Rain-Producing Systems for the SAMS

and their Moisture Sources

Ed Zipser

University of Utahand

Paola Salio and Matilde Nicolini

University of Buenos Aires

OUTLINE

Precipitation Systems over South America• contrasting properties north and south of ~20°S• 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.

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 > 2000km2

and Area of 85 GHz Tb below 250K > 2000km2

and at least one pixel with 85GHz Tb < 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 <250K):

TRMM Radar Algorithm Annual Rain in mm/month

Fractional rain (%) contributed by large MPFs vs. small PFsVery large

Small

Fractional rain (%) contributed by PFs with specified properties

Fractional rain (%) contributed by MPFs with 40 dBZ height </> 6 km

Fractional rain (%) contributed by MPFs with convective fraction </> 50%

Conclusions from TRMM Statistics

• SESA (south of ~23°S) 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

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

Number and Location of MCSs in relation to SALLJ Events

Chaco Jets (LLJs extending south of 25°S) favor MCSs south of 23°S

From Salio and Nicolini

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NCJE

LLJ-ARG

NO-LLJ

CJE

Fill symbols represent nocturnal MCS (0-12]

open symbols represent diurnal MCS (12-00]

MCSs Centroids during SALLEX MCSs Centroids during SALLEX (Nov 15, 2002 – Feb 15, 2003) (Nov 15, 2002 – Feb 15, 2003)

Initiation

UTC Time

Subtropical MCSs between 23S-40S – 65W-52WSubtropical MCSs between 23S-40S – 65W-52W

Decay

UTC Time

Relationship between SALLJ and MCSs

• When SALLJ extends south of 25°S (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?

19Dec 2002 MCS near B.A. used by C. Saulo in modeling study

(Passive microwave- ice scattering) (Passive microwave- ice scattering)(IR cold cloud tops massiveand colder than -80°C)

Radar structure is that of a classicleading line-trailing stratiform Sq. line

(ignore- artifacts)

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x

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x

Notice d/dt (theta-e) from tropical Atlantic to ArgentinaMCS Case Study

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x

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x

Notice d/dt (theta-e) from tropical Atlantic to ArgentinaDec-Jan-Feb MEAN conditions

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x

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x

MCS Case Study (SALLJEX 22 Jan 03) Notice d/dt (theta-e) from tropical Atlantic to Argentina

x

x

NCEP: Wind 330° 6 m/s Theta-e: 348°KP-3: Wind 360° 10 m/s Theta-e: 358°K

Reanalysis will benefit from high resolution data assimilation including aircraft data!

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

January 19, 2005 Department of Meteorology Seminar Series, University of Utah

Thank you!Thank you!

January 26, 2005 Department of Meteorology seminar series, University of Utah

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Initiation Maximum extent Decay

MCSs Centroids trajectoriesMCSs Centroids trajectories

Night Day00 12 black03 15 red06 18 blue09 21 green

SALLJEX wind data during CJE events

Diurnal cycle

Maximum extent

UTC Time

Hours

Lifetime

Maximum extent size

*103 km2