Dr. Eric Wilcox, Research Professor of Atmospheric Science at the Desert Research Institute
Research Interests: Climate, climate change, and the atmospheric phenomena that influence regional to global climate, including the interactions of aerosols and clouds and their impacts.
Research Highlights: Twenty-four hours a day, seven days a week, the atmosphere is brimming with aerosols—tiny solid and liquid particles that include everything from windblown dust and water vapor to smoke from wildfires and industrial pollutants. These aerosols drift from the stratosphere to the surface, and range in size from the width of the smallest virus to the diameter of a human hair.
Yet, while they might be small, there’s no question that aerosols have a big impact on Earth’s climate thanks to their ability to absorb or scatter sunlight. How an aerosol interacts with light depends primarily on its composition and color. In general, bright-colored or translucent aerosols tend to scatter sunlight, cooling the atmosphere; darker aerosols absorb sunlight, warming the atmosphere.
Aerosols also affect climate by promoting the formation of clouds. In clean air, clouds are composed of a relatively small number of large droplets, which results in clouds that are somewhat dark and translucent. Conversely, in air with high concentrations of aerosols, water can more easily condense on the particles to create a large number of small droplets. Clouds consisting of numerous small droplets are dense, bright white, and highly reflective.
Although the effects of aerosols on cloud formation are well understood, the impacts of these aerosol-related processes on the heating or cooling of the atmosphere and, in turn, Earth’s climate, are not. Among the scientists working to better understand them is Dr. Eric Wilcox, research professor of atmospheric science in the Desert Research Institute’s Division of Atmospheric Sciences.
“[My] focus has been light-absorbing aerosols, including dust, smoke from wildfires and agricultural burning in the tropics and sub-tropics, and the combustion of fossil fuels like diesel, coal, and biomass for energy,” said Wilcox. “In the developing world, a lot of the combustion processes are somewhat incomplete and the dark, sooty particles they produce absorb sunlight and heat the atmosphere. So, a big area of focus has been trying to understand what the consequences of that might be.”
Included among these consequences is the effect of sooty aerosols on the development of low clouds.
“If you have a plume of wildfire smoke coming off of a continent in sort of a broad, regional sense and that absorbs a bit of sunlight and heats the atmosphere over a large area, are there circulation responses to that? Does that affect the dynamics of the boundary layer that creates low clouds?” Wilcox said. “Then, if you change the number of low clouds, you really change the amount of sunlight coming through. That’s where these knock-on climate effects can become more significant and that’s what we’re trying to understand.”
To investigate the atmospheric impacts of sunlight-absorbing aerosols, Wilcox and his colleagues have focused their attention on areas where high concentrations of light-absorbing aerosols are common, such as over the Southeastern Atlantic Ocean off the west coast of Africa, over the Java Sea near Indonesia, and over the Northern Indian Ocean.
“We’ve been to the Maldives to do field work and during the winter there are a lot of emissions from the combustion of fossil and biofuels, as there are in many rapidly industrializing parts of the world,” Wilcox said. “There are also some agricultural fires that happen [there] and the mixture of carbonaceous aerosols blows from the [Indian] subcontinent south over the ocean. That makes the region a useful place to study the effects of aerosols on the clouds that form over the ocean.”
To conduct his research in these areas, Wilcox relies on several datasets from the Moderate Resolution Imaging Spectroradiometer (MODIS) instruments aboard NASA’s Aqua and Terra satellites, which he gets from NASA’s Level 1 and Atmosphere Archive and Distribution System Distributed Active Archive Center (LAADS DAAC).
LAADS DAAC is part of the Terrestrial Information Systems Laboratory at NASA's Goddard Space Flight Center in Greenbelt, Maryland. As one of the 12 DAACs within NASA's Earth Observing System Data and Information System (EOSDIS), LAADS DAAC archives and distributes Level 1 data (geolocation, Level 1A, and radiance Level 1B) and Atmosphere (Level 2 and Level 3) data products from MODIS and several other satellite instruments, including the Visible Infrared Imaging Radiometer Suite (VIIRS) instruments aboard the joint NASA/NOAA Suomi National Polar-orbiting Partnership (Suomi NPP) and NOAA-20 satellites. The data from these instruments are essential for monitoring Earth’s atmosphere as well as for studying how aerosols and clouds influence climate, weather, and air quality.
