Research report - cyclone Debbie PDF

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Research report Tropical cyclone debbie. Question 2....

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Question 2: Severe Tropical Cyclone Debbie Abstract This report investigates the synoptic situation around severe tropical cyclone Debbie, which was officially named and formed as a category 1 on the 22 nd March 2017. At time of formation the sea surface temperature was ~30°C, and humidity ~82% which was favourable for cyclonic formation. The vertical wind shear at the time was 1-7 m/s -1, which is very weak and optimal for cyclone development. Overall cyclone Debbie followed Piekle’s (1990) criteria involved in formation and development of tropical cyclones.

Introduction A tropical cyclone is a dangerous, destructive weather event that forms from a low-pressure system that has a cloud cluster where a Mesoscale Convective System (MSC) is present (Gray, 1998). They have a warm core in a lowpressure system in the middle and upper troposphere. After being formed, their intensity is consistent until the heat and moisture source is reduced, and / or dry and cool air is introduced into the system (Sturman and Tapper, 2006). The official definition of a tropical cyclone from the Bureau of Meteorology is: A non-frontal low pressure system of synoptic scale developing over warm waters having organised convection and a maximum mean wind speed of 34 knots or greater extending more than half-way around near the centre and persisting for at least six hours (Bureau of Meteorology, 2021). The Australian region has a high potential for cyclogenesis as the continent is surrounded by tropical waters. The monsoon shear line hosts the formation of almost all tropical cyclones in Australia, 95% of them formed within the 9-19° S latitude zone. They tend to move westward due to the easterly flow and its own circulation. Once tropical cyclones move inland, they generally decay due to the loss of moisture but bring heavy rains with them (Sturman and Tapper, 2006). The Australian region sees about 13% of the world’s total - ~11 cyclones each season (Bureau of Meteorology, 2021). The Bureau of Meteorology use Pielke’s (1990) six major criteria for classifying the formation and development of a tropical cyclone: 1. The presence of a pre-existing synoptic-scale region of low-level convergence and low surface temperature. 2. A warm, moist tropical atmosphere conductive to overturning with air saturation as air rises in the cumulonimbus clusters, condensation occurs, releasing latent heat and adding to thermal instability. 3. Surface ocean temperature must be >26-27°C so that cumulonimbus development can be sustained with sufficient heat and moisture.

Murdoch University 4. A weak vertical shear of the horizontal wind between the lower and upper troposphere within a radius of 4° of latitude from the centre of convection. This keeps the latent heating of the atmosphere above the area of lowest surface pressure and maximum convergence. 5. Distance must be 4-5° of latitude away from the equator, ensuring the Coriolis deflection is sufficient to spiral air inwards at low levels and outwards from upper level high pressure systems. 6. Development of a large-scale anticyclone in the upper troposphere above surface low. This allows a sufficient rate of removal of air mass allowing the surface pressure of the cyclone to fall. Severe tropical cyclone Debbie was first spotted on the 22 nd of March 2017, as a tropical low in Papua New Guinea. It was named Debbie on the 25 th when it was upgraded to a Category 1. Debbie then drifted southsouth west during favourable conditions, and hit Hamilton Island as a Category 4 on the morning of the 28th. At 12:40pm AEST Debbie hit Airline Figure 1: Image of the track Debbie took 24 th March to 1st April 2017, in AEST (Bureau of Meteorology, 2021). beach on the Queensland coast as a Category 3, moving inland before it was downgraded to an intense lowpressure system, moving south-east towards Brisbane and back out towards the ocean (2).

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Methods This analysis will be based on Pielke’s (1990) tropical cyclone formation and development criteria. The data graphs and plots were retrieved from various sites using their public access software available (Tab.1). Table 1: Piekle’s (1990) environmental variables – data used in analysis. Data

Retrieved

Sea Surface Temperature Monthly Mean

Image provided by the NOAA-ESRL Physical Sciences Laboratory, Boulder Colorado retrieved from https://psl.noaa.gov/cgibin/db_search/SearchMenus.pl Image provided by the NOAA-ESRL Physical Sciences Laboratory, Boulder Colorado retrieved from https://psl.noaa.gov/cgibin/db_search/SearchMenus.pl Image provided by the NOAA-ESRL Physical Sciences Laboratory, Boulder Colorado retrieved from https://psl.noaa.gov/cgibin/db_search/SearchMenus.pl Image provided by the University of Wisconsin – CIMSS retrieved from http://tropic.ssec.wisc.edu/archive/

Geopotential Height

Relative Humidity

Vertical Wind Shear

Results Sea Surface Temperature Sea surface temperature on the coast of Queensland was overall above the threshold for cyclone formation. SST at site of formation was ~30°C, and ~29°C when Debbie hit the coastline as a category 4.

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Figure 2: Sea surface temperature in the South Pacific Ocean for March 2017. Data retrieved from NOAA-ESRL Physical Sciences Laboratory.

Geopotential Height A low high pressure system is clearly present from 850 hPa to 200 hPa as the strong cyclonic system can still be seen in the upper troposphere over the Queensland coast.

a)

b)

c)

d)

Figure 3: Surface and upper level pressure charts for 4pm AEST, in the South Pacific Ocean for 28th March 2017. (a) 850 hPa contour height analysis; (b) 700 hPa contour height analysis; (c) 500 hPa contour height analysis; and (d) 200 hPa contour height

Murdoch University analysis. Data retrieved from NOAA-ESRL Physical Sciences Laboratory.

