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Characteristics of Warm Season Precipitation in the Australian Region: 

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


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 Australia extends form mid-latitudes to tropics Maritime Continent Region significant topography

US Seven Season Climatology: 

US Seven Season Climatology

Laing and Fritsch 1997: 

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


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


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 < -15,-25,-35,-45,-55K Analysis generally employs frequency of TBB data < -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 Correlation > 0.4 a” fit” (span > 100km, duration > 3h, speed >2.5ms-1)


Great Divide Flinders Ranges Atherton Table Land Cape York Great Divide Carnarvon MacDonnell \Musgrave R. Hamersley R. Kimberleys Arnham Escarpment SW Aust 30-40S 20-30S 10-20S


FHC <-55C FHC<-35C FHC <-15C 30-40S


Sensitivity of streak characteristics to IR temperature threshold (1996-2001 all cases-spring-summer) 20-40 IR Sensistivity Low IR T sensitivity tropics Low IR T sensitivity N-S gradient 20-400 Fast (US 13ms-1) 2-4 h greater cw US 4.5h Low IR T sensitivity tropics



Environment-Vertical Cross Section of Zonal Winds-NCEP : 

Environment-Vertical Cross Section of Zonal Winds-NCEP 30-40S 20-30S 10-20S SPRING SUMMER




SPRING MCV CEA A 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 insufficient and topography not nearly as influential as found in the US and China *20-40N employed in China MCV


CED CEG 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% > 30-40S Summer > 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 Occur on larges scale -opposite sides of the continent Thermal forcing from an elevated heat source by itself not enough to generate the convection-Synoptic forcing


10-20 S CEI CEJ CEK CEL CEM 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.


FREQ35 -40 to -30 S FREQ35 -30 to -20 S FREQ35 -10 to -20 S SUMMER


FREQ35 -40 to -30 S FREQ35 -30 to -20 S FREQ35 -20 to -10 S SPRING


Diurnal Cycle in Sub Tropics 20 20 20 20 20 20 40 40 40 40 40 40 110 135 160 110 135 160 110 135 160 110 135 160 Latitude Latitude Latitude Longitude Longitude SUMMER SPRING e) 2000 d)1600 c)0900 b)2000 1600 f) 0900 a)1600


0632 UTC 1832 UTC 12 Nov 13 Nov 14 Nov


A B C D E F G H I A1 A2


A B C 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 Steering ~300mb *Moves across eastern boundary


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 >12 ms-1


Periods of regular forcing coincident with deep northerly flow-shifts with abrupt trough movement Propagating modes into Unfavourable environments


Latitude 0 20 S 110 130 150 110 130 150 Longitude 110 130 150 110 130 150 Longitude Diurnal Cycle in the Tropics d) 1300 a) 1900 b) 0000 c) 0700 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). . Latitude 0 20 S 20 S 0 Latitude FHC


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. 13ms-1

Eastward and Westward Modes 10-20 S: 

Eastward and Westward Modes 10-20 S PHC 55 Eastward Westward Suumary :10 to 20 S


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

Comparison of IR Thresholds : 

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.


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).


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 Duration (h) 4.8 19.2 6 36* 50.4* Span(km) 320 823 366 2194 2358 Speed(ms-1) 17.6 16.5 17.3 14.9 11.4 Trough(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

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