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Subcloud-Layer Feedbacks Driven by the Mass Flux of Shallow Cumulus Convection over Land

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  • 1 Meteorology and Air Quality Section, Wageningen University, Wageningen, Netherlands
  • | 2 Max Planck Institute for Meteorology, Hamburg, Germany
  • | 3 Meteorology and Air Quality Section, Wageningen University, Wageningen, Netherlands, and Max Planck Institute for Chemistry, Mainz, Germany
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Abstract

The processes and feedbacks associated with the mass flux of shallow cumulus clouds over land are studied by analyzing the results from large-eddy simulations and a mixed-layer model. The primary focus is to study the development of the (well mixed) subcloud layer and understand the four primary feedbacks between the subcloud-layer dynamics and cumulus mass flux. Guided by numerical experiments in large-eddy simulations that show the transition from clear to cloudy boundary layers at midlatitudes over land, the feedbacks introduced by shallow cumuli are first conceptually described. To study the complex interplay between the subcloud and cloud layer, a mixed-layer model is proposed and validated with large-eddy simulations for the Atmospheric Radiation Measurement Southern Great Plains case. The mixed-layer model is shown to identify and reproduce the most relevant feedbacks in the transition from clear to cloudy boundary layers: a reduced mixed-layer growth and drying of the subcloud layer by enhanced entrainment and mass flux transport of moisture to the cloud layer. To complete the study, the strength of the different feedbacks is further quantified by an analysis of the individual contributions to the tendency of the relative humidity at the top of the mixed layer.

Current affiliation: Max Planck Institute for Meteorology, International Max Planck Research School on Earth System Modelling, Hamburg, Germany.

Corresponding author address: Bart van Stratum, Max Planck Institute for Meteorology, Bundesstraße 53, 20146 Hamburg, Germany. E-mail: bart.vanstratum@mpimet.mpg.de

Abstract

The processes and feedbacks associated with the mass flux of shallow cumulus clouds over land are studied by analyzing the results from large-eddy simulations and a mixed-layer model. The primary focus is to study the development of the (well mixed) subcloud layer and understand the four primary feedbacks between the subcloud-layer dynamics and cumulus mass flux. Guided by numerical experiments in large-eddy simulations that show the transition from clear to cloudy boundary layers at midlatitudes over land, the feedbacks introduced by shallow cumuli are first conceptually described. To study the complex interplay between the subcloud and cloud layer, a mixed-layer model is proposed and validated with large-eddy simulations for the Atmospheric Radiation Measurement Southern Great Plains case. The mixed-layer model is shown to identify and reproduce the most relevant feedbacks in the transition from clear to cloudy boundary layers: a reduced mixed-layer growth and drying of the subcloud layer by enhanced entrainment and mass flux transport of moisture to the cloud layer. To complete the study, the strength of the different feedbacks is further quantified by an analysis of the individual contributions to the tendency of the relative humidity at the top of the mixed layer.

Current affiliation: Max Planck Institute for Meteorology, International Max Planck Research School on Earth System Modelling, Hamburg, Germany.

Corresponding author address: Bart van Stratum, Max Planck Institute for Meteorology, Bundesstraße 53, 20146 Hamburg, Germany. E-mail: bart.vanstratum@mpimet.mpg.de
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