Abstract
Abstract. Aerosol–cloud interactions remain largely uncertain with respect to predicting their impacts on weather and climate. Cloud microphysics parameterization is one of the factors leading to large uncertainty. Here, we investigate the impacts of anthropogenic aerosols on the convective intensity and precipitation of a thunderstorm occurring on 19 June 2013 over Houston with the Chemistry version of Weather Research and Forecast model (WRF-Chem) using the Morrison two-moment bulk scheme and spectral bin microphysics (SBM) scheme. We find that the SBM predicts a deep convective cloud that shows better agreement with observations in terms of reflectivity and precipitation compared with the Morrison bulk scheme that has been used in many weather and climate models. With the SBM scheme, we see a significant invigoration effect on convective intensity and precipitation by anthropogenic aerosols, mainly through enhanced condensation latent heating. Such an effect is absent with the Morrison two-moment bulk microphysics, mainly because the saturation adjustment approach for droplet condensation and evaporation calculation limits the enhancement by aerosols in (1) condensation latent heat by removing the dependence of condensation on droplets and aerosols and (2) ice-related processes because the approach leads to stronger warm rain and weaker ice processes than the explicit supersaturation approach.
Highlights
Deep convective clouds (DCCs) produce copious precipitation and play important roles in the hydrological and energy cycle as well as regional and global circulation (e.g., Arakawa, 2004; Houze, 2014)
DCCs and associated precipitation are determined by water vapor, vertical motion of air, and cloud microphysics that could be affected by aerosols through aerosol–radiation interactions (ARI), aerosol–cloud interactions (ACI), or both
We look at the effects of anthropogenic aerosols on the deep convective storm simulated with the spectral bin microphysics (SBM) and Morrison microphysics schemes
Summary
Deep convective clouds (DCCs) produce copious precipitation and play important roles in the hydrological and energy cycle as well as regional and global circulation (e.g., Arakawa, 2004; Houze, 2014). For aerosol–DCC interactions, a well-known theory is that increasing aerosol concentrations can suppress warm rain as a result of increased droplet numbers but reduced droplet size This allows more cloud droplets to be lifted to altitudes above the freezing level, inducing stronger ice microphysical processes (e.g., droplet freezing, riming, and deposition) which release larger latent heating, thereby invigorating convective updrafts (referred to as “cold-phase invigoration”; Khain et al, 2005; Rosenfeld et al, 2008). Rosenfeld et al (2008) showed that the buoyancy restores and increases after the precipitation of the ice hydrometeors that form upon freezing of the high supercooled liquid water content into large graupel and hail Another theory is that increasing aerosols enhances droplet nucleation, secondary nucleation, after warm rain initiates, which promotes condensation because of the larger integrated droplet surface area associated with a higher number of small droplets (Fan et al, 2007, 2013, 2018; Koren at al., 2014; Lebo, 2018; Sheffield et al, 2015; Chen et al, 2020). The simulated storm case is the same as the case for the Aerosol–Cloud–Precipitation– Climate (ACPC) model intercomparison project (Rosenfeld et al, 2014; http://www.acpcinitiative.org/, last access: 29 January 2021)
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