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Synoptic-Dynamic Evaluation of the ECMWF Nature Run: African Monsoon & Tropical Atlantic

Insights and assessments of the ECMWF Nature Run focusing on mid-latitude cyclones, African Monsoon, and tropical Atlantic activity. Detailed analysis of cyclone characteristics, jet streams, and weather system behaviors.

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Synoptic-Dynamic Evaluation of the ECMWF Nature Run: African Monsoon & Tropical Atlantic

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  1. Synoptic-dynamic evaluation of the ECMWF Nature RunSummary of some previous assessmentOreste Realewith contributions from Michiko Masutani, Lars Peter Riishojgaard, Erik Andersson,Joe Terry, Juan Carlos Jusem

  2. General Assessment of T511 ECMWF Nature Run • It is important to analyze a nature run not only from the statistical point of view, but also with a `phenomenological’ approach targeting relevant weather events • Mid latitude cyclones • Tropics: focus on the African Monsoon and tropical Atlantic activity • Some results published on: • Reale, O., J. Terry, M. Masutani, E. Andersson, L. P. Riishojgaard, J. C. Jusem, 2007: Preliminary evaluation of the European Centre for Medium-Range Weather Forecasts (ECMWF) Nature Run over the Tropical Atlantic and African Monsoon region. Geophysical Research Letters, 34, L22810, doi:10.1029/2007GL31640.

  3. ET Cyclones center pressure distribution Joe Terry

  4. Extra tropical cyclones deepening rates

  5. Extra tropical cyclones lifespans

  6. Extra tropical cyclones propagation

  7. ET cyclone track variabilitya qualitative assessment Comparable degree of latitudinal asymmetry between western and eastern hemispheres. Note the presence of lee-cyclones downstream the Andes. Comparable areas void of baroclinic activity

  8. Searching for realistic tropical weather systems in the Tropical Atlantic region • In this section we are trying to understand if fundamental dynamical features associated to the climate-weather interface generally present in the tropical atmosphere, are reproduced in the Nature Run. Focus is the African Monsoon region and the tropical Atlantic • Until few years ago, to search for evidence of tropical weatherin a global model was quite pointless and only large-scale, diluted large-scale features could be detected

  9. The African Easterly Jet (AEJ) before and at the monsoon onset • `Traditional’ ECMWF AEJ depition: the AEJ appears at higher elevation and is slightly more to the north with respect to climatology and analyses, as already pointed out for the operational model (e.g. Tompkins et al., 2005). • Intensity (11m/s) compares well with observed climatology (e.g Burpee 1972 and operational analyses for the period) • Realistic clear separation between AEJ and low-level Harmatthan flow • Realistically well-defined low level westerly flow • Moderate horizontal cyclonic shear on the southern flank of the AEJ, increasing towards the end of the analyzed period

  10. The AEJ and the peak of the African Monsoon • July and August means show a northward displacement of the AEJ, an intensifcation of the Harmatthan flow and of the low-level monsoonal flow. • Realistically well-defined low level westerly monsoonal flow in agreement with obs (e.g. Asnani 2005) • Realistically intense horizontal cyclonic shear on the southern flank of the AEJ in July and Aug • Tropical Easterly Jet at 200-150 hPa is stronger than climatology, thus creating higher-than-observed vertical easterly shear, which appears to inhibit the early development of AEWs attempting to become vortices

  11. The African Easterly Jet (AEJ) in September • The AEJ appears at the `perfect’ climatological elevation (650hPa) but is still slightly more to the north with respect to climatology and analyses, as pointed out for the operational model (e.g. Tompkins et al., 2005). • Intensity (11m/s) compares well with observed climatology (e.g Burpee 1972 and operational analyses for the period) • Realistic clear separation between AEJ and low-level Harmatthan flow at 20E • Weaker than climatology low-level westerly monsoonal flow • Reduced vertical easterly shear due to reduction of the TEJ at 150 hPa with respect to Jul-Aug • Strong horizontal cyclonic shear on the southern flank of the AEJ, leads to condition much more favorable to tropical development

  12. The AEJ in October • October shows a realistic receding of the African monsoon • A realistic weakening of the AEJ, but also of the easterly vertical shear is evident • Tropical Easterly Jet at 200-150 hPa follows climatology, reducing vertical shear and allowing several AEWs to become vortices in spite of reduced horizontal shear in the southern flank of the AEJ

  13. AEJ Horizontal Structure at the monsoon onset The AEJ has a realistic maximum of 11 m/s at 600 hPa but wind speed are too low at 750 and 700 hPa consistently with the altitude bias. Meridional shear of zonal wind is realistic and supportive of barotropic instability.

  14. AEJ horizontal structure in September and October The AEJ has a perfectly realistic maximum of 11 m/s at 600 hPa in September and gradually weakens in October following climatology.

  15. African Easterly Waves (AEWs) AEWs show a realistic propagation speed of about 5-9 deg/day Moreover, unlike July and August, there is a period of about six weeks in which the majority of waves present signs of development. This is similar to what happens in very active seasons. The disappearance from the Hovm relates to changes in latitude.

  16. AEWs (continue) 700hPa u,v wind The appearance of pulses of westerly flow in the Hovm is an evident sign of closed circulations and increasing latitudes (only the southern part of the circulation remains in thelatitude band). Strong development of some systems in late September and October can be seen.

