Day 304: Tropical Storm Tomas Bears Down on Haiti
Tropical Storm Tomas is still spinning in the Caribbean Sea. The intensity is weaker than it was yesterday (when it was a Hurricane), but it still a menace. In fact the National Hurricane Center (NHC) has Tomas regaining hurricane strength soon.
I know I mentioned Tomas last night, but the forecast track is such that I felt it necessary to discuss again. Tomas is forecast to impact Haiti as a hurricane in the next few days. The high winds, and extremely heavy rain will be extremely detrimental to the country as it still tries to recover from the major earthquake in early 2010.
All humanitarian organizations should be prepared for the possibility of a major humanitarian crisis.
Day 303: Hurricane Tomas
An unusual late-season hurricane is currently spinning in the far eastern Caribbean Sea. Although hurricanes in November aren’t particularly rare, having one this far east is.
Below is the forecast track from the National Hurricane Center (NHC). Previous NHC forecasts had Hurricane Tomas becoming a Major Hurricane (category 3 or greater), but this latest forecast has backed off on the intensity slightly.
Day 302: The Morning Cold
Today is my birthday, so I’m taking it easy with respect to the blog tonight. Above are the morning low temperatures observed by the Oklahoma Mesonet. I love crisp mornings!
Day 301: The Cold Aftermath
After a series of relatively in depth meteorological posts, tonight’s post is a bit shorter. In the way of such a strong cyclone, colder air has surged south across the central United States. Frost Advisories and Freeze Warnings have been issued for a large portions of the south-central United States. If you are in or near a Frost Advisory or a Freeze Warning, please remember to cover outside plants that are sensitive to the cold.
Day 300: The Tropopause Fold (aka Mid-Tropospheric Front)
Tonight’s post contains fairly complicated meteorology, but I’ve tried to simplify it as much as possible for all to enjoy.
For the most part humans are constrained to two dimensions when trying to capture and display images in a digital media. (I say “for the most part” because of the rise in popularity of 3D televisions may change this.) Because of this, meteorologists are (typically) forced to interrogate the atmosphere in 2D — there is software available to allow users to simulate a 3D fields, but these require user interaction and aren’t widely available on the web.
Meteorologists tend to use “plan view” plots. These are quasihorizontal planes roughly parallel to the ground (in the X-Y directions for the mathematically inclined). These are horizontal slices through the atmosphere. Be evaluating these plots at various heights, meteorologists can try to mentally create a 3D representation of the 3D atmosphere. Sometimes meteorologists will use a plot known as a cross section to interrogate the atmosphere with a 2D plane that cuts through the vertical, but in only one horizontal direction (X-Z or Y-Z planes). These cross sections are vertical slices through the atmosphere.
The first two plots below are cross sections from 12 UTC this morning (7 AM CDT) from Glasgow, MT to Lake Charles, LA.
The first thing that should stand out is the big red blob in the middle of the cross section. This big red blob (and the entire blue, yellow, red color fill) is the same mid-level jet streak discussed on Day 298. Jet streaks tend to be the strongest near the top of the troposphere, just below the tropopause, which is the boundary between the troposphere below and the stratosphere above.
The thin red lines represent lines of potential temperature, or isentropes. (The word isentrope is comprised of “iso” meaning “equal” and “trope” derived from “entropy”. So, for the physics majors out there [of which I am one], an isentrope is also know as a line of constant entropy! It is beyond the scope of this post to discuss potential temperature per se, so please read the links above to get a better idea if you are confused!)
For our purposes, the tropopause can be identified as a strong vertical packing of the isentropes near the jet streak. To the south and east (right in this image) of the jet streak, this vertical packing is found fairly high in the troposphere, whereas to the north and west (left) it is found roughly near the same level as the jet streak. This is typical behavior of the tropopause. A quick and dirty explanation of this can be made by considering the average temperature of the atmosphereic columns either side of the jet streak. On the south side of the jet, the atmosphere is warm (think back to the Day 298 blog post and the thermal gradients associated with the jet streak!); a warm troposphere expands. On the north side of the jet streak, the atmosphere tends to be cold; a cold troposphere contracts. Thus, the positioning of the isentropes around the jet streak make sense!
