Wednesday, November 2, 2011

Expect more dry days

At Gan, we have been experiencing a few days with little rain, after some rainy days that Eric and Daehyun had discussed.
I did not expect this lack of rain on my flight here. There are other interesting things going on, though. Every 15 minutes, clouds over the sky are recorded by the four cameras facing the four directions on board the S-Pol. This is today's animation from sunrise to sunset from the camera facing east. Most of the clouds are not raining. They are moving toward the camera, because of the prevailing wind blowing west at upper levels. Around the noon time, there are some big raining cloud.
This was a picture facing northeast from my camera at the moment. This raining cloud did not move toward us, but moved northeastwards. It didn't bring any rain.
It turns out that this cloud is a tiny one on the radar screen, single convective cells. On the mobile Doppler radar screen, the Smart-R, it was a few kilometers away from us and tiny over a 300x300 area of the radar scope. They are within red circles in this radar image.
This cell was not organized, it did not grow high to 10 km, and it lasted only one and half hour. So this was not something particularly interesting for storm-chasers. It was something can be easily ignored. But our radar here captured its full life cycles. The radar is going to be running for 6 months. It will capture many, many of these single convective cells, also capture something much bigger, like cloud systems hundreds of kilometers, often called mesoscale convective systems.
The long period of radar observations during DYNAMO will give precious statistics of cloud systems over this part of Indian ocean. This is very important for developing numerical models for predicting global tropical weather and climate. Surely, it will take many years.
Another thing I want to mention. So far I haven't seen any cloud spinning. This is unlike what I observed at east coast of US or in Texas, where circulations can fire up convective systems. In the tropics, these clouds are more influenced by local vertical temperature and moisture profiles. Some people would say thermodynamics largely control these clouds; now I tend to agree with them. I do wish to do some research to find out how important is the regional circulation dynamics (e.g., gravity waves) for the clouds in the tropics.
What we see locally is only a little piece of the big MJO puzzle. In the big picture, this lack of rain is believed to caused by the suppressed phase of the MJO. The MJO phase diagram that Eric discussed looks like this:
Now the convective phase has moved to eastern Indian ocean and is expected to move further east. So there might be more days ahead with little rain. (These are good days for diving and snorkeling.) In the tropics, we also have many tropical waves. I hope to see some mesoscale scale systems from those waves locally soon.

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