Evaluating Large-Eddy Simulations of Boundary-Layer Cloud Response to Climate Change using MAGIC Observations

Technical Report · Tue Jan 29 00:00:00 EST 2019

DOI:https://doi.org/10.2172/1492729· OSTI ID:1492729

Bretherton, Christopher S.  ^[1]

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1. Univ. of Washington, Seattle, WA (United States)

Over the cool waters off California, a persistent and extensive deck of low-lying clouds forms atop a shallow, turbulent layer of moist air extending 500-2000 meters above the ocean, above which the air is much warmer and drier. Its west edge varies with weather and season and is typically half way to Hawaii. This cloud deck and similar cloud decks elsewhere reflect sunlight back to space, cooling the underlying ocean and indeed the entire planet. They must be accurately represented by a climate model to correctly simulate our current climate or future global warming. Most climate models suggest these cloud decks will thin and break up in a warmer CO₂-enriched climate, so extra sunlight would reach the ocean surface and amplify warming worldwide. But these models have difficulty accurately simulating the cloud thickness and extent because the turbulent cloud-forming air currents are much too small to be simulated by their coarse computational grid. In 2012-2013, the DOE Atmospheric Radiation Measurement MAGIC program instrumented a container ship transiting between California and Hawaii to observe clouds and rain, small particles (aerosols), and weather conditions. Data were obtained from 20 4-6 day cruises in each direction. This study tested whether a sophisticated ‘large-eddy simulation’ (LES) of the turbulent air motions that create the clouds in this region accurately predicts the variations in cloud properties seen across the different cruises. If so, that model is more believable for predicting cloud changes in this region in a future warmer climate. Our LES used a numerical mesh over a 6.4x6.4 km region with 50 m horizontal and 5 m vertical grid spacing to simulate the cloud-forming eddies and the sharp overlying transition between moist and dry air. To better compare with MAGIC observations, we configured our model to simulate a region that moved along with the ship. We used a weather forecast model to specify effects of the large-scale air flow. We compared our results with balloon ascents, a radar and other instruments pointed upward at the clouds, measurements of downward sunlight and infrared radiation, aerosols, and weather variables for all suitable cruise periods. Our LES was remarkably skillful in predicting the cloudiness variations measured by the ship. MAGIC also sampled mesoscale cells, regions of cloud thickening and thinning over distances of 10-50 km. An LES run over a larger region 100 km on a side shows these cells form spontaneously, enhanced by strong radiative heat loss at the cloud tops.

• Research Organization: Univ. of Washington, Seattle, WA (United States)
• Sponsoring Organization: Office of Science (SC), Biological and Environmental Research (BER). Earth and Environmental Systems Science Division
• Contributing Organization: European Centre for Medium Range Weather Forecasts (ECMWF) Brookhaven National Laboratory
• DOE Contract Number: SC0011602
• OSTI ID: 1492729
• Report Number(s): DOE-UWASHINGTON-11602
• Resource Relation: Related Information: McGibbon, J., and Bretherton, C. S., 2017: Skill of ship-following large-eddy simulations in reproducing MAGIC observations across the Northeast Pacific stratocumulus to cumulus transition region. J. Adv. Model. Earth Syst., 9, 810–831, https://doi.org/10.1002/2017MS000924.Bretherton, C. S., and P. N. Blossey, 2017: Understanding mesoscale aggregation of shallow cumulus convection using large-eddy simulation. J. Adv. Model. Earth. Syst., 9, 2798–2821, https://doi.org/10.1002/2017MS000981.Zhou, X., and C. S. Bretherton: Simulation of mesoscalecellular convection in marine stratocumulus. Part II: Non-drizzlingconditions. J. Adv. Model. Earth. Syst., 10, https://doi.org/10.1029/2018MS001448.
• Country of Publication: United States
• Language: English

Related Subjects

54 ENVIRONMENTAL SCIENCES
Clouds
Climate
Turbulence
Large-Eddy Simulation
Aerosols