Friday, November 25, 2011

Cyclone and westerly wind burst

As we get deeper into the active phase, the flow over the Indian ocean has come to be dominated by the developing tropical cyclone that is just north of the equator and east of the Maldives. In satellite imagery the clouds are beginning to have a hint of telltale swirl, but still the convection associated with this storm is not yet that well organized. Just looking at the latest Meteosat water vapor image for example, you might just say it looks like an active MJO event.
If you look at the low-level flow on the other hand - here in a very short-term forecast from the French Arpege model - you clearly see the circulation, just south of India. The colors in the image show geopotential at 850 hPa; think of this as a measure of the pressure field just a little ways above the surface. The blue indicates the center of the low pressure system.
The various operational weather centers that track tropical cyclones in this region are just now starting to take an active interest in this system. The US Navy's Joint Typhoon Warning Center's current warning says "THE POTENTIAL FOR THE DEVELOPMENT OF A SIGNIFICANT TROPICAL CYCLONE WITHIN THE NEXT 24 HOURS IS HIGH." The Chennai regional office of the India Meteorological department has a statement on their front page that reads "A well marked low has formed over Comorin area and neighbourhood on 25.11.11. Fishermen are advised not to venture into open sea along and off Kerala, Lakshadweep and South Tamil Nadu coast".
This is a very low latitude storm, with a center currently diagnosed at about 4 degrees latitude - much closer to the equator and tropical cyclone formation becomes more or less impossible. On the equator, or just south of it as here in Addu, it's hard to say to what extent the low-level westerly winds are associated with the cyclone, and to what extent they are part of the MJO westerly wind burst. Maybe the distinction isn't even meaningful. What's for sure is that the westerlies are strong. Yesterday the P3 flew in the area and dropped many "dropsondes" - basically similar to radiosondes, except instead of going up with a balloon, they are dropped from a plane. Here is one very near the equator south of the cyclone, showing westerlies as strong as 40 knots in a deep layer. (The picture below is a little too fuzzy, you can see a better version here.) Look at the wind barbs on the lower right; a long barb is 10kt, a short one is 5; you have to make an effort to read them as they are stacked very close together, indicating that the sonde made many measurements close together in height.
That's a westerly wind burst for you!


  1. This is exciting to watch, massive lower-mid level convergence and the westerlies! Very impressive stuff!

  2. This comment has been removed by the author.

  3. Hi Adam,

    What have the surface winds been like on Gan?


  4. Surface winds at Gan have been westerly to southerly, up around 10 m/s or even a little higher for a day or day and a half around the 23rd, then fluctuating between 2-10 for last couple days, with average probably about 5. You can see daily time series on the catalog, research products -> Gan products (near bottom) -> meteogram. Bunch of other fun things there too like the rain gauge at the ARM site (there are a couple but the one that is just called "precipitation" seems the best, it's a tipping bucket).

    It's hard to bike across the Fedu bridge in 10 m/s westerlies...