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

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 freerunning 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.

Quantifying the impact of sub-grid surface wind variability on sea salt and dust emissions in CAM5

“…The horizontal winds were nudged to the ERA-Interim reanalysis with a relaxation timescale of 6 h. Temperature and humidity were not nudged, since earlier experiences have shown that directly constraining these two fields might lead to substantially different model climate (Zhang et al, 2014;Jeuken et al, 1996). Details of the nudging implementation are described in Zhang et al (2014).…”

“…The horizontal winds were nudged to the ERA-Interim reanalysis with a relaxation timescale of 6 h. Temperature and humidity were not nudged, since earlier experiences have shown that directly constraining these two fields might lead to substantially different model climate (Zhang et al, 2014;Jeuken et al, 1996). Details of the nudging implementation are described in Zhang et al (2014). Nudging effectively constrains the large-scale circulation and suppresses the noise caused by natural variability, thus allowing for detection of changes in the simulated aerosol characteristics and their climate effects with substantially shorter simulations (see, e.g., Kooperman et al, 2012).…”

Can nudging be used to quantify model sensitivities in precipitation and cloud forcing?

“…On the other hand, as nudging introduces an artificial forcing term to the prognostic equations of the model, it can significantly change the basic characteristics of the model climate, resulting in unintentional impacts on the simulated atmospheric physics and/or chemistry. For example, Lohmann and Hoose [2009] found that ECHAM5-HAM produced more convection and precipitation in the tropics when it was nudged toward the European Centre for Medium-Range Weather Forecasts (ECMWF) ERA40 reanalysis, while Kooperman et al [2012] and Zhang et al [2014] noted that when temperature and winds in CAM5 were nudged toward the ERA-Interim reanalysis, changes were seen in the top-ofatmosphere (TOA) energy budget, convective precipitation, and the liquid and ice water paths. These studies suggest that, although nudging can reduce the impact of natural internal variability, it could also potentially cause biases.…”

The Chemistry Climate Model ECHAM6.3-HAM2.3-MOZ1.0

“…The weaker cloud radiative effects can be explained by a lower cloud cover and a smaller liquid water path in the ECHAM-HAMMOZ simulation. Analog differences were found by Zhang et al (2014) for ECHAM6.1-HAM2.2 due to differences in nudging techniques. Zhang et al (2014) investigated the behavior of aerosol-climate models when different nudging techniques are applied compared to free running simulations.…”

“…Analog differences were found by Zhang et al (2014) for ECHAM6.1-HAM2.2 due to differences in nudging techniques. Zhang et al (2014) investigated the behavior of aerosol-climate models when different nudging techniques are applied compared to free running simulations. In particular the additional nudging of temperature decreased the cloud radiative effects by about 5, resp.…”