Aerosol activation and cloud processing in the global aerosol-climate model ECHAM5-HAM

Abstract

Abstract.A parameterization for cloud processing is presented that calculates activation of aerosol particles to cloud drops, cloud drop size, and pH-dependent aqueous phase sulfur chemistry. The parameterization is implemented in the global aerosol-climate model ECHAM5-HAM. The cloud processing parameterization uses updraft speed, temperature, and aerosol size and chemical parameters simulated by ECHAM5-HAM to estimate the maximum supersaturation at the cloud base, and subsequently the cloud drop number concentration (CDNC) due to activation. In-cloud sulfate production occurs through oxidation of dissolved SO 2 by ozone and hydrogen peroxide. The model simulates realistic distributions for annually averaged CDNC although it is underestimated especially in remote marine regions. On average, CDNC is dominated by cloud droplets growing on particles from the accumulation mode, with smaller contributions from the Aitken and coarse modes. The simulations indicate that in-cloud sulfate production is a potentially important source of accumulation mode sized cloud condensation nuclei, due to chemical growth of activated Aitken particles and to enhanced coalescence of processed particles. The strength of this source depends on the distribution of produced sulfate over the activated modes. This distribution is affected by uncertainties in many parameters that play a direct role in particle activation, such as the updraft velocity, the aerosol chemical composition and the organic solubility, and the simulated CDNC is found to be relatively sensitive to these uncertainties.

Evaluation of simulated aerosol properties with the aerosol-climate model ECHAM5-HAM using observations from the IMPACT field campaign

“…In the moist period sulfate maximizes at 2200 m altitude coinciding with the maximum in the accumulation mode particle concentration and humidity. This sulfate originates mostly from in-cloud chemical processes in aerosol water associated with activated modes, simulated as described by Roelofs et al (2006). The model also associates significant sulfate amounts with the dust plume that reaches Cabauw at the end of the month.…”

“…The bulk cloud chemistry scheme in ECHAM5-HAM has been replaced with a cloud processing parameterization (Roelofs et al, 2006). First, the cloud drop number concentration is estimated through an empirical approach.…”

Processes limiting the emergence of detectable aerosol indirect effects on tropical warm clouds in global aerosol-climate model and satellite data

“…This is because i) sulphuric emissions are high just off the African coast, and ii) this region is known to be dominated by low-level clouds, which are captured by the host model (Nam, 2011), thus enabling efficient in-cloud production of SU. In-cloud production of SU leads to a shift of the aerosol size distribution towards larger sizes, which typically increases CCN concentrations (Roelofs et al, 2006). Therefore, aqueous oxidation cannot contribute SU mass to aerosol particles in the main area of interest, which is located further south, possibly limiting the emergence of AIEs.…”

Importance of tropospheric volcanic aerosol for indirect radiative forcing of climate

“…However, our approach and our choice of updraught velocities have been shown to provide a good approximation of mean CDNC (Fountoukis et al, 2007;Pringle et al, 2009;Karydis et al, 2011). As with many other aerosol activation schemes (e.g., Chen and Penner, 2005;Roelofs et al, 2006), the employed scheme does not account for droplet collision-coalescence (i.e. no droplet loss rate), thus CDNCs are shown at cloud-base altitude.…”