Sunday, November 20, 2011

Gust fronts and dragonflies

Hi, this is Zhiming, the last of our modeling group to set foot on the Gan island.

I got here in the evening of Nov. 19th. It was a long flight but I slept reasonably well on the airplanes. And I was advised not to go to bed before 10pm to get over the jet lag sooner. So after dinner, I had a beer with the folks that were already here, Adam Sobel, Chidong Zhang, Jean Philippe Duvel, Deanna Hence, and Hannah Barnes. We talked about the interesting weathers that were going on, and Adam has been writing about these in other posts of this blog. In those conversations, I came across this idea about dragonflies giving the radar returns for gust fronts. "That sounds pretty exotic", I thought.

The next morning, I went to the S-Pol site. It is an impressive radar (see pictures in earlier posts). With 1 MWatt of power, 0.9 degree of beam width, and dual polarization, it can see a lot of things. Tammy Weckwerth, a NCAR scientist working with S-Pol, kindly showed me some of the data they have been collecting.

Here is a snapshot of the ZDR data from the radar (left figure below), where ZDR is the ratio of the returns in the two polarizations. It gives a measure of the aspect ratio of the objects that are scattering back the radar signal. Higher ZDR values tell us the objects are wide in the horizontal and short in the vertical. Such values are useful to tell apart different types of objects: for example, whether they are snow, hail, or raindrops.

This figure is 300km across in each direction. Note the red circles between 11 and 12 o'clocks and also between 6 and 8 o'clocks. It's a scan at 0.5 degree angle. At 50km radius, the radar is looking at roughly between the surface and 1km. These circular features were spawns by small showers.

"That's pretty cool", I said. The features also reminded me about pictures that I had seen like this one (right figure above) from NASA's Multi-angle Imaging SpectroRadiometer (MISR), where the cold pool expands and the gust fronts spawn new lines of convection.

The question now is what's giving the radar returns in the S-Pol image? Cloud droplets are nearly spherical and should have a ZDR close to zero.

"Dragonflies." said Tammy.

I didn't have too much beer the night before and as I said I slept pretty well during the flights. So I remembered this exotic idea from the conversations that I had the night before. Now I know where it came from.

I was intrigued. Swarms of dragonflies telling a radar scientist where the gust fronts are... This is not unusual over land where reflection off insects is common. But over the open ocean... Hmmm. "The ZDR is too high for cloud droplets and for clear sky turbulence", Tammy reasoned. " And "there is this annual migration of dragonflies from India to the Maldives and Africa and back", she quoted a recent paper, "the maximum numbers are here in November and December". Apparently, the dragonflies reproduce in fresh water pools whenever possible and it takes 4 generations for them to migrate. I would encourage you to contact Tammy to learn more about the dragonflies. And if you are an entomologist reading this blog (for whatever reason), you might be interested in knowing that the dragonflies might have helped Tammy to track the gust fronts.

It would be nice to actually see the swarms of dragonflies by eye. Tammy is still waiting to hear from the P3 crew on whether they saw the dragonflies when the plane flew through a gust front. It would also be interesting to see if this return signal from the radar becomes more rare when the migration season is over.

That was my first day at S-Pol. Among discussions of waveguide, beam width, extratropical intrusions and of course MJO initiations, was the story of dragonflies and gust fronts.

Oh, before I go, a couple pictures of rainbow in the rain shaft and the sunset.

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