MCTEX: December Summary

1 December 1995 : IOP #9

• Synoptic Situation: MT trough to north ( 8 S)with east to south easterly flow extending across the MCTEX domain. Prog maintain and enhance easterly flow over the next 36 hours. Regional analysis shows low level SE flow over islands with convergence zone to south.
• Mission Priorities: Convective Dynamics/SBM/CI OD S. Rutledge, RC J. Wilson, LAS T. Keenan
• Picker profiler down, aerosonde down until pm. Monash kite not in operation. Franklin to report at 1130 CST. Will be stationed on NW coast of Bathurst at this stage since development expected on northern side of islands.
• 0930 CST DWN profiler shows enhancement of SE flow over last few hours.  Bathurst report low-level northerlies on island.
• 0955 CST No clearly defined PBL signature on BTH radar so update on possible SBM mission will be deferred until 1015 CST.
• 1025 CST Overcast persist over island and AWS stations indicate temperatures of 30C - less than SST.  Will defer Cessna take-off time to 1100 CST in the hope of better definition of situation.
• 1055 CST VAD radar data indicate low level westerly flow.  SBM with 073/69-089/76 off SE coast off Melville for Cessna.  Take off nominally 1100 CST.  No parallel-to-SB legs.
• 1110 CST Cessna due to take off 1120 CST.  Should be on station at 1145 CST.  First indications of fine line echo on south coast Melville.
• 1130 CST Cirrus anvil, from satellite imagery, appears to cover the Tiwis; from radar, all convection appears to be suppressed as a result.  Franklin is near Rocky Point on west coast of Bathurst Is. and was advised to stay in this vicinity until convection situation becomes clearer.
• 1150 CST: Radar provided new way points for the Cessna, namely:   054/42-026/60 on northern coast Melville in vicinity of Radford Pt.  Some evidence of enhancement in this area.
• 1155 CST Stronger southerly SB flow evident at BTH as island starts to heat.  Rolls evident.
• 1205 CST Cessna completed first SBM.  Reports SBF detected with 8-10 kt southerlies with NR above.  Cloud base ~ 1 km.  Diverting to second SBM.
• 1213 CST:  Jorg Hacker / Cessna reports more convective activity east of the way points provided.  BI radar:  OK to divert as they have no information on radar.  Unable to make immediate contact with aircraft for Jorg to use his judgement.
• 1240 CST: Weak SBF on the north coast ...
• 1245: New way points provided by BI radar for the Cessna: 010 / 51 and 010 / 23 At completion of this leg Cessna will have been airborne approx 2 h.
• 1300 Examination of 0930 CST sounding shows very dry air at 900 mb (Dew point depression approximately 15 C).
• 1320 CI pattern started way points 346/39 011/38 022/21 336/21
• 1400 Still very quiet without much activity.
• 1630 After significant delay assocaited with dry air a cell is propagating w-e along the SB convergence line with a good gust front ahead.
• 1715 CST System in decay state but still visually impressive.

SUMMARY: Late development case with unfavourable conditions associated with cirrus and very dry air at low levels probably. Drying a result of mesoscale descent after passage of majoe disturbance and previous Hector?

