Imaging method devised to study cloud behaviour
Lawrence Berkeley National Laboratory scientists David Romps and Rusen Oktem have devised a new imaging method to study cloud behaviour. By using two digital cameras situated around 1km apart, the researchers were able to generate 3D data on how fast clouds rise, which could allow scientists to improve global climate models.
The work is based on stereophotogrammetry, whereby images are used to make 3D measurements of cloud boundaries. This technique has been used before to study cloud behaviour, but generally only over land where there are reference points. The work carried out at Lawrence Berkeley National Laboratory allows scientists to study clouds over the open ocean.
‘We want to answer a very basic question: with what speeds do clouds rise through the atmosphere? This is very difficult to answer by any technology other than stereophotogrammetry,’ Romps said. ‘Knowing their speeds is important for several reasons; the important one is that we lack a really basic understanding of what processes control these clouds, the levels they peter out at, and how buoyant they are.’
The technique was detailed in a paper in the Journal of Atmospheric and Oceanic Technology, with the cameras positioned looking out over Miami’s Biscayne Bay. Romps and his team have devised algorithms to automate the 3D reconstruction, quickly find feature points and match them. The accuracy of the technique was validated with lidar and radiosondes.
With images taken every 10 to 30 seconds, ‘we can really start to look at the full lifecycle of clouds’, said Romps, who has a joint appointment in UC Berkeley’s Department of Earth and Planetary Science. His technique also offers far higher spatial and temporal resolution than other technologies.
The US Department of Energy’s Atmospheric Radiation Measurement (ARM) programme has funded a second set of cameras at its Southern Great Plains site in Oklahoma, the largest and most extensive climate research site in the world.
Using stereophotogrammetry, Romps has measured the speeds of shallow clouds rising through the atmosphere at 1 to 3 metres per second and of deeper clouds rising at speeds in excess of 10 metres per second. ‘The updraft speeds play an important role in the microphysics, general precipitation, and aerosol processing, which all impact climate simulations,’ he said.
The largest source of uncertainty in today’s climate models is clouds. ‘We are still seeking a fundamental theory for moist convection, or what we call convection with phase changes. Without that theory, it is difficult to construct more accurate parameterisations [or models] of clouds that go into global climate models,’ Romps said. ‘Stereophotogrammetry can provide very useful information in this quest.’
Romps and Oktem are also developing new techniques to automate the reconstruction of three-dimensional cloud features.