This isn't the first time I've written about this. In fact, it's the fourth: 2009, 2011, 2013, and now again in 2015.
The latest episode(s) ...
THING #1
On the morning of Monday, July 27, the more I looked at the thing that was in the northeastern Gulf of Mexico, the more it seemed the circulation was that of a low pressure system with tropical characteristics. I was going to tweet that it was a "tropical low."
Then I reviewed the official posted definition of a tropical cyclone, and I was unable to find anything within that which the low pressure system did not have. Yet not only was it not officially a tropical cyclone, or even given a low probability of becoming one, or even, as is sometimes done, identified while given a zero percent chance of becoming one, in the tropical weather outlook this system was not even acknowledged to exist.
Which moved me to tweet this:
And then this:
Quite a bit of info and messaging can actually be fit in a tweet via images along with the 140-character limit, but let's take a closer look at that system, and the definition of tropical cyclone and tropical depression.
The American Meteorological Society glossary of meteorology definition:
"The general term for a cyclone that originates over the tropical oceans. This term encompasses tropical depressions, tropical storms, hurricanes, and typhoons. "
Paradoxically, while very generic, it's also much too restrictive -- geographically, that is, as many tropical cyclones originate outside the tropics, and some even outside the subtropics. (And that's not even including such phenomena as "Medicanes" over the Mediterranean Sea or polar lows in high latitudes.)
But the basic idea of that definition, that it's simply a cyclone which is of a tropical, not extratropical or subtropical, nature, is right on.
And the AMS glossary definition of cyclone is:
"An atmospheric cyclonic circulation, a closed circulation."
So isn't any closed circulation, that's not an extratropical or subtropical cyclone, a tropical cyclone (even if it doesn't have convection)?
The AMS glossary definition of a tropical cyclone goes on to say:
"Tropical cyclones are initiated by a large variety of disturbances, including easterly waves and monsoon troughs. Once formed, they are maintained by the extraction of latent heat from the ocean at high temperature and heat export at the low temperatures of the tropical upper troposphere."
Note that it describes convective processes required for maintenance and intensification, not necessarily formation, and that tropical cyclones are initiated by a large variety of incipient atmospheric disturbances. Note also there's no temporal duration requirement.
Now let's look at the government's official definition that I cited in my tweet:
"A warm-core non-frontal synoptic-scale cyclone, originating over tropical or subtropical waters, with organized deep convection and a closed surface wind circulation about a well-defined center." And a tropical depression is defined as a tropical cyclone with maximum 1-minute sustained winds of 38 mph or less.
And let's apply that to what transpired on July 27 ...
Warm core? ✓
Not super-deeply, but this analysis indicated it was on that side of the fence rather than cold-core.
Image source: Robert Hart and Jenni Evans; my annotation.
And it wasn't under an upper low/trough which would imply being a subtropical cyclone or another type of hybrid or even extratropical, it was on the southern periphery of a hot high pressure ridge.
Non-frontal? ✓
It had evolved from a mid-latitude frontal system, which several days earlier had produced lines of thunderstorms moving offshore of Florida into the Atlantic and unfortunately resulting in a tragedy at sea; but while there was still a convergence zone along which the low spun up, a primarily baroclinic front no longer existed.
Image sources: Wright-Weather.com (left), NOAA / Colorado State University (right)
Synoptic-scale, originating over tropical or in this case subtropical waters, with a closed surface wind circulation about a well-defined center? ✓
Image source: coolwx.com
Organized deep convection? ✓
There were curved rainbands rotating in the circulation, and while not super deep, it wasn't just shallow, with cloud tops of -50° to -70°C and radar echo tops of 40K'+, and by the following morning cloud tops of -80°C.
This is not the way satellite and radar loops look over, say, Minnesota in January.
Image source: SimuAWIPS
Image source: Gibson Ridge
And it was over very warm water.
The convection was asymmetric due to northerly and northeasterly shear around that hot ridge, but does the organized deep convection need to be symmetric or at least completely around the center? At best, the wording and sentence structure of the definition is ambiguous about that, but let's say for the sake of argument that that's what was meant to be specified in the criteria.
And let's apply that to all systems equally.