  2 December 1995 : IOP #10

• 0845: Maxwells Ck operational status -- all "up" except for the Monash kite.
• 0850: Report from aerosonde, now on sea breeze mission: currently between land and ocean grid points -- even now, significant thermal activity at approx. 250 m AGL
• 0900: Discussion with Brian Sanderson on the Franklin --- they are cruising between Caution Point and Cape Fourcroy at the extreme west of Bathurst Is. and will be prepared to monitor any convection that make take place later in the day.
• 0915: From BI radar -- there is now east to northeast flow over the Tiwis. Could be a good idea to check on Franklin's readiness to do some flux measurements in conjunction with the aircraft at about 1100 local time. Contacted Franklin again -- this should be OK for 1100 local --- they would like at least 30 minutes notice.
• 0930: Suggested to Hacker that we would do the flux intercomparison and will update positions and times in 30 minutes or so. He will be ready for take off at 1030 or so.
• 1000: Franklin position just off Caution Pt. Radar requests Cessna delay til 11 AM. No boundaries evident over island at this time. Will go with a flux mission at the start of the Cessna flight.
• 1010: Aircraft way points from BI radar: (1) south of BI at 260 deg / 35; (2) middle of BI at 290 deg / 33; (3) over the Franklin at 307 deg / 67 km.
• 1010: Aerosonde has just crossed the coast on the revised way points heading northwest. Alt: 200 m.
• 1105: Aerosonde has completed two complete circuits; going for a third. Has just passed the southeasternmost point.
• 1120: Talked to Franklin. No sign of Cessna yet. Cessna should be on station right about this time. Small disturbance passing to immediate north of islands. Small Q evident with this feature. Good NE to east flow over islands. TK reports boundary layer rolls in the clear air pattern at this time.
• 1200: New waypoints for Cessna. 335/16 and 219/15 for seabreeze mapping. Altitudes of 100 and 500 m. Second waypoints to study convective rolls are 335/16 and 064/14. Altitudes of 100 nad 700 m. Suggested location for R/V Franklin to be halfway between Rocky Point and Cape Helvitius.
• 1230: Aerosonde to now work SE Melville coast with waypoints of 070/37 and 104/40.
• 1240: Cessna will do a convective initiation study after the horizontal roll mission. Waypoints are 354/21, 310/24, 242/15, 116/03. Altitudes to be 100 m AGL and cloud base. Cessna will try to penetrate the sea breeze front on the southern leg.
• 1255: New Aerosonde waypoints to be 039/55, 062/44. Getting good sea breeze mapping from the Aerosonde today.
• 1330: Cessna starting on upper leg of convective initiation pattern. This will complete their research time for the day. Good convective initiation reported by radar (in box). Reflectivities to 50 dBZ.
• 1350: Cessna in bound Darwin now. This was an excellent convective initiation mission by the Cessna.
• 1400: Communications with R/V Franklin have been fairly poor today. We cannot reach them on radio or telephone. They can call into us but we cannot dial them up.
• 1425: Appears that we have lost communications to the island. The repeater may have been hit by lightning. Radar reports (via the blower) that there has been lots of lightning around the hill that has the repeater on it.
• 1515 or so ... Radio comms back on the air (thanks!!) - it may have been a simple power outage that has been restored. 

3 December 1995

VAD derived flow in the lowest km revealed winds backing slightly to NNW at 3-4 m/s. The 0100 Z Maxwell Creek sounding concurred with flow out of the NNW at 2.3 m/s at 1000 mb. Steering level flow was at 700 mb with an east- southeasterly jet of 13.9 m/s. Development was fairly slow up to 0330 Z with scattered cumulus, especially over the northern half of Bathurst Island and the western third of Melville Island. Tops were under 5 km and the Cu were highly sheared up to 0330 Z. A fine line associated with a sea-breeze was evident along the southern coast of the Islands as expected with the northerly low level flow (opposing flow on southern coast) by 0220 Z. As a result, activity was expected to begin along the southern coast with a preference to the southwestern coast of Melville.

As seen in both C-pol radar and GMS IR data, rapid development occurred between 0400 Z and 0430 Z fairly close to C-pol along the southern coast of Bathurst and southwest Coast of Melville Island. Light rain with intermittent moderate to heavy rain occurred at the radar between 0420 Z and 0530 Z. As gauged by radar, IR satellite, and total flash rate measurements, explosive convective development occurred with a first order merger process between 0430 Z and 0530 Z. Total lightning flash rates increased from 5 per minute to over 50 per minute during this period. Both radar and GMS IR show an east-west line primarily over southwestern Melville Island and southern Bathurst (just north of radar) by 0530 Z. As seen in the C-pol reflectivity data, a re-orientation from an east-west line to a north-south oriented line centered over Bathurst Island occurred between 0550 Z and 0615 Z. During this time flash rates remained high (between 45 and 55 per minute). By 0630Z, the total lightning flash rate began to rapidly decrease as the IR cold cloud shield surface area rapidly increased over Bathurst Island. From 0630 Z to 0730 Z, the infrared cold cloud shield reached its maximum area, primarily over Bathurst Island. Flash rates continued to decrease to about 3 per minute. Radar showed a north-south oriented convective line advecting to the west coast of Bathurst with a small area of trailing stratiform over the central portion of the island. Hector continued to dissipate after 0730Z.