“I’m an extensive user of the MODIS data for aerosols and clouds. These datasets are of such high quality that you can really do detailed, quantitative science on cloud physics and aerosols,” said Wilcox, who is also a member of the LAADS DAAC User Working Group (UWG). “Now we have more than 20 years of data because these instruments are so robust, and they’ve lasted for so long. This allows for really powerful statistical analysis. You can take 20 years over a region and build a really robust statistical understanding of the variations in clouds and aerosols and how they might relate.”
He also supplements MODIS data with data from Aqua’s Atmospheric Infrared Sounder (AIRS) instrument to measure the impact of light-absorbing aerosols on atmospheric temperatures.
“Those dark particles are absorbing sunlight and heating the atmosphere, but can we actually detect that signal with AIRS temperature retrievals, which profile temperatures in the atmosphere? Because [AIRS] is on [Aqua] along with MODIS, we have these co-located measurements, so you can build a really robust statistical relationship because we have 20 years of temperatures co-located with cloud and aerosol data,” Wilcox said.
In addition to data from AIRS, Wilcox also uses data from Aqua’s Advanced Microwave Scanning Radiometer for EOS (AMSR-E) instrument, which provides measurements of liquid water in clouds even when dense layers of carbonaceous aerosols are present.
“If you have a thick layer of smoke above the cloud, it interferes with the visible and infrared radiation you’re using to estimate the cloud’s properties,” Wilcox said. “But if you use AMSR-E, which is a microwave instrument, it sees through the smoke and observes the cloud.”
Wilcox used this combination of MODIS and AMSR-E data in a 2012 paper published in the journal Atmospheric Chemistry and Physics that investigated how the layers of light-absorbing dark carbonaceous aerosols appearing above the clouds in the Southeast Atlantic Ocean’s marine boundary layer impact radiative forcing (i.e., atmospheric warming). He and his colleagues found that the carbonaceous aerosols from the smoke resulted in a small increase in atmospheric warming, which was significant given that satellites have traditionally not provided good measurements of the effect light-absorbing aerosols have on the atmosphere.
“MODIS doesn’t really distinguish which particles are dark and absorbing sunlight and which particles are bright and scattering sunlight,” Wilcox said. “We don’t really have a good measure of the composition or the optical properties of these aerosols from a polar-orbiting satellite, so one of the motivations for doing this was, ‘Can we add temperature measurements from the satellites and combine them with the MODIS observations of aerosol optical depth to get some idea of when light-absorbing aerosols are actually having an impact on the atmosphere?’”
In another paper slated for publication in the journal Atmospheric Measurement Techniques Discussions later in 2023, Wilcox and his colleagues discuss their development of a database of deep convective clouds throughout the global tropics and subtropics and spans the entire MODIS data record from Aqua and Terra.
By matching individual clouds with Version 2 of the Modern-Era Retrospective analysis for Research and Applications (MERRA-2) dataset and brightness temperatures from AMSR-E, the database explores the relationships among the horizontal scale of cloud systems, the thermodynamic environment in which the clouds reside, aerosol concentrations, and indicators of the microphysical structures of clouds. It also provides a means for the empirical study of the factors determining the spatial structure and coverage of convective cloud systems, which are strongly related to the overall radiative forcing by clouds. This allows users to then compare these relationships with atmospheric models to evaluate the representation of clouds and convection in climate projections. This database is available from NASA’s Goddard Earth Sciences Data and Information Services Center (GES DISC).
“What we’re starting to see, on a broad scale, is that there are some relationships between the thermodynamic structure of the atmosphere and really big clouds that are different from those of small clouds,” Wilcox said. “We’re hoping that this database will be useful in comparisons with climate models, because getting the statistics of these really intense convective events right is not something models have traditionally done very well.”
