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    Original Research Papers

    Convection forced by a descending dry layer and low-level moist convergence

    Authors:

    Andrew Russell ,

    Centre for Atmospheric Science, University of Manchester, GB
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    Geraint Vaughan,

    Centre for Atmospheric Science, University of Manchester, GB
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    Emily G. Norton,

    Centre for Atmospheric Science, University of Manchester, GB
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    Hugo M. A. Ricketts,

    Centre for Atmospheric Science, University of Manchester, GB
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    Cyril J. Morcrette,

    Met Office, Exeter, GB
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    Tim J. Hewison,

    Met Office, Exeter, GB
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    Keith. A. Browning,

    School of Earth and Environment, University of Leeds, GB
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    Alan M. Blyth

    School of Earth and Environment, University of Leeds, GB
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    Abstract

    A narrow line of convective showers was observed over southern England on 18 July 2005 during the Convective Storm Initiation Project (CSIP). The showers formed behind a cold front (CF), beneath two apparently descending dry layers (i.e. sloping so that they descended relative to the instruments observing them). The lowermost dry layer was associated with a tropopause fold from a depression, which formed 2 d earlier from a breaking Rossby wave, located northwest of the UK. The uppermost dry layer had fragmented from the original streamer due to rotation around the depression (This rotation was also responsible for the observations of apparent descent—ascent would otherwise be seen behind a CF). The lowermost dry layer descended over the UK and overran higher θw air beneath it, resulting in potential instability. Combined with a surface convergence line (which triggered the convection but had less impact on the convective available potential energy than the potential instability), convection was forced up to 5.5 km where the uppermost dry layer capped it. The period when convection was possible was very short, thus explaining the narrowness of the shower band. Convective Storm Initiation Project observations and model data are presented to illustrate the unique processes in this case.

    How to Cite: Russell, A., Vaughan, G., Norton, E.G., Ricketts, H.M.A., Morcrette, C.J., Hewison, T.J., Browning, K.A. and Blyth, A.M., 2009. Convection forced by a descending dry layer and low-level moist convergence. Tellus A: Dynamic Meteorology and Oceanography, 61(2), pp.250–263. DOI: http://doi.org/10.1111/j.1600-0870.2008.00382.x
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      Published on 01 Jan 2009
    Peer Reviewed
     CC BY 4.0
     Accepted on 12 Nov 2008            Submitted on 25 Jun 2008

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