The global aerosol-climate model ECHAM-HAM, version 2: sensitivity to improvements in process representations

Abstract

This paper introduces and evaluates the second version of the global aerosol-climate model ECHAM-HAM. Major changes have been brought into the model, including new parameterizations for aerosol nucleation and water uptake, an explicit treatment of secondary organic aerosols, modified emission calculations for sea salt and mineral dust, the coupling of aerosol microphysics to a two-moment stratiform cloud microphysics scheme, and alternative wet scavenging parameterizations. These revisions extend the model's capability to represent details of the aerosol lifecycle and its interaction with climate. <br><br> Sensitivity experiments are carried out to analyse the effects of these improvements in the process representation on the simulated aerosol properties and global distribution. The new parameterizations that have largest impact on the global mean aerosol optical depth and radiative effects turn out to be the water uptake scheme and cloud microphysics. The former leads to a significant decrease of aerosol water contents in the lower troposphere, and consequently smaller optical depth; the latter results in higher aerosol loading and longer lifetime due to weaker in-cloud scavenging. <br><br> The combined effects of the new/updated parameterizations are demonstrated by comparing the new model results with those from the earlier version, and against observations. Model simulations are evaluated in terms of aerosol number concentrations against measurements collected from twenty field campaigns as well as from fixed measurement sites, and in terms of optical properties against the AERONET measurements. Results indicate a general improvement with respect to the earlier version. The aerosol size distribution and spatial-temporal variance simulated by HAM2 are in better agreement with the observations. Biases in the earlier model version in aerosol optical depth and in the Ångström parameter have been reduced. The paper also points out the remaining model deficiencies that need to be addressed in the future

Aerosol indirect effects from shipping emissions: sensitivity studies with the global aerosol-climate model ECHAM-HAM

“…The version of HAM used in this study is based on the original model presented in Stier et al (2005) with several new developments. In their recent work, Zhang et al (2012) present the changes applied to the original model version and evaluate these changes with respect to observations. Here, we will not delve into the details of this model evaluation, but it should be stated that the model-simulated aerosol size distribution and spatio-temporal variance of the aerosol population have improved compared to observations.…”

“…ECHAM is coupled to HAM, a microphysical aerosol module which calculates the evolution of an aerosol population represented by seven interacting internally and externally mixed log-normal aerosol modes (Zhang et al, 2012). In the setup applied here, HAM treats sulfate (SU), black carbon (BC), particulate organic matter (POM), sea salt (SS) and dust (DU) aerosol.…”

“…Here, we utilise the global aerosol-climate model ECHAM-HAM (Zhang et al, 2012) to thoroughly investigate the effect of shipping emissions on clouds. In the simulations discussed here, cloud cover is computed following a relative humidity-based approach (Sundqvist et al, 1989) and the treatment of convective clouds and -transport is based on the mass-flux scheme of Tiedtke (1989) with modifications for penetrative deep-convection according to Nordeng (1994).…”

A simple model for the time evolution of the condensation sink in the atmosphere for intermediate Knudsen numbers

“…2. As was mentioned 25 earlier, the correction leads to a decrease in the growth rate as compared to the kinetic regime, and the distribution shape changes in time if the correction is applied. Note that the solution obtained numerically employing the full FS coefficient (shown with symbols in Fig.…”

“…This method is not too expensive computationally, and is at the same time rather accurate (Whitby and McMurry, 1997). A 'pseudomodal' approach has been 10 used to develop fast and efficient aerosol microphysics modules for large-scale atmospheric modelling purposes (Vignati et al, 2004;Stier et al, 2005;Mann et al, 2010;Pringle et al, 2010;Aquila et al, 2011;Zhang et al, 2012;Liu et al, 2016).…”