Preprint Article Version 1 This version is not peer-reviewed

The use of Semigeostrophic Theory to Diagnose the Behaviour of an Atmospheric GCM

Version 1 : Received: 20 July 2018 / Approved: 20 July 2018 / Online: 20 July 2018 (13:51:21 CEST)

How to cite: Cullen, M. The use of Semigeostrophic Theory to Diagnose the Behaviour of an Atmospheric GCM. Preprints 2018, 2018070386 (doi: 10.20944/preprints201807.0386.v1). Cullen, M. The use of Semigeostrophic Theory to Diagnose the Behaviour of an Atmospheric GCM. Preprints 2018, 2018070386 (doi: 10.20944/preprints201807.0386.v1).

Abstract

A diagnostic method is presented for analysing the large-scale behaviour of the Met Office Unified Model, which is a comprehensive atmospheric model used for weather and climate prediction. Outside the boundary layer, on scales larger than the radius of deformation, semigeostrophic theory will give an accurate approximation to the model evolution. In particular, the ageostrophic circulation required to maintain geostrophic and hydrostatic balance against prescribed forcing and a rate of change of the geostrophic pressure can be calculated. In the tropics the balance condition degenerates to the weak temperature gradient approximation. Within the boundary layer the semigeostriptic approximation has to be used because friction and rotation are equally important. Assuming the calculated pressure tendency and ageotriptic circulation match the observed model behaviour, the influence of the large-scale state and the nature of the forcing on the model response can be deduced in a straightforward way. This process is illustrated by comparing predictions of the ageotriptic circulation from the theory and the model. It is then used to show that the effects of latent heat release can be included by modifying the static stability, and to show the effect of an idealised tropical heat source on the subtropical jet. Finally the response of the ageotriptic flow to boundary layer heating in the tropics is demonstrated. These illustrations show that the model behaviour on large scales conforms with theoretical expectations, so that the results of the diagnostic can be used to aid the development of further improvements to the model.

Subject Areas

Unified Model; geostrophic balance; geotriptic balance; ageotriptic circulation.

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