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The Sensitivity of HadGAM1 Dynamics to the Vertical Structure of Tropical Heating

The Sensitivity of HadGAM1 Dynamics to the Vertical Structure of Tropical Heating

Chris Dearden HadGEM1a Configuration Manager
Climate Model Development and Evaluation Group
Met Office Hadley Centre for Climate Prediction and Research
FitzRoy Road Exeter EX1 3PB United Kingdom
Tel: +44 (0)1392 886392 Fax: +44 (0)1392 885681
E-mail: chris.dearden@metoffice.gov.uk http://www.metoffice.gov.uk

Investigation of the atmospheric response to localised tropical diabatic heating using a linear, steady-state model (such as A. E. Gill?s ?shallow water? approach) has proven to be a useful technique for elucidating the essential features of the large-scale tropical circulation. More recently, this method has been expanded and applied to more sophisticated primitive equation models (e.g. Wu et al. 2000, Amer. Meteor. Soc.), showing that the response of the atmosphere depends highly on the vertical distribution of diabatic heating.

Given that low-level tropical circulation errors exist in the current Hadley Centre AGCM (HadGAM1), it is of relevance to establish the sensitivity of the model dynamics to the tropical heating profile, and also to diagnose how well the model parametrization schemes represent the vertical structure of the heating. To achieve these aims, the technique of studying heat-induced tropical circulations is applied to the dry dynamical core of HadGAM1. This idealised model atmosphere is initialised with a motionless basic state in hydrostatic balance, with no horizontal temperature gradients. A circulation is then driven exclusively by a localised tropical heat source which is centred along the equator at some arbitrary longitude. Results are shown which suggest that HadGAM1 dynamics respond to changes in the vertical structure of heating in a way that is consistent with theory; however, analysis of the diabatic heating profile taken from a HadGAM1 aqua-planet simulation show that the model physics produces a more bottom-heavy heating profile than is expected. The excessive low level heating may be due to a lack of representation of older, decaying convective activity which typically produces stratiform precipitation. This missing physical process also has wider implications for coupled atmosphere-ocean configurations, where the excessive heating produces overly-strong easterly wind stresses which contribute to cold SST biases along the equatorial Pacific and the poor simulation of ENSO.