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Can a reanalysis using a frozen model (fixed CO2) have an anthropogenic trend ? (Cai and Kalnay, submitted to J. of Climate) Intuition may suggest that Reanalysis would at most show only a watered down trend of greenhouse warming, but it is wrong.

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## Can a reanalysis using a frozen model (fixed CO2) have an anthropogenic trend ? (Cai and Kalnay, submitted to J. of C

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**Can a reanalysis using a frozenmodel (fixed CO2) have an**anthropogenic trend?(Cai and Kalnay, submitted to J. of Climate) • Intuition may suggest that Reanalysis would at most show only a watered down trend of greenhouse warming, but it is wrong. • Cai and Kalnay (2003, subm. to J of Cl) show analytically that essentially the full trend present in the observations appears in the Reanalysis after a short transient.**An analogy of our reasoning**versus Observation Model (a constant warming) (a constant drift) Q: What does a data assimilation system do with observations? A: In nut shell, at each observational time, a data assimilation system does a weighted average of the observation with the model forecasts started from previous analysis.**= 50/%**< 36% Step 1 Step 2 The bleaching glass represents the model adjustment to the imbalance between the model physics and the assimilated temperature. < 22% < 2% Step 3 Step N Weights assigned to observation and to model = 0.5 An analogy of our reasoning (cont.)**Observation:**• Model without data assimilation: “Watered-down effect” • Analysis (weight assign to model a ): Analysis step/model adjustment time step RESULTS**0.995**0.99 0.98 0.95 0.90 0.75 0.5 0.25 Analysis Step N Ratio of the trend between two consecutive analysis cycles to the trendin the observation as a function of N, the analysis step starting from N = 1, and (1 – a), the weight assigned to the observation in the data assimilation procedure for Dt/t = 0.01. (Cai and Kalnay, subm. to J. of Clim., 2003) 1-a

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