Limits to the Impact of Empirical Correction on Simulation of the Water Cycle

Paul A. Dirmeyer Center for Ocean–Land–Atmosphere Studies, Calverton, Maryland

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Timothy DelSole George Mason University, Fairfax, Virginia, and Center for Ocean–Land–Atmosphere Studies, Calverton, Maryland

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Mei Zhao Centre for Australian Weather and Climate Research, Melbourne, Victoria, Australia

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Abstract

Empirical correction is applied to wind, temperature, and soil moisture fields in a climate model to assess its impact on simulation of the water cycle during boreal summer. The empirical correction method is based on the biases in model forecasts only as a function of the time of year. Corrections are applied to the prognostic equations as an extra nudging term. Mean fields of evaporation, precipitation, moisture transport, and recycling ratio are all improved, even though humidity fields were not corrected. Simulation of the patterns of surface evaporation supplying rainfall at locations over land is also improved for most locations. There is also improvement in the simulation of evaporation and possibly rainfall, as measured by anomaly correlation coefficients and root-mean-square errors of the time series of monthly anomalies. However, monthly anomalies of other water cycle fields such as moisture transport and recycling ratio were not improved. Like any statistical adjustment, empirical correction does not address the cause of model errors, but it does provide a net improvement to the simulation of the water cycle. It can, however, be used to diagnose the sources of error in the model. Since corrections are only applied to prognostic variables, shortcomings due to physical parameterizations in the model are not remedied.

Corresponding author address: Paul Dirmeyer, Center for Ocean–Land–Atmosphere Studies, 4041 Powder Mill Rd., Suite 302, Calverton, MD 20705-3106. E-mail: dirmeyer@cola.iges.org

Abstract

Empirical correction is applied to wind, temperature, and soil moisture fields in a climate model to assess its impact on simulation of the water cycle during boreal summer. The empirical correction method is based on the biases in model forecasts only as a function of the time of year. Corrections are applied to the prognostic equations as an extra nudging term. Mean fields of evaporation, precipitation, moisture transport, and recycling ratio are all improved, even though humidity fields were not corrected. Simulation of the patterns of surface evaporation supplying rainfall at locations over land is also improved for most locations. There is also improvement in the simulation of evaporation and possibly rainfall, as measured by anomaly correlation coefficients and root-mean-square errors of the time series of monthly anomalies. However, monthly anomalies of other water cycle fields such as moisture transport and recycling ratio were not improved. Like any statistical adjustment, empirical correction does not address the cause of model errors, but it does provide a net improvement to the simulation of the water cycle. It can, however, be used to diagnose the sources of error in the model. Since corrections are only applied to prognostic variables, shortcomings due to physical parameterizations in the model are not remedied.

Corresponding author address: Paul Dirmeyer, Center for Ocean–Land–Atmosphere Studies, 4041 Powder Mill Rd., Suite 302, Calverton, MD 20705-3106. E-mail: dirmeyer@cola.iges.org
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