Evaluation of CESM1 (WACCM) free-running and specified-dynamics atmospheric composition simulations using global multi-species satellite data records
Authors: Froidevaux Lucien , Kinnison Douglas E. , Wang Ray , Anderson John , Fuller Ryan A. 
Published: June 2018

Abstract. We evaluate the recently delivered Community Earth System Model version 1 (CESM1) Whole Atmosphere Community Climate Model (WACCM) using satellite-derived global composition datasets, focusing on the stratosphere. The simulations include free-running (FR-WACCM) and specified-dynamics (SD-WACCM) versions of the model. Model evaluations are made using global monthly zonal mean time series obtained by the Aura Microwave Limb Sounder (MLS), as well as longer-term global data records compiled by the Global Ozone Chemistry and Related Trace gas Data Records for the Stratosphere (GOZCARDS) project. A recent update (version 2.20) to the original GOZCARDS merged ozone (O3) data set is used. We discuss upper atmospheric climatology and zonal mean variability using O3, hydrogen chloride (HCl), nitrous oxide (N2O), nitric acid (HNO3), and water vapor (H2O) data. There are a few significant model/data mean biases, such as for lower stratospheric O3, for which the models overestimate the mean observed values and seasonal amplitudes. Another clear difference occurs for HNO3 during recurring winter periods of strong HNO3 enhancements at high latitudes; this stems from the known omission of ion chemistry relating to particle precipitation effects, in the global models used here. In the lower stratosphere at high southern latitudes, the variations in polar winter/spring composition observed by MLS are generally well matched by SD-WACCM, the main exception being for the early winter rate of decrease in HCl, which is too slow in the model. In general, the latitude/pressure distributions of annual and semi-annual oscillation amplitudes derived from the MLS data are properly captured by the corresponding model values. Nevertheless, detailed aspects of the interactions between the quasi-biennial, annual, and semi-annual ozone variations in the upper stratosphere are not as well represented by FR-WACCM as by SD-WACCM. One of the evaluation diagnostics we use represents the closeness of fit between the model/data anomaly time series, and we also consider the correlation coefficients. Not surprisingly, SD-WACCM, which is driven by realistic dynamics, generally matches observed deseasonalized anomalies better than FR-WACCM does. Other results indicate that the root mean square variability is sometimes found to be significantly smaller in FR-WACCM than in SD-WACCM and the observations. Most notably, FR-WACCM underestimates the observed interannual variability for H2O by ~ 30 %, typically, and by as much as a factor of two in some regions; this has some implications for the time needed to detect small trends. We have derived trends using a multivariate linear regression (MLR) model, and there is a robust signal in both MLS observations and WACCM of an upper stratospheric O3 increase from 2005 to 2014 by ~ 0.2–0.4 %/yr (±0.2 %/yr, 2σ), depending on which broad latitude bin (tropics or mid-latitudes) is considered. In the lower stratosphere, while some decreases are indicated for 1998–2014 (based on merged GOZCARDS O3), we find near-zero or positive trends when using MLS O3 data alone for 2005–2014, albeit with no robust statistical significance. SD-WACCM results track such positive tendencies (albeit with no statistical significance). For H2O, the most statistically significant trend result for 2005–2014 is an upper stratospheric increase, peaking at slightly more than 0.5 %/yr in the lower mesosphere, in fairly close agreement with SD-WACCM trends, but with smaller values in FR-WACCM. For HCl, while the lower stratospheric vertical gradients of MLS trends are duplicated to some extent by SD-WACCM, the model trends (decreases) are always on the low side of the data trends. There is little model-based indication (in SD-WACCM) of a significantly positive HCl trend derived from the MLS tropical series at 68 hPa; this deserves further study. For N2O, the MLS-derived trends (for 2005–2012) point to negative trends (of up to about −1 %/yr) in the NH mid-latitudes and positive trends (of up to about +3 %/yr) in the SH mid-latitudes, in good agreement with the asymmetry that exists in SD-WACCM trend results. The small observed positive N2O trends of ~ 0.2 %/yr in the 100 to 30 hPa tropical region are also consistent with model results (SD-WACCM in particular), which in turn are very close to the known rate of increase in tropospheric N2O. In the case of HNO3, MLS-derived lower stratospheric trend differences (for 2005–2014) between hemispheres are opposite in sign to those from N2O and in reasonable agreement with both WACCM results, despite large error bars. The data sets and tools discussed here for the evaluation of the models could be expanded to additional comparisons of species not included here, as well as to model intercomparisons using a variety of CCMs, keeping in mind that there are different parameterizations and approaches for both free-running and specified-dynamics simulations.