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Estimating uncertainty of reanalyses using energy budget diagnostics

Estimating uncertainty of reanalyses using energy budget diagnostics. Michael Mayer. Aim of this work. Evaluation of climatology, anomalies and trends of the atmospheric energy budget 1979-2011 Data: Reanalyses : ERA-Interim, MERRA, CFSR Additionally : CERES, ISCCP, GPCP

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Estimating uncertainty of reanalyses using energy budget diagnostics

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  1. Estimating uncertainty of reanalyses using energy budget diagnostics Michael Mayer Michael Mayer - April 2012

  2. Aim of this work • Evaluation of climatology, anomalies and trends of the atmospheric energy budget 1979-2011 • Data: • Reanalyses: ERA-Interim, MERRA, CFSR • Additionally: CERES, ISCCP, GPCP • Changes in the budgets: • True climate signal? • Changes in the observing system? Michael Mayer - April 2012

  3. Divergence of horizontal energy transport Direct method Indirect method • Calculation of the divergence as a residual • Use of 12-hourly accumulated vertical fluxes • Energy tendency from 12-hourly forecasts • No problems with time resolution • Mass inconsistencies as well Michael Mayer - April 2012 Calculation of the divergence from u, v, q, T Analysed fields with 6-hourly resolution Problems with mass consistency

  4. Mass budget in ERA-Interim • Systematically deficient surface pressure forecasts • (Up to -600hPa/year!) • Instantaneous mass flux divergence also erroneous • Mass correction is necessary!

  5. Climatology at TOA • Too little energy input in the Tropics • Too weak meridionalRadTOA gradient Michael Mayer - April 2012

  6. Mean poleward energy transport • Much too weak total poleward energy transport (atmosphere + ocean) • Atmospheric transport in better agreement with reference estimate Michael Mayer - April 2012

  7. Variability: Zonal mean budget (20N/20S) • Tropical energy export is varying very weakly • Direct/Indirect method show different anomalies Michael Mayer - April 2012

  8. Fluxes at TOA and at the surface • Radiative fluxes quite stable in time • Trends in latent heat flux are compensated by trends in energy tendency forecasts Michael Mayer - April 2012

  9. Analysis increments • Defined as the difference between the analyzed and the forecasted state of the model, using 12-hourly forecasts • Total energy tendency analysis increments: • Increments can change in time due to • Changes in the observation system • Imperfectly forecasted changes of the atmospheric circulation Michael Mayer - April 2012

  10. Analysis increments in the Tropics • Strong shifts in analysis increments • Precipitation and moist analysis increments highly correlated Michael Mayer - April 2012

  11. Zonal mean latent heat analysis increments • Strongest shifts related to SSM/I in 1992 and 2008 • Apparently relatively weak anomalies related to climate anomalies • On overall, no improvement over time Michael Mayer - April 2012

  12. Zonal structure of tropical moisture flux divergence Michael Mayer - April 2012

  13. Zonal structure of tropical latent heat analysis increments • Use different climatologies to get rid of inhomogeneities • Simple algorithm reducing RMS-value of analysis increments • ENSO-related anomalies Michael Mayer - April 2012

  14. Further reduce inhomogeneities • 2nd iteration: remove either 2008 shift or stripes Michael Mayer - April 2012

  15. Result: fairly homogeneous anomaly timeseries Michael Mayer - April 2012

  16. Trends in zonal mean transport Indirect estimate Held and Soden (2006): ΔF [PW] • IPCC AR4 models predict increase in total poleward energy transport + stronger Hadley cell • Indirect ERA-I estimate shows opposite trends Michael Mayer - April 2012

  17. Direct estimate Held and Soden (2006): ΔF [PW] • Direct estimate slightly more stable in time • Both estimates show unrealistically strong trends • Reliable trend estimation of energy transports not possible Michael Mayer - April 2012

