TropicalCyclonesAffectingDarwin

TropicalCyclonesAffectingDarwin


DARWIN

TROPICAL CYCLONES AFFECTING DARWIN

Historical tropical cyclone information of a general nature is available for Darwin as early as 1910, but specific tropical cyclone data, containing storm tracks, wind and pressure data, etc., are only available starting in 1961.Table XI-17 contains a descriptive history of all 38 tropical cyclones and hurricanes passing within 180 nmi of Darwin during the 39-year period 1961-1999. Unless otherwise indicated, all of the tropical cyclone statistics utilized in this report for storms passing within 180 nmi of Darwin are based on the data set used to compile Table XI-18. Statistically, the speeds and the headings of the tropical cyclones are variable with a range of speeds at CPA between 1.4 to 21.6 kt and headings ranging from 036° to 179° to 280° T.

The tropical cyclone season for Australia is considered to extend from November through May. Note that tropical cyclones in the Southern Hemisphere have a clockwise wind circulation. Table XI-18 shows that December through March had the greatest tropical cyclone activity, with 29 of 38 storms (78%) occurring during those four months. Of the 38 storms in the data set, only four were of hurricane strength (64 kt) within 180 nmi of Darwin. Table XI-18 also shows the average direction towards which storms were moving when they were at CPA to Darwin. The average movement for all storms is 239° at 7 kt. However, there is a large variation in direction and speed for the individual tropical cyclones.

Figure XI-32 shows the annual distribution of the 38 tropical cyclones that entered the 180 nmi threat radius around Darwin in seven-day increments. The period of maximum activity, from November 5th through April 15th, is clearly shown.

Figure XI-33 depicts the chronology of the 38 tropical cyclones that passed within 180 nmi of Darwin during the 39-year period 1961-1999. As shown in the figure, there is an average occurrence rate of 0.97 storms per year for all tropical cyclones, regardless of intensity. The recurrence interval of all tropical cyclones is 1 year. However, for hurricane strength (64 kt) storms, the average occurrence rate is 0.10 storms per year, with a recurrence rate of 9.8 years. There have been several multiple-year periods since 1961 when no tropical cyclone entered Darwin's 180 nmi threat radius. The four-year period from the1992-1993 season through the 1995-1996 season had no storms passing within 180 nmi of Darwin.

Figure XI-34 shows the octants from which the 38 tropical cyclones were approaching when they were at CPA to Darwin. As shown in the figure, the predominant threat direction is from the east through northeast. However, tropical cyclones have approached Darwin from a myriad of directions. It must also be remembered that the figure represents tropical cyclone movement at CPA, and may not represent the tropical cyclone's initial movement towards Darwin. The threat direction is an important consideration for evasion at sea.

As shown in Figure XI-35, the movement of the 38 tropical cyclones that entered Darwin's 180 nmi radius can be described as variable. The dark circles in the figure indicate the locations of the 38 tropical cyclones when they were 72 hours from CPA to Darwin. Note that the dark circles are not grouped together and the tropical cyclones follow erratic tracks. The existence and location of the monsoon trough influence their formation and movement. Tropical cyclones that form in the monsoon trough can move either east or west. If they form north of the monsoon trough axis they tend to move southeast, and if south of the axis tend to move southwest. The monsoon trough axis can shift closer or further away from the equator. The tracks of the tropical cyclones indeed show movements that vary between easterly, northerly, and westerly. The northwest monsoon (prevailing winds from the northwest) further contributes to hot and humid weather, daily showers, frequent thunderstorms, and variable winds. The southeast monsoon (prevailing winds from the southeast) occurs during the Australian winter and is referred to as the "Dry Season."

Figure XI-36 depicts the track of four tropical cyclones that contained at least hurricane force (64 kt) sustained winds while within Darwin's 180 nmi threat radius. The dashed portion of the tracks indicates maximum winds below hurricane force. Numbers adjacent to the track correspond to the index numbers given on Table XI-17 with number 19 being hurricane Tracy that virtually destroyed Darwin December 24-25, 1974. The dark circles in the figure indicate the location of three of the tropical cyclones when they were 72 hours from CPA to Darwin. Severe hurricane Tracy was not in existence 72 hours before CPA.

Figure XI-37 is the statistical summary of threat probability based on tropical cyclone tracks for the period 1961-1999. The data is presented with solid lines representing "percent threat" for the 180 nmi radius circle surrounding Darwin. The heavy dashed lines represent approximate approach times to Darwin based on the climatological speeds of movement. For example, a tropical cyclone located near 9°S 137°E has approximately a 40% probability of passing within 180 nmi of Darwin. In addition, this tropical cyclone would reach Darwin in 3-4 days if the speed remains close to the climatological speeds of tropical cyclones passing within 180 nmi of Darwin. But, as noted, the direction and speed of advance of tropical cyclones is extremely variable in the Darwin area.

Wind

The Port of Darwin weather instruments are located on the roof of the port office building at Fort Hill Wharf. A complimentary set of gauges is planned for the new East Arm berths in due course.

Port authorities state that a container crane, when it is in its storm tie-down position, adversely affects the accuracy of the wind gauge for winds from the east-southeast through south-southeast. Unfortunately, this is the wind direction during part of the approach time of cyclones that may threaten the port. Other than this factor, winds are representative of those experienced by ships in the port.

Tropical cyclones around Darwin tend to have a smaller radius of strong winds than for those in the Northern Hemisphere. For example, for tropical cyclone Tracy, which hit Darwin directly, the swath of destructive winds 62.5 kt (100 km/hour) was approximately 15.6 miles (25 km) wide.

The land and sea breeze regime is established year-round at the port. It is strong and regular during the Dry Season (May to October) when both land (peaking about 1130) and sea (peaking about 1730) breezes may exceed 20 knots. Because sea breeze winds start picking up around 0700 during the Dry Season, it is recommended that ships enter/depart early in the morning.

During the Wet Season (November to April) the land breeze is more variable and the sea breeze is less powerful. During the changes of season (April/May and November/December) the weather can be variable and squally.

Waves

Maximum wave motion noted in the wharf area was about 6.6 ft (2 m) during the approach of tropical cyclone Gretel (1984). Port Authorities also reported wave motion nearing 13.1 ft (4 m) in the port approaches during tropical cyclone Max (1981). However, these heights are anecdotal and not supported by instrumental records. Under normal conditions, wave action rarely exceeds 4.6 ft (1.4 m) at the commercial berths or anchorages used by U. S. Naval ships.

Storm Surge

Tidal data has been sparse until recent years. Port Authorities noted that the passage of tropical cyclone Tracy, which destroyed the town in December 1974, was well recorded and a clear storm surge of about four meters was noted (Kevin Murphy, 1984). However, the storm center crossed the coast at about half flood of a neap tide and while a bulge was recorded the following high water was not significantly affected. Port Authorities state that considerable damage occurred within the port during the passage of Tracy but nothing could be solely attributed to storm surge. Because the tidal range is 25.9 ft (7.9 m), a combination of high tide and surge could have been significantly higher and even more destructive.

Additionally, no records exist to show what changes to currents in the approaches, channels, and port itself that might be generated by the passage of a tropical cyclone storm surge.

During the peak of the northwest monsoon season (February) a variable surge may become visible in the port. This takes the form of a ground swell about 0.66 ft (20 cm) in height, 164.1 ft (50 m) to 229.7 ft (70 m) in length, and with a 25-30 second period. Ships alongside the wharves are not affected.

Source: http://www.nrlmry.navy.mil/port_studies/thh-nc/australi/darwin/text/sect7.htm