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 Jet in MERRA and 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 MERRA provide different 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,

5. Comparison of the AEJ structure in MERRA, ERA-40, NCEP-R2, and JRA-25
Comparison of the AEJ structure 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,

6. From Wu et al. (2010) Fig 2 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,
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 than discrepancies 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-5 both 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: /2011JD

21. 7-day moisture transport across the entire forecast initialized at 00z22July Reale et al. (2012)
Contour:
Vertically integrated
7-day mean total
Moisture flux
Shaded:
7-day RET minus
RAD meridional
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. Huge 600hPa zonal wind difference affects the entire tropical Pacific in 2010 involving all 3 data sets Speeds 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 2006 Obs 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. COMPLETELY FLAT PRESSURE FIELD
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
COMPLETELY FLAT PRESSURE FIELD
800x600km
Contours
every 1hPa
WINDS DO NOT FORM A CLOSED CIRCULATION
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: /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
CNTRL Analysis (above)
And forecast (below): No Cyclone
Accurate landfall is produced in the forecasts initialized with AIRS: (Reale et al., 2009, Geophys. Res. Lett.)

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 tropics does 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 weather to 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