The climatology of the atmospheric boundary layer

Richard Davy: Researcher at the Nansen CenterRichard Davy: Researcher at the Nansen CenterThe new paper from Richard Davy “The climatology of the atmospheric boundary layer in contemporary global climate models” out now in Journal of Climate is the first to quantify the climatology of the atmospheric boundary layer in the full ensemble of climate models in CMIP5 – the climate model inter-comparison project which is widely used for the IPCC reports.


-It shows that, while the models do a reasonably good job of capturing the climatological-mean state, they have strong biases in the diurnal and seasonal cycles. These biases in the boundary layer depth can lead to large biases in how the surface climate responds to changes in forcing because the warming is amplified most strongly in the seasonal and diurnal minima, Davy at the Nansen Center says.


The atmospheric boundary layer depth is a very important quantity within the climate system, and it is therefore also very important for climate models and the projections they make for how the climate will change in the future. The boundary layer is the thin layer of air close to the ground. The turbulent dynamics inside this thin layer of air governs the exchange of heat, moisture, carbon, momentum, and aerosols between the surface and the atmosphere. For example, weak turbulent mixing in the boundary layer can limit how much pollutants emitted from engines are spread in the atmosphere, resulting in dangerous air quality hazards for pedestrians.


But in addition to trapping pollutants close to the ground, weak turbulent mixing in the atmosphere can also mean that heat at the surface is not dissipated through the atmosphere but can be trapped in a thin layer of air, leading to strong warming. This is part of the reason scientists have observed more rapid warming at night compared to during the day, and in the Arctic compared to in the tropics.


The depth of the atmospheric boundary layer, which is a measure of how much turbulent mixing there is in this layer, strongly controls the amount of warming you get at the surface. It is therefore of high importance that global climate models accurately capture the distribution of boundary layer depths observed in reality if they are to make accurate projections as to how the surface climate will change in response to additional forcing – from the buildup of atmospheric carbon dioxide.


Illustration: “The seasonal cycle in the atmospheric boundary layer depth from A the ensemble mean of CMIP5 models and B an atmospheric reanalysis product, ERA-Interim. On average climate models greatly under-estimate the strength of the seasonal, mostly due to a positive bias in the seasonal minima which can not easily be explained by other meteorological quantities."

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