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Keywords:

  • stratocumulus;
  • entropy;
  • energetics;
  • CTEI;
  • thermodynamic diagrams

Abstract

A moist entropy potential temperature–denoted by θs–is defined analytically in terms of the specific entropy for moist air. The expression for θs is valid for a general mixing of dry air, water vapour and possible condensed water species. It displays the same conservative properties as the moist entropy, even for varying dry air or total water content. The moist formulation for θs is equal to the dry formulation θ if dry air is considered, and it displays new properties valid for the moist air cases, both saturated or unsaturated ones. Exact and approximate versions of θs are evaluated for several stratocumulus cases, in particular by using the aircraft observation datasets from the FIRE-I experiment. It appears that there is no (or only a small) jump in θs at the top of the planetary boundary layer (PBL). The mixing in moist entropy is almost complete in the PBL, with the same values observed in the clear air and the cloudy regions, including the very top of the entrainment region. The Randall–Deardorff Cloud-Top Entrainment Instability analysis may be interpreted as a mixing in moist entropy criterion. The iso-θs lines are plotted on skew T–log p and conserved variable diagrams. All these properties could suggest some hints on the use of moist entropy (or θs) in cloud modelling or in mixing processes, with the marine stratocumulus considered as a paradigm of moist turbulence. Copyright © 2011 Royal Meteorological Society