Aerosol–cloud interactions in mixed-phase convective clouds – Part 1: Aerosol perturbations

Abstract

Abstract. Changes induced by perturbed aerosol conditions in moderately deep mixed-phase convective clouds (cloud top height ∼ 5 km) developing along sea-breeze convergence lines are investigated with high-resolution numerical model simulations. The simulations utilise the newly developed Cloud-AeroSol Interacting Microphysics (CASIM) module for the Unified Model (UM), which allows for the representation of the two-way interaction between cloud and aerosol fields. Simulations are evaluated against observations collected during the COnvective Precipitation Experiment (COPE) field campaign over the southwestern peninsula of the UK in 2013. The simulations compare favourably with observed thermodynamic profiles, cloud base cloud droplet number concentrations (CDNC), cloud depth, and radar reflectivity statistics. Including the modification of aerosol fields by cloud microphysical processes improves the correspondence with observed CDNC values and spatial variability, but reduces the agreement with observations for average cloud size and cloud top height.Accumulated precipitation is suppressed for higheraerosol conditions before clouds become organised along the sea-breeze convergence lines. Changes in precipitation are smaller in simulations with aerosol processing. The precipitation suppression is due to less efficient precipitation production by warm-phase microphysics, consistent with parcel model predictions.In contrast, after convective cells organise along the sea-breeze convergence zone, accumulated precipitation increases with aerosol concentrations. Condensate production increases with the aerosol concentrations due to higher vertical velocities in the convective cores and higher cloud top heights. However, for the highest-aerosol scenarios, no further increase in the condensate production occurs, as clouds grow into an upper-level stable layer. In these cases, the reduced precipitation efficiency (PE) dominates the precipitation response and no further precipitation enhancement occurs. Previous studies of deep convective clouds have related larger vertical velocities under high-aerosol conditions to enhanced latent heating from freezing. In the presented simulations changes in latent heating above the 0 • C are negligible, but latent heating from condensation increases with aerosol concentrations. It is hypothesised that this increase is related to changes in the cloud field structure reducing the mixing of environmental air into the convective core.The precipitation response of the deeper mixed-phase clouds along well-established convergence lines can be the opposite of predictions from parcel models. This occurs when clouds interact with a pre-existing thermodynamic environment and cloud field structural changes occur that are not captured by simple parcel model approaches.

The effects of cloud-aerosol-interaction complexity on simulations of presummer rainfall over southern China

“…By capturing both high-and low-aerosol concentrations in the same domain, the aerosol-processing experiment is able to combine aspects of both the low-20 and high-intensity regimes and therefore gives surface rainfall properties that are intermediate between the clean and polluted extremes. The opposing tendencies in f and I are consistent with Miltenberger et al (2018) who found that convective-cell size increased (indicating more intense rainfall) whereas the number of cells decreased with increasing amounts of aerosols (indicating a smaller in the rainy area). Finally, in Fig.…”

“…Although the 1 https://doi.basic hypotheses that cloud-droplet number concentrations can alter the brightness, longevities and amounts of clouds are wellestablished (Twomey , 1977;Albrecht , 1989;Rosenfeld et al , 2008), how these processes combine to determine the responses of systems of clouds has been found to depend on both the system under consideration (Kaufman et al , 2005;Rosenfeld et al , 2008) and the model being used (Hill et al , 2015;Johnson , 2015). For deep-convective clouds in particular, uncertainty abounds because increased droplet numbers are associated with both increased and decreased rainfall in manner that appears 5 sensitive to several factors including: ice formation, the large-scale environment and history of the evolving aerosol-cloud system (Khain et al , 2008;Miltenberger et al , 2018).A range of complexities are also involved in aerosol schemes. Speciated models treat the population of aerosols as composed of physically distinct species, for example salts of sulphuric acid or sodium, organic-and inorganic-carbon compounds.…”

Aerosol midlatitude cyclone indirect effects in observations and high-resolution simulations

“…The Cloud-AeroSol Interacting Microphysics (CASIM) two-moment microphysics scheme (Hill et al, 2015;Shipway and Hill, 2012;Grosvenor et al, 2017;Miltenberger et al, 2018) was used for all clouds in the convection-permitting simulation and for largescale cloud cover in the GCM-surrogate simulation. The CASIM microphysics scheme is described in Shipway and Hill (2012).…”

How important are aerosol–fog interactions for the successful modelling of nocturnal radiation fog?

“…CASIM has been specifically developed to simulate and investigate aerosol–cloud interactions in the MetUM (e.g. Field et al, ; Grosvenor et al , ; Miltenberger et al , ; Stevens et al , ) and MONC (Dearden et al , ). This study attempts to address two key objectives: Evaluate how well MONC coupled with the CASIM scheme can simulate an optically thin nocturnal fog case; Investigate the influence of aerosol properties on fog development. …”