Saturday, December 24, 2011

On a long-lived MJO event, and another Sri Lankan cyclone

We really seem right back in an active MJO phase in the Indian ocean.  Is it a new MJO event or did the old one never really leave as it was supposed to?  This comes down to a matter of what the MJO really is.  If it's something that goes eastward at a certain phase speed, then this is a new event.  But if we allow a little more flexibility - if we had a definition in terms of MJO dynamics rather than just statistical properties - then it's not so clear.  Maybe some MJO events just don't do so well at the propagating eastward part.

In any case, the satellite images from the water vapor IR channel of Meteosat as well as the MIMIC total column water vapor product both show the jellyfish structure, with the head of the jellyfish pushing up against Sumatra from the west:

The jellyfish is often a precursor to the formation of tropical cyclones.  Indeed the northern blob of clouds just east of Sri Lanka has been declared an "invest" by the JTWC.  Their current statement on the system says that the potential for the development of a significant tropical cyclone is "medium".  After my earlier experience, I now pay close attention to what happens here.  The Sri Lanka Meteorological Department does, to their credit, have a bad weather warning out on this system.  They politely say that "the fishing and naval community are kindly advised to refrain from their activities" in the region of the storm.  They do not, however, make any mention of the fact that there is a significant probability of this system becoming a proper tropical cyclone.  They have a specific area of their web page for cyclone warnings, which is currently blank.

As usual our information about what will happen in the future is in part based on numerical models.  Here is the 24-hour forecast of low-level (850 hPa) wind and relative humidity from the NOAA GFS model, valid 12Z Christmas day.  Note the swirling winds just east of Sri Lanka:
Also note the consistency with the GFS run of a few days ago shown in the previous post (valid the preceding day, today).  I didn't talk about this Sri Lankan system in that post but it was there in the model.  The consistency from run to run does remind me of the November storm.

I don't understand why the Sri Lankan forecasters (and as far as I can tell, the Indian ones as well - the IMD has no cyclone warning that I can find on this system either) think it would be so wrong to tell the public that there is a good chance a cyclone might form soon.  Surely they know that the potential is there;  they just seem not to want to use the word "cyclone" until one actually exists.  But that word gets people's attention more than just "bad weather" does, and is more likely to change people's actions.  Yes, there is a chance of over-warning, but isn't it better to take that risk if it could save lives?

Astute observers will notice that the GFS model is also still spinning up the sytem off the top end of Australia.  That system is not impressive yet, but JTWC statement on it rates its potential for becoming a significant cyclone within the next 24 hours as "high".  The Australian Bureau of Meteorology's forecast is here.  It isn't forecast to hit Darwin at this point, but still, the timing is spooky, for those who know Darwin's history with tropical cyclones.

Tuesday, December 20, 2011

The latest active phase cyclones

Tropical storm Washi formed around December 13, in the West Pacific (6N, 145E). At this point the MJO amplitude was weak. About a week earlier, though, the MJO had been quite strong - around 2 on the RMM scale - somewhere on the boundary between phase 4 and 5, which means active phase near the region of Washi's formation. This is pretty typical. The active MJO phase creates an environment favorable for tropical cyclone formation, but it often takes a few days or a week for the cyclones to actually get going. So the peak of cyclogenesis is not in the midst of the active phase, but a little later. It's fair to say that Washi probably owed its existence in part to the MJO.

Washi went on to do horrible damage on the island of Mindanao in the Philippines. The death toll now stands just below 1000. This is more than half the number of people who died in Hurricane Katrina. Yet this is not a story featured prominently in the news, at least here in the US.

In the meantime, at least some of the models are predicting a cyclone off the Top End of Australia, not far from the city of Darwin, in the next few days. Here's the GFS model's prediction of wind and relative humidity at 850 hPa (lower troposphere) for Christmas Eve:
The Australians living in this part of the Northern Territory have some experience with cyclones forming on Christmas Eve.

