Rain-Producing Systems for the SAMS and their Moisture Sources

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Rain-Producing Systems for the SAMS and their
Moisture Sources

• Ed Zipser
• University of Utah
• and
• Paola Salio and Matilde Nicolini
• University of Buenos Aires

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OUTLINE Precipitation Systems over South
America contrasting properties north and south
of 20S comparison with systems over global
tropics MCSs during SALLJEX using 30-minute IR
data relationship with LLJ diurnal cycle
example of 19 Dec 02 Moisture Source
moisture source vs. theta-e (CAPE) source
shortcomings of NCEP and ERA40 reanal.
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Definition of PF (Precipitation Feature)and MCS
(Mesoscale Convective System)
PF is any contiguous precipitation area with 2A25
near-surface rain (can be any size,
and most are small, weak, and shallow) Original
PF with MCS definition (Nesbitt et al., 2000)
A PF with 2A25 Near Surface Rain area gt 2000km2
and Area of 85 GHz Tb below
250K gt 2000km2 and at least
one pixel with 85GHz Tb lt 225K During 5 years
(1/1998-11/2000 12/2001-12/2003), Total number
of PFs 17,253,615 Total number of
PFs with MCS 113,246 (0.66) Total number of
PFs with ice 1,289,366 (7.47)
(i.e, Min 85 GHz pct lt250K)
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TRMM Radar Algorithm
Annual Rain in mm/month
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Fractional rain () contributed by large MPFs vs.
small PFs
Very large
Small
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Fractional rain () contributed by PFs with
specified properties
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Fractional rain () contributed by MPFs with 40
dBZ height lt/gt 6 km
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Fractional rain () contributed by MPFs with
convective fraction lt/gt 50
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Conclusions from TRMM Statistics

• SESA (south of 23S) dominated by MCSs
• and(Amazonia and SACZ are not)
• Rainfall comes mostly from large systems in
  SESA
• More of the rainfall is from small systems
  north of SESA
• Systems with intense convection contribute
  highest of rainfall in SESA.and(less in
  Amazonia and SACZ)
• Fraction of convective rain highest on Andes
  slopes of Argentina
• Fraction of stratiform rain highest in western
  Amazonia

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Number and Location of MCSs in relation to SALLJ
Events Chaco Jets (LLJs extending south of
25S) favor MCSs south of 23S
SALLJEX Days Number of MCS Number of MCS south of 23S Days with convection not meeting the MCS criteria Days with no convection south of 20S
CJE 45 61 22 15 3
NCJE 16 31 8 2 2
LLJ-ARG 14 18 2 12 2
NO-LLJ 17 29 2 4 9
From Salio and Nicolini
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MCSs Centroids during SALLEX (Nov 15, 2002 Feb
15, 2003)
CJE
NCJE
LLJ-ARG
NO-LLJ
Fill symbols represent nocturnal MCS (0-12 open
symbols represent diurnal MCS (12-00
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Subtropical MCSs between 23S-40S 65W-52W
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Relationship between SALLJ and MCSs

• When SALLJ extends south of 25S (definition of
  the Chaco Jet, CJE), MCSs are favored in SESA
• Large MCSs reach peak extent during the night
  (but initiate in late afternoon-evening)
• SALLJ wind speed peaks during the night
• Issue to be resolved Are the diurnal cycles of
  MCSs and LLJs closely linked?
• MCSs are favored by high CAPE and low-level
  shear
• MCSs reach peak extent at night wherever they
  occur
• Is the main role of the LLJ to provide the
  high-CAPE environment, or is it also necessary
  that a strong LLJ be present at the same
  time/location?

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19Dec 2002 MCS near B.A. used by C. Saulo in
modeling study
(IR cold cloud tops massive and colder than -80C)
(Passive microwave- ice scattering)
(Passive microwave- ice scattering)
Radar structure is that of a classic leading
line-trailing stratiform Sq. line
(ignore- artifacts)
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x
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Notice d/dt (theta-e) from tropical Atlantic to
Argentina
MCS Case Study
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Notice d/dt (theta-e) from tropical Atlantic to
Argentina
Dec-Jan-Feb MEAN conditions
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MCS Case Study (SALLJEX 22 Jan 03)
Notice d/dt (theta-e) from tropical Atlantic to
Argentina
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NCEP Wind 330 6 m/s Theta-e 348K P-3
Wind 360 10 m/s Theta-e 358K Reanalysis
will benefit from high resolution data
assimilation including aircraft data!
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Summary

• There are major, systematic, regional differences
  in the structure, intensity, and diurnal cycle of
  rainfall systems
• The LPB has a particularly extreme domination by
  large and intense MCSs
• Satellite databases can provide important
  statistical constraints on convective intensity
  as well as rainfall
• Modelers should accept this reality as a
  challenge
• The role of the SALLJ in MCSs is important-- is
  it mostly as a provider of high-CAPE air, or also
  direct mass flux?
• Reanalyses should assimilate high-resolution
  databases and also pay close attention to
  thermodynamic variables

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MCSs Centroids trajectories
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SALLJEX wind data during CJE events
Diurnal cycle
00 12 black 03 15 red 06 18 blue 09 21 green
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