Tuesday, November 29, 2011
Monday, November 28, 2011
Sunday, November 27, 2011
Friday, November 25, 2011
Wednesday, November 23, 2011
Now for a change let's look at the time series of surface wind observations for today, below. The blue is wind direction, with the scale on the left, in degrees. So zero degrees is wind from the east, 90 is from the north, 180 is from the west, etc. Wind speed is on the right, in meters per second. So what this plot shows is that at about 730 UTC - 1230 local time in Addu - our winds spiked up and shifted from southwesterly closer to westerly. Since then we have had winds of 10-12 m/s, which is 22-25 mph. At least from this one island, on this one day, it looks and feels like a "westerly wind burst", which is a classic indication of the MJO active phase. The fact that it keeps pouring doesn't do anything to diminish one's impression that that's what's going on.
Monday, November 21, 2011
Sunday, November 20, 2011
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.
Saturday, November 19, 2011
Friday, November 18, 2011
Thursday, November 17, 2011
The P3 then flew through a region of active convection near Diego Garcia. This was quite a large convective complex. Here it is on the IR satellite image, look around the letters "JDG" in the southern hemisphere:
Tuesday, November 15, 2011
Monday, November 14, 2011
Sunday, November 13, 2011
While scientists are converging on the Indian Ocean, a small team of forecasters from the NOAA Climate Prediction Center (CPC), University of Albany (SUNY), the Centre for Australian Weather and Climate Research (CAWCR), and the Cooperative Institute for Climate and Satellites (CICS) are busy monitoring and creating short-term climate forecasts for the global tropics. This weekly assessment helps the DYNAMO campaign prepare for MJO events, so accurate measurements can be gathered.
Led by Jon Gottschalck, the team of Paul Roundy, Matt Wheeler, Carl Schreck, Matt Rosencrans, Augustin Vintzileos, Michelle L’Heureux, and others come together each Monday via teleconference to discuss the latest observations over the Indian Ocean DYNAMO array and the rest of the global tropics (primarily from 30°N to 30°S). The goal is to generate a short assessment of which tropical climate patterns, or “modes,” are influencing regions of precipitation and tropical cyclogenesis during the next one-to-two week period. This discussion leads to the generation of the “Global Tropical Hazards/Benefits Outlook.”
In order to understand the phenomenon and their impacts, the team considers the observations, such as outgoing longwave radiation (OLR) and low-level and upper-level winds, and examines them from several different perspectives (see the previous posting “On the Hovmoeller plot”). They also consider many different forecast tools, based on dynamical models (observations are ingested and equations are solved on large supercomputers) and statistical models (conceptually simpler models based on historical observed relationships). Many scientists on the team have developed web-based resources to help guide the forecast discussion:
The MJO can affect the circulation over the entire globe including the higher latitudes, such as the United States, which is sometimes discussed during the teleconference. Many of the forecasters are currently working with other researchers to investigate how they can use information from the MJO and other tropical waves to improve short-term climate prediction (weeks out to a month) across the globe. Even after the DYNAMO field campaign is over, the MJO briefing and Global Tropical Hazards/Benefit Outlook will continue, and so the team is excited about the potential for the DYNAMO field campaign to new generate insights and breakthroughs that will help improve climate prediction.