Characteristics of Warm Season Precipitation in the Australian Region

1
Characteristics of Warm Season Precipitation in
the Australian Region

• T.D. Keenan1 and R. Carbone2
• 1Bureau of Meteorology Research Centre , GPO Box
  1289K, Melbourne, Australia 3001
• 2National Center for Atmospheric Research,
  Boulder, CO, 80307-3000

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• Rationale
• US radar-based climatology warm season
  precipitation episodes
• of Carbone et al. (2002) showed coherent
  rainfall events of order 1000 km zonal
• span and 1-day duration with high frequency.
• Events were complex events made up of a coherent
  succession of
• convective systems.
• The phase speed often exceeded the phase speed
  of upper
• tropospheric anomalies and zonal low-mid
  tropospheric steering winds.
• Exhibited a phase locking with thermal and
  topographic forcing.
• Studies showed that models with current
  parameterisation schemes
• do not reproduce the natural space time
  distribution of precipitation
• WWRP Global Analysis of Characteristics of Warm
  season Precipitation
• East Asia
• Africa
• Europe
• South America
• Australia/Indonesian Region
• Australia relatively flat with main ranges on
  leeward coast

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US Seven Season Climatology
4
Laing and Fritsch 1997
Convection regularly self-organizes to very large
scales, observed over all non-polar continents.

5
140E
150E
120E
130E
110E
Arnham Escarpment
10S
Kimberley Plateau
Cape York Atherton Table Land
20S
Hamersley R
MacDonnell Ranges
Great Divide Carnarvon
Flinders Ranges
Musgrave Ranges
30S
Great Divide
SW Australia
40S
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Data and Method

• Climatology derived using GMS IR Data 1996-2001
  spring-summer
• (November-March inclusive)
• Domains 110-160E
• 30-40S(Midlatitude-subtropical)
• 20-30S(subtropical-tropical)
• 10-20S(tropical)
• Hourly 4 km GMS TBB data put on to a 0.2 by 0. 2
  degree lat-long grid
• Files created with frequency of TBB lt
  -15,-25,-35,-45,-55K
• Analysis generally employs frequency of TBB data
  lt -35K
• in each 0.2x0.2 lat-long grid
• Analysis method follows that of Carbone et al.,
  2002
• 2D Rectangular cosine weighting function
• Stepped through all grid points(space-time)at
  1degree increments

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Great Divide
Flinders Ranges
SW Aust
30-40S
MacDonnell \Musgrave R.
Great Divide Carnarvon
Hamersley R.
20-30S
Atherton Table Land Cape York
Arnham Escarpment
Kimberleys
10-20S
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FHC lt-15C
FHClt-35C
FHC lt-55C
30-40S
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Sensitivity of streak characteristics to IR
temperature threshold (1996-2001 all
cases-spring-summer)
Low IR T sensitivity N-S gradient 20-400 Fast
(US 13ms-1)
Low IR T sensitivity tropics
2-4 h greater cw US 4.5h
20-40 IR Sensistivity Low IR T sensitivity tropics
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FHC
FHC 35 OVERVIEW SUMMER ( Jan-Feb)
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Environment-Vertical Cross Section of Zonal
Winds-NCEP
SPRING
30-40S 20-30S
10-20S
SUMMER
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30-40 S
20-30 S
10-20 S
CED
CEC
CEA
CEI
CEK
CEL
MCV
F
E
D
A
H
J
SPRING
CEE
E1
B
CEM
CEJ
CEH
G
CEG
CEF
SUMMER
I
C
CEB
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CEA

• Minimal interannual variation, eastward moving
• cloud streaks on average once every 2.3 days
• Streaks embedded as multiple events within long
  lived
• envelopes or forcing zones
• Envelope clusters typically have phase speeds in
  the range 4-6 ms-1 (comparable with the US
  values)
• Diurnal modulation and enhancement of the streak
  occurs
• in regions of elevated topography at time of max
  solar heating
• (A , C -SW Australia D,E, F,G Flinders R . H,I
  Great Divide
• J atypical-fixed geographically
• Medium span, duration, speed 550km, 8.5h and
  19ms-1
• -greater than US and China
• e.g.China sat averages 375.5km, 7.9h and 13.2
  ms-1
• Aust averages are 805km, 11.6h and 21.4 ms-1
• Spring events have larger span, duration
• Medium summer (spring) span, duration and speeds
• 600(520) km, 8.75(8.0) and 19.4(19.6) ms-1
• Spring more favourable
• Not necessarily generated exclusively in regions
  of elevated
• terrain suggesting thermal forcing alone is
  often