For example, Guillermo recently in the central Pacific:
Image source: NOAA/NESDIS
In fact, by the the time Guillermo was classified as a post-tropical remnant low, it had been three days since there was much convection, deep or otherwise, much less that being all the way around the center.
Arguably it shouldn't matter that the lack of convection was at the end of Guillermo's life rather than the beginning if the definition is to be applied equally, but let's say for the benefit of the doubt that in reality there's a difference, if not meteorologically then in communication and messaging, of initiation of the tropical cyclone/depression/storm label as opposed to cessation of it.
Well, this is when Andrea was operationally designated the first tropical storm of the season a couple years ago:
Image source: Naval Research Laboratory
And it was reasonable for it to have been designated a tropical storm. The fact of the matter is that tropical cyclones come in different sizes, strengths and structures, and some are asymmetric and sheared.
Back to the system on Monday July 27, a long radar loop from the morning into mid-afternoon showed rainbands continuing to circulate ...
Image source (and next two radar loops): College of DuPage
And then that evening into the wee hours of Tuesday July 28 ...
While crossing the northern Florida peninsula during the day the low-level circulation became less tight, then reorganized as it emerged over the Atlantic offshore of Jacksonville during the late afternoon.
During the next couple of days the system meandered over the subtropical Atlantic and dissipated, finally succumbing to the northerly shear.
THING #2
Then the following Sunday-Monday it was, as the great philosopher Yogi Berra would say, déjà vu all over again.
That Sunday, August 2, I was looking at satellite imagery, and this caught my eye ...
Image source: GREarth
And on radar ...
Image source (of this and next radar loop): Gibson Ridge
This time, the system's existence was officially acknowledged, as was its possession of a well-defined surface low pressure system, but nevertheless given only a 10% chance of becoming a tropical cyclone, even though by evening it looked like this on radar imagery, and seeming to meet the definition of a tropical depression.
Deep convection flourished through the night and into the next morning ...
Image credit: NASA Earth Science Office
During that afternoon and evening, like with the previous system, the center crossed the northern Florida peninsula and became strung out and less well-defined, but then by morning as the center reorganized along the coast of the Carolinas there was a big blowup of deep convection.
Image source: NRL/JPSS NexSat
That prompted a special statement with a sudden raising of the tropical cyclone "development" probability to 40% and deployment of aircraft reconnaissance. The plane found tropical storm / gale force winds.
But by then the system was in its final stage, and during the afternoon and evening the convection had become ragged and elongated, and the surface center along the coast had dissipated in favor of a low pressure trough inland in hot air.
Image source: GREarth
I respect and appreciate the pressure which those who have to issue official tropical cyclone advisories are under, and the issues involved in coordination with other agencies, public perception, etc., and how if something is called even a tropical depression, vs. not, there's such a difference in reaction, especially in this viral age.
Given that only the government is allowed to classify systems as tropical depressions, imagine if I had tweeted for either Thing #1 or Thing #2, "A tropical depression has formed." A storm would have developed alright -- a {bleep}storm!
If a snowstorm meets wind and visibility criteria for being a blizzard, I or other meteorologists could call it that, and it'd be, to quote Carl Spackler, no big deal. The term derecho has at times caused confusion and even sarcasm, but I have been able to address that upon presenting observed data vis-à-vis accepted peer-reviewed criteria (which removes a criterion still posted on an official website).
But I can't call a warm-core non-frontal synoptic-scale cyclone, originating over tropical or subtropical waters, with organized deep convection and a closed surface wind circulation about a well-defined center, a tropical cyclone or tropical depression?
This, despite the fact that it's not like there was any expectation of intensification into a hurricane, or need for evacuations or anything like that. Just a matter of simply calling it a tropical cyclone or tropical depression.
Nor, conversely, is it like there weren't any effects on land, i.e. raising awareness of the existence of the system would have been pointless or even counterproductive in that regard. As Thing #1 was developing there were multi-day rainfall totals as high as nearly a foot, and with Thing #2 there were 12-hour rainfall measurements as high as 7", with major flooding.
So, if effects were the same either way, whether or not called a tropical cyclone/depression, why is this even worth writing about?
Well, it's a matter of principle. If something looks, swims and quacks like a duck, based on an objective meteorological assessment such as presented here, should I have to call it a goose?
