Diagnostic computation of moisture budgets in the 1 ERA - Interim Reanalysis with reference to analysis of 2 CMIP - archived atmospheric model data

5 The diagnostic evaluation of moisture budgets in archived atmosphere model data is examined. 6 Sources of error in diagnostic computation can arise from the use of numerical methods dif7 ferent to those used in the atmosphere model, the time and vertical resolution of the archived 8 data and data availability. These sources of error are assessed using the climatological mois9 ture balance in the European Centre for Medium Range Weather Forecasts Interim Reanalysis 10 (ERA-I) that archives vertically integrated moisture fluxes and convergence. The largest single 11 source of error arises from the diagnostic evaluation of divergence. The chosen second order 12 accurate centered finite difference scheme applied to the actual vertically integrated moisture 13 fluxes leads to significant differences from the ERA-I reported moisture convergence. Using 14 daily, instead of 6 hourly, data leads to an underestimation of the patterns of moisture diver15 gence and convergence by mid-latitude transient eddies. A larger and more widespread error 16 occurs when the vertical resolution of the model data is reduced to the 8 levels that is quite 17 common for daily data archived for the Coupled Model Intercomparison Project (CMIP). Di18 viding moisture divergence into components due to the divergent flow and advection requires 19 bringing the divergence operator inside the vertical integral which introduces a surface term 20 for which a means of accurate evaluation is developed. The analysis of errors is extended to 21 the case of the spring 1993 Mississippi Valley floods, the causes of which are discussed. For 22 future archiving of data (e.g. by CMIP) it is recommended that monthly means of time step 23 resolution flow-humidity co-variances be archived at high vertical resolution. 24