4 December 1995 :IOP #11

• Mission Priorities Convective Lifecycle/ SBM/CI with Cessna OD Larry Carry RC Dave Kingsmill Aerosonde 0700 Launch, Cessna 1100 launch Weaker easterly flow and indication of northerly flow across islands although light and variable.
• 0930 Early VAD reports indiccte westerly flow across island so action anticipated on eastern end of Melville. Aeosonde way points for monitoring SE and SW SB flow on Melville, 123/34 097/33 109/51 Possible second aersonde will be used for mapping SB on north coast.
• 1130 Second aerosonde has engine trouble. Will be delayed. Cessna airborne for 30 minutes and in SBM at eastern end of the island with way points 062/114 061/87 @ 50,300 1000 m.
• 1145 New SBM for cessna are 076/64092/71 on SE coast of Melville.
• 1210 Bathurst Hector going well. Cell evident on extreme eastern end of Melville.
• 1215 New aersonde way points 1-3/27- 063/27 are N/S transects in anticipated area of convective growth.
• 1235 Way points for Cessna CI pattern are as follows 028/32 055/48 066/44 039/20. Almost east west line of shallow convection on both islands.
• 1245 Cessna in CI flight pattern.
• 1320 Good CB's on eastern Melville Is. and over Bathurst with congestus starting to bubble in between.
• 1339 Cessna completes mission.
• 1407 E-W flight pattern for aerosonde to monitor inflow to "bridging" region. 090/22 090/53
• 1415 Bridging in process with 53 dBZ echoes apparent. Extensive Anvil from storms back towards Darwin.
• 1515 Re orientation occurring
• This case is an excellent example of a 2nd order merger between an east coast Melville Island complex and a west coast Melville Island complex with a subsequent re-orientation.
• Early morning VAD analyses and both the 2215 Z and 0100 Z Maxwell creek soundings indicated weak westerly flow near the surface (250-270 deg). Steering level flow at 700 mb was out of the east but very weak ( Aerosonde and Cessna way points are included in the Darwin summary. Several C-pol and cloud radar intercomparisons were accomplished while scanning cloud rolls developing into cumulus congestus to the north of C-pol. Approximate times and locations of intercomparisons are as follows:

  •      Time (UTC)     C-pol relative locations

    1. 0045Z          R=43 km, Az=298 to R=45 km, Az=323

    2. 0100Z          R=39 km, Az=342

    3. 0130Z          R=36 km, Az=338

These convective rolls were rapidly developing into Cb's with the first Cb occuring about 0200Z with 8 km tops just to the northwest of the radar on Bathurst Island. First lightning flashes accompanied this initial convective burst. By 0300Z, activity was located over southern Bathurst Island, northwestern Melville, and the beginnings of congestus over eastern Melville. The total flash rate rapidly increased to over 40 per minute by 0320 Z associated primarily with the cell to the NW of the radar. By 0330 Z, activity was more widely scattered over Bathurst and western Melville with the cells over eastern Melville continuing to grow to the Cb stage. By 0430Z, there were three primary areas of activity: 1. northern Bathurst, 2. western to central Melville, and 3. eastern Melville that were all the result of 1st order mergers. By 0515, the Bathurst island complex began to weaken and the Melville Island cells under went a 2nd order merger to form a large, broad, east-west oriented line over the eastern 2/3 of Melville Island. As seen in C-pol data, the Melville Island complex underwent a re-orientation to N-S from 0515 Z to 0540 Z. During this time, the total lightning flash rate began to decrease. The complex over Melville dissipated by 0630 Z and a new cell formed over the southern coast of Bathurst. This cell was accompanied by a momentary resurgence in the flash rate. Large anvil cloud remained over Melville Island from 0630 Z to 0730 Z. The cell over Bathurst was short lived and dissipated by 0720 Z.

5 December 1995 :Non IOP Day

Satellite imagery at 1000 CST indicated considerable monsoonal like convective activity surrounding the islands with convection and cirrus anvils located over both the continent and oceans. Extensive cirrus was evident over much of the islands and it persisted until 1300 CST. North south oriented cloud bands were evident moving towards the west in the Beagle Gulf and extended across the south coast of Melville Is.

By 1030 CST weak cells were evident on the radar on the southern coast of Melville Is. associated with the westward propagating external disturbance near Cape Gambier in the Beagle Gulf. By 1045 CST 40 dBZ cells were evident in this region. From 1100-1200 CST convection continued to develop along the southern coast of Melville Is. Cells tops were generally in the 7-10 km range. The centre of Melville Is. showed clear air "measle type" echoes at this time. By 1200 CST convection was concentrated along the southern half of Melville with new development along the extreme eastern coast.

At ~1300 CST new shallow cell development was evident along a convergence zone running along the centre of Bathurst Is. eastward through the centre of Melville Island (north of the main precipitation on Melville Is.). A northward propagating gust front from the early convection may have been responsible. By 1400 CST most convection on Melville Is. had ceased although there was evidence for continued propagation of the previous gust front across Maxwells Ck followed by a collision with the north coast SBF. At this time the initial convection located over Bathurst Island had developed into the main convective complex with 12 km tops.