  17. The Tropical Easterly Jet (TEJ) • The TEJ at about 200-100 hPa is a planetary scale flow, related to the Indian Monsoon • It is the main responsible of the vertical easterly shear which is one of the factors controlling suppression or development at the early stages of development • Fluctuations on interannual scale are known (Chen and van Loon, 1987) but intraseasonal variations, despite their importance, are still little known • Only certain fact is the eastward retreat of strong easterly speed throughout the monsoon season, produces decreased easterly shear over the Atlantic and an increasingly favorable environment for development • The presence of strong vertical easterly shear could be perhaps related to early recurving systems, since only the systems that recurve immediately can survive the vertical shear • The NR shows a stronger than climatology TEJ, but a very credible and realistic decrease of easterly speed with the progress of the monsoon season. As a consequence, the environment becomes conducive to more development from August onwards

  18. Tropical Easterly Jet (TEJ)

  19. Diurnal Cycle of the TEJ? In the NR, in addition to the realistic changes on a monthly scale, the TEJ displays oscillations (intensity and latitude of the core) on a 3-6day scale (possibly realistic) and a perplexingly strong diurnal cycle. Diurnal cycle (DC) at single points is much stronger than when it is latitudinally averaged. DC is stronger at some distance from the Jet core and is very evident on one flank of the TEJ whenever the jet core experiences a trend. No observational evidence so far to either confute or confirm these fluctuations

  20. Example of realistic nondeveloping AEW due to Easterly Shear In the early stages, 850 hPa vort. increases and vort max becomes aligned with 650hPa circulation center. Eventually upper-level easterly shear suppresses development.

  21. Example of realistic nondeveloping AEW due to Easterly Shear (cont.) The potentially favorable situation induced by a vertically aligned structure between 800 and 500 hPa at 12-14N is counteracted by easterly vertical shear of the order of 20 m/s.

  22. Longitude bands in which recurving of disturbances out of AEWs occur: September 700hPa u,v wind Latitude Upward, Time horizontal In both months (Sep and Oct) the model manifests a tendency to create several early recurvers (around or east of 45W) or system that remain in the Eastern Atlantic

  23. Longitude bands in which recurving of disturbances occur: October 700hPa u,v wind Latitude Upward, Time horizontal However, the most intense systems occur in the western Atlantic. The number of early recurvers is higher than climatology (about 50% against observed less than 20%).

  24. However, early recurvers do occur in packs Example of a number of early recurving systems in an active season, 2004.

  25. At least 9 `major’ tropical cyclone-like systems,more weaker systems. Only TCLs having a center pressure of less than 1000 hPa in the 1x1 fields and 900 hPa vorticity greater than 3x10-4 s-1 are considered

  26. HL vortices: vertical structure Vertical structure of a HL vortex shows, even at the degraded resolution of 1 deg, a distinct eye-like feature and a very prominent warm core.

  27. HL vortices: horizontal structure Pressure center and low-level wind are realistic, but little evidence of bands is seen, either in the low-level vorticity or in the specific humidity fields.

  28. More HL vortices: horizontal structure Pressure center and 850hPa wind are very realistic (970 hPa and 50 m/s) in the major 20 October 2005 Gulf of Mexico `Hurricane’

  29. `Major’ hurricane over the Gulf: vertical structure Wind speed (m/s) Temp (oC) Vertical structure of a HL vortex shows, even at the degraded resolution of 1 deg, a distinct eye-like feature and a very prominent warm core. Structure even more impressive than the system observed in August. Low-level wind speed exceeds 55 m/s

  30. EC T511: Realistic Variability of Atl. TC tracks Looping and Binary vortex interaction 4 systems: Looping, Binary vortex Interaction, Extratropical Transitions and Extra-tropical Re-intensification Singuarities, binary vortex Interactions, Intensity fluctuations Due to large-scale forcing fluctuations A long simulation must produce complex tracks

  31. EC T511: Multiple simultaneous tropical cyclones can be present in the Atlantic in very active seasonsAnother important –realistic- capability of the ECMWF NR 500 hPa geop (m) and 900 hPa rel vort (s-1)

  32. Remarkable days in which several processes related to Tropical Cyclones happen together:21-24 September Although weaker systems than at other times, the period 21-24Sep is extremely interesting. Multiple tropical cyclogenesis, cyclolysis,subtropical cyclogenesis extratropical transition, looping, binary vortex interaction, center jump, rejuvenation, extratropical reintensification. Up to four tropical or at least partially tropical systems simultaneously present. Tropical-Extratropical interaction is very evident Given the resolution limitations, this is an extraordinary realistic representation of a very active period 500 hPa geop (m) and 900 hPa rel vort (s-1)

  33. Concluding remarks on the Nature Run with respect to midlatitude cyclones, African Monson region and tropical Atlantic activity • Baroclinic activity in winter hemispheres appears convincing in terms of cyclone numbers, tracks, lifespans, deepening rates, intensity • A synoptic assessment of the NR over the AM region and the tropical Atlantic shows an overall very realistic African Monsoon, AEJ and wave activity • At least 9 `strong’ systems develop during September and October • Several weaker tropical and sub-tropical systems are also present • In spite of a tendency of creating several early recurvers, it can be stated that the NR, given the resolution limitation, does have a good representativeness of tropical cyclone track variability in the Atlantic, as it would occur in an active season • This Nature Run represents a powerful tool to perform OSSEs targeting tropical cyclones over the tropical Atlantic

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