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

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(Very) preliminary synoptic evaluation of the
ECMWF Nature Run at NASA GSFCOreste Reale

• The African Monsoon Region and the Tropical
  Atlantic
• Period May August 1 degree fields

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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

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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

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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.
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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
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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.
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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.
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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.
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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. ).
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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.
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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.
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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

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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