# Presentation: 369414

## 
Period: September-November; 1 degree fields

Collaborators: Joe Terry, Juan Carlos Jusem

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

- Preliminary synoptic-dynamic evaluation of the ECMWF Nature Run: the African Monsoon region and the tropical AtlanticPart IIOreste Reale

## 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

## 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

## 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

## 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.

## 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.

## AEWs (continue)

- 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.

- 700hPa u,v wind

## 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

## Tropical Easterly Jet (TEJ)

## 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

## Longitude bands in which recurving of disturbances out of AEWs occur: September

- 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

- Latitude
- Upward,
- Time
- horizontal

- 700hPa u,v wind

## Longitude bands in which recurving of disturbances occur: October

- 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%).

- Latitude
- Upward,
- Time
- horizontal

- 700hPa u,v wind

## However, early recurvers do occur in packs

- Example of a number of early recurving systems in an active season, 2004.

## 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

## 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’

## `Major’ hurricane over the Gulf: 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. Structure even more
- impressive than the system observed in August. Low-level wind speed exceeds 55 m/s

- Wind speed (m/s)
- Temp (oC)

## 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

## Multiple simultaneous tropical cyclones in the Atlantic

- 500 hPa geop (m) and 900 hPa rel vort (s-1)

## 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)

## 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

## 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