What controls the vertical distribution of aerosol? Relationships between process sensitivity in HadGEM3–UKCA and inter-model variation from AeroCom Phase II

Abstract

Abstract. The vertical profile of aerosol is important for its radiative effects, but weakly constrained by observations on the global scale, and highly variable among different models. To investigate the controlling factors in one particular model, we investigate the effects of individual processes in HadGEM3–UKCA and compare the resulting diversity of aerosol vertical profiles with the inter-model diversity from the AeroCom Phase II control experiment. In this way we show that (in this model at least) the vertical profile is controlled by a relatively small number of processes, although these vary among aerosol components and particle sizes. We also show that sufficiently coarse variations in these processes can produce a similar diversity to that among different models in terms of the global-mean profile and, to a lesser extent, the zonal-mean vertical position. However, there are features of certain models' profiles that cannot be reproduced, suggesting the influence of further structural differences between models. In HadGEM3–UKCA, convective transport is found to be very important in controlling the vertical profile of all aerosol components by mass. In-cloud scavenging is very important for all except mineral dust. Growth by condensation is important for sulfate and carbonaceous aerosol (along with aqueous oxidation for the former and ageing by soluble material for the latter). The vertical extent of biomass-burning emissions into the free troposphere is also important for the profile of carbonaceous aerosol. Boundary-layer mixing plays a dominant role for sea salt and mineral dust, which are emitted only from the surface. Dry deposition and below-cloud scavenging are important for the profile of mineral dust only. In this model, the microphysical processes of nucleation, condensation and coagulation dominate the vertical profile of the smallest particles by number (e.g. total CN > 3 nm), while the profiles of larger particles (e.g. CN > 100 nm) are controlled by the same processes as the component mass profiles, plus the size distribution of primary emissions. We also show that the processes that affect the AOD-normalised radiative forcing in the model are predominantly those that affect the vertical mass distribution, in particular convective transport, in-cloud scavenging, aqueous oxidation, ageing and the vertical extent of biomass-burning emissions.

An Evaluation of the CHIMERE Chemistry Transport Model to Simulate Dust Outbreaks across the Northern Hemisphere in March 2014

“…An increase of the fine fraction of dust is modeled in the equatorial area particularly over the Atlantic Ocean with an increase of the altitude of the maximum concentrations certainly in relation to an increase of the magnitude of precipitations (Figure 7), which are peaking during this season in equatorial areas. Indeed, precipitations remove the biggest particles by impaction processes by sub-cloud scavenging while the smallest ones will be less influenced [104] except by Brownian motions, this behavior observed over the Atlantic Ocean at 0-5 • N close to the equator is consistent with the patterns for the Pacific Ocean with a slight shift towards northern latitudes (about 10 • N). This behavior is highlighted in Figure 7 with an increase of the Ω w ratio for wet deposition close to the equator in relation with an increase of precipitations.…”

“…The dust concentrations peak are linked to the lowest contributions of fine particles (with diameter lower than 2.5 µm), in background areas of the atmosphere the fine fraction contribution is generally higher due to the sedimentation and dry deposition of the biggest particles. In the lower troposphere, the lowest concentrations are simulated near the polar easterlies area (about 60 • N) with a second concentrations peak between 2 and 3 km in altitude in the polar cell above 80 • N; this specific location over the poles is confirmed during the AeroCom experiment [104]. A close look at vertical wind speeds shows slight positive vertical winds around 2-3 km in altitude ( Figure S11 in Supplementary Materials) upper in altitude, which can explain this feature, generating stagnant conditions leading to higher concentrations.…”

Climate Feedback on Aerosol Emission and Atmospheric Concentrations

“…Aerosol particles are transported from their surface sources to higher altitudes by turbulent mixing and convection. Kipling et al [70] emphasized the importance of convective mixing controlling the aerosol vertical profile for all types except dust. Simulations using a plume model to assess future changes in volcanic aerosol transport suggest that a future increase in tropopause height due to strengthened convection will lead to reduced transport of volcanic aerosol into the stratosphere reducing its lifetime and cooling effect [71].…”

Cloud albedo changes in response to anthropogenic sulfate and non-sulfate aerosol forcings in CMIP5 models

“…decrease the albedo, in a way that is not well represented in the models. Model evaluations of the vertical distribution of aerosols have indicated that aerosol amount and absorption above clouds, and consequently atmospheric heating, are often underestimated in models (Peers et al, 2016;Myhre and Samset, 2015), and Kipling et al (2016) point at several factors including convective transport, emission height, vertical mixing, and deposition processes to which a global model's vertical distribution of aerosol may be sensitive.…”