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Major discrepancies in the analyzed wind in the tropics

Major discrepancies in the analyzed wind in the tropics. Oreste Reale GESTAR-USRA and NASA GSFC Earth Sciences Division. Motivation. Operational analyses and state-of-the-art reanalyses show increasing agreement in the extra-tropical atmosphere.

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Major discrepancies in the analyzed wind in the tropics

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  1. Major discrepancies in the analyzed wind in the tropics Oreste Reale GESTAR-USRA and NASA GSFC Earth Sciences Division

  2. Motivation • Operational analyses and state-of-the-art reanalyses show increasing agreement in the extra-tropical atmosphere. • However, large discrepancies still occur in major features of the tropical atmosphere. • These discrepancies are present in both seasonal climatologies and instantaneous snapshots, and indicate the imperative need of additional wind data

  3. Outline • African Easterly Jet (climatology of structure and instability properties) in state-of-the-art reanalyses (ERA-40, NCEP-R, JRA-25 and MERRA) • AEJ representation on weather-time-scales in operational analyses during SOP-3 NAMMA (2006) • Somali JetinMERRAand NCEP operational(2010) • Mid-tropospheric flow over the entire tropical Pacific in August 2010 in NCEP operational, ECMWF operational,and MERRA • Mis-representation or absence of observed Tropical cyclones in analyses (Indian Ocan, Atlantic, away from HH flights) in operational analyses • Conclusions

  4. AEJ and its instability properties in state-of-the-art reanalyses • New publication in 2012(Wu et al., 2012) compares the climatological representation of the African Easterly Jet (AEJ)structure and its instability properties in reanalyses across a 22-year average • Despite revealing some instability property of the AEJ that appears data-independent,ERA-40, NCEP-R2, JRA-25 and MERRAprovidedifferent descriptions of the AEJ horizontal structure,intensity, and of some properties that control wave growth on a seasonal scale (JAS). M.-L. C. Wu, Reale, O., S. Schubert, M. Suarez, C. Thorncroft, 2012: African Easterly Jet: barotropic instability, waves and cyclogenesis. J. Climate, 25, 1489-1510.

  5. Comparison of the AEJstructure in MERRA, ERA-40, NCEP-R2, and JRA-25. Speed, vorticity and barotropic instability (Fig 2) Wu, M.-L, O. Reale, S. Schubert, M. J. Suarez, C. Thorncroft, 2012:African Easterly Jet: barotropic instability, waves and cyclogenesis.Journal of Climate, 25, 1489-1510.

  6. The analyses differ in terms of strength of the low-level monsoonal flow, slope of the barotropically unstable part of the AEJ, horizontal shear distribution. All Figures show a 22-year JAS average From Wu et al. (2010) Fig 2

  7. Large discrepancies between snapshots of analyzed representation of the African Monsoon-Eastern Tropical Atlantic regions • The African Easterly Jet at about 600hPa,the low-level monsoonal flow (predominantly southwesterly between 1000 and 800 hPa)and the Tropical Easterly Jet (between 200 and 100 hPa)are the critical players in Atlantic tropical development. • Comparison between operational NCEP analyses and GEOS-5-produced analyses reveal serious discrepancies • Validation against vertical sounding in the area at Cape Verde (15N, 23.5W) produced during the 2006 NAMMA campaign, show that both analyses have large errors

  8. Huge discrepancies between GEOS-5 and NCEP operational analyses Wind at 5-15N, 500-600 hPa, has opposite direction! Only in the tropics the two analyses differ substantially Section at 23.5W

  9. Largest differences between reanalyses are in the tropics, at about 15N (on the order of 12m/s)larger even thandiscrepancies in the southern hemisphere jet stream NCEP GEOS-5

  10. Huge differences in the entire tropical zonal flow from 20S to 20N at all levels

  11. Largest mid-tropospheric wind difference is in the tropics, at 0-10N GEOS-5 analyses produce a weaker easterly flow than NCEP GEOS-5 NCEP

  12. Largest low-tropospheric wind difference is in the tropics, between 10S and Equator Opposite-sign discrepancy with respect to previous slide: GEOS-5 analyses produce stronger easterly flow than NCEP) NCEP GEOS-5

  13. Additional vertical soundings at Cape Verde during SOP-3 (2006) provide the chance to validate operational analyses and hindcast experiments Both NCEP and GEOS-5 miss the AEJ maximum at 600hPa. Error larger than 10 m/s at AJE level!!! One of the rare cases in which NCEP and GEOS-5 differ less than 5 m/s) obs NCEP vs GEOS-5 obs

  14. Catastrophic non-systematic differences NCEP provides a good representation of low-level and upper-level flows but misses the AEJ. GEOS-5 has huge errors at all levels except at 600hPa. NCEP and GEOS-5both miss the low-level flow, with NCEP having larger errors. obs NCEP vs GEOS-5 obs

  15. Catastrophic non-systematic differences NCEP has a stronger AEJ. GEOS-5 has a stronger AEJ. NCEP vs GEOS-5

  16. Large differences in the atmospheric circulation over the tropical Indian Ocean • The Somali Jet, a localized, very shallow jet partly controlled by the Indian monsoonal low and by the orography of Eastern Africa, appears to be very sensitive to different data assimilation systems or data assimilation strategies • Its representation is of great importance in prediction of the Monsoon breaks, development of tropical cyclones, intensity of low-level moist flow controlling precipitation • The circulation over the entire northern Indian Ocean is strongly impacted by observations

  17. Somali Jet: comparison between MERRA and NCEP operational 5-day average

  18. Somali Jet core position in MERRA and NCEP:500 km of displacement,very large height and shape differences Somali Jet Somali Jet