By 1500 CST the Bathurst Is. complex had re-oriented into a north south aligned complex which was propagating to the west off the island. A quasi-stationary SBF zone was evident along the north coast of Melville Island. By 1535 CST a cell had developed on this SBF. A significant gust front developed from this complex and propagated south across the Maxwells Ck area but failed to trigger any convection. By this time Bathurst Is. was covered by an extensive stratiform deck as the major convective area entered its decay phase.

From 0845 CST to 1225 CST PBL surveillance scans were employed. At 1255 a surveillance scan followed by three 120 degree rain map sector scans were employed. At 1305 CST the sequence was changed to consist of a surveillance scan, a 210 degree rain map scan followed by successive 60 degree rain map sectors set to cover the Maxwells Ck area. With non development of rain in the Maxwells Ck area at 1500 CST, a basic polarimetric surveillance scan was employed through to 1700 CST.

Summary; Early initiation occurred on southern coasts forced by an external disturbance. Extensive stratiform cloud persisted to the north and delayed development. A gust front developed from the initial southern Melville Is. system and interacted with the north coast SBF but failed to produce significant convection. In general weaker storms were evident producing gust fronts which subsequently did not produce significant convection. The major storm development was on Bathurst Is. and it propagated off to the west. Rainfall was not concentrated in the Maxwells Creek area.

  6 December 1995
Non IOP day

Rainfall mission day.

All fixed sites were operational and soundings were performed at Maxwell's Creek at 0900 and 1400 with westerly winds above 700 hPa and easterlies below.

The developing monsoon trough to the north was still evident. Overnight a SSW squall line moved north and dissipated 30-40 km south of the radar site. Oceanic convection to the north and west was also present overnight and these combined to produced an extended, quite thick layer of cirrus evident on the satellite imagery. The presence of a mid-latitude trough acted to produce a southward shift of the convection.

Morning VAD from radar showed low level NW'lies with a SE overlying this flow with a return to westerlies above 4 km. The Maxwell's Ck profiler showed the low level winds moving more westerly after 0930 with easterlies above 1 km. Around 1000 LT there was some convection on north side of the island moving to the north. Echo tops around 11-12 km. By 1130 cells were decaying and brightband evident indicating mostly stratiform rain. This had dissipated by 1230.

Cells were developing along south coast by 1240, and isolated weak cells over much of the islands by 1410. Tops were less than 5 km. Still a stratus layer at 8-10 km over the island, but weakening. More cells developing at eastern end of Melville at 1430 and weak cells over the d-scale by 1530.

Major development over NE Melville by 1630 with tops of 15-16 km and gust front propagating out. Note also the cirrus had cleared substantially by this time. Southerly sea breeze onset was clear at Maxwells from 1530 extending up to ~800 m. New development along north coast with 2 main complexes by 1824. The eastern complex may have developed from an interaction between outflows from the earlier development along southern sea breeze and the northern sea breeze. S-band profiler data shows tops over 16 km with possible small hail at upper levels around 1730 and stratiform over Garden Point from 1900 to 2330. The eastern complex just entering the d-scale at 1824. By 1911 45 dBz echoes over the d-scale. By 1934 the eastern complex weakened into a more stratiform pattern while western complex developed N-S orientation. System was mostly stratiform by 2030 and the total complex was decaying by 2108.

Some oceanic convection offshore around 0300.

In summary, weak to moderate very late developing Hector over northern half of the islands with delayed onset due to extensive overcast from primarily oceanic convection associated with monsoon type activity.

7 December 1995
Non IOP day

Objective Rainfall Mapping

Synoptic Situation:

Equipment Status;

Cloud Radar    UP
Lidar     UP
PSU/ARM Radiometers UP
S band Profiler     UP
GP Profiler    UP
CSIRO NFOV Sensor UP
Lightning Interferometer ?
All Sky Camera ?
Piker Profiler/RASS UP
Aerosonde DOWN
C-POL     UP
Maxwells Profiler/RASS UP
Maxwells Soundings  UP
FIAMS Cessna   DOWN

The 1000 CST satellite image indicated significant oceanic convection to the north west, west and south west of the islands. Associated with this monsoonal like convection were extensive anvil regions. To the north and west of the Tiwi Islands, a narrow south west to north east oriented cloud band was evident. The radar data at this time indicated this band was moving to the south east across the islands.

From 1025 CST 1050 CST coordinated scans were undertaken between the MU cloud radar and C-POL investigating convective initiation as new cells were forced by the disturbance moving across the islands. Shallow cumulus echoes were evident in a SSW-NNE oriented band at this time extending across Bathurst Is. and the north coast of Melville Is.

