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Modeling Shallow Cloud Effects on Global Climate
One of the crucial influences on global climate fluctuations is also the least understood. Shallow clouds, those that form near the Earth’s surface (in the atmospheric boundary layer), are known to significantly impact the energy balance of the ocean and atmosphere in the tropics and sub-tropics. As they form, persist and dissipate, these clouds play an important role in mixing energy and moisture from the Earth’s surface into the upper atmosphere.
At the University of British Columbia (UBC), Philip Austin and his students are working on a range of research projects aimed at better understanding the processes that determine the transport and radiative processes taking place in cloud fields.
“Modeling the impact of cloud mixing at global scales requires high spatial resolution simulations across a region large enough to contain a large number of small clouds, over time periods long enough for the cloud field to fully adjust to changes in the surrounding environment,” said Austin, an Associate Professor with UBC’s Department of Earth, Ocean and Atmospheric Sciences. “This makes the problem tractable only with the kind of large-scale, high performance computing resources provided by WestGrid and Compute Canada.”
Measuring and studying the behaviour and influence of cloud fields comes with some interesting challenges. Open questions include:
- the relationship between sub-cloud turbulence and the cloud field convective mass flux;
- the role of environmental humidity in determining the basic character of the convection;
- the differences between cloud transport of energy, moisture and the particles on which cloud droplets form.
- “nature vs. nurture” – the degree to which convection is shaped by cloud base properties or mixing over the cloud lifecycle.
It has recently become possible to use high resolution (“large eddy simulation”) models to directly measure the cloud-environment mixing (“entrainment/detrainment”) in an evolving cloud field. Austin and his team are using WestGrid and Compute Canada resources to perform three-dimensional simulations on domains of 25 x 25 km extent, across a range of atmospheric and ocean conditions that generate shallow clouds.
Last year they collaborated in a long-term intercomparison of the ability of 15 global climate models to simulate the effects of cloud fields, comparing high resolution cloud simulations to the global models. The deficiencies identified in that work spurred the organization of a special session on cloud mixing at the December 2012 meeting of the American Geophysical Union.
The results of Austin’s research hold exciting promise. His team’s findings will not only improve scientists’ global climate forecasting abilities, it will also increase our understanding of how human-induced climate change could affect Canada in the coming decades. For more information on Austin’s work, click here.