Small Earth, deep atmosphere, and hypohydrostatic models

[knocker]

I've put this in a new thread as I can't find anywhere else to put it. And in my opinion anything Isaac Held posts is worth a read and I say that without reservation.

Quote

A key problem in atmospheric modeling is the large separation in horizontal scales between the circulations that contain the bulk of the kinetic energy and dominate the horizontal transport of heat, momentum, and moisture, and the much smaller convective eddies that provide much of the vertical transport, especially in the tropics.  We talk about the aspect ratio of a flow, the ratio of its characteristic vertical scale to its horizontal scale.  The large-scale eddies dominating the horizontal transport have very small aspect ratios.  These eddies (extratropical storms and even tropical cyclones) are effectively pancakes.  In contrast, the small-scale convective motions have aspect ratios of order one. You have to get the horizontal grid size down well below the vertical scale of the atmosphere — the tropopause height or the scale height — to begin to resolve these small-scale motions. They are not resolved in current global climate models, a fact that colors all of climate modeling. We try to develop theories (closure schemes) for the vertical fluxes by unresolved eddies, but that’s hard and success is limited.  So groups around the world are developing global models with horizontal resolution of a few kilometers.  (The animation in post #19 is produced by a model of this resolution but in a very small domain a few hundred kilometers on a side.)  While these high resolution models don’t resolve all of the vertical transports, global models with horizontal grid size of 1km or so will clearly help a lot.  But we are still pretty far from being able to utilize global models with such high resolution as a flexible tool in climate research.  They are too slow on available computing resources.  So we look for shortcuts.

http://www.gfdl.noaa.gov/blog/isaac-held/2015/12/18/65-small-earth-deep-atmosphere-and-hypohydrostatic-models/