Description and evaluation of a new four-mode version of the Modal Aerosol Module (MAM4) within version 5.3 of the Community Atmosphere Model

Abstract

Abstract. Atmospheric carbonaceous aerosols play an important role in the climate system by influencing the Earth's radiation budgets and modifying the cloud properties. Despite the importance, their representations in large-scale atmospheric models are still crude, which can influence model simulated burden, lifetime, physical, chemical and optical properties, and the climate forcing of carbonaceous aerosols. In this study, we improve the current three-mode version of the Modal Aerosol Module (MAM3) in the Community Atmosphere Model version 5 (CAM5) by introducing an additional primary carbon mode to explicitly account for the microphysical ageing of primary carbonaceous aerosols in the atmosphere. Compared to MAM3, the four-mode version of MAM (MAM4) significantly increases the column burdens of primary particulate organic matter (POM) and black carbon (BC) by up to 40 % in many remote regions, where in-cloud scavenging plays an important role in determining the aerosol concentrations. Differences in the column burdens for other types of aerosol (e.g., sulfate, secondary organic aerosols, mineral dust, sea salt) are less than 1 %. Evaluating the MAM4 simulation against in situ surface and aircraft observations, we find that MAM4 significantly improves the simulation of seasonal variation of near-surface BC concentrations in the polar regions, by increasing the BC concentrations in all seasons and particularly in cold seasons. However, it exacerbates the overestimation of modeled BC concentrations in the upper troposphere in the Pacific regions. The comparisons suggest that, to address the remaining model POM and BC biases, future improvements are required related to (1) in-cloud scavenging and vertical transport in convective clouds and (2) emissions of anthropogenic and biomass burning aerosols.

A production-tagged aerosol module for earth system models, OsloAero5.3 – extensions and updates for CAM5.3-Oslo

“…2.2, for the MSA contribution to SOA, 20% and 80% of the MSA mass is added to the SOAGLV and SOAGSV tracers, respectively. The exact LV to SV ratio is unknown, but we find a quite low sensitivity of the 10 anthropogenic change in cloud effective radiative forcing (i.e., the indirect effect, which is the most important in a climate change perspective) to assumed apportionment of MSA: test simulations indicate that the total short-wave and long-wave (Fig 5 a), and that it now compares better with the HIPPO observations in the Pacific (Fig 5 b), although the concentrations are still too high in the upper troposphere and lower stratosphere for this region, similar to the findings for CAM5.3-MAM4 in Liu et al (2016). This is probably related to the way aerosols are transported and scavenged in deep convective clouds in the model.…”

“…being nudged to a meteorology produced by the model itself (e.g., the AMIP_PD simulation) instead of the ERA meteorology. In a similar comparison by Liu et al (2016), they obtain as much as ca. 20% lower BC and OM burdens with nudged (towards 1 year recurrent meteorology) vs. a free simulation (10 years).…”

“…They partly attribute this to inter-annual 30 variability, but mainly to (climatological) differences in the meteorological conditions between the free and nudged model simulations, which affect aerosol transport and cloud processing. Unlike the nudging procedure applied here, Liu et al (2016) also nudged the model meteorology to reanalyzed temperatures (Tilmes et al, 2015), which may explain the larger difference in simulated aerosol burdens between their nudged and free AMIP simulations. A similar effect was found in an older version of CAM5.3-Oslo as we went from nudging temperatures, specific humidity, as well as U, V, PS and some surface fields, to only nudging U, V and PS: the difference in globally averaged LWP between the nudged and free simulations was reduced by an order of magnitude.…”

Seasonal variation and light absorption property of carbonaceous aerosol in a typical glacier region of the southeastern Tibetan Plateau

“…Yulong area. The four-mode modal aerosol scheme of CAM5 recently developed by Liu et al (2016) is used here, in which BC and primary OC particles are emitted into a primary-carbon mode. Then they grow through condensation of gas-phase precursors (e.g., sulfuric and organic gases) and move to the accumulation size mode, where hygroscopic aerosol particles, including carbonaceous aerosols, are subject to wet removal by precipitation.…”

“…However, light absorption by OC has not yet been taken into consideration in many climate models, e.g., various versions of the Community Earth System Model (CESM) (Flanner and Zender, 2006;Wang et al, 2013;Qian et al, 2015;Liu et al, 2016), which causes certain uncertainties in precisely evaluating climate/radiative forcing of carbonaceous aerosol in the atmosphere and snow/ice. The radiative forcing of carbonaceous aerosol remains one of the great challenges in climate simulation (Jacobson, 2001;IPCC, 2013).…”