Quantifying and Parameterizing the Transport of Sub-Cloud Layer Moisture and Reactants by Shallow Cumulus Clouds over Land
Abstract
We investigate the vertical transport of moisture and atmospheric chemical reactants from the sub-cloud layer to the cumulus cloud layer related to the kinematic mass flux that is driven by shallow convection over land. The dynamical and chemical assumptions needed for mesoscale and global chemistry-transport model parameterizations are systematically analysed using numerical experiments performed by a Large-Eddy Simulation (LES) model. First, we identify and discuss the four primary feedback mechanisms between sub-cloud layer dynamics and mass-flux transport by shallow cumulus clouds for typical mid-latitude conditions. These feedbacks involve mixed-layer drying and heating, changing the moisture variability at the sub-cloud layer top and adjusting entrainment. Based on this analysis and LES experiments, we design parameterizations for cloud properties and mass-flux transport of air and moisture that can be applied to large-scale models. As an intermediate step, we incorporate the parameterizations in a conceptual mixed-layer model, which enables us to study these interplays in more detail. By comparing the results of this model with LES case studies, we show for a wide range of conditions that the new parameterizations enable the model to reproduce the sub-cloud layer dynamics and the four aforementioned feedbacks. However, by considering heterogeneous sensible and latent heat fluxes at the surface, we demonstrate that the parameterizations are sensitive to specific boundary conditions due to changes in the boundary-layer dynamics. Second, we extend the investigation to determine whether the parameterizations are suitable for tropical conditions and to represent the transport of reactants. The numerical experiments in this analysis are inspired by observations over the Amazon during the dry season. Isoprene, a key atmospheric compound over the tropical rain forest, decreases by 8.5 % hr-1 on average and 15 % hr-1 at maximum due to mass-flux induced removal. The new mass-flux parameterizations for the transport of chemical reactants agree satisfactorily with the transport that is numerically resolved by LES, except for some reactants like O3, NO and NO2. The latter is caused by the local partitioning of reactants, influenced by UV radiation extinction by clouds and small-scale variability of ambient atmospheric compounds. By considering the longer lived NOx (NO + NO2), the transport is well represented by the parameterization.
- Publication:
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AGU Fall Meeting Abstracts
- Pub Date:
- December 2013
- Bibcode:
- 2013AGUFM.A31A0001O
- Keywords:
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- 3314 ATMOSPHERIC PROCESSES Convective processes;
- 3310 ATMOSPHERIC PROCESSES Clouds and cloud feedbacks;
- 3307 ATMOSPHERIC PROCESSES Boundary layer processes;
- 3323 ATMOSPHERIC PROCESSES Large eddy simulation