The importance of surface pressure changes in the response of the atmosphere to zonally-symmetric thermal and mechanical forcing
Haynes, P. H. and Shepherd, T. G. (1989) The importance of surface pressure changes in the response of the atmosphere to zonally-symmetric thermal and mechanical forcing. Quarterly Journal of the Royal Meteorological Society, 115 (490). pp. 1181-1208. ISSN 1477-870X
To link to this item DOI: 10.1002/qj.49711549002
he classical problem of the response of a balanced, axisymmetric vortex to thermal and mechanical forcing is re-examined, paying special attention to the lower boundary condition. The correct condition is DΦ/Dt = 0, where Φ is the geopotential and D/Dt the material derivative, which explicitly accounts for a mass redistribution as part of the mean-flow response. This redistribution is neglected when using the boundary condition Dp/Dt = 0, which has conventionally been applied in this problem. It is shown that applying the incorrect boundary condition, and thereby ignoring the surface pressure change, leads to a zonal wind acceleration δū/δt that is too strong, especially near the surface. The effect is significant for planetary-scale forcing even when applied at tropopause level. A comparison is made between the mean-flow evolution in a baroclinic life-cycle, as simulated in a fully nonlinear, primitive-equation model, and that predicted by using the simulated eddy fluxes in the zonally-symmetric response problem. Use of the correct lower boundary condition is shown to lead to improved agreement.