Newswise – Argonne’s new algorithm for capturing drizzle-turbulence interactions could improve predictions of future climate conditions.
From space, large bridges of closely spaced stratocumulus clouds appear as brilliant cotton balls hovering above the ocean. They cover large areas – literally thousands of miles of subtropical ocean – and linger for weeks, if not months.
Because these marine clouds reflect more solar radiation than the ocean surface, cooling the Earth’s surface, the lifespan of stratocumulus clouds is an important component of the Earth’s radiation budget. It is therefore necessary to accurately represent the lifetimes of clouds in models of the Earth system (MY) used to predict future climatic conditions. Turbulence – the movements of air occurring on a small scale – is primarily responsible for the longevity of marine stratocumulus clouds.
“Analysis of the developed dataset allowed us to show that drizzle decreases turbulence under stratocumulus clouds, which was only shown by model simulations in the past. The wealth of data developed will allow us to answer several fundamental questions concerning drizzle-turbulence interactions in the future. “- Virendra Ghate, atmospheric scientist in Argonne
Drizzle – precipitation made up of water droplets less than half a millimeter in diameter – is constantly present in and below these marine cloud systems. Because these tiny drops affect and are affected by turbulence under marine clouds, scientists need to know more about how drizzle affects turbulence in these clouds to enable more accurate climate predictions.
A team led by Virendra Ghate, an atmospheric scientist, and Maria Cadeddu, senior atmospheric research engineer in the environmental sciences division of the US Department of Energy (DOE) Argonne National Laboratory, studies the impact of drizzle inside sea clouds since 2017. Their unique data set caught the attention of researchers from DOELawrence Livermore National Laboratory.
About three years ago, a collaborator from Livermore, who led national efforts to improve cloud representation in climate models, commissioned observational studies focusing on drizzle-turbulence interactions. Such studies did not exist at the time due to the limited number of observations and the lack of techniques to derive all the geophysical properties of concern.
“Analysis of the developed dataset allowed us to show that drizzle decreases turbulence under stratocumulus clouds – something that was only shown by model simulations in the past, âsaid Ghate.“The wealth of data developed will allow us to answer several fundamental questions concerning drizzle-turbulence interactions in the future. “
The Argonne team set out to characterize the properties of clouds using observations measuring atmospheric radiation (ARM)‘s site in the eastern North Atlantic, a DOE Office of Science User Facility and data from instruments aboard geostationary satellites and in polar orbit. The instruments collect engineering variables, such as voltages and temperatures. The team combined measurements from different instruments to derive the properties of water vapor and drizzle in and under clouds.
Ghate and Cadeddu looked at geophysical variables, such as cloud water content, drizzle particle size, and others. They therefore developed a new algorithm which synergistically recovered all the necessary parameters involved in the drizzle-turbulence interactions. The algorithm uses data from several ARM instruments – including radar, lidar and radiometer – to derive the geophysical variables of interest: size (or diameter) of precipitation drops, quantity of liquid water corresponding to cloud drops and precipitation drops. Using data from ARM, Ghate and Cadeddu derived these parameters, subsequently publishing three observational studies that focused on two different spatial organizations of stratocumulus clouds to characterize the drizzle-turbulence interactions in these cloud systems.
Their results led to a collaborative effort with modelers at Livermore. In this effort, the team used observations to improve the representation of drizzle-turbulence interactions in DOEEarth system model at the exascal scale of energy (E3SM).
“The observational references of the Ghate and Cadeddu recovery technique helped us to determine this version. 1 of E3SM produces unrealistic misting processes. Our collaborative study implies that comprehensive reviews of modeled cloud and drizzle processes with observational references are needed for current climate models, âsaid Xue Zheng, researcher in the Atmosphere, Earth and Energy division at Livermore. .
Said Cadeddu:“Usually, the unique expertise here in the lab is attributable to our ability to move from raw data to physical parameters and from there to physical processes in clouds. The data and the instruments themselves are very difficult to use as they are mostly remote sensors that do not directly measure what we need (e.g. rainfall rate or liquid water path) ; instead, they measure electromagnetic properties such as backscattering, Doppler spectra, and radiance. Additionally, the raw signal is often affected by artifacts, noise, aerosols, and precipitation. The raw data are either directly related to the physical quantities that we want to measure through sets of well-defined equations, or indirectly related. In the latter case, to derive the physical quantities means to solve mathematical equations called‘inverse problems âwhich in themselves are complicated. The fact that we have been able to develop new ways to quantify the physical properties of clouds and extract reliable information about them is a major achievement. And that has put us at the forefront of research on these types of clouds. “
Because they focused only on the few aspects of the complex drizzle-turbulence interactions, Ghate and Cadeddu plan to continue their research. They also intend to focus on other regions such as the North Pacific and South Atlantic Oceans, where the properties of clouds, drizzle and turbulence differ significantly from those of the North Atlantic.
The work of Ghate and Cadeddu was funded by DOEScience office. Their research has been published in several scientific journals, including their Collaborative Modeling Study from Livermore calling for observational studies of drizzle-turbulence interactions; their study of improved recovery technique; their studies which quantified the drizzle-turbulence interactions during two different marine stratocumulus organizations (study 1 and study 2); and a study by Livermore collaborators that used their recoveries to evaluate and improve model simulations.
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