March 16, 2012

Atlantic Hurricanes: The Long and the Short of It

Filed under: Climate Extremes, Hurricanes

Last May, we reviewed a paper on Atlantic basin tropical cyclone trends by Gabriele Villarini and colleagues that focused on a breed of storms called they called “shorties”—small tropical storms that lasted less than two days. The authors concluded that while the number of identified “shorties” has been increasing with time, the increase was primarily the result of changing (improving) observational practices not a changing climate. Now, we review a new paper that looks at the other end of the spectrum of Atlantic tropical cycles—“biggies” (our term)—intense Category 4 and 5 hurricanes. In a new paper, Andrew Hagen (University of Miami) and Chris Landsea (National Hurricane Center) conclude that changing observational practices have resulted in more Cat 4&5 hurricanes being identified in recent decades compared to past ones. Again, the increase is not due to a changing climate but changing detection technologies.

Whether talking about the total number of tropical cyclones (which is increasing because of detection technology) or their intensity (which is increasing because of detection technology) only a person unaware of this important research would say that there has been a climate-related trend.

There are other yet-to-be-determined repercussions of this work, but we would hypothesize that it is going to make work of computer modelers look like they simulated something that hasn’t happened. Fancy that!

Figure 1 illustrates the major changes in how Atlantic tropical cyclones—tropical storms and hurricanes— have been observed and measured over the past 150 years or so. Big changes in observational capabilities occurred in the mid-1940s when aircraft reconnaissance became common practice and again in the mid-1960s with the launch of geostationary satellites. And all along, technological improvements have been occurring which have enhanced our ability to get an increasingly detailed look at evolving storm systems. As a consequence, we are continually able to observe much smaller scale features, which have led both to an increasing number of tropical systems being identified and also an increase in the apparent storm intensity.


Figure 1. Changes in tropical cyclone observing technologies since the mid-1850s. The vertical blue bar represents the period under study by Hagen and Landsea (figure source: Hagen and Landsea, 2012).

In their paper, Hagen and Landsea set out to attempt to determine how the strongest hurricanes (Category 5 storms) which were identified during the period 1992 through 2007 would have been described using the observational network and technologies available in the period 1944-1953—the beginning of the period of aircraft reconnaissance.

To do so, they first excluded all the data on the 1992-2007 category 5 hurricanes that would not have been available during the 1944-53 period:

1) They excluded all satellite observations;
2) All data from buoys and weather stations installed after 1953 were excluded;
3) No aircraft observations of pressures lower than 950 millibars were included from 1944-49 and lower than 940 mb from 1950-53 (during the early years of aircraft reconnaissance, the planes would not penetrate the eyewall of major hurricanes because the winds speeds and turbulence were too great for the equipment);
4) No nighttime aircraft observations were included (during the early years of aircraft reconnaissance, low-level flights required the pilot to be able to see the ocean’s surface when navigating);
5) No radar data were included.

Analyzing the data that still remained, Hagen and Landsea determined as best they could (i.e., using standard procedures) the track and intensity of the ten Cat 5 hurricanes that were observed in the Atlantic during the 1992-2007.

Figure 2 shows an example of their analysis as applied to 2003’s hurricane Isabel—a storm that reached Cat 5 out in the western Atlantic Ocean and eventually came ashore as a strong Cat 2 hurricane along the central North Carolina coast. The blue line (and comments in the yellow boxes) are derivations of the storm’s peak winds speeds using all the data collected in 2003. The light blue line and magenta lines (and comments in the white boxes) are the peak winds derived using data limited by the technology of the early 1950s and late 1940s, respectively. Notice that had Isabel occurred in the late 1940s or early 1950s it would not have been deemed to have reached Cat 5 status, and its peak winds would have been estimated to be some 20 to 30 knots lower then they actually were.


Figure 2. Analysis and reanalysis of 2003’s Hurricane Isabel. The dark blue line (and comments in the yellow boxes) are derivations of the storm’s peak winds speeds using all the data collected in 2003. The light blue line and magenta lines (and comments in the white boxes) are the peak winds derived using data limited by the technology of the early 1950s and late 1940s, respectively. (source: Hagen and Landsea, 2012)

Table 1 summarizes Hagen and Landsea’s results for all ten of the most recent Cat 5 storms. What they found was astounding—of the ten Cat 5 storms observed from 1992-2007 only 2 or 3 of them would have been categorized as Cat 5 storms had they occurred during the 1944-1953 time period.

Hagen and Landsea also identified three Category 4 storms in recent years (2003-2010) that would not have been classified as major hurricanes using the observational network and technology of the 1944-1953 period.


Table 1. Ten most recent category 5 hurricanes observed in the Atlantic Basin. The “Best Track Peak” column gives the strength of the peak winds as observed, the “Early 1950s Peak” column given the peak wind as best determined using technologies and observational network available from 1950-1953, and the “Late 1940s Peak” column given the peak wind as best determined using technologies and observational network available from 1944-1949 (adapted from Hagen and Landsea, 2012).

In summary, Haden and Landsea wrote:

This research suggests that the counts of Category 4 and 5 hurricanes (at least through 1953 and likely beyond that year) are not nearly as reliable as they are today. Future studies that discuss frequency trends of Atlantic Basin Category 4 and 5 hurricanes must take into account the undercount biases that existed prior to the geostationary satellite era due to the inability to observe these extreme conditions.

The long and short of it is this— if you are doing studies investigating the changes in the number or intensity of tropical cyclones and you don’t take into account those that are the result of improvements in observational practices over time, you will get the wrong answer. And most likely, the wrong answer will be that Atlantic tropical cyclones are increasing in numbers and growing in strength as the planet warms. You will also get on CNN, NPR, and NBC Nightly News, and anyone that says you are wrong will be accused by Michael Mann of being part of a vast fossil-fuel funded conspiracy to deceive the world.

Reference

Hagen, A. B., and C. W. Landsea, 2012. On the classification of extreme Atlantic hurricanes utilizing mid-20th century monitoring capabilities. Journal of Climate, doi:10.1175/JCLI-D-11-00420.1, in press.




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