Period: September-November; 1 degree fields Collaborators: Joe Terry, Juan Carlos Jusem

1. Preliminary synoptic-dynamic evaluation of the ECMWF Nature Run: the African Monsoon region and the tropical AtlanticPart IIOreste Reale Period: September-November; 1 degree fields Collaborators: Joe Terry, Juan Carlos Jusem

2. Searching for realistic tropical weather systems in the Tropical Atlantic region • Statistical properties of midlatitude cyclone activity have been proved very satisfactory (Joe Terry) • Precipitation in the tropics is still far from being realistic (Jusem) which is somewhat to be expected from a global model, but does represent an improvement with respect to previous Nature Runs • In this presentation 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 AMMA region and the tropical Atlantic • Until few years ago, to search for evidence of tropical weather in a global model was quite pointless and only large-scale, diluted large-scale features could be detected

3. The African Easterly Jet (AEJ) in September • Improved AEJ depition with respect to July and August: 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

4. 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

5. AEJ (cont.) The AEJ has a perfectly realistic maximum of 11 m/s at 600 hPa in September and gradually weakens in October following climatology.

6. African Easterly Waves (AEWs) AEWs in Sep/Oct show again 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.

7. 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.

8. 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

9. Tropical Easterly Jet (TEJ)

10. 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 already discussed strong diurnal cycle. Diurnal cycle (DC) at single points is much stronger than when it is latitudinally averaged (cfr. Previous discussion). 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

11. 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

12. 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%).

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

14. At least 9 `major’ tropical cyclone-like 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

15. More HL vortices: horizontal structure As observed in July and August, the 1 degree fields do not allow a full evaluation of HL vortices structure, since little evidence of bands is seen. However, pressure center and 850hPa wind are very realistic (970 hPa and 50 m/s) in the major 20 October 2005 Gulf of Mexico `Hurricane’

16. `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

17. Realistic Variability of TCL system tracks in the Atlantic 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

18. Multiple simultaneous tropical cyclones in the Atlantic 500 hPa geop (m) and 900 hPa rel vort (s-1)

19. Remarkable days in which almost all possible 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)

20. Preliminary concluding remarks on the African Monson region and tropical Atlantic • 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 • In addition to the 2 `major’ systems with a full hurricane-like development up to Aug 31st , at least other 9 `strong’ systems (for 1x1 res.) 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 seems to represent a very promising tool to perform OSSEs over the tropical Atlantic

21. CURRENT and FUTURE WORK at NASA GSFC (GLA and SIVO) on the NR validation • Comprehensive statistics on midlatitude cyclone activity have been performed (J. Terry) • Comprehensive statistics on precipitation (J.C. Jusem) are in progress and will be presented in the next meeting together with an assessment of the South American monsoon • Analysis of tropical weather over Indian Ocean and Asian Monsoon regions with emphasis on the Somali Jet, monsoon onset and breaks, tropical depressions, is being completed • An assessment of the Eastern Pacific tropical cyclone season is also being completed • Southern Hemisphere and Western Pacific tropical weather systems will be investigated in the near future