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

Abstract

Abstract. In May 2008, the measurement campaign IM-PACT for observation of atmospheric aerosol and cloud properties was conducted in Cabauw, The Netherlands. With a nudged version of the coupled aerosol-climate model ECHAM5-HAM we simulate the size distribution and chemical composition of the aerosol and the associated aerosol optical thickness (AOT) for the campaign period. Synoptic scale meteorology is represented realistically through nudging of the vorticity, the divergence, the temperature and the surface pressure. Simulated concentrations of aerosol sulfate and organics at the surface are generally within a factor of two from observed values. The monthly averaged AOT from the model is 0.33, about 20% larger than observed. For selected periods of the month with relatively dry and moist conditions discrepancies are approximately −30% and +15%, respectively. Discrepancies during the dry period are partly caused by inaccurate representation of boundary layer (BL) dynamics by the model affecting the simulated AOT. The model simulates too strong exchange between the BL and the free troposphere, resulting in weaker concentration gradients at the BL top than observed for aerosol and humidity, while upward mixing from the surface layers into the BL appears to be underestimated. The results indicate that beside aerosol sulfate and organics also aerosol ammonium and nitrate sigCorrespondence to: G. J. Roelofs (g.j.h.roelofs@uu.nl) nificantly contribute to aerosol water uptake. The simulated day-to-day variability of AOT follows synoptic scale advection of humidity rather than particle concentration. Even for relatively dry conditions AOT appears to be strongly influenced by the diurnal cycle of RH in the lower boundary layer, further enhanced by uptake and release of nitric acid and ammonia by aerosol water.

Technical Note: On the use of nudging for aerosol–climate model intercomparison studies

“…If the unconstrained terms approximate atmospheric processes reasonably, the resulting simulations should produce modeled features that can be compared to observation for specific weather episodes (Feichter and Lohmann, 1999;Dentener et al, 1999;Coindreau et al, 2007;Schulz et al, 2009;Roelofs et al, 2010). Because the meteorological features are strongly constrained, nudging eliminates one source of model variability, reduces error and uncertainty in other terms, and thus facilitates detection of signatures of changes in process representations (parameterizations) in simulations that might otherwise require multiple decades of simulation time in order to clearly discriminate between signal and noise (Lohmann and Hoose, 2009;Lohmann and Ferrachat, 2010;Kooperman et al, 2012).…”

Estimation of aerosol water and chemical composition from AERONET Sun–sky radiometer measurements at Cabauw, the Netherlands

“…On some days the observed hourly variability, represented by the black line, appears to be captured by the model (6, 13 May). A significant overestimation occurs on 4 May (to a lesser extent also on 2 May and during one measurement in the afternoon of 12 May; however these are already marked as "suspect"), when traces of Sahara dust were present over the Cabauw region (Roelofs et al, 2010). Since mineral dust is not taken into account in this study, the associated scattering and absorption is now attributed to the other compounds in our model.…”

“…During the second half of the month the weather was more unsettled, with cloudy and rainy periods (e.g., Roelofs et al, 2010;, and as a result AERONET observations are relatively sparse. Therefore we focus our analysis on 1-14 May.…”

“…Further, observed Cl − concentrations are very low ( 1 µg m −3 ). Traces of Sahara dust were present over the Cabauw region only on 4 May (Roelofs et al, 2010). To reduce the number of parameters in the optimization we assumed specific ammonium : sulfate and nitrate : sulfate ratios, based on .…”

Improving the simulation of global aerosol with size-segregated anthropogenic number emissions

“…Past simulations performed with the ECHAM-HAM include an extensive analysis of particle nucleation (Makkonen et al, 2009(Makkonen et al, , 2014Kazil et al, 2010), aerosol properties (Roelofs et al, 2010), and emission data set implementation (Zhang et al, 2012). Although the ECHAM-HAM has a detailed microphysics module for describing the aerosol size distribution (Vignati et al, 2004), previous studies have not included an exhaustive module for the input particle number size distribution.…”