The spectral bridge between meso- and micro-scale processes

Harm Jonker
Technische Universiteit te Delft

Based on early work by e.g. van der Hoven (1957) it has become a conventional view that processes associated with the mesoscale range are separated from microscale processes (boundary layer turbulence) by a so-called "spectral gap": a distinct minimum in the spectral energy.
However, more often than not satellite observations show that boundary layer clouds are organized in patterns with a typical length scale far exceeding the boundary layer depth, even though we associate the origin of these clouds with boundary layer processes and therefore expect the boundary layer to emerge as a dominating length-scale. Aircraft observations in convective (cloud-topped) boundary layers tend to tell the same story: prominent contributions from mesoscale fluctuations to the variance of the measured variables, with no hint of a spectral gap.

In the lecture we will focus on the spectral connection between the
meso- and microscale and discuss more in general methods with which one can quantify spectral interactions between different scales, and identify upscale and downscale energy transfer. As an example we apply such methods to Large Eddy Simulations of clear and cloudy boundary layers. These simulations, devoid of explicit mesoscale forcings, reveal that buoyancy driven convection autonomously creates spatial fluctuations with a lateral size far exceeding the boundary layer depth, i.e. the initially installed spectral void gets filled up rapidly. By analysing and quantifying the spectral interaction between scales it will be possible to get insight into the underlying mechanism responsible for bridging the spectral gap.

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