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Technical Note: on the Use of Nudging for Aerosol–climate Model Intercomparison Studies : Volume 14, Issue 16 (26/08/2014)

By Zhang, K.

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Book Id: WPLBN0003994090
Format Type: PDF Article :
File Size: Pages 15
Reproduction Date: 2015

Title: Technical Note: on the Use of Nudging for Aerosol–climate Model Intercomparison Studies : Volume 14, Issue 16 (26/08/2014)  
Author: Zhang, K.
Volume: Vol. 14, Issue 16
Language: English
Subject: Science, Atmospheric, Chemistry
Collections: Periodicals: Journal and Magazine Collection, Copernicus GmbH
Publication Date:
Publisher: Copernicus Gmbh, Göttingen, Germany
Member Page: Copernicus Publications


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Wan, H., Liu, X., Ghan, S. J., Rasch, P. J., Kooperman, G. J., Lohmann, U.,...Neubauer, D. (2014). Technical Note: on the Use of Nudging for Aerosol–climate Model Intercomparison Studies : Volume 14, Issue 16 (26/08/2014). Retrieved from

Description: Pacific Northwest National Laboratory, Richland, Washington, USA. 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.

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

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