August 27, 2013
Using ‘Dropsondes’ To Get Detailed Weather Information
We’re planning to fly over the vast Atlantic with our Global Hawk - once again looking at the dust and dry air from the Sahara. Takeoff on Saturday morning and landing on Sunday morning. We’ll fly from Wallops to a point near the Cape Verde Islands (just off of Africa) and back in about 25 hours. It took Columbus 5 weeks to sail from the Canary Islands to the New World!One of our key instruments is called the Airborne Vertical Atmospheric Profiling System or AVAPS. Actually, nobody here at HS3 actually calls it by this formal name, we all just call it the “dropsondes”. Now a sonde is a device that measures winds, temperature, and humidity. Sondes that are carried on balloons are called “radiosondes” because they transmit their information over radio frequencies. The NOAA National Weather Service launches about 200 balloon-borne radiosondes every day from weather stations around the country. The information is transmitted to the ground, quality controlled, and then the information is ingested into computer models that forecast our weather. Our instrument is called a dropsonde because we drop it out of the tail of the Global Hawk.
The dropsonde is an 11-inches long tube - about the size of the cardboard paper tube for a paper towel roll (see the attached picture of Drs. Gary Wick and Terry Hock holding a dropsonde). It’s fairly light and flimsy, and it includes a little parachute to slow it down. When we eject it from the Global Hawk, it drifts downward measurig and transmitting information about the temperature and humidity. As the wind moves the sonde, the little global positioning computer chip transmits the position. By comparing positions every few seconds, we can estimate how far the wind moved the sonde, and thereby get a wind speed and direction.
AVAPS is actually like a soda-machine with a total of 89 “cans” or dropsondes. Terry and Gary will upload a “load” command to the plane over the satellite communication system. This “load” command moves the dropsonde out of the rack, activates the sonde, and puts it into the tube. The pilots check to see if there is any air traffic below us, and if the air is clear, they give the go-ahead to “drop”. Terry and Gary then send a 2nd command to launch the dropsonde, and a plunger pushes it out the tube and it’s on its way to the ocean surface. Like the radiosonde, the dropsonde transmits its information back to the plane, and then that information is sent via our satellite communication back to us here at Wallops. After a quick check of the data, the dropsonde information is sent off to all of the world’s meteorological services, and into their computer models.
The Atlantic is a vast region that is well covered by satellites, but there is no radiosonde information for our data-hungry computer models. The dropsondes fill this huge data gap with detailed weather information.
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