Relative Humidity Relative humidity recorded at 72.5-80% while Debbie was a category 2 and 3, moving into the coast where the humidity increased to 87.5%.

Figure 4: Relative humidity (%) daily average in the South Pacific Ocean for March 2017. Data retrieved from NOAA-ESRL Physical Sciences Laboratory.

Vertical Wind Shear The vertical wind shear around Debbie when it hit the coastline was very low, about 1-7 m/s-1, providing optimal conditions for the continuation of the cyclone. Further inland and south-east the wind shear increased to ~35-50 m/s-1, which would support why Debbie reduced in categories to a low-level system as it moved towards Brisbane.

Murdoch University Figure 5: Vertical wind shear (m/s-1) in the South Pacific Ocean for March 2017. Data retrieved from University of Wisconsin – CIMSS.

Discussion According to Pielke’s (1990) criteria, sea surface temperature (SST) of more than 26°C is optimal for cyclone formation. Tropical cyclone Debbie was formed in SST of ~30°C near Papua New Guinea, and its pathway to the Queensland coast was over waters that were at ~29-30°C (Fig.2). These conditions during Debbie’s formation and development follow Pielke’s (1990) criteria where the higher heat and moisture sustains cumulonimbus development. After Debbie downgraded to a tropical low inland and made its way back to the ocean, the SST was ~26°C (Fig.2), not an ideal temperature for tropical cyclone development and may be a factor in Debbie not being reformed. Surface and upper level charts present different densities in the atmosphere along the contour lines; the pressure level height is determined by that density reading of the air below it (NOAA). Anticyclonic systems are usually high pressure systems, whereas low pressure systems are cyclonic. Figure 3 presents the evidence of pre-existing low level convergence and pressure, in accordance to Pielke’s (1990) tropical cyclone development criteria. Cyclone Debbie is visible on all upper level charts of 850 – 200 hPa (Fig3 a, b, c and d).

Relative humidity is defined as the vapour content in the air as a percentage of the amount that can saturate the air at the same temperature. Saturated air is warmer than non-saturated, and is therefore more likely to be buoyant producing atmospheric instability (Sturman and Tapper, 2006). Debbie’s genesis was in a moist atmosphere of ~72.5-80% (Fig.4), where cumulonimbus development is sustained, condensations and releasing latent heat all factor in to thermal instability in the atmosphere (Pielke, 1990).

Wind shear is defined as a variation of wind speed / direction with height by Sturman and Tapper (2006). Low level (weak) vertical wind shear is typically present below an upper-level inversion, providing optimal grounds for cyclonic activity as the pressure changes very little within the area of a cyclone. A strong vertical wind shear inhibits cyclonic activity by interrupting the localized heat and moisture in the air, pushing it away, reducing the fuel for the cyclone

Murdoch University (Sturman and Tapper, 2006). Wind shear of about 2-4 m/s -1 is optimal conditions for rapid intensification of a tropical cyclone (Paterson et al, 2005). The wind shear reading when Debbie hit the coastline was roughly 1-7 m/s -1, optimal in the succeeded development of Debbie from a category 2 to a category 4 when it hit the coast. When Debbie drifted south east, inland and back towards the coast, the vertical wind shear increased significantly to ~3550 m/s-1, interrupting that parcel of heated and moisture rich air, pushing it away from the localised area of the cyclone, leaving Debbie with no fuel to sustain any category.

Conclusion Cyclone Debbie followed Pielke’s (1990) criteria of large-scale environmental variable involved in formation and development of tropical cyclones. The SST was more than optimal for cyclone formation, at 30°C (Fig.2), providing the thermal energy needed for cyclogenesis. The upper level pressure charts showed Debbie’s formation high up into the troposphere (Fig.3 a, b, c and d) and relative humidity quite high ~72-80% (Fig.4), keeping the air warm and moist within the cyclonic area. Vertical wind shear was quite low, 1-7 m/s -1, optimal for cyclone formation as there is no wind within the vertical atmosphere to disturb the humid parcels of air fuelling the cyclone. Conclusively, Debbie follows Pielke’s (1990) criteria satisfactory in formation, development and eventual reduction to a low-level pressure system. Word Count: 1365 References:

1. http://www.bom.gov.au/cyclone/history/debbie17.shtml 2. Bureau of Meteorology. (2021). Tropical cyclone Debbie technical report. Retrieved: http://www.bom.gov.au/cyclone/history/database/Tropical-Cyclone-Debbie-Technical-Report-Final.pdf

3. Gray, W. M. (1998). The formation of Tropical Cyclones. Meteorology and Atmospheric Physics, 67, 37-67. https://link.springer.com/content/pdf/10.1007/BF01277501.pdf 4.

Pielke, R. A. (1990). The Hurricane. International Journal of Climatology, 11 (4), 462-463.

5.

Sturman, A., & Tapper, N. (2006). The weather and climate of Australia and New Zealand (2nded). Oxford University Press. ISBN 9780195584660

6.

Paterson, L. A., Hanstrum, B. N., Davidson, N. E., & Weber, H. C. (2005). Influence of environmental vertical wind shear on the intensity of hurricane-strength tropical cyclones in the Australian region. Monthly Weather Review, 133(12), 3644-3660.

7.

Bureau of Meteorlogy. (2021). Tropical cyclone climatology. Retrieved: http://www.bom.gov.au/cyclone/tropical-cyclone-knowledge-centre/history/climatology/

8.

National Oceanic and Atmospheric Administration (NOAA). Common features of constant pressure charts. Retrieved: https://www.weather.gov/jetstream/common...