Evaluation of the European Centre for Medium-Range Weather Forecasts

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Evaluation of the European Centre for
Medium-Range Weather Forecasts (ECMWF) Nature
Run midlatitude cyclone activity tropical
Atlantic and African Monsoon

• Reale, O, J. Terry, M. Masutani, L.P.
  Riishojgaard, E. Andersson, and J.C. Jusem

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Assessing the NR realism midlatitude systems
versus tropical weather

• Statistical properties of the atmospheric flow,
  particularly midlatitude cyclone activity, are
  investigated
• In the tropics however, it is crucial the assess
  the presence of fundamental features of tropical
  weather systems
• Until few years ago, to search for evidence of
  tropical weather in a global model was not
  possible and only large-scale, diluted
  large-scale features could be detected

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Midlatitude Cyclone Diagnostics

• Diagnostics produced
• Tracks
• Lifespan
• Distribution by central pressure
• Deepening rate
• Mean direction and speed (total, zonal and
  meridional speed)
• Cyclone, genesis and lysis density

• Goals
• Identify any trends or biases
• Identify any conspicuous anomalies and
  determine source
• Assess realism

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Cyclone Tracking

• Utilize Goddards cyclone tracking software
  (Terry and Atlas, AMS conf, Aug 1996)
• Identifies and tracks mostly extratropical
  cyclones (cutoff at 20 deg N/S latitude)
• Interfaces with GrADS contouring algorithm
• Uses SLP field at 4hPa contour interval
• Finds centroid of inner-most closed isobar
• Tracks the centers using extrapolation and
  500hPa steering

• Cyclone tracks generated
• Nature run at one degree for Jun 2005 to May
  2006 (each month and season)
• NCEP operational analysis at one degree for
  2000 to 2006 (each month, 68 of 84 months were
  available)

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DJF
Cyclogenesis density from NCEP reanalysis over
Southern Hemisphere. (Simmonds and Keay, Journal
of Climate, March 2000)
JJA
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Searching for realistic tropical weather systems
in the Tropical Atlantic regiona
phenomenological approach

• Statistical properties of midlatitude cyclone
  activity have been proved very satisfactory
• Fundamental dynamical features associated to the
  climate-weather interface generally present in
  the tropical atmosphere, are searched in the
  Nature Run. Focus is the AMMA region and the
  tropical Atlantic
• African Easterly Jet, African Waves, Tropical
  Cyclones track (and their complexity), Tropical
  Cyclone structure are investigated
• 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,
  doi10.1029/2007GL31640.

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The African Easterly Jet (AEJ)

• 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 Harmattan 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 seasonal
evolution

• July and August means show a northward
  displacement of the AEJ, gradual 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

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The African Easterly Jet (AEJ) in September

• The AEJ appears at the perfect climatological
  elevation (650hPa) but is slightly more to the
  north with respect to climatology and analyses
• 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

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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

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Comparison with NCEP op. Analyses for Sep 2005
Nature Run
NCEP operational analyses
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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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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.
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African Easterly Waves (AEWs)
AEWs show a realistic propagation speed of about
5-9 deg/day, comparable to analyses. Moreover,
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.
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Tropical Easterly Jet (TEJ) at 150hPa

• The TEJ controls the easterly vertical shear that
  may inhibit early stages of development.
• It is a planetary scale feature connected with
  the Asian Monsoon
• Fluctuations on interannual scale are known (Chen
  and van Loon, 1987) but intraseasonal variations,
  despite their importance, are still little known
• Eastward retreat of strong easterly speed (40
  m/s or higher) throughout the monsoon season,
  produces decreased vertical easterly shear over
  the Atlantic and an increasingly favorable
  environment for development
• 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

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Tropical Easterly Jet (TEJ)
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Example of nondev. AEW due to Easterly Shear
850hpa Vort (sh)
650 hPa flow
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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Nature Run Atlantic tropical cyclone season

• First Nature Run to simulate one entire season
• Twelve tropical cyclones develop
• Realistic variability of tracks
• Most intense reaches 957 hPa
• Binary vortices, looping and singularities are
  observed (good from OSSE perspective)

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Tropical Cyclones in the Nature Run
FiveEarly recurvers appear in the season. Three
central-Atlantic TCs with convincing
Extra-tropical transitions, and 3 systems of the
Gulf. Overall, very realistic track variability.
Early recurvers are more than climatology but not
unseen.
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Early recurvers do occur in packs
Four early recurving systems in an active season,
2004 (NHC).
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Tropical cyclone vortices vertical structure
Vertical structure of a TC 2 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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Major TC over the Gulf vertical structure
Wind speed (m/s) Temp (oC) Vort (s-1)
Vertical structure of TC11 shows another example
of eye-like feature and a very prominent warm
core. Structure even more impressive than the
TC2. Low-level wind speed exceeds 55 m/s
vorticity maxIn the lower levels.
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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
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Multiple simultaneous tropical cyclones in the
Atlantic
500 hPa geop (m) and 900 hPa rel vort (s-1)
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Remarkable days in which almost all possible
processes related to Tropical Cyclones happen
together21-24 September
Although weaker systems than at other times, the
period 21-24Sep is extremely interesting. Multipl
e tropical cyclogenesis, cyclolysis, subtropical
cyclogenesis extratropical transition,
looping, binary vortex interaction, center jump,
rejuvenation, extratropical reintensification. U
p 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)
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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
• Several weak tropical and sub-tropical systems
  are present, together with major tropical
  cyclones
• In spite of a tendency of creating several early
  recurvers, it can be stated that the NR, given
  the resolution limitation, does have a very good
  representativeness of tropical cyclone track
  variability in the Atlantic, as it would occur in
  an active season
• This Nature Run represents a very promising tool
  to perform OSSEs over the tropical Atlantic

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CURRENT and FUTURE WORK at NASA GSFC (GLA, GMAO,
JCSDA and SIVO) on the NR validation

• Comprehensive statistics on midlatitude cyclone
  activity have been performed
• Analysis of tropical weather over Indian Ocean
  and Asian Monsoon regions with emphasis on the
  Somali Jet, monsoon onset and breaks, tropical
  depressions, together with an assessment of the
  Eastern Pacific tropical cyclone, Southern
  Hemisphere and WPacific seasons is being
  completed
• Results from this comprehensive assessment to be
  submitted as a journal article during 2008