles/kossin_velden_MWR.pdf
Mark A. Lander, Charles P. Guard, Arthur N.L. Chiu, METEOROLOGICAL ASSESSMENT FOR SUPER TYPHOON PONGSONA AT LANDFALL ON GUAM — DECEMBER 8, 2002. http://ams.confex.com/ams/pdfpapers/75323.pdf