Tropical Cloud Research  [28.02.20]

How fast will global warming progress in the coming decades? A decisive factor for this has hardly been researched so far: tropical fair weather clouds, which form over the ocean day after day in the trade-wind regions near the equator and act as a cooling protective shield for the atmosphere. If the cloud cover were to become thinner under the influence of climate change, this could significantly accelerate the further rise in temperature. The worldwide unique European field study EUREC4A with participation of more than 30 national and international partners took a closer look than ever before at the complex interactions between ocean and atmosphere using the region around Barbados as an example. Five climate researchers and high-power laser technology from the University of Hohenheim were also involved. The ARTHUS (Atmospheric Raman Temperature and Humidity Sounder) lidar system, which was used on board a research vessel, is regarded worldwide as the best remote sensing system for measuring water vapor and temperature in the lower atmospheric layers

 

 

Four research aircraft, four ocean-going research vessels, five autonomous underwater vehicles, state-of-the-art ground-based remote sensing, and a new generation of sophisticated satellite remote sensing methods and high-resolution climate modelling: The German-French-led campaign EUREC4A (Elucidating the role of clouds-circulation coupling in climate) is currently considered to be the most ambitious European field study in the area of atmospheric and marine sciences.

"The overall objective is to make more accurate predictions about the further course of global warming. In recent years there has been growing evidence that the equatorial trade-wind regions are particularly sensitive to climate change, i.e. that they play an important role in the global climate system. Small changes here could trigger feedback effects that significantly accelerate the rise in temperature," explained Prof. Dr. Volker Wulfmeyer from the Institute of Physics and Meteorology at the University of Hohenheim, who conducted a measurement campaign on board the research vessel Maria S. Merian as part of EUREC4A in recent weeks.

 

Trade-wind clouds cool the atmosphere

The research interest focuses on cloud formation in the trade-wind regions. Like all fair weather clouds, tropical trade-wind clouds form day after day at a uniform height above the ocean and dissipate again in the evening. They are particularly light and flat and rarely cause precipitation.

But as fleeting as these cloud formations are, their role in the global climate could be highly significant: "The cloud cover in the trade-wind regions reflects more sunlight than the ocean. It therefore has a cooling effect on the atmosphere. However, climate researchers suspect that the trade-wind cloud cover could decrease considerably as a result of global warming. This could further accelerate the future rise in temperature," said Prof. Dr. Wulfmeyer.

 

Verifying complex interactions

Hypotheses on this feedback effect were developed over a period of decades from several rounds of international comparative climate model studies (CMIP). The aim of the measurement campaign EUREC4A is now to check whether these models are correct.

"To do this, it is necessary to better understand how clouds form, spatially organize, and dissolve. Important factors for this are temperature, humidity, and upwinds and downdrafts that occur when air masses in the atmosphere collide or drift apart. Little research has been done so far on the complex interaction of the ocean with the atmosphere. For example, what influence do ocean currents, ocean temperature, rising water vapor, etc. have on cloud formation," said Prof. Dr. Wulfmeyer.

There are already models that can depict the highly complex interactions. However, these still need to be checked and adapted on the basis of measurements. For the first time in the EUREC4A campaign, small and large scale observations of atmospheric and oceanic processes will be linked together.

 

Laser systems from the University of Hohenheim in use

In recent weeks, high-power laser technology from the University of Hohenheim has also been used on one of the four research ships: The Atmospheric Raman Temperature and Humidity Sounder ("ARTHUS") is one of three lidar systems from the Institute of Physics and Meteorology. It is regarded worldwide as the best remote sensing system for measuring water vapor and temperature in the lower atmospheric layers up to 4 km altitude in extremely high resolution.

"A special feature of the Hohenheim method is that it also enables three-dimensional representations to be made. For this purpose, a third laser is tilted like a radar during the measurement process, so that not only the air column directly above the laser can be captured, but also a complete section of several square kilometres. This allows, for example, the wind profile to be measured up to a height of several kilometers," said Dr. Andreas Behrendt, head of the remote sensing team.

The measurements of the lidar systems on board the research vessel Maria S. Merian were carried out by the Remote Sensing Department of the Institute of Physics and Meteorology (IPM) at the University of Hohenheim, with the participation of scientists Dr. Diego Lange, Dr. Andreas Behrendt, Dr. Florian Späth, and Timo Keller. The campaign was headed by Prof. Dr. Volker Wulfmeyer.

 

Background: "Grand Science Challenges on Clouds, Circulation and Climate Sensitivity"

The EUREC4A field study, which ran from 20 January to 20 February 2020, is a highlight of the "Grand Science Challenges on Clouds, Circulation and Climate Sensitivity" of the World Climate Research Programme (WCRP). It builds on a decade of measurements in the tropical Atlantic, which began in 2010 with the construction of a cloud observatory on Barbados and continued with two measurement campaigns with the research aircraft HALO in 2013 and 2016.

 

Text: Leonhardmair / Translation: Neudorfer