Weather Related Factors of the Adelaide floods ; 7 th to 8 th November 2005

Transcription

1 Weather Related Factors of the Adelaide floods ; th to th November 2005 Extended Abstract Andrew Watson Regional Director Bureau of Meteorology, South Australian Region 1. Antecedent Weather 1.1 Rainfall of previous month Rainfall over most of South Australia during October 2005 was above average. Much of the Mount Lofty Ranges region received in excess of 0 millimetres (mm) for the month, which was well above average rainfall for October. For many locations rainfall was in the highest % of October recordings. This significant antecedent rainfall meant that catchments within the Mt Lofty Ranges at the beginning of November 2005 were much wetter than normal for that time of year. In fact, much of the catchment area was still close to saturation, meaning that any further rainfall would lead to direct run-off into creeks and rivers. 1.2 Weather situation leading up to the flood event The evolution of the Mean Sea Level Pressure (MSLP) pattern from the 5 th to the th of November can be seen in figure 1. On the morning of the 5 th, a large high pressure system extended from just east of Tasmania into the Tasman Sea. A trough of low pressure extended north to south through eastern parts of Western Australia, preceding a cold front which was traversing the southwest corner of the continent. Figure 1a : MSLP Analysis, 30 CDST, 5 //05 Figure 1b : MSLP Analysis, 30 CDST, 6 //05

2 Figure 1c : MSLP Analysis, 30 CDST, //05 Figure 1d : MSLP Analysis, 30 CDST, //05 By the morning of 6 th November, the cold front had interacted with the trough, forming a slowmoving cell of low pressure to the west of Kangaroo Island. An associated active cold front extended north to south through central parts of South Australia. On the morning of th November, the low pressure cell had moved only slowly eastward, and was centred just south of Kangaroo Island. The associated the cold front had traversed the state, lying over western parts of New South Wales and Victoria. During the following 24 hours, the low followed an unusual movement. It retrogressed temporarily to the northwest, then moved to the northeast for a brief period, whereafter it moved southeastward and steadily weakened in intensity, to be positioned south of Mount Gambier by the morning of the th November. Satellite imagery (figure 2) depicts the evolution of the cloud features associated with the MSLP pattern, from the morning of 6 th to the evening of th November. In figure 3a, north-south orientated bands of showers and thunderstorms can be seen through central and eastern South Australia, associated with the cold front. Figure 2a : Satellite Image, 0900 CDST, 6 //05 Figure 2b : Satellite Image, 0900 CDST, //05 2

3 Figure 2c : Satellite Image 00 CDST, //05 Figure 2d : Satellite Image 2300 CDST, //05 Figure 2b shows that by the morning of th November, the frontal showers and storms had progressed into New South Wales and Victoria. A northwest to southeast oriented band of cloud south of Kangaroo Island was associated with a frontal occlusion 1 to the south of the low centre, and an associated band of rain. As illustrated in figure 2c, by 00 CDST on th, the frontal activity had progressed further into eastern New South Wales, with the occlusion rainband wrapping around the southern sector of the low and extending northward toward central South Australia. The centre of the low was just to the east of Kangaroo Island, as depicted by the configuration of the cloud features. By 2300 CDST, the occlusion rainband, whilst having weakened somewhat, had wrapped around the northern sector of the low and progressed into eastern districts of South Australia. The low centre, depicted by the comma shaped cloud pattern in figure 2d, had moved into the upper southeast district. From that point, the low subsequently moved southeastward and weakened. 2. Weather Situation of th and th November 2.1 Evolution of the fine-scale synoptic situation The fine scale synoptic pattern over southern South Australia (figure 3a) at 0900 CDST on th November depicts the detail of the low pressure feature which was centred near the southern coast of Kangaroo Island at that time. An eastward moving wind change line marking a northwest to west surface wind discontinuity lay through central Eyre Peninsula. A southward moving northwest to northeast wind change lay through the southeast district. Over the Adelaide region winds were generally between and 20 knots in strength, and temperatures in the mid teens. The lower levels of the atmosphere were exceedingly moist, with dewpoints generally within one or two degrees of the air temperature, indicating that the airmass was close to saturation. However, little significant rainfall was occurring over the Mount Lofty Ranges at the time. By 00 CDST (figure 3b), the low centre had undertaken an unusual movement, tracking slowly to the northwest, to be near the southwest cape of Kangaroo Island. It had increased slightly in intensity, with a central pressure near 00 hectopascals (hpa). Winds over the Adelaide region had 1 An occlusion often develops around the flank of a maturing low pressure system, as the cold front wraps around the low, to become an occluded front. It is a zone of steady airmass lift, and is often associated with a band of rain and strong winds. 3

4 increased slightly in speed, but maintained a northwesterly direction. The relative humidity of the lower atmosphere had reduced marginally, due to daytime heating of the airmass. However, the moisture content remained very high, with dewpoints in excess of 0 C. A few light showers had fallen in Adelaide in the period from 0900 CDST, amounting to 3 to 4 mm L Figure 3a : MSLP analysis, 0900 CDST, //05. Arrows indicate wind direction at automatic weather stations, with wind speed in knots. Air temperature ( 0 C) is indicated in blue text to the top left of the location, dewpoint is bottom left in green, and MSLP (hpa) is at top right. Isobars are indicated by fine broken lines. Significant wind changes depicted by a bold broken lines. 4

