Sunday, August 21, 2011

Tropical Depression Harvey & TS Irene: August 21, Update A

Ok, I’m now armed with a new bottle of window cleaner and can see things more clearly. We have what’s left of Harvey and Irene to chat about.

TD Harvey:
Let’s just get Harvey out of the way shall we? His convection started to deteriorate yesterday evening as he moved away from the bad influence of the Caribbean waters, and he is now just a tropical depression. Currently he is at around 17N, 92.6W (a little south of the official location), heading W(ish – with a bit of a southerly direction in there) at 15mph. Winds are 30mph and central pressure is 1006mb. He’ll probably stay over land (Mexico) and the plan is that he will fizzle away there. However, I see his circulation is still very good in the lowest half of the troposphere. If his center is a little south of the official location (17.7N, 92.6W) then there is a very very small chance that he might cross over into the Pacific and decide to have another go. If the center is at the official location, there’s a very small chance he will remerge into the Gulf before hitting Mexico again. This will be my last entry on Harvey, unless he does something else.

TS Irene:
“I can see clearly now the rain has gone” (who sang that first?). It pretty much sums up Irene (well, it also helps that it is daylight now). The dry, dusty air took its toll and the convection decreased a bit – it is still not very cohesive. This is why Tom on St. Thomas reported: “as of 6am not a single drop of rain here in Peterborg… and the trees are as still as a painting.” and then again about three hours later “we had a 5 minute rain about 45 min ago...still no wind yet but I think if the NHC track is correct things should get interesting later today as the VI will be in the northeast quad where the winds should be brisk.”

The NHC center was in the wrong place last night, but not to the south as I was guessing. It ‘redeveloped’ to the north. At the last advisory they had her at 17N, 63.2W, moving WNW at 20 mph. This means her track is closer to the north-eastern islands of the Caribbean (it might actually pass over all of them and skirt the northern side of Puerto Rico and the Dominican Republic).

They currently have her as a mid-level Tropical Storm with winds at around 50mph (TS range:39-73mph) and a central pressure of 1005mb. I think the winds might be stronger than this. Her circulation is very good in the lower half of the troposphere, and there is also a signal in the upper troposphere (the first clear signal up there that I’ve seen all year)– this is an indication of her being close to hurricane strength. However, because her rainbands are not very well developed, I don’t know if they will classify her as a hurricane until she has had a chance to improve her convection as well. The forecast calls for a hurricane as she gets to the Dominican Republic, but it will be interesting to see what call the NHC will make if she doesn’t manage to get the convection going. The main convection is to the north side of the center, so I agree with Tom’s analysis that they should get something more ‘interesting’ later today. Sea surface temperatures are around 29-30 deg C in this area, the upper 100m of the water is warmer than 26 deg C, and wind shear is not too strong – these are factors that will help her to develop. But working against her is the SAL and her interactions with the islands (although they are small, they still have a bit of an impact).

Another indication that she has good bone structure is the upper level divergence and lower level convergence, which are both looking good. I hear you ask: “But what gobbly-de-gook is this that she babbling about now? It’s Sunday morning… surely too early for a glass of wine already?” ;-)

<Science Alert> (yay! J).

So, in addition to the other things I mention regularly, I also look at something called upper level divergence and lower level convergence. Although we all look at maps or satellite images of tropical storm, we know that tropical storms aren’t only 2-dimensional and on the surface but also have a structure that extends up vertically through the troposphere (the lowest level of the atmosphere – see much earlier blog for definition). At the surface (where we live), we know there is a low pressure at the center. You can imagine a low pressure being like a dip or valley and high pressure being a hill (I think I’ve mentioned this before when I talked about storm steering). When you have a fluid, like rainwater, and there’s a dip in the land, that’s where the water flows towards. Well the air is the same. Where there is low pressure, it flows towards that, and where there is high pressure, it flows away. So one indication of a strong storm is the central low pressure – the lower it is (i.e. the deeper the dip), the faster the air ‘rushes’ in and the stronger the winds are. Of course as it doesn’t just flow straight ‘in’, it spirals inwards because of the Coriolis effect (because the earth is rotating). But basically, lower level convergence is the convergence (or inflow) of this air near the surface.

Now you can’t have all this air rushing in from all directions without it going somewhere… so where does it go? It can’t go down because that’s where the earth is, so it has to flow upward. But at the top of the troposphere (called the tropopause) there is essentially a barrier in the atmosphere that divides the troposphere from the next level up (the stratosphere), which stops the air just carrying on upward all the time like we would see with smoke from a chimney or steam from a steam train (if you are into those). So there is only one way it can go when it reaches the tropopause, and that is out from the center. Because all this air is being ‘pushed’ upwards and flows outwards at the top, instead of low pressure, we have a high pressure at the top of a tropical storm. So upper level divergence is the divergence (or outflow) of this air at the top of the storm.

If the lower convergence and upper divergence are good, it means that the vertical structure of the storm is good.

<End of Science Alert>

Here are maps that show Irene’s lower convergence (left) and upper divergence (right).

You can see a nice concentric circular(ish) pattern in both images near the storm, which is what I expect to see with something that has a nice structure. If it didn’t have good structure, the pattern would not be as circular, and there would not be many concentric contours. The more contours we see, the stronger the storm.

Now that I’ve given you all something to think about, I’ll go and put my feet up. J I’ll be back later today with another update!

Oh, and with Gert, Harvey and now Irene, Kevin in Florida wants to know if these are tropical storm names or somehow we got confused with the roster for the members of a shuffleboard club. ;-) If it’s any help, the next two are Jose and Katia.

Ciao for now!

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DISCLAIMER: These remarks are just what I think/see regarding tropical storms - not the opinion of any organization I represent. If you are making an evacuation decision, please heed your local emergency management and the National Hurricane Center's official forecast and the National Weather Service announcements. This is not an official forecast. If I "run away, run away" (Monty Python), I'll let you know.

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