Most of the data that we collect on AMT4SentinelFRM are water profiles or continuous surface water measurements. However, every day at midday we also collect atmospheric data when Werenfrid Wimmer from the University of Southampton launches a weather balloon aft of the Conductivity, Temperature and Density CTD winch gantry.
These balloons carry a device, which is about the size of your hand, through the lower atmosphere. It collects atmospheric temperature, humidity, pressure, wind speed, wind direction and geographic position data as it ascends to about 30 km above the ship and travels up to 50 km away from it.
The device carried by the balloon is known as a radiosonde and is powered by batteries (6 AA Alkaline) that transmit the data by radio to a ground receiver at a frequency of 403 MHz.
On land, hundreds of radiosondes are launched everyday all over the world, and the data forms an essential component of numerical weather prediction. There are very few of these launches over the open ocean, so the data that Werenfrid collects on this year’s AMT4SentinelFRM campaign is not only very rare, but is essential to help to model and correct for atmospheric effects in satellite data over the open ocean.
The balloons that lift the radiosondes through the lower atmosphere are made of latex and are filled with helium. The balloon ascent is determined by the thickness and diameter of the balloon, and grade and purity of the helium. On AMT4SentinelFRM we use 350 g balloons and the maximum height reached so far on this campaign over the Atlantic has been 29770 m.
We will use five 50 litre bottles of grade A Helium for approximately 42 launches during this year’s campaign.
As the balloon travels upwards through the lower atmosphere, the pressure decreases, which causes the balloon to expand and eventually burst after which a small parachute carries it back down to the sea surface. The data are transmitted by radio to a computer, where the data is stored and can be downloaded.
Globally, there are approximately 1300 radiosonde launch sites and launches are coordinated at the same time every day (00:00 UTC and 12:00 UTC).
This effort provides a worldwide perspective of atmospheric conditions and is used to improve and refine numerical models for weather forecasting. There are very few sites over remote ocean locations, especially in the Southern Ocean.
AMT4SentinelFRM and Werenfrid’s work contribute to filling in these gaps.