Editorial Type:
Article
Article Type:
Research Article

Organization and Representation of Boundary Layer Clouds

Peter Bechtold Royal Netherlands Meteorological Institute, De Bilt, the Netherlands

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Pier Siebesma Royal Netherlands Meteorological Institute, De Bilt, the Netherlands

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Print Publication:
01 Mar 1998
DOI:
https://doi.org/10.1175/1520-0469(1998)055<0888:OAROBL>2.0.CO;2
Page(s):
888–895
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Abstract

This study is intended to summarize and to simplify the complicated processes in boundary layer cloud regimes using a single parameter, Q1, the normalized saturation deficit.

With the aid of large eddy simulation (LES) data from different boundary layer cloud regimes it is illustrated i) that the in-cloud buoyancy flux is maximized when the fractional cloudiness approaches zero; ii) that the ensemble average buoyancy flux possesses two maxima, one for the trade wind cumulus case and one for the stratocumulus case; and iii) that the preferred mode for boundary layer clouds is either small cumuli or high values of cloudiness, and that cloudiness transitions from one regime to the other are difficult to represent numerically as in the transition regime the cloud-water-related variables are very parameter sensitive.

In addition, the importance of the contribution of the liquid water flux to the in-cloud and total buoyancy flux is outlined, and simple analytical and empirical methods are presented to compute the liquid water flux as a function of the fluxes of conserved variables for different boundary layer cloud regimes.

* Current affiliation: Laboratoire d’Aérologie, Observatoire Midi-Pyrénées, Toulouse, France.

Corresponding author address: Dr. Peter Bechtold, Laboratoire d’Aérologie, Observatoire Midi-Pyrénées, 31400 Toulouse, France.

Abstract

This study is intended to summarize and to simplify the complicated processes in boundary layer cloud regimes using a single parameter, Q1, the normalized saturation deficit.

With the aid of large eddy simulation (LES) data from different boundary layer cloud regimes it is illustrated i) that the in-cloud buoyancy flux is maximized when the fractional cloudiness approaches zero; ii) that the ensemble average buoyancy flux possesses two maxima, one for the trade wind cumulus case and one for the stratocumulus case; and iii) that the preferred mode for boundary layer clouds is either small cumuli or high values of cloudiness, and that cloudiness transitions from one regime to the other are difficult to represent numerically as in the transition regime the cloud-water-related variables are very parameter sensitive.

In addition, the importance of the contribution of the liquid water flux to the in-cloud and total buoyancy flux is outlined, and simple analytical and empirical methods are presented to compute the liquid water flux as a function of the fluxes of conserved variables for different boundary layer cloud regimes.

* Current affiliation: Laboratoire d’Aérologie, Observatoire Midi-Pyrénées, Toulouse, France.

Corresponding author address: Dr. Peter Bechtold, Laboratoire d’Aérologie, Observatoire Midi-Pyrénées, 31400 Toulouse, France.

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