From NOT A LOT OF PEOPLE KNOW THAT

By Paul Homewood

Back to the question of the rising number of Atlantic tropical storms, which we discussed last week.

To recap, this is what the BBC reported two years ago:

Over the past 10 to 15 years, though, named storms have formed prior to the official start about 50% of the time.

And the way they are defined and observed has changed significantly over time.

“Many of these storms are short-lived systems that are now being identified because of better monitoring and policy changes that now name sub-tropical storms,” Dennis Feltgen, meteorologist at the US National Hurricane Center (NHC) told BBC Weather.

The number of named storms has increased over the decades, but there is no real evidence this is the result of a warming world.

The overall increase from 1961 is also likely to be due to better technology, along with observations over the Atlantic Ocean.

Since satellites came along in the 1980s, we can spot and monitor the development of tropical cyclones and name them when they meet the threshold.

We are simply able to record more.

Increasing Hurricane Frequency Due To Better Observation, Not Climate Change–BBC

Dr Neil Frank, who was Director of the US National Hurricane Center from 1974 to 1987 goes further, maintaining that many of the storms now named would not have been in his day.

He made two particular complaints about current methods in 2021:

1) Many named storms are actually winter storms, not tropical storms. He states that the first six tropical storms in 2020 would not have been counted in his time.

2) Nowadays the NHC rushes to name a storm, simply based on wind speeds. His team would have waited until the central pressure dropped to confirm that it really was a tropical storm, and not just a thunderstorm. This often explains why named storms are often so short lasting now.

I can now confirm that there was indeed a policy change in 2002, and it concerned subtropical storms. The US National Weather Service explain:

The National Hurricane Center began naming subtropical storms in 2002. Between 1968 and 2001, subtropical storms were simply given numbers (“One”, “Two”, etc). Before 1968, subtropical storms were never classified as such, but were sometimes called “Unnamed storm”. A landmark study performed by Herbert and Poteat (1975) led to a substantial increase in the identification and naming of subtropical storms in the late 1970s and early 1980s. However, when Bob Sheets became director of the National Hurricane Center between 1987 and 1995, he declared that subtropical storms should not be recognized, and very few subtropical storms were classified during this period. Prior to 1968, there are many systems that were subtropical in the Atlantic that should have been included in the official HURDAT database.

https://www.weather.gov/source/zhu/ZHU_Training_Page/tropical_stuff/sub_extra_tropical/subtropical.htm

They go to define what subtropical are:

It is often difficult to tell from looking at forecast model data whether a low that is expected to develop near the U.S. coast will be tropical, subtropical, or extratropical. The difference is important, since tropical systems have the potential to quickly grow into hurricanes, while extratropical or subtropical storms do not. So, here’s a quick meteorology lesson on the normal progression one sees from extratropical cyclone, to subtropical cyclone, to tropical cyclone.

1) An extratropical cyclone forms. Extratropical cyclones have cold air at their core, and derive their energy from the release of potential energy when cold and warm air masses interact. These storms always have one or more fronts connected to them, and can occur over land or ocean. An extratropical cyclone can have winds as weak as a tropical depression, or as strong as a hurricane. Examples of extratropical cyclones include blizzards, Nor’easters, and the ordinary low pressure systems that give the continents at mid-latitudes much of their precipitation.

2) If the waters under the extratropical cyclone are at least 21C (70F), thunderstorm activity will gradually build inside the storm and moisten and warm the lower levels. Over time, the core of the storm may gradually go from cold to warm, and the storm will start getting some of its energy from “latent heat”, which is the energy released when water vapor that has evaporated from warm ocean waters condenses into liquid water. Latent heat is what powers tropical cyclones. At this point, the storm is called subtropical. If the winds are already more than 39 mph (as happened in the case 2007’s Subtropical Storm Andrea), it is called a subtropical storm. If the winds are less than 39 mph, then it is called a subtropical depression. So, you don’t need to start with a subtropical depression in order to get a subtropical storm.

A subtropical storm typically has a large, cloud free center of circulation, with very heavy thunderstorm activity in a band removed at least 100 miles from the center. The difference between a subtropical storm and a tropical storm is not that important as far as the winds they can generate, but tropical storms generate more rain. There is no such thing as a subtropical hurricane. If a subtropical storm intensifies enough to have hurricane force winds, than it must have become fully tropical. The definition of a subtropical storm, according to the National Hurricane Center: A non-frontal low pressure system that has characteristics of both tropical and extratropical cyclones.

This change of policy is more than just an academic debate about the naming of storms. It also affects the calculation of ACE, which is widely used as a metric of the energy released by tropical cyclones. NOAA, for instance, always include this chart in their annual Tropical Cyclone Report:

It is calculated by summing the square of a tropical cyclone’s maximum sustained winds, measured every six hours.

ACE however is calculated not just from hurricanes, but from all named storms, even subtropical ones:

The “Normal” is based on the 1991-2020 climatology, so is clearly grossly underestimated and a worthless measure.

There is a very simple solution – simply exclude all tropical and subtropical storms, and only report on tropical cyclones (i.e. hurricanes).