The Sensitivity of HadGAM1 dynamics to the vertical structure of tropical heating

Christopher Dearden

    Research output: ThesisMaster's Thesis

    Abstract

    A series of experiments are performed with the dry dynamical core of the current Hadley Centre CGCM, HadGEM1, to investigate the dependence of the atmospheric circulation on the vertical structure of tropical heating. The model atmosphere is initialised such that it is motionless and in hydrostatic balance, with a basic state that contains no horizontal temperature gradients. Circulations are driven exclusively by a three-dimensional, steady localised heating anomaly that is imposed along the equator. Over a certain timescale the temperature increments from the heating anomaly are approximately balanced by a Newtonian cooling term, which serves to relax the model back to the initial theta profile, such that an approximate steady-state is reached and one can analyse the response. The vertical structure of the imposed heating anomaly is based on an idealisation of the heating from mescoscale cloud systems (MCSs). Results indicate that the response of the HadGEM1 atmosphere is sensitive to the vertical structure of the diabatic heating produced by these cloud systems. Typically they can be partitioned into a ‘convective’ region, containing the towering cumulus cloud, and a ‘stratiform’ region, which describes the cloud that forms from the old remnants of convective cells. The temperature profile associated with the stratiform anvil region of tropical cloud in particular plays an important role in re-distributing the heating in the vertical, and this has significant consequences for the surface circulation and indeed, for the circulation throughout the depth of the atmosphere. Additional idealised experiments are performed which explore the sensitivity to the depth of the heating profile. Heating that is concentrated at levels near the surface is much more efficient at driving strong surface winds than deep heating. Analysis of the tropical heating profiles from a HadGAM1 aqua-planet simulation reveal that the model is deficient at producing stratiform anvil cloud, and this missing process leads to a tropical heating profile that has too much heating at lower levels. The effect of this is to produce excessive convergence near the surface, and therefore easterlies that are too strong. In the coupled atmosphere-ocean configuration, this leads to sea-surface temperatures that are too cold along the equatorial Pacific due to the increased wind stress; this has further implications for the simulation of the El-Nino Southern Oscillation.
    Original languageEnglish
    QualificationMaster of Science
    Awarding Institution
    • The University of Reading
    Supervisors/Advisors
    • Martin, Gill, Supervisor, External person
    • Inness, Pete, Supervisor, External person
    Publication statusPublished - 2006

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