This is not necessarily a problem with the models. Rather, it's a matter of computational limitations, for as Wilcox notes, when running a century-scale climate simulation, it’s difficult to run models at very high resolution. Therefore, he and his colleagues have had to rely on statistical approximations to account for all the detailed physics that they can’t explicitly model in their projections. The database Wilcox and his colleagues developed gives atmospheric researchers a new tool for evaluating those approximations.
“This database can provide some utility in comparisons with climate models to understand how well they’re doing in terms of the statistics of the occurrence, size, and structure of these cloud systems,” Wilcox said. “Maybe down the road the database will be helpful in refining some of the representation of the physics in the models.”
The database may also provide a means of exploring the impacts of aerosols on the micro- and macrophysical properties of clouds.
“Several hypotheses have been proposed for mechanisms by which aerosols may alter the structure and size of deep convective clouds. Such changes in the coverage of these clouds may contribute important impacts on the radiative forcing of cloud systems beyond those that might be attributed just to changes in cloud microphysics,” Wilcox and his coauthors write in the paper. “The dataset described here was constructed in part to test these ideas by capturing not just estimates of the aerosol loading in the vicinity of clouds, but also the thermodynamic properties . . . that strongly influence the scale of cloud systems.”
Yet even though he is a self-described “extensive user” of MODIS satellite data and champions the utility of the 20-plus years of MODIS data, Wilcox is quick to point out that advances in scientific understanding usually require a multifaceted, multi-method approach.
“In some places, there’s a strong diurnal cycle in the development of [deep convective] clouds that I can’t capture with data from MODIS, so we have to augment what we learn from polar-orbiting satellites with observations from geostationary satellites, from field measurements, ground remote sensing, and so on,” Wilcox said. “As a community, the real understanding we arrive at comes from combining all of these different methods.”
Wilcox contributes to that community—a community made up of scientists engaged in all those various methodologies—by doing work that, as he puts it, “tries to see if we’re all coming to a consistent story.” Undoubtedly, the decades’ worth of satellite data he gets from NASA’s LAADS DAAC plays a critical part in helping him make that assessment.
Representative Data Products Used or Created:
Available through LAADS DAAC:
- MODIS Level-1B Calibrated Radiances - 1 km (MOD021KM)
- MODIS/Terra Level-2 Clouds 5-Minute Swath 1 km and 5 km (MOD06_L2)
- MODIS/Terra Aerosol Cloud Water Vapor Ozone Daily Level-3 Global 1 Degree CMG (MOD08_D3)
Other data products used:
- Aqua/AIRS Level-2 Standard Physical Retrieval (AIRS2RET)
- AMSR-E/Aqua L2A Global Swath Spatially-Resampled Brightness Temperatures, Version 4 (AE_L2A)
- MERRA-2 inst6_3d_ana_Np Hourly, Instantaneous, Pressure-Level, Analysis, Analyzed Meteorological Fields V5.12.4 (M2I6NPANA)
- MERRA-2 inst3_2d_gas_Nx Hourly, Instantaneous, Single-Level, Assimilation, Aerosol Optical Depth Analysis V5.12.4 (M2I3NXGAS)
Read about the Research:
Wilcox, E.M., Yuan, T., & Song, H. (2023). Deep convective cloud system size and structure across the global tropics and subtropics [preprint; in review]. Atmospheric Measurement Techniques Discussions. doi:10.5194/amt-2023-6
Wilcox, E.M., Thomas, R.M., Praveen, P.S., Pistone, K., Bender, F.A.M., & Ramanathan, V. (2016). Black carbon solar absorption suppresses turbulence in the atmospheric boundary layer. Proceedings of the National Academy of Sciences, 113(42): 11794-11799. doi:10.1073/pnas.1525746113
Wilcox, E. M. (2012). Direct and semi-direct radiative forcing of smoke aerosols over clouds, Atmospheric Chemistry and Physics, 12(1): 139-149.
Wilcox, E. M. (2010) Stratocumulus cloud thickening beneath layers of absorbing smoke aerosol, Atmospheric Chemistry and Physics, 10(23): 11769-11777.