Changes in the climate are created by complex interactions between temperature, clouds, aerosols, wind, moisture and a host of other factors. To get a better understanding of the specific impact of numerous low thin clouds on climate, a study at the Cloud-Aerosol-Rain Observatory (CAROb), based on Virginia Key, Florida, was conducted with an MPL system and other instruments over a 10-week period from 20 December 2014 to 1 March 2015, focusing on the characterization of low-optical-depth clouds residing near the surface.
The optical depth and height of clouds influences climate. Subtropical trade wind regions, like southern Florida, exhibit a higher incidence of low thin clouds than in other regions, and these clouds are connected to cooling.
“We know low clouds help cool the surface of the Earth; on a cloudy day, the temperature is cooler,” explains Dr. Paquita Zuidema, Professor at University of Miami. “The question is, how will the cloud distributions change within a warmer climate. We are looking at the processes for how small clouds adapt to climate change. A consensus view is slowly emerging in which a warmer climate will likely include fewer low clouds overall, and the ones that are there, will likely be thinner. The climate models operate on a large scale which don’t show the clouds in detail. We know less about this cloud type from space observations than of others because thin clouds are difficult to characterize. This is where the MPL becomes useful. It is able to see and even profile thin clouds. This was the topic of our study.” Figure A below describes one scenario of how clouds may be altered by climate change.
Low, shallow clouds are often too small for robust characterization from space; however, the high incidence of these clouds makes them radiatively important. MPL data characterizes low clouds by creating a detailed vertical profile, enhanced by the MPL’s exceptional signal-to-noise ratio and depolarization capabilities. When used in conjunction with other measurements, the vertical structure helps explain the interaction taking place between clouds, aerosols, wind, etc., which is crucial for improving models.
“The surface-based MPL provides a more accurate characterization of the clouds due to the narrow field of view of the LiDAR beam,” says Zuidema. “Also, its proximity to the cloud circumvents the issues with sensor detection, spatial resolution and obstruction, and three-dimensional radiative transfer that affect space-based studies.”
The MPL at Miami is also used for characterizing the vertical structure of dust. Due to Miami’s location at the southern end of a peninsula, bordered on the west by the Gulf of Mexico and on the east by the Atlantic Ocean, the dominant aerosols in the area are sea salt and dust. Both types are made up of large particles; however, sea salt is round, especially with moisture attached, while dust is angular. Dust has a significant impact on the climate primarily because of its interaction with sunlight; dust mostly scatters sunlight back to space, reducing the amount that can reach the Earth’s surface and thereby cooling the planet, but dust can also absorb some sunlight, changing the local atmospheric temperature profile. Large dust particles can also absorb some of the Earth’s warmth, providing a slight greenhouse effect.
MPL technology is particularly well suited for differentiating between sea salt and dust because its transmitted beam alternates between co-polarized and cross-polarized states. By tracking increases in dust intensity in the Earth’s atmosphere, MPL observations help assess how well dust events can be predicted.
If there are more thin clouds, even if overall climate is warming, will they still help cool the Earth? More needs to be known about the extent of this influence. Continued studies using MPL and the resulting measurements of dust and the optical properties of clouds will help refine weather forecasts and climate models.
For more information about Dr. Zuidema’s work, go to https://cloud-aerosol.rsmas.miami.edu/index.html. Details of the study in question can be found in Delgadillo, R., K. Voss and P. Zuidema, 2018: Characteristics of optically-thin coastal Florida cumuli from surface-based lidar measurements. J. Geophys. Res., 123, p. 10,591-10,605, doi:10.1029/2018JD028867.