Transcript 091027SaltLakeCity_PMMSciTeamMtgV4_Romatschke.ppt

Extreme Convection in South America as Seen by TRMM
Ulrike
1,
2
Romatschke
1University
and Robert A. Houze,
of Washington,
2University
1
Jr.
of Vienna
Introduction
Conclusions
Tropical Rainfall Measuring Mission (TRMM) Precipitation
Radar (PR) data are used to indicate mechanisms responsible
for extreme summer convection over South America. Three
types of extreme radar echo structures are defined and the
synoptic and diurnal forcings associated with their occurrence
in the complex topography of South America are investigated.
Deep convective cores are associated with
•Trough position
•SALLJ in the vicinity of the Andes
•Solar heating and lifting over foothills and coastal mountains
Wide convective cores are associated with:
•Trough position
•SALLJ in the vicinity of the Andes
•Lifting over lower slopes and foothills (daytime)
•Downslope flow (nighttime)
•Squall lines triggered by sea breeze
Broad stratiform regions
•Follow wide convective cores in time
•Coincide with climatological precipitation maxima
Definition and distribution of extreme
convection and regions of interest
From the summer months (DJF) of 10 years of TRMM data
(products 2A23 and 2A25) three types of extreme radar echo
structures are defined:
•Deep convective cores: contiguous convective echo ≥ 40 dBZ
extending ≥ 10 km in height.
•Wide convective cores: contiguous convective echo ≥ 40 dBZ
extending ≥ 1000 km2 in area.
•Broad stratiform regions: contiguous stratiform echo
extending ≥ 50,000 km2 in area.
0.0072
0.0072
Synoptic forcing
Composite plots show the enhancement of convection in
relation to the position of a mid-latitude trough in the 500 mb
geopotential height and surface pressure anomalies in
different sub-regions. Shown are composites for days when
wide convective cores were observed in the FHS, PLB, FHN
(representative for FHN, AMZ, NEC, and BHL), and ATL regions
which are representative for all echo structure types observed
in the respective sub-regions. Wind composites are shown for
~02 LT (06 UTC), the time when the South American Low-Level
Jet (SALLJ), a near surface northerly flow which transports
moisture along the eastern Andes foothills, is strongest.
500 mb geopot. height anomalies
0°
48
10°S
20°S
30°S
40°S
80°W
70°W
60°W
50°W
40°W
5°S
0.006
25°S
Deep
convective
cores
0.0036
0.0024 25°S
11°S
35°S
40°S
35°S
0
80°W
70°W
5°S
15°S
25°S
12
60°W
50°W
40°W
[Probability]
0.036
13
Broad
stratiform
regions
0
0.03
80°W
70°W
60°W
14
15°S
126
127
128
129
0.018 130
6
28
50°W
40°W
3
[Probability]
0
80°W
70°W
60°W
50°W
40°W
[mm
50°W
40°W
0°
80°W
70°W
60°W
50°W
40°W
0°
20 Jan
00:00
20 Jan
02:00
-3.6
70°W
60°W
50°W
40°W
d-1]
0°
4.8
10°S
2.4
-28
0
-1.2
20°S
30°S
-3.6
70°W
60°W
50°W
40°W
-4.8
[mb]
30°W
0°
4.8
24 10°S
2.4
12
1.2
0
20°S
-12
0
-1.2
-24
-2.4
-36 30°S
-3.6
-48
[m]
-4.8
[mb]
30°W
40°S
80°W
70°W
60°W
50°W
40°W
0°
60°W
50°W
40°W
30°W
0°
10°S
20°S
30°S
10 ms-1
40°S
80°W
70°W
60°W
50°W
40°W
30°W
0°
10°S
2.4
0
-12
0
-1.2
30°S
-36
10°S
30°S
FHN
40°S 80°W
70°W
60°W
50°W
40°W
-48
[m]
40°S
80°W
30°W
-3.6
70°W
60°W
50°W
40°W
10 ms-1
80°W
70°W
60°W
50°W
40°W
30°W
ATL
[km]
20°S
0
20°S
-0.9
-1.8
30°S
30°S
-2.7
0°
40°S
10°S
20°S
30°S
-4.8
[mb]
40°S
30°W
80°W
10 ms-1
70°W
60°W
50°W
40°W
30°W
The “Atlantic Trough Regime” (ATL): Convergence and
enhanced convection in the ATL region.
BHL
1.8
10°S
0.9
-2.4
30°S
3.6
2.7
1.2
20°S
0°
20°S
3.6
10°S
Daytime convergence/nighttime divergence over the Andes
influence the diurnal cycle of convection:
•Systems with deep and wide convective cores form in the
afternoon in the FHS, as low-level easterlies are lifted over the
foothills, and are advected over the PLB where they evolve to
contain nocturnal broad stratiform regions, sustained by
downslope flow and the SALLJ.
•Daytime divergence in the FHN leads to suppression of wide
convective cores and broad stratiform regions resulting in
their nighttime/morning maximum.
•As a late response to the convergence over the Andes the
convergence over the ATL is weakened in the evening leading
to minima in wide convective cores and broad stratiform
regions.
Surface winds and divergence
0°
40°S
30°W
4.8
-24
AMZ
FHS
70°W
-2.4
3.6
24
20°S
NEC
PLB
10 ms-1
1.2
12
Time 
30°S
3.6
36
10°S
20 Jan
04:00
20°S
-4.8
[mb]
40°S
30°W
80°W
36
48
0
19 Jan
22:00
-36 30°S
48
4
Seven sub-regions with maxima
in one or more echo structure
types are further investigated: the
northern and southern foothills of
the Central Andes (FHN and FHS),
the Amazon and Plata Basins
(AMZ and PLB), the northeastern
coast (NEC), the southern
Brazilian Highlands (BHL), and
the western South Atlantic (ATL).
-2.4
Diurnal Forcing
10°S
The “East Coast Trough Regime” (FHN): Weak SALLJ and
enhanced convection in the FHN, AMZ, NEC, and BHL regions.
21
0
-24
0°
The “Plata Basin Trough Regime” (PLB): Strong SALLJ and
enhanced convection in the PLB region.
40°S
35°S
60°W
60°W
9
25°S
0.012 Radar
Millimeter Cloud
8
70°W
30°S
Precip.
20
80°W
20°S
132
0
-1.2
-48
[m]
40°S
30°W
80°W
12
131
0
20°S
-12
-48
[m]
40°S
30°W
80°W
10°S
15
0.024
12
70°W
[Probability]
21
18
0.006
80°W
40°W
5°S
16
35°S
50°W
1.2
-36
0.0024
0.0012
12
-24
30°S
0.0012
2.4
0
-12
0.0036
dBZ
24 10°S
12
20°S
Wide
convective
cores
3.6
24
0.0048
15°S
4.8
36
10°S
15°S
0°
36
48
0.006
0.0048
Surface winds
The “West Coast Trough Regime” (FHS): Strong SALLJ and
enhanced convection in the FHS region.
0°
5°S
Surface pressure anomalies
Romatschke and Houze, 2009: Extreme summer convection in
South America. J. Climate, submitted.
~14 LT
80°W
70°W
60°W
50°W
40°W
30°W
40°S
~02 LT
80°W
70°W
60°W
50°W
40°W
[s-1]
10 ms-1
Other diurnal mechanisms:
•Afternoon squall lines containing wide convective cores are
triggered in the NEC region and propagate over the AMZ,
where they merge with systems triggered farther southwest
and evolve into a nocturnal maximum of systems with broad
stratiform regions.
•Deep and wide convective cores form in the afternoon in the
BHL as low-level easterlies are drawn landward over the
coastal mountains.