Abstract
Aerosol indirect effects on precipitation were investigated in this study using a Global/Regional Integrated Model system (GRIMs) linked with a chemistry package devised for reducing the heavy computational burden occurring in common atmosphere–chemistry coupling models. The chemistry package was based on the Goddard Chemistry Aerosol Radiation and Transport scheme of Weather Research and Forecasting with Chemistry (WRF-Chem), and five tracers that are relatively important for cloud condensation nuclei (CCN) formation were treated as prognostic variables. For coupling with the cloud physics processes in the GRIMs, the CCN number concentrations derived from the simplified chemistry package were utilized in the cumulus parameterization scheme (CPS) and the microphysics scheme (MPS). The simulated CCN number concentrations were higher than those used in original cloud physics schemes and, overall, the amount of incoming shortwave radiation reaching the ground was indirectly reduced by an increase in clouds owing to a high CCN. The amount of heavier precipitation increased over the tropics owing to the inclusion of enhanced riming effects under deep precipitating convection. The trend regarding the changes in non-convective precipitation was mixed depending on the atmospheric conditions. The increase in small-size cloud water owing to a suppressed autoconversion led to a reduction in precipitation. More precipitation can occur when ice particles fall under high CCN conditions owing to the accretion of cloud water by snow and graupel, along with their melting.
Highlights
Atmospheric aerosols indirectly affect the earth’s energy budget by modifying the microphysical structure, lifetime, and coverage of clouds by serving as cloud condensation nuclei (CCN).The importance of aerosol indirect effects has been emphasized more as an aspect of the earth’s radiation budget with a high impact and high uncertainty compared to a direct effect [1]
A simplified chemistry package is first proposed to reduce the computational burden, which was implemented in the Global/Regional Integrated Model system (GRIMs) [10] along with derived CCN effects in precipitation algorithms for both cumulus parameterization and microphysics schemes
The WRF double-moment 6-class (WDM6) microphysics scheme (MPS) [30], which deals with six water species, namely, water vapor, cloud water, rain, cloud ice, snow, and graupel, as well as the number concentrations of the CCN, cloud droplets, and rainwater, was implemented in the present study to consider the aerosol indirect effect by CCN, because a single-moment microphysics scheme does not consider the number concentration of the hydrometeor, which requires the CCN effect in microphysics
Summary
Atmospheric aerosols indirectly affect the earth’s energy budget by modifying the microphysical structure, lifetime, and coverage of clouds by serving as cloud condensation nuclei (CCN). For weather forecasting models on a regional scale, aerosols effects have been coupled with precipitation algorithms [6,7] In such studies, the incorporation of aerosol CCN effects into weather or climate models has been accomplished by focusing on the microphysics. Few studies have been carried out on trying to couple the aerosols with precipitation algorithms for both cumulus parameterization and microphysics schemes applied to global weather forecasting models. We intended to consider aerosol indirect effects in a global weather forecasting model via linking with a chemistry process For this purpose, a simplified chemistry package is first proposed to reduce the computational burden, which was implemented in the Global/Regional Integrated Model system (GRIMs) [10] along with derived CCN effects in precipitation algorithms for both cumulus parameterization and microphysics schemes. The indirect aerosol effects in this framework are discussed, focusing on precipitation
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