MCV
MCV
A
SPRING
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• Considerable inter and intra-seasonal variability
• Spring CEE and CED slowly propagating envelopes (
  3.5 and 5.8 ms-1 )
• with embedded streaks to those observed in at
  30-40S
• Spring, slower, shorter than 30-40S
• span, duration is 490 km, 7.9 h and 18.1 ms-1,
• cw 30-40S 550km, 8.5h and 19ms-1
• Spring streaks 3 events per day, 30 gt 30-40S
• Summer gt frequency of diurnally forced quasi
  stationary events
• (CEE and CEG)
• Envelope of enhanced connective activity can
  slowly migrate
• or switch rapidly from one region to another
  (CEE)
• Summer streak generation is typically 3 per day
  (same as spring)
• Summer span, duration, speed 425km, 6.8h and 17.5
  ms-1.
• i.e. 10-15 decrease on the spring values
• Environmental conditions change considerably from
  the
• 30-40 S to 20-30 S band and especially from
  spring to summer
• Slow westward propagating envelope indicated
  (CEH)-2 ms-1
• Increased importance of diurnal forcing
  especially during the summer
• Regimes of stationary suppressed /enhanced
  convection

CED
CEG
16
CEI
CEK
10-20 S
CEL

• Increase in the importance of thermally induced
  forcing
• with a significant increase in total cloudiness
• CEI geographically fixed and diurnally forced
  convection repeated for
• at least 15 days.
• Slow eastward and westward propagating envelopes
  of convection.
• Westward-CEK and CEJ -4(-5) ms-1, eastward CEL
  7ms-1
• Active to break periods evident in the cloudiness
  20-30 days,
• implying a MJO impact.
• Westward(eastward)streaks 2.6(2.8) per day in
  spring
• to 4.2 (3.2) per day in summer-increase
• Double 20-30S numbers
• Span larger for westward moving streaks
• 400km (westward)220 km (eastward)
• Faster propagation for westward
• Medium speed westward moving streaks is are 16
  ms-1
• Medium speed 13.8 ms-1 for the eastward moving
  streaks.

CEM
CEJ
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SUMMER
FREQ35 -40 to -30 S
FREQ35 -30 to -20 S
FREQ35 -10 to -20 S
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SPRING
FREQ35 -30 to -20 S
FREQ35 -20 to -10 S
FREQ35 -40 to -30 S
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Diurnal Cycle in Sub Tropics
SUMMER
SPRING
110 135 160
110 135 160
20
20
Latitude
d)1600
a)1600
40
40
1600
20
20
Latitude
e) 2000
b)2000
40
40
20
20
Latitude
f) 0900
c)0900
40
40
110 135 160
110 135 160
Longitude
Longitude
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1832 UTC
0632 UTC
12 Nov
13 Nov
14 Nov
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E
F
G
H
A2
I
A
C
A1
B
D
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A
C
B
A3
Event BOX A3 Event A B C Duration(h) 26.4 24
4.8 . Span(km) 1005 1590 548
Speed(ms-1) 9.9 18.9 29.4 Trough(300mb) Nmax
Zero (300mb) (ms-1) 9.0 7.7 Envelope 9.1 St
eering 300mb Moves across eastern boundary
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14 Jan 2000 Duration 43.2 h, span 3975 km speed
23.0 ms-1. From behind to ahead of trough,
Multiday diurnal, streak, steering 350mb, Prop
gt12 ms-1
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Periods of regular forcing coincident with deep
northerly flow-shifts with abrupt trough movement
Propagating modes into Unfavourable environments
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Diurnal Cycle in the Tropics
110 130
150
110 130
150
FHC
0
Latitude
a) 1900
b) 0000
20 S
0
0
Latitude
Latitude
d) 1300
c) 0700
20 S
20 S
110 130
150
110 130
150
Longitude
Longitude
Diurnal Cycle of FHC at -55 C Tbb from equator to
20 S. a) mature convection over land resulting
from diurnal heating b) dissipation and early
transition toward lowlands and coastal oceanic
convection c) mature oceanic convection
associated with offshore flows d) excitation of
convection over Cape York peninsula amidst
oceanic convection of nocturnal origin.
Southward propagation of oceanic convection from
New Guinea is coincident with the excitation of
early-day convection over Cape York. Westward
propagation from Cape York (climatologically)
assumes the shape of an organized bow cloud.
Note the especially high amplitude of diurnal
variation associated with convection over the
Indonesian region (5S, 110-115E). .
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13ms-1
30 20 Latitude 10
0 30
20 Latitude 10 0
Meridional Hovmoller diagrams of FHC in the
tropics for the five season (N-M, 1996-2001)
period of record. Left, Western Austral-Asia.
Right, Eastern Austral-Asia. Coherent patterns,
suggestive of propagation and phase-locked
convection are apparent and these are identified
by dashed or dotted lines.
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Eastward and Westward Modes 10-20 S
Suumary 10 to 20 S
PHC 55
Eastward
Westward
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Summary