Monday, December 19, 2011

Can't let go

Here are the current Meteosat water vapor channel and MIMIC total precipitable water satellite images:
Looking at these, you might say that while by now the MJO should have moved on across the Pacific, it seems it hasn't; the center of convection seems to be somewhere in the Maritime Continent (aka Indonesia), with the eastern Indian ocean still being somewhat active, despite the title of my last post. The RMM index would agree with you. By that measure, the MJO continues to tool around somewhere between phase 4 and 5, unable to decide if it wants to move on east, go backwards, or die altogether:
Consistent with this the westerly wind anomalies are persisting - actually picking up again - over the Indian ocean:
And the recent soundings at Addu Atoll, such as the latest one, look not too far off what one would expect with the active phase - something like what we saw in the buildup. Not really busting loose, but not far from it. High humidity (consistent with the MIMIC picture), with dew point not much less than temperature, and healthy westerlies in the lower troposphere: Why can't the MJO make up its mind? What's going on?
This is not too uncommon really. In fact the spectacular behavior we had for the first two months of DYNAMO - with two healthy MJO cycles coming through like clockwork in two months - was somewhat unusual. What's interesting is that the forecast models reasonably well predicted this current slowdown (though perhaps the amplitude is staying a little stronger than predicted). What did they see? What atmospheric conditions cause the MJO to either move forward or stall, or to strengthen or die?

Wednesday, December 7, 2011

Dried out

The MJO has moved into phase 4 or even maybe 5 by now, with the active convection over the "Maritime Continent" (Indonesia) and making its way towards the Pacific. The Indian ocean has been suppressed for around a week now. Here's the current infrared satellite image:

There is some deep convection in the IO, but it's spotty. In the east, it's focused way in the southern hemisphere, in the form of two tropical cyclones. The eastern one has been named "Alenga", and is the stronger of the two (though still a tropical storm, below category 1 on the Saffir-Simpson scale used in the US), while the weaker western one has been dubbed, poetically "Two" - it will get a better name if it gets intense enough to deserve it. North of the equator, there is a well-defined but not particularly intense ITCZ in the west, off the east African coast. The northeast quadrant is pretty quiet up until Sumatra. Convection in the Bay of Bengal and Arabian Sea has completely shut down, perhaps for the season as we head into northern winter and the sea surface temperatures there are cooling rapidly. The southeastern IO is also completely dry. This can't be ascribed to the season, but perhaps can be blamed in part on the suppressed phase of the MJO.

A lot of us working on DYNAMO have become fond of the "morphed" precipitable water product called "MIMIC" produced at the University of Wisconsin. This is a product based on passive microwave satellite observations. ("Passive" means that the radiation measured is just what's naturally emitted from the planet, as opposed to e.g. a radar that sends out its own signal; "microwave" refers to the low frequency of the radiation measured.) The quantity shown is total column-integrated water vapor. That is, if you took all the water vapor in the atmosphere overhead at any given point and condensed it to liquid, this is how deep a layer of liquid water it would make. If the whole column were saturated - 100% relative humidity from the surface into the stratosphere - this depth would be a little over 70 mm, typically, in the tropics. (This maximum saturation value depends on the temperature, and so is less at higher latitudes.) Not much water in the atmosphere, right?

In any case, the atmosphere is never completely saturated, so the values you see are less everywhere than 70 mm (see the color scale on the right) though they get close in some places. A couple interesting things to notice in this picture:

1. The two tropical cyclones, Alenga and Two, show up as round red blobs swirling gracefully in the south, wrapping dry air around them on their western flanks.

2. The tongue of yellow and green pushing into the orange and red area - just south of the equator in the western IO, and extending about as far east 80E, roughly parallel with the tip of India. This is dry subtropical and continental air that has been pulled in by low-level westerlies associated in part with the late active and early suppressed phases of the MJO. It seems that the active phase to some extent puts an end to itself because the convection causes the westerlies (by fluid dynamics I'm not explaining today) but then the westerlies bring in dry air which shuts down the convection.

Going briefly to the local picture, here is a recent radiosonde trace from Gan plotted as a "skew-T log-P diagram". This is a very strange graph used only by meteorologists. I will not explain it in detail today, we'll leave that for another post. For now I just want you to notice that the green and red traces are relatively far from each other for most of the way up. This means that the dew point is much less than the temperature, which means the relative humidity is low. Not that low really, by the standards of the earth as a whole - the column water vapor is 45 mm (4.5 cm) as shown in the inset plot on the upper right. Notice, incidentally, that this value derived from in situ balloon measurements is in pretty good agreement with the satellite-retrieved value. In the MIMIC picture above, look at 73E, 1S - the location of Addu Atoll - and notice the color is yellowish there, which is around 45 mm on the scale at the right.
45 mm of column water vapor would be a large value - a humid atmosphere - in some places, for example New York City in winter. But for Addu Atoll in November, this is a relatively low value, lower than any observed during the 17 days I was just there during the buildup and active phase.