Rethinking Buoyancy, at the Surface and Aloft

Abstract:

The Archimedean buoyancy is the standard measure of buoyant acceleration, but fails to account for the back-reaction of the environment on accelerating parcels. For parcels that are wide and/or near the surface, this back-reaction can produce an `effective buoyancy' quite different from the Archimedean one. We quantify these effects by computing exact analytical expressions for the effective buoyancy of idealized buoyant parcels at the surface and aloft. These results depend strongly on both aspect ratio as well as surface proximity, and in particular show that a surface parcel accelerates much slower than the same parcel aloft. We illustrate these effects with large-eddy simulations, and understand them using a view of buoyancy in which vertical accelerations are driven by horizontal hydrostatic pressure gradients, similar to how a chimney works. These analytical formulae may be useful in convective parameterizations, as well as for thinking about the “grey zone” of horizontal resolution in large-scale numerical models.