Photo credit: Stu Ostro
The latest episode(s) ...
THING #1
On the morning of Monday, July 27, the more I looked at the thing that was in the northeastern Gulf of Mexico, the more it seemed the circulation was that of a low pressure system with tropical characteristics. I was going to tweet that it was a "tropical low."
Then I reviewed the official posted definition of a tropical cyclone, and I was unable to find anything within that which the low pressure system did not have. Yet not only was it not officially a tropical cyclone, or even given a low probability of becoming one, or even, as is sometimes done, identified while given a zero percent chance of becoming one, in the tropical weather outlook this system was not even acknowledged to exist.
Which moved me to tweet this:
And then this:
Quite a bit of info and messaging can actually be fit in a tweet via images along with the 140-character limit, but let's take a closer look at that system, and the definition of tropical cyclone and tropical depression.
The American Meteorological Society glossary of meteorology definition:
"The general term for a cyclone that originates over the tropical oceans. This term encompasses tropical depressions, tropical storms, hurricanes, and typhoons. "
Paradoxically, while very generic, it's also much too restrictive -- geographically, that is, as many tropical cyclones originate outside the tropics, and some even outside the subtropics. (And that's not even including such phenomena as "Medicanes" over the Mediterranean Sea or polar lows in high latitudes.)
But the basic idea of that definition, that it's simply a cyclone which is of a tropical, not extratropical or subtropical, nature, is right on.
And the AMS glossary definition of cyclone is:
"An atmospheric cyclonic circulation, a closed circulation."
So isn't any closed circulation, that's not an extratropical or subtropical cyclone, a tropical cyclone (even if it doesn't have convection)?
The AMS glossary definition of a tropical cyclone goes on to say:
"Tropical cyclones are initiated by a large variety of disturbances, including easterly waves and monsoon troughs. Once formed, they are maintained by the extraction of latent heat from the ocean at high temperature and heat export at the low temperatures of the tropical upper troposphere."
Note that it describes convective processes required for maintenance and intensification, not necessarily formation, and that tropical cyclones are initiated by a large variety of incipient atmospheric disturbances. Note also there's no temporal duration requirement.
Now let's look at the government's official definition that I cited in my tweet:
"A warm-core non-frontal synoptic-scale cyclone, originating over tropical or subtropical waters, with organized deep convection and a closed surface wind circulation about a well-defined center." And a tropical depression is defined as a tropical cyclone with maximum 1-minute sustained winds of 38 mph or less.
And let's apply that to what transpired on July 27 ...
Warm core? ✓
Not super-deeply, but this analysis indicated it was on that side of the fence rather than cold-core.
Image source: Robert Hart and Jenni Evans; my annotation.
And it wasn't under an upper low/trough which would imply being a subtropical cyclone or another type of hybrid or even extratropical, it was on the southern periphery of a hot high pressure ridge.
Non-frontal? ✓
It had evolved from a mid-latitude frontal system, which several days earlier had produced lines of thunderstorms moving offshore of Florida into the Atlantic and unfortunately resulting in a tragedy at sea; but while there was still a convergence zone along which the low spun up, a primarily baroclinic front no longer existed.
Image sources: Wright-Weather.com (left), NOAA / Colorado State University (right)
Synoptic-scale, originating over tropical or in this case subtropical waters, with a closed surface wind circulation about a well-defined center? ✓
Image source: coolwx.com
Organized deep convection? ✓
There were curved rainbands rotating in the circulation, and while not super deep, it wasn't just shallow, with cloud tops of -50° to -70°C and radar echo tops of 40K'+, and by the following morning cloud tops of -80°C.
This is not the way satellite and radar loops look over, say, Minnesota in January.
Image source: SimuAWIPS
Image source: Gibson Ridge
And it was over very warm water.
The convection was asymmetric due to northerly and northeasterly shear around that hot ridge, but does the organized deep convection need to be symmetric or at least completely around the center? At best, the wording and sentence structure of the definition is ambiguous about that, but let's say for the sake of argument that that's what was meant to be specified in the criteria.
And let's apply that to all systems equally.
