Abstract
Abstract. Nudging as an assimilation technique has seen increased use in recent years in the development and evaluation of climate models. Constraining the simulated wind and temperature fields using global weather reanalysis facilitates more straightforward comparison between simulation and observation, and reduces uncertainties associated with natural variabilities of the large-scale circulation. On the other hand, the forcing introduced by nudging can be strong enough to change the basic characteristics of the model climate. In the paper we show that for the Community Atmosphere Model version 5 (CAM5), due to the systematic temperature bias in the standard model and the sensitivity of simulated ice formation to anthropogenic aerosol concentration, nudging towards reanalysis results in substantial reductions in the ice cloud amount and the impact of anthropogenic aerosols on long-wave cloud forcing. In order to reduce discrepancies between the nudged and unconstrained simulations, and meanwhile take the advantages of nudging, two alternative experimentation methods are evaluated. The first one constrains only the horizontal winds. The second method nudges both winds and temperature, but replaces the long-term climatology of the reanalysis by that of the model. Results show that both methods lead to substantially improved agreement with the free-running model in terms of the top-of-atmosphere radiation budget and cloud ice amount. The wind-only nudging is more convenient to apply, and provides higher correlations of the wind fields, geopotential height and specific humidity between simulation and reanalysis. Results from both CAM5 and a second aerosol–climate model ECHAM6-HAM2 also indicate that compared to the wind-and-temperature nudging, constraining only winds leads to better agreement with the free-running model in terms of the estimated shortwave cloud forcing and the simulated convective activities. This suggests nudging the horizontal winds but not temperature is a good strategy for the investigation of aerosol indirect effects since it provides well-constrained meteorology without strongly perturbing the model's mean climate.
Highlights
Further analysis suggested that this lack of sensitivity to temperature nudging in ECHAM6-HAM2 is possibly attributable to the smaller contribution of homogeneous nucleation to ice crystal number concentration, and the weaker sensitivity of ice formation to aerosol abundance in the parameterization of Kärcher and Lohmann (2002) and Lohmann and Kärcher (2002)
In this paper we discussed the impact of nudging on the characterization of aerosol indirect effects in two climate models
The motivation for using nudging in such an investigation is to allow for comparison with observations in a particular time period, to reduce uncertainties associated with natural variabilities in the large-scale flow, and to facilitate comparison among results from different models that participate in the AeroCom Phase III activities
Summary
Nudging ( called Newtonian relaxation) of meteorological fields towards estimates from weather analyses has been used in various studies concerning climate model development and evaluation (e.g., Jeuken et al, 1996; Feichter and Lohmann, 1999; Machenhauer and Kirchner, 2000; Ghan et al, 2001; Hauglustaine et al, 2004; Kerkweg et al, 2006; Schmidt et al, 2006; Telford et al, 2008; Kooperman et al, 2012). Because the meteorological features are strongly constrained, nudging eliminates one source of model variability, reduces error and uncertainty in other terms, and 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) Because of these benefits, the AeroCom aerosol–climate model intercomparison initiative (http://aerocom.met.no/) explicitly requires nudged simulations for several projects of its Phase III activities on assessing the aerosol indirect effect (https://wiki.met.no/aerocom/ indirect).
Sign-in/Register to view highlights & summary Sign-in/Register to access full text options 
Free) 
Talk to us
Join us for a 30 min session where you can share your feedback and ask us any queries you have
Schedule a call