• Long episodes occur at all latitudes, but with
  increasing frequency poleward
• Diurnal modulation evident at all latitudes, but
  decreasing poleward
• Synoptic scale influence markedly increases
  poleward
• Triggering by elevated heat sources is prominent,
  but less influential when compared with other
  continents.
• Lifecycles of episodes and propagation relative
  to mean flow is similar to observations over N.A.
  and East Asia.
• Steering winds (and shear) seem critical to
  occurrence of long propagating events both in
  easterlies and westerlies.
• Northerly meridional wind markedly increases the
  liklihood of long events.
• Coherent sequences of convective systems are
  observed less regularly when compared to N.A. and
  East Asia.
• Diminished presence of phase-locked behaviour
  in the diurnal cycle, compared to N.A. and East
  Asia, is attributed to lower and smaller elevated
  heat sources, where the main cordillera is
  located on the leeward side of the continent
• Complex sequence of events in Maritime
  Continent-New Guinea impact on N. Australia

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Comparison of IR Thresholds

• Jan (summer) tropopause temperatures
• 30-40S 20-30S 10-20S
• -68C -78 -83
• (Albany 35S ) (Giles 25S) (Darwin12S)
• Heights(km)
• -55 12 13 13
• -35 9 10 10
• -15 6 7 8
• With opaque clouds an IR blackbody temperature of
  -55 reflects deep tropospheric convection or
  partially opaque deep cirrus near the tropopause,
  
• -35 mid troposheric convection and partially
  opaque anvil cloud and
• -15 relatively shallow convection
• Warm precipitation?
• Various threshold have been used in previous
  studies. -35C closest to Arkin 238K.

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• Above statistics imply faster phase speeds (4-6
  ms-1) and longer durations (3-4 h) in the
  Australian data.
• Statistics on span and duration are influenced
  by characteristics of satellite data ie capturing
  blow off cirrus and anvil
• Satellite tracking may not represent systems
  with rainfall reaching the ground.
• Requires exmaination of the correspondance
  between radar and satellite observed
  characteristics.
• Tuttle has undertaken an exmination of 4 months
  of US satellite and radar data.
• This preliminary initial examination shows that
• Radar observed speeds are 4 ms-1 less than the
  satellite speeds
• Satellite duration (medium) 2 h less than radar
  (radar 7.7 h sat 5.7 h),
• Satellite span slightly greater than radar (med
  values, radar 371, sat 398).

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Event BOX A1 Event A B C D Duration(h) 9.6 19.2
31.2 24 Span(km) 823 1280 2559 1371 Speed(ms-
1) 38 29 23 16 Trough(ms-1) Nmax Zero (300mb
trough used) 12 12-13 Envelope(ms-1) 15 300mb u
25-30 ms-1 Event BOX A2 Event E F G H I Duratio
n (h) 4.8 19.2 6 36 50.4 Span(km) 320 823 366 2
194 2358 Speed(ms-1) 17.6 16.5 17.3 14.9 11.4 Tr
ough(300mb) Nmax Zero (300mb) (ms-1) 3 5.9 (3-6)
Envelope 7.5 (ms-1) Steering near
300mb Extends out of eastern boundary Of
Oceanic Origin