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Atmospheric Chemistry and Physics An interactive open-access journal of the European Geosciences Union
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© Author(s) 2019. This work is distributed under
the Creative Commons Attribution 4.0 License.
© Author(s) 2019. This work is distributed under
the Creative Commons Attribution 4.0 License.

Submitted as: research article 05 Aug 2019

Submitted as: research article | 05 Aug 2019

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This discussion paper is a preprint. A revision of the manuscript was accepted for the journal Atmospheric Chemistry and Physics (ACP).

The diurnal stratocumulus-to-cumulus transition over land

Xabier Pedruzo-Bagazgoitia1, Stephan R. de Roode2, Bianca Adler3, Karmen Babić3, Cheikh Dione4, Norbert Kalthoff3, Fabienne Lohou4, Marie Lothon4, and Jordi Vilà-Guerau de Arellano1 Xabier Pedruzo-Bagazgoitia et al.
  • 1Meteorology and Air Quality Group, Wageningen University and Research, Wageningen, the Netherlands
  • 2Delft University of Technology, Delft, the Netherlands
  • 3Institute of Meteorology and Climate Research, Karlsruhe Institute of Technology (KIT), Karlsruhe, Germany
  • 4Laboratoire d'Aérologie, Université de Toulouse, CNRS, UPS, France

Abstract. The misrepresentation of the diurnal cycle of boundary-layer clouds by large scale models strongly impacts the modeled regional energy balance in southern West Africa. In particular, recognizing the processes involved in the maintenance and transition of the nighttime stratocumulus to diurnal shallow cumulus over land remains a challenge. This is due to the fact that over vegetation, surface fluxes exhibit a much larger magnitude and variability than on the more researched marine stratocumulus transitions. An improved understanding of the interactions between surface and atmosphere is thus necessary to improve its representation. To this end, the DACCIWA measurement campaign gathered a unique dataset of observations of the frequent stratocumulus to cumulus transition in southern West Africa. Inspired and constrained by these observations, we perform a series of numerical experiments using Large Eddy Simulation. The experiments include interactive radiation and surface schemes where we explicitly resolve, quantify and describe the physical processes driving such transition. Focusing on the local processes, we quantify the transition in terms of dynamics, radiation, cloud properties, surface processes and the evolution of dynamically relevant layers such as subcloud layer, cloud layer and inversion layer. We further quantify the processes driving the stratocumulus thinning and the subsequent transition initiation by using a liquid water path budget. Finally, we study the impact of mean wind and wind shear at cloud top through two additional numerical experiments. We find that the sequence starts with a nighttime well-mixed layer from surface to cloud top, in terms of temperature and humidity, and transitions to a prototypical convective boundary layer by the afternoon. We identify radiative cooling as the largest factor for the maintenance leading to a net thickening of the cloud layer of about 18 g m−2 h−1 before sunrise. Four hours after sunrise, the cloud layer decouples from the surface through a growing negative buoyancy flux at cloud base. After sunrise, the increasing impact of entrainment leads to a progressive thinning of the cloud layer. While the effect of wind on the stratocumulus layer during nighttime is limited, after sunrise we find shear at cloud top to have the largest impact: the local turbulence generated by shear enhances the boundary layer growth and entrainment aided by the increased surface fluxes. As a consequence wind shear at cloud top accelerates the breakup and transition by about 2 hours. The quantification of the transition and its driving factors presented here sets the path for an improved representation by larger scale models.

Xabier Pedruzo-Bagazgoitia et al.
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Xabier Pedruzo-Bagazgoitia et al.
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Publications Copernicus
Short summary
Using a high resolution model we simulate the transition from night to day clouds on southern West Africa using observations from the DACCIWA project. We find that the radiative effects of clouds help mantain a thick cloud layer in the night, while the mixing of cloud air with air above during the day, aided by moisture and heat fluxes at the surface, thins this layer and promotes its transition to other clouds. The effect of changing wind with height accelerates the transition.
Using a high resolution model we simulate the transition from night to day clouds on southern...