# Presentation: 474981

## (Very) preliminary synoptic evaluation of the ECMWF Nature Run at NASA GSFCOreste Reale 

- The African Monsoon Region and the Tropical Atlantic
- Period: May – August ; 1 degree fields

## The African Easterly Jet (AEJ)

- `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

## The AEJ and 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 perhaps stronger than climatology, thus creating higher-than-observed vertical easterly shear, which appears to inhibit the early development of AEWs attempting to become vortices

## AEJ (cont.)

- 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.

## African Easterly Waves (AEWs)

- AEWs in July/Aug show a realistic propagation speed of about 6-9 deg/day
- Realistic change in wave speed and structure at transition (about 15W)
- Realistic amplitudes and scales
- Zonal wind over the continent subject to strong (perhaps excessive?) diurnal cycle

## Tropical Easterly Jet (TEJ)

- The TEJ (200-150hPa) displays a perplexingly strong diurnal cycle over land but
- also over ocean. It also appear to produce a stronger-than-observed vertical easterly
- shear which may inhibit the early stages of development.

## Example of nondev. 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.

## Example of nondev. 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.

## Hurricane-like (HL) vortices 

- Only two relatively strong HL vortices are observed until Aug 31st.
-    (slp mins of about 970 hPa and 975 hPa in the 1deg res. ).

## HL vortices: horizontal structure

- The 1 degree fields do not allow a full evaluation of HL vortices. Pressure center and
- low-level wind are realistic, but no evidence of bands is seen, neither in the low-level
- vorticity or in the specific humidity fields. Full-resolution fields need to be analyzed.

## 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.

## Preliminary concluding remarks

- A preliminary synoptic assessment of the NR over the AM region and the tropical Atlantic shows an overall realistic African Monsoon, AEJ and wave activity
- However, only 2 systems appear to have acquired a full hurricane-like development up to Aug 31st
- More evaluations need to be done and the entire season has to be investigated prior to any conclusive statement on the NR representiveness of the Atlantic tropical cyclone activity
- Full-resolution fields need to be analyzed to assess the realism of the HL vortices structure

## IN PROGRESS and FUTURE WORK

- Investigate the entire African Atlantic Season and establish realism of AEW evolutions, perform statistics of developing versus non-developing systems, intensity/track of developing systems, ET transitions.
- Investigate Indian Ocean and Asian Monsoon with emphasis on the Somali Jet, monsoon onset and breaks
- Investigate easterly waves on a planetary scale assessing scales of motion, development statistics, interaction with baroclinic systems, transitions
- Investigate mid-latititude activity, jet stream structure and maintenance, cyclogenesis and cyclolysis, perform cyclone development and scale diagnostics