In the presence of strong jets, however, the tropopause becomes relatively ill-defined very near the jet streak. In the image above we find a strong vertical packing of the isentropes both above and below the jet streak. Does this mean we have two tropopauses? Yes! The tropopause actually folds on itself around the jet streak, leading to two tropopauses! Thus, if you were to travel vertically through one of these tropopause folds, you would actually travel from troposphere to stratosphere back to troposphere back to stratosphere! These tropopause folds are sometimes referred to as “mid-tropospheric fronts” because the horizontal change in potential temperature is very similar to what we find with surface fronts!
Tropopause folds are very important because they allow stratospheric air to descend to levels typically found well in the troposphere. This may not sound like a bad thing, but it actually is. Because of the high static stability of the stratospheric air, pollutants such as ozone — AND NUCLEAR FALLOUT — become trapped. Thus, tropopause folds increase the chances of these pollutants interacting with the troposphere and those living in the troposphere!
Also, take a look at the change in wind speed at the tropopause fold. The wind speed increases from about 50 kts at the base of the tropopause fold (boundary of troposphere to stratosphere) to over 100 kts at the top of the tropopause fold (boundary of stratosphere back to troposphere). This is incredible vertical wind shear and cases all sorts of problems for those flying through one of these phenomena!
Below I have included the soundings from Norman, OK and Dodge City, KS. As is indicated in the cross section above, the tropopause fold (stratospheric air) is found closer to the ground in the Norman sounding than the Dodge City sounding.
Day 299: The Extratropical Cyclone
They say a picture’s worth a thousand words; I’m not sure that will be enough to describe what had to happen in the atmosphere to create what is shown above. This is a textbook example of an extratropical cyclone.
Sometimes it is best not to overanalyze something and simply stand in awe of its beauty. Tonight, I’ll do just that.
Day 298: The Developing (Meteorological) Bomb — UPDATE
To all my meteorologist friends, the definition of a bomb cyclone is one where the central (or lowest) pressure decreases by at least 24-millibars in 24-hours or less. You cannot use the pressure at any random point; it must be the central pressure.
Here’s a thought exercise. Say we have a cyclone that has already “bombed” meaning it has a fairly deep central pressure In fact, let’s say the cyclone is actually weakening with time (meaning the pressure is rising). As the cyclone moves, by definition, the pressure downstream of the cyclone should fall. Thus, as the cyclone moves it is possible for a location downstream to experience a 24-millibar pressure drop in 24-hours with a weakening cyclone! This would hardly be a bomb cyclone anymore!
You can also construct situations where a modest cyclone is rapidly moving into an area the is being vacated by a strong surface high pressure. Again, in this situation it would be possible for a location to experience a 24-millibar pressure drop in 24-hours entirely due to the ambient condition and not because the surface cyclone is rapidly strengthening!
You can read the original post on the developing “bomb” here:
Day 298: The Developing (Meteorological) Bomb
As noted by Harold Brooks (NSSL), Sanders and Gyakum defined a bomb as deepening at a rate of 1 Bergeron-24 mb fall in 24 hours at 60 N. Multiply the pressure fall by sin (60)/sin(lat). So, technically at 42.5 degrees, you need ~19 mb fall in 24 hours to achieve “bomb” status.
About Patrick
Patrick Marsh is a Ph.D. candidate at the University of Oklahoma's School of Meteorology. He earned a Bachelors of Science in Mathematics and Physics (cum laude) from the University of Arkansas in 2005 and a Masters of Science in Meteorology from the University of Oklahoma in 2007. In addition to working on his doctorate, Patrick serves as the National Severe Storms Laboratory Liaison to the Hazardous Weather Testbed where his research interests lie in working with operational forecasters, data mining, data visualization, and all things high-impact weather. For more about Patrick's work, check out his complete CV!
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