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Use of Nudging for Aerosol–Climate Model Intercomparison Studies

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Nudging meteorological fields toward estimates from weather analyses is a popular method used in various studies for climate model development and evaluation. This technique introduces extra terms into the equations that govern the evolution of temperature, horizontal winds and sometimes mass fields, to nudge them toward observed values. Nudging has seen an increased use as an assimilation technique in recent years to develop and evaluate climate models. However, nudging introduces forcing that can be strong enough to change the basic characteristics of the model climate. A team led by U.S. Department of Energy researchers at Pacific Northwest National Laboratory found that when the Community Atmosphere Model version 5 (CAM5) applied nudging toward reanalysis, it results in substantial reductions in the ice-cloud amount and the impact of anthropogenic aerosols on long-wave cloud forcing. To reduce discrepancies between the nudged and unconstrained simulations, and also take the advantage of nudging, the team evaluated two alternative experimentation methods. The first constrains only the horizontal winds. The second method nudges both winds and temperature, but replaces the long-term climatology of the reanalysis with that of the model. They found 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. Wind-only nudging not only provides very good correlations between model simulation and reanalysis for the large-scale dynamical fields, such as wind and geopotential height, but also indirectly improves the simulated specific humidity.

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Zhun Guo
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