For example, Guillermo recently in the central Pacific:
Image source: NOAA/NESDIS
In fact, by the the time Guillermo was classified as a post-tropical remnant low, it had been three days since there was much convection, deep or otherwise, much less that being all the way around the center.
Arguably it shouldn't matter that the lack of convection was at the end of Guillermo's life rather than the beginning if the definition is to be applied equally, but let's say for the benefit of the doubt that in reality there's a difference, if not meteorologically then in communication and messaging, of initiation of the tropical cyclone/depression/storm label as opposed to cessation of it.
Well, this is when Andrea was operationally designated the first tropical storm of the season a couple years ago:
Image source: Naval Research Laboratory
And it was reasonable for it to have been designated a tropical storm. The fact of the matter is that tropical cyclones come in different sizes, strengths and structures, and some are asymmetric and sheared.
Back to the system on Monday July 27, a long radar loop from the morning into mid-afternoon showed rainbands continuing to circulate ...
Image source (and next two radar loops): College of DuPage
And then that evening into the wee hours of Tuesday July 28 ...
While crossing the northern Florida peninsula during the day the low-level circulation became less tight, then reorganized as it emerged over the Atlantic offshore of Jacksonville during the late afternoon.
During the next couple of days the system meandered over the subtropical Atlantic and dissipated, finally succumbing to the northerly shear.
THING #2
Then the following Sunday-Monday it was, as the great philosopher Yogi Berra would say, déjà vu all over again.
That Sunday, August 2, I was looking at satellite imagery, and this caught my eye ...
Image source: GREarth
And on radar ...
Image source (of this and next radar loop): Gibson Ridge
This time, the system's existence was officially acknowledged, as was its possession of a well-defined surface low pressure system, but nevertheless given only a 10% chance of becoming a tropical cyclone, even though by evening it looked like this on radar imagery, and seeming to meet the definition of a tropical depression.
Deep convection flourished through the night and into the next morning ...
Image credit: NASA Earth Science Office
During that afternoon and evening, like with the previous system, the center crossed the northern Florida peninsula and became strung out and less well-defined, but then by morning as the center reorganized along the coast of the Carolinas there was a big blowup of deep convection.
Image source: NRL/JPSS NexSat
That prompted a special statement with a sudden raising of the tropical cyclone "development" probability to 40% and deployment of aircraft reconnaissance. The plane found tropical storm / gale force winds.
But by then the system was in its final stage, and during the afternoon and evening the convection had become ragged and elongated, and the surface center along the coast had dissipated in favor of a low pressure trough inland in hot air.
Image source: GREarth
I respect and appreciate the pressure which those who have to issue official tropical cyclone advisories are under, and the issues involved in coordination with other agencies, public perception, etc., and how if something is called even a tropical depression, vs. not, there's such a difference in reaction, especially in this viral age.
Given that only the government is allowed to classify systems as tropical depressions, imagine if I had tweeted for either Thing #1 or Thing #2, "A tropical depression has formed." A storm would have developed alright -- a {bleep}storm!
If a snowstorm meets wind and visibility criteria for being a blizzard, I or other meteorologists could call it that, and it'd be, to quote Carl Spackler, no big deal. The term derecho has at times caused confusion and even sarcasm, but I have been able to address that upon presenting observed data vis-à-vis accepted peer-reviewed criteria (which removes a criterion still posted on an official website).
But I can't call a warm-core non-frontal synoptic-scale cyclone, originating over tropical or subtropical waters, with organized deep convection and a closed surface wind circulation about a well-defined center, a tropical cyclone or tropical depression?
This, despite the fact that it's not like there was any expectation of intensification into a hurricane, or need for evacuations or anything like that. Just a matter of simply calling it a tropical cyclone or tropical depression.
Nor, conversely, is it like there weren't any effects on land, i.e. raising awareness of the existence of the system would have been pointless or even counterproductive in that regard. As Thing #1 was developing there were multi-day rainfall totals as high as nearly a foot, and with Thing #2 there were 12-hour rainfall measurements as high as 7", with major flooding.
So, if effects were the same either way, whether or not called a tropical cyclone/depression, why is this even worth writing about?
Well, it's a matter of principle. If something looks, swims and quacks like a duck, based on an objective meteorological assessment such as presented here, should I have to call it a goose?
Photo credit: Stu Ostro
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