By 1130 CST cells developed on the SBF along the south east coast of Melville Is. SW to NE oriented HCR's were evident across the centre of Melville Is. and on the western coast of Bathurst Is. However the most significant convection was concentrated over northern Bathurst Is. and off the north coast of Melville Is. although cells were less than 10 km deep.

By 1230 CST a SBF was evident on the south coast of Melville Is. and gust fronts were evident propagating south from the thunderstorms over Bathurst Is. and along the north coast of Melville Is. Convection along the SE coast of Melville Is. continued to develop and by 1300 CST had cell tops to 13 km with a northward propagating GF originating from this complex. At this time shallow cumulus was developing in the centre of Melville Is. as the original northern coast. convection decayed. At 1400 CST one storm complex had developed over the centre of Melville Is., significant convection to 15 km height existed on the southern coast of Melville and new 8 km deep convection developed at the eastern coast of Melville Is. This latter convection merged with the rest of the Melville Is. convection by 1420 CST. By 1430 CST a northward propagating GF was evident originating again from the southern Melville Is. coastal complex. This continued to move north but failed to produce significant convection except on its eastern flank.

At 1500 CST the main convection was concentrated in two complexes over western and eastern Melville Is. The western complex had cirrus extending over the Maxwells Ck area. Storm tops of 16-17 km were observed along the south east coast of Melville Is.

At 1600 CST the island convection was in a state of decay although new convection did develop over waters to the south of Apsley St. By 1600 CST an extensive cirrus anvil remained over the islands.

Extensive precipitation was not observed over the Maxwells Ck D-scale raingauge network.

At 0900 CST PBL surveillance scans were employed with polarimetric surveillance scans starting at ~1130 CST. Numerous RHI scans were undertaken from 1025-1050 CST during the period of coordinated scanning with the MU cloud radar. At 1200 CST the rainfall mapping scan sequence was initiated using 210 degree sector scans. At 1235 CST 60 degree sector scans over the Maxwell s Ck area were employed in anticipation of precipitation development in this region. With the failure to achieve significant convection the 210 degree sector rain mapping scan sequence was re-introduced at 1340 CST. This basic scan sequence was then employed for the rest of the day.

Summary: This day was a case with external triggering of convection in a pre-monsoonal environment. Significant convection was triggered along the southern coast of Melville and Bathurst Islands. This was not a day for major re-orientation of the storm complexes. Again the major areas of rainfall were not concentrated over the D-scale raingauge network.

  8 December 1995
Non IOP day

Rainfall monitoring day.

This was last day of CPol operations and much of the remote sensing equipment packing began. Last sounding at Maxwells was 1030 am, profiler, cloud radar and lidar at Garden point ceased operations in the morning. RASS ceased at Maxwells and Picker in the afternoon. The Maxwells profiler showed northerly flow up to 3 km for most of the day.

The only sounding was at 1030 and showed weak easterlies above 700 with a southerly jet between 250 and 150 hPa, indicating the monsoon is pretty much there.

Overnight lines of cells of both north and south coasts around 1am, which may have been associated with land breeze or there was some evidence of larger scale organisation in long range radar scans. This had largely decayed by 0230. Weak echoes off north coast persisted until 0500.

Some significant cells developed at NE end of Melville and also off Cape Forcroy about 0800. and a north-south oriented line of cells extending of the NE corner of Melville indicating an outside disturbance. Extensive oceanic convection to the north and east is evident in the satellite imagery. A multi-cellular complex covered the eastern third of Melville by 10 am and reorientation and merging of the cells produced a N-S aligned system with some evidence of new growth on a gust front by 11 am. These moved slowly westward and the line began to decay as new growth on the outflow increased by 1120. The system decayed rapidly into a stratiform type pattern by 1200.

New development occurred on the SW coast of Melville and SE tip of Bathurst Island around 1230. By 1300 a N-S line had formed over central Bathurst Island and the coastal development began to decay. The early system was stratiform and covered the eastern half of Melville with still some active cells off-shore to the NE. By 1330 there was a major N-S oriented squall line just about to move off the coast of Bathurst Island. The system began to decay quite rapidly as it moved off-shore.

The only rain at this time over the d-scale was associated with some remnants of the stratiform remains of the eastern system around 1530. There was extensive development off the north coast at this time and the area of convection built slowly southward. Convection was widespread and complex. This included the NW coastlines by 1630 and by 1900 there was 40 dBz echoes over much of the d-scale. By 2030 the convection over the island had decayed to stratiform although there were still active elements off shore to the west and NW.

In summary, a disturbed day with a Hector development over Bathurst Island, but for the most part dominated by monsoonal type convection as would be expected from the sounding. There was monsoonal rain over the d-scale in the late afternoon/early evening.