5 L Figure 3b : MSLP Analysis, 00 CDST, //05 By 20 CDST the fine scale synoptic pattern (figure 3c) had evolved such that periods of moderate rain had been falling over the greater Adelaide area for around 5 hours. The onset of the rainfall had occurred as the low pressure centre altered its previous westward direction of movement, and tracked steadily to the northeast to a position near Cape Jervis. The approach of the low centre, and the associated wind change, meant that surface winds over Adelaide shifted towards the west, increasing in both strength and moisture content. Lifting of the airmass had increased, due to a combination of the topography of the Mount Lofty Ranges, and airmass lift within the occlusion rainband, which was now wrapped around the low from its western to northern flank. This evolution lead to protracted periods of rain from around 00 to 2300 CDST. It was during this period that nearly 35 mm was recorded at the Bureau of Meteorology in central Adelaide. During the same period, more than double that amount was recorded in the higher reaches of the Mount Lofty Ranges, which ultimately lead to the flooding of creeks and rivers. From its position near Cape Jervis, the low then commenced moving in a southeasterly direction, so that by 0300 CDST (figure 3d) it was located to the north of Cape Jaffa. Whilst it began to slowly weaken, the very moist northwest to westerly surface winds over the Adelaide Region were maintained for several hours, as was the occlusion rainband, with periods of rain continuing. With the eventual progression of the low to the southeast, the occlusion gradually contracted from the Adelaide region, and cleared during the early hours of th November. 5

6 L Figure 3c : MSLP Analysis, 20 CDST, // L Figure 3d : MSLP Analysis, 0300 CDST, //05 6

7 2.2 Weather Radar Data Weather radar imagery (figure 4a) from the Buckland Park radar (near Two Wells), indicates that at 30 CDST areas of light to moderate rain were traversing the Adelaide area. These were associated with the occlusion rain band, with the patchiness indicative of the relatively weak lifting of the airmass. In the ensuing two hours, whilst there had been generally eastward progression of the rain areas, the central Mount Lofty Ranges region had remained under steady rain. The anchoring of this rain area was due to the in situ lifting mechanism of the topography. By 2030 CDST (figure 4b), some progression to the northeast of the rain area had ensued, as the occlusion progressed eastward. However, a new rain area, still within the occlusion, had become evident over southern Gulf St Vincent, moving northeastward towards Adelaide. By 20 CDST (figure 4c) this rain area was positioned over southern parts of the Mount Lofty Ranges, and due to topographic lift, had expanded in both area and intensity. Figure 4a : Weather radar imagery, Buckland Park radar 30 CDST, //05 Figure 4b : Weather radar imagery, 2030 CDST, //05

8 Figure 4c : Weather radar imagery 20 CDST, //05 Figure 4d : Weather radar imagery, 2230 CDST, //05 This rain area then remained quasi-stationary over the ranges until after 2230 CDST (figure 4d), delivering a further period of steady rainfall. By 2330 CDST the rain area had weakened, with only small, light rainfall patches remaining. This was consistent with the eastward contraction of the occlusion, accompanied by the weakening and southeastward movement of the associated low pressure cell. 2.3 Rainfall analysis Rainfall recordings over the greater Adelaide area for the seven day period up to 0900 CDST on the th November are illustrated spatially in figure 5a. Rainfall in excess of 60 mm was recorded in much of the elevated area, peaking at around 0 mm in the higher reaches of the Mount Lofty Ranges. In areas apart from the Mt Lofty Ranges, the bulk of the rainfall was received in the 4 hour period up to the morning of the th, with showers and thunderstorms associated with the frontal system which preceded the low pressure centre being the main contributing factors.

9 43 66 Figure 5a : Seven day rainfall with totals at selected official Bureau stations, to 0900 CDST, //05, greater Adelaide area The rainfall distribution over the greater Adelaide Area during the 24 hour period to 0900 CDST on the th (figure 5b, based on the Bureau s official daily rainfall network) illustrates a clear focus of the heaviest falls over the higher terrain of the Mount Lofty Ranges. With rainfall in areas of lower elevation generally less than 30 mm, the influence of elevation on rainfall quantity during the period is clear. Rainfall analysed according to the records from the hydrology rainfall network, shown in figure 5c, illustrates more clearly how the heaviest rainfall was focused around the high terrain in the Mount Lofty Heathfield Cherryville Ashton zone. Twenty-four hour totals in excess of 0 mm were recorded at all those localities. With reference to the depicted creeks and rivers, it is clear from the analysis how the heaviest rainfall would have fed copious quantities of water to the upper reaches of many of the water courses which flow down the western slopes of the Mount Lofty Ranges, and into the eastern and southern suburbs of Adelaide. 9

10 Figure 5b : Twenty-four hour rainfall to 0900 CDST, //05, Greater Adelaide Area Figure 5c : Twenty-four hour rainfall to 0900 CDST, November, hydrology stations. The main creeks and rivers are also depicted.

11 Reference to figure 5d, which shows 24 hour rainfall totals superimposed on topography, provides an indication of the correlation between terrain elevation and rainfall quantity. Falls in excess of 0 mm were confined to elevations above 500 metres, with falls of more than 5 mm confined to elevations higher than 