Rain catchers out at sea

2 October 2017, by Franziska Neigenfind

Photo: UHH/C. Klepp

On board the Meteor: Geoscientist Christian Klepp doing maintenance work on the precipitation-measuring device.

Eighty percent of global precipitation falls over the oceans, and plays an essential role in our climate. But how can we measure rain- and snowfall on the open seas? Geoscientist Christian Klepp is developing a new dataset based on extensive measurements, which he is gathering on board research ships with the help of specially designed equipment. This approach closes an important research gap, while also providing a new way to make satellite data more precise.

In the words of a crew member on board the research ship Meteor off the coast of Guinea during a heavy tropical rain shower: “It’s like a wall of water! It’s raining so hard I can’t see my outstretched hand.” At the time, September 14, 2015, one of my measuring devices was also on board: It recorded 40 liters of rain per square meter in just 20 minutes. It must have been an incredible cloudburst. For comparison: In Germany, severe weather warnings are issued for rainfalls of eight liters per square meter in the same time.

That extreme measurement is part of a new, world-ocean dataset that I am developing at Universität Hamburg’s Cluster of Excellence for climate research. My OceanRAIN project fills an important scientific gap, as to date precipitation over the sea has not been accurately and comprehensively measured on board ships – even though around 80 percent of all precipitation falls over the oceans. As such, the oceans play a key role for the water cycle in the climate system – including evaporation, which we also measure.

Special satellites – which indirectly determine the precipitation over a measured radiation field – provide a global picture. These require mathematical rules – algorithms that convert the radiation field into precipitation. Around the globe, numerous research institutes have created such algorithms, but until now they lacked high-quality, shipboard measurements of rainfall over the oceans. Now, for the first time the satellite data can be verified, calibrated and improved: If they reproduce what our equipment has measured, we know that the satellites are correctly adjusted. Climate models can also be improved in this way, and in turn verified using satellite data.

Until now there were no measurements over the oceans, because there was no suitable equipment for use on board ships. While land-based gauges collect rain – and less accurately, snow – shipboard measurements call for a different method, since here the wind often blows the rain away from the collection containers and they remain empty. On board a ship the containers are also exposed to more movement – for example stronger airstreams and more turbulence due to structures on deck.

The solution is an electronic device that allows me to determine the amount and type of precipitation: an optical distrometer. This records every individual raindrop and snowflake, creating a shaded area for each. Together with the manufacturer, I have further optimized the device for automatic operation. It offers an improvement on the previous satellite data, because it can differentiate between rain, snow and mixed phases. Using a distrometer we were able to accurately measure snowfall at minus forty degrees. It measures precipitation with 128 different particle sizes, from 0.1 to 22 millimeters in diameter.

There are now fourteen ships around the globe with our distrometers on board, with the longest continuous dataset being collected by the icebreaker Polarstern: For seven years, the distrometer recorded rain, snow and mid-phase precipitation from the Arctic Ocean to the Antarctic. Before I started my project, there were only a total of about 10, 000 rain measurements collected over the world’s oceans – and they were of comparatively poor quality. In the meantime we have taken more than seven million measurements, ten percent of which relate to precipitation. These are available for remote-sensing and climate research – and the dataset is still growing.

This content was first published as a guest article in the newspaper Hamburger Abendblatt on 20th September 2017.

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