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
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 PFs
Very large
Smal
l
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
Number and Location of MCSs in relation to SALLJ Events
Chaco Jets (LLJs extending south of 25°S) favor MCSs south of 23°S
SALLJEX Number Number of
of MCS MCS south
Days
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
LLJARG
14
18
2
12
2
NOLLJ
17
29
2
4
9
From Salio and Nicolini
MCSs Centroids during SALLEX (Nov 15, 2002 – Feb 15, 2003)
-10
-15
CJE
NCJE
LLJ-ARG
-20
NO-LLJ
-25
-30
-35
-75
-70
-65
-60
-55
-50
Fill symbols represent
nocturnal MCS (0-12]
open symbols represent
diurnal MCS (12-00]
-45
Subtropical MCSs between 23S-40S – 65W-52W
Initiation
UTC Time
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)
Radar structure is that of a classic
leading line-trailing stratiform Sq. line
(Passive microwave- ice scattering)
(ignore- artifacts)
(IR cold cloud tops massive
and colder than -80°C)
x
x
x
x
MCS Case Study
Notice d/dt (theta-e) from tropical Atlantic to Argentina
x
x
x
x
Dec-Jan-Feb MEAN conditions
Notice d/dt (theta-e) from tropical Atlantic to Argentina
x
x
x
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°K
P-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
Thank you!
January
19, 2005
January
26, 2005
of Meteorology
DepartmentDepartment
of Meteorology
seminar series, Seminar
UniversitySeries,
ofUtahUniv ersity of Utah
MCSs Centroids trajectories
-10
-15
-20
-25
-30
-35
-75
-70
Initiation
-65
-60
-55
-50
Maximum extent
-45
Decay
SALLJEX wind data during CJE events
Diurnal cycle
Night
Day
00
03
06
09
12
15
18
21
black
red
blue
green
Maximum extent
UTC Time
Lifetime
Hours
Maximum extent size
*103 km2