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Vaughan Phillips

Research in the Area of Clouds, Aerosols and Climate

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Improved mixing representation in Emanuel's convection scheme

Author

  • JY Grandpeix
  • Vaughan Phillips
  • R Tailleux

Summary, in English

Recent empirical and modelling studies suggest that mid-tropospheric relative humidity (RH) is an important controlling factor of deep atmospheric convection, which appears to be underestimated in present cumulus parametrizations. This indicates the possible presence of shortcomings in the way that entrainment is represented in such parametrizations. This matter was explored in the European Cloud Systems project (EUROCS) by means of an idealized humidity experiment in which the main controlling parameter is RH. In the latter study, cloud-resolving model (CRM) experiments suggested that a shallow/deep convection transition occurs when RH crosses a threshold value that ranges from about RH = 50% to RH = 60%. In this paper, we seek to increase the responsiveness of Emanuel's convection scheme to RH, and to reproduce the threshold behaviour of the idealized humidity case, by replacing the original uniform probability density function (PDF) for mixing fractions by a more flexible two-parameter bell-shaped function that allows a wider range of behaviour. The main result is that the parameters of this PDF can be tuned to allow a regime transition to occur near a threshold value of RH approximate to 55%. In contrast to CRM results, however, this transition is between two different regimes of deep convection rather than between a shallow and deep regime. Possible ways to obtain a shallow-to-deep transition with Emanuel's scheme are discussed.

Publishing year

2004

Language

English

Pages

3207-3222

Publication/Series

Quarterly Journal of the Royal Meteorological Society

Volume

130

Issue

604

Document type

Journal article

Publisher

Royal Meteorological Society

Topic

  • Physical Geography

Keywords

  • cumulus parametrization
  • entrainment
  • mixing
  • tropospheric humidity

Status

Published

ISBN/ISSN/Other

  • ISSN: 0035-9009