Toward a minimal representation of aerosols in climate models: description and evaluation in the Community Atmosphere Model CAM5

Abstract

Abstract.A modal aerosol module (MAM) has been developed for the Community Atmosphere Model version 5 (CAM5), the atmospheric component of the Community Earth System Model version 1 (CESM1). MAM is capable of simulating the aerosol size distribution and both internal and external mixing between aerosol components, treating numerous complicated aerosol processes and aerosol physical, chemical and optical properties in a physically-based manner. Two MAM versions were developed: a more complete version with seven lognormal modes (MAM7), and a version with three lognormal modes (MAM3) for the purpose of long-term (decades to centuries) simulations. In this paper a description and evaluation of the aerosol module and its two representations are provided. Sensitivity of the aerosol lifecycle to simplifications in the representation of aerosol is discussed.Simulated sulfate and secondary organic aerosol (SOA) mass concentrations are remarkably similar between MAM3 and MAM7. Differences in primary organic matter (POM) and black carbon (BC) concentrations between MAM3 and MAM7 are also small (mostly within 10 %). The mineral dust global burden differs by 10 % and sea salt burden by 30-40 % between MAM3 and MAM7, mainly due to the different size ranges for dust and sea salt modes and different standard deviations of the log-normal size distribution for sea salt modes between MAM3 and MAM7. The model is able to qualitatively capture the observed geographical and temporal variations of aerosol mass and number concentrations, size distributions, and aerosol optical properties. However, there are noticeable biases; e.g., simulated BC concentrations are significantly lower than measurements in the Arctic. There is a low bias in modeled aerosol optical depth on the global scale, especially in the developing countries. These biases in aerosol simulations clearly indicate the need for improvements of aerosol processes (e.g., emission fluxes of anthropogenic aerosols and precursor gases in developing countries, boundary layer nucleation) and properties (e.g., primary aerosol emission size, POM hygroscopicity). In addition, the critical role of cloud properties (e.g., liquid water content, cloud fraction) responsible for the wet scavenging of aerosol is highlighted.

Assessment of Madden–Julian oscillation simulations with various configurations of CESM

“…The revised cloud macrophysics scheme provides a more transparent treatment of cloud processes and imposes a pseudo condensation-evaporation process to remove the inconsistency between stratus fraction and in-stratus cloud condensate (Neale et al 2010a). The aerosol treatment in the model uses a modal-based prognostic scheme similar to that in Easter et al (2004), but with only three modes (Aitken, accumulation and coarse) (Liu et al 2012), whereas CAM4 uses a prescribed mass-based scheme with direct radiative effects and has no interaction with clouds.…”

Impact of moisture divergence on systematic errors in precipitation around the Tibetan Plateau in a general circulation model

“…CAM5 features (1) a new shallow convection scheme and a new moist turbulence scheme ) developed by the University of Washington ; (2) a two-moment cloud microphysics scheme Gettelman et al 2008) and a cloud macrophysics scheme (Park et al 2014) for the parameterizations of clouds; and (3) an open source Rapid Radiative Transfer Model for GCM (RRTMG) package developed by the Atmospheric and Environmental Research (AER) incorporation is used as the radiation module (Mlawer et al 1997;Iacono et al 2008). A new modal aerosol module is also implemented in CAM5 (Liu et al 2012). …”

The parametric sensitivity of CAM5's MJO

“…For large-scale winds associated with the MJO, we use reanalysis data for the years 1980 to 2011 from the European Centre for Medium-Range Weather Forecasts (ERA-Interim) [Dee et al, 2011]. [Morrison and Gettelman, 2008;Gettelman et al, 2010] and diagnostic cloud fraction scheme Aerosols Modal aerosol model [Liu et al, 2012] Journal of Geophysical Research: Atmospheres…”