Abstract
AbstractThis study presents results from a model intercomparison project, focusing on the range of responses in deep convective cloud updrafts to varying cloud condensation nuclei (CCN) concentrations among seven state-of-the-art cloud-resolving models. Simulations of scattered convective clouds near Houston, Texas, are conducted, after being initialized with both relatively low and high CCN concentrations. Deep convective updrafts are identified, and trends in the updraft intensity and frequency are assessed. The factors contributing to the vertical velocity tendencies are examined to identify the physical processes associated with the CCN-induced updraft changes. The models show several consistent trends. In general, the changes between the High-CCN and Low-CCN simulations in updraft magnitudes throughout the depth of the troposphere are within 15% for all of the models. All models produce stronger (~+5%–15%) mean updrafts from ~4–7 km above ground level (AGL) in the High-CCN simulations, followed by a waning response up to ~8 km AGL in most of the models. Thermal buoyancy was more sensitive than condensate loading to varying CCN concentrations in most of the models and more impactful in the mean updraft responses. However, there are also differences between the models. The change in the amount of deep convective updrafts varies significantly. Furthermore, approximately half the models demonstrate neutral-to-weaker (~−5% to 0%) updrafts above ~8 km AGL, while the other models show stronger (~+10%) updrafts in the High-CCN simulations. The combination of the CCN-induced impacts on the buoyancy and vertical perturbation pressure gradient terms better explains these middle- and upper-tropospheric updraft trends than the buoyancy terms alone.
Highlights
One of the primary and most studied pathways in which aerosol particles interact with deep convective clouds is via their ingestion into convective updrafts
The majority of cloud condensation nuclei (CCN) are ingested through the bases of deep convective clouds within the atmospheric boundary layer, several studies have shown that some fraction of CCN in the middle troposphere can become entrained within deep convective updrafts, form cloud droplets, and subsequently impact the cloud development (Fridlind et al 2004; Lebo 2014; Marinescu et al 2017)
As part of the Aerosol–Cloud– Precipitation–Climate international working group (ACPC) initiative, a model intercomparison project (MIP) was organized and completed in order to assess the consistency of CCN impacts on deep convective clouds among seven state-of-the-art cloudresolving models
Summary
One of the primary and most studied pathways in which aerosol particles interact with deep convective clouds is via their ingestion into convective updrafts. Quantify the spread in the CCN effects on convective updrafts from various community-developed cloud-resolving models in their typical, standard model configurations and subsequently to present the consistent and inconsistent updraft trends Despite these model differences, all the models produce scattered deep convective clouds near Houston in both the High-CCN and Low-CCN simulations during the 12-h period (Fig. 6), albeit with different timing and frequencies, which can be partly explained by the varying environmental conditions among the models. The COSMO, UM, and NUWRF models produce less and/or generally weaker convection than the other models, while the RAMS, MesoNH, and WRFSBM models produce longer periods of and more intense deep convection (Fig. 6), consistent with the models’ environmental stability (Fig. 5) Using these identified deep convective cloud updrafts, their differences under the High-CCN and Low-CCN conditions will be assessed. To better understand the physical processes that are associated with these updraft responses to the varying CCN concentrations, the terms of the vertical velocity tendency equation are assessed
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