Abstract
Simulating the complex aerosol microphysical processes in a comprehensive Earth System Model can be very computationally intensive and therefore many models utilize a modal approach, where aerosol size distributions are represented by observations-derived lognormal functions. This approach has been shown to yield satisfactory results in a large array of applications, but there may be cases where the simplification in this approach may produce some shortcomings. In this work we show specific conditions under which the current approximations used in modal approaches might yield some incorrect answers. Using results from the Community Earth System Model v1 (CESM1) Geoengineering Large Ensemble (GLENS) project, we analyze the effects in the troposphere of a continuous increasing load of sulfate aerosols in the stratosphere, with the aim of counteracting the surface warming produced by non-mitigated increasing greenhouse gases concentration between 2020–2100. We show that the simulated results pertaining to the evolution of sea salt and dust aerosols in the upper troposphere are not realistic due to internal mixing assumptions in the modal aerosol treatment, which in this case reduces the size, and thus the settling velocities, of those particles and ultimately changes their mixing ratio below the tropopause. The unnatural increase of these aerosol species affects, in turn, the simulation of upper tropospheric ice formation, resulting in an increase in ice clouds that is not due to any meaningful physical mechanisms. While we show that this does not significantly affect the overall results of the simulations, we point to some areas where results should be interpreted with care in modeling simulations using similar approximations: in particular, the evolution of upper tropospheric clouds when large amount of sulfate is present in the stratosphere, as after a large explosive volcanic eruption or in similar stratospheric aerosol injection cases. Finally, we suggest that this could be avoided if sulfate aerosols in the coarse mode, the predominant species in these situation, are treated separately from other aerosol species in the model.
Highlights
A comprehensive representation of aerosol processes in Earth system models is crucial for a variety of reasons
In particular, we focus on the Community Atmosphere Model version 5.0 (CAM5) and its implementation in the Community Earth System Model (CESM), using the Modal Aerosol Module with three modes (MAM3, Liu et al, 2012)
In this work we have identified the presence of some weaknesses in the three-mode modal approach (MAM3) used in Community Earth System Model v1 (CESM1)(WACCM) when a large amount of aerosols settles down from the stratosphere, as under stratospheric sulfate aerosol injection, which results in some artificial changes to cirrus clouds
Summary
A comprehensive representation of aerosol processes in Earth system models is crucial for a variety of reasons. Kuebbeler et al (2012) and Visioni et al (2018a) found in two different climate models (ECHAM-HAM5 and ULAQ-CCM, using a modal and a sectional aerosol approach, respectively) that the change in the vertical temperature gradient resulting from the stratospheric heating would reduce the formation of tropical cirrus ice clouds by less than 10 %, contributing to surface cooling This effect is tied to the fact that, in both models, the amount of water vapor reaching the upper troposphere and which is necessary for cloud formation is directly tied to the available turbulent kinetic energy, which is a function of the vertical temperature gradient, and is a purely dynamical effect.
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