  18. Conclusions • Reanalyses have too weak mean meridionalRadTOAgradient and thus too weak poleward total energy transports • Weak variation in zonal mean budgets, true climate anomalies hard to identify • Strong increase of evaporation over time -> compensated by analysis increments • For investigation of interannual variability split of climatologies is helpful • 30-year trends in zonal mean poleward transport are too strong and in disagreement with climate models Michael Mayer - April 2012

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  22. Correction methods I Michael Mayer - April 2012 • Indirect method

  23. Correction methods II • Direct method Michael Mayer - April 2012

  24. Mass budget in ERA-Interim II • Time-averaged mass flux divergence unrealistic • Indirect estimation from mass tendency and E-P much better Michael Mayer - April 2012

  25. Mass budget in ERA-Interim • Systematically deficient surface pressure forecasts () Up to -600hPa/year! Forecast error: Michael Mayer - April 2012

  26. Results so far – Direct method • Manyimportantclimatologicalfeatures, but noisy Michael Mayer - April 2012

  27. Indirect method • Much clearer and less noisy Michael Mayer - April 2012

  28. Poleward energy transports– different results FT08…Fasullo und Trenberth (2008) • Mass correction reduces cross-equatorial energy transports Michael Mayer - April 2012

  29. Interannualvariationsofthetransports Michael Mayer - April 2012 Whichanomaliesaretrueclimatesignals (e.g. due to ENSO)? Whichanomaliesareartefacts?

  30. COSMIC AIRS AMSU-A D. Dee(2009) • Changes in theobservingsystemaffectanalysisincrements SSM/I Michael Mayer - April 2012

  31. Next steps Negative lag of total exportanomalies Positive Lag of total exportanomaly ~Zero lag (moist/dry/total) Michael Mayer - April 2012 Understanding the different lagsofmoistand dry anomalies

  32. Next steps Michael Mayer - April 2012 Fast propagatinganomalies (continousblue) in agreementwithTrenberth et al. 2002 We find a much larger pattern (dashedblue) Slowlypropagatinganomalies (red) linkedto ENSO? -> Reproducibilitywithotherdatasets? -> Objectivemethodfordeterminingphasespeedofthe transient featuresrequired (e.g. wavelets) -> propagating SLP anomalies in Peterson and White (1998) – connection?

  33. Next steps Michael Mayer - April 2012 Reliable ‚independent‘ datasetsarerequired ISCCP isbeingreprocessed More homogeneousdatasetsexpected

  34. Next steps Michael Mayer - April 2012 • Prolongation ofthe ERA-Interim-datasets back to 1979 isbeingproduced • extensionofthestudyperiod • Long-termtransporttrendestimatesbecomepossible • Try toverifyexpectedincreaseofpolewardenergytransports (Held and Soden, 2006) • Employmentof a GCM to • Study varyingpartitionbetweenmoistand dry anomalies • Study effectsofElNiño on energytransportswithoutdatainhomogeneities • verifyfeaturesfound in thereanalysisdata

  35. Summary – Scientific goals Michael Mayer - April 2012 A newestimateofthepolewardatmosphericenergytransport A betterunderstandingoftheinterannualvariabilityoftheenergytransports Understanding ofthemechanismsleadingtotheobservedeastwardmovinganomalypatterns Feedback regarding data homogeneity and consistency to the producers of the used datasets

  36. Work plan andfinancing Thankyouforyourattention Michael Mayer - April 2012 1st year: Visit CAS at NCAR (02/2011-07/2011) -> access to new datasets (GCM output, reprocessed satellite data, ocean data) and expertise of the group 2nd year: access to extended ERA-Interim dataset and investigation of long-term trends 3rd year: Employment of GCM results and investigation of the described questions Thesis work is funded within FWF project “Global historic in situ upper air data for climate change research” (running at least until 06/2012) Stay at NCAR mainly funded by ASP

  37. Literature Michael Mayer - April 2012

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