  19. Large, non-systematic differences in the definition of the Somali Jet and in the wind over the Indian Ocean NCEP Operational vs MERRA Somali Jet Somali Jet Five-day average, meridional section at 60E from 65S to 65N, at 900hPa and 800 hPa respectively

  20. Pakistan Floods (2010) • Catastrophic floods in late July and August 2010 • 3 sets of 45-day data assimilation runs: July 15 to August 31 with different AIRS data sets • 3 sets of 7-day forecasts at 00z of every day for every set of analyses • Assessment of Global Skill and evaluation of Precipitation analysis and forecast over region show that the moist southerly flow against Pakistan is strongly affected by the assimilation strategy • Impact assessment on dynamics of the mechanism • Reale, O., W.K. Lau, J. Susskind, R. Rosenberg, (2012), AIRS Impact on Analysis and Forecast of an Extreme Rainfall event (Indus River Valley, Pakistan, 2010) with a global data assimilation and forecast system.’’J. Geophys. Res., 117, D08103, doi:10.1029/2011JD017093.

  21. 7-day moisture transport across the entire forecastinitialized at 00z22July Reale et al. (2012) Contour: Vertically integrated 7-day mean total Moisture flux Shaded: 7-dayRET minus RAD meridional Moisture flux Net increase of Northward merid. Transport towards Northern Pakistan Consequent to AIRS v5 retrieval assimilation

  22. Large differences between operational ECMWF, NCEP and MERRA over the entire tropical Pacific during strong La Nina conditions (Aug 2010) • Weather prediction over the tropical Pacific is controlled by a good representation of the predominantly easterly flow and periodic westerly bursts along the Equator • Large errors in the equatorial flow propagate away from the Equator affecting TC genesis prediction, and TC track forecast as far as 30N/S

  23. Huge600hPa zonal wind difference affects the entire tropical Pacific in 2010involving all 3 data setsSpeeds are very comparable away from the tropics

  24. The largest 600hPa wind difference at 165W occurs in the tropics, between 20S and 10N ECMWF NCEP MERRA

  25. In addition to differences in the general circulation in the Tropics, state-of-the-art operational systems can completely miss existing Tropical Cyclones. • Analyses are particularly deficient in the depiction of developing, deepening and transitioning tropical cyclones • Analyses are deficient in representing cyclogenesis and existing deepening cyclones in the eastern Atlantic • Analyses are particularly deficient in representing even fully-developed TCs over the Indian Ocean

  26. TS Debby (2006) at 06z 24 Aug 2006Obs center slp 999 hPa; Max wind 22 m/s NCEP analyses do not produce a closed circulation GEOS-5 An. 200km displacement error for center (obs. center X) Wind speed m/s

  27. Complete miss of TC Nargis (2008) in both operational NCEP and MERRA analyses at a time when is declared having hurricane-level winds by the JTPC and IMC COMPLETELYFLAT PRESSUREFIELD 800x600km Contours every 1hPa WINDS DONOT FORMA CLOSEDCIRCULATION 800x600km Contours every 1hPa X observed cyclone’s center WINDS DO NOT REACH 12m/s WINDS DO NOT FORM A CLOSED CIRCULATION

  28. Published study on the impact of AIRS, focused on tropical cyclone Nargis (2008)emphasizes the difficulty of analysing TCs over the Indian Ocean • Work published in 2009 shows some improvements in analysis over the tropics in in the GEOS-5 DAS and forecasting model consequent to assimilation of AIRS-derived information in CLOUDY areas. Case chosen: catastrophic cyclone Nargis which hit Burma causing devastating loss of life • Tropical Cyclones in the Northern Indian Oceans are extremely difficult to analyze: operational global analyses often do not represent these cyclones’ position (or even the TCs’ very existence) accurately. Forecasts are penalized by these poor analyses Reale, O., W. K. Lau, J. Susskind, R. Rosenberg, E. Brin, E. Liu, L.P. Riishojgaard, M. Fuentes, R. Rosenberg, 2009: AIRS impact on the analysis and forecast track of tropical cyclone Nargis in A global data assimilation and forecasting system. Geophys. Res. Lett., 36, L06812, doi: 10.1029/2008GL037122

  29. Some improvement with AIRS v5 retrievals Analysis obtained assimilating AIRS cloudy retrievals (version 5) Well-defined Cyclone Green: Observed Track 108-hour forecast (slp) initialized from improved analyses Green: Observed Track Accurate landfall is produced in the forecasts initialized with AIRS: (Reale et al., 2009, Geophys. Res. Lett.) CNTRL Analysis (above) And forecast (below): No Cyclone

  30. Conclusions • State of the art reanalyses (ERA-40, JRA-25, NCEP-R2 and MERRA) show susbtantial differences in the seasonally-averaged representation of the African Easterly Jet and more generally of the circulation in the African Monsoon and tropical Atlantic regions • Operational analyses or reanalyses differ also at instantaneous times in the tropical region. On the contrary, away from the tropics, different analyses provide almost identical representations of the wind field • However, it appears evident that the global tropical wind field is a very deficient aspect of current tropospheric analysis • Despite changes in models and assimilation systems, and increase in resolution, the representation of wind in the tropicsdoes not show much improvement from 2006 to 2010 • Major discrepancies appear on all 3 basins:Atlantic, Indian and Pacific Oceans on scales spanning from storm-scale to planetary, from weatherto seasonal • To reduce the errors in the analyzed wind field on different scales in the tropics is of paramount importance and should be the goal for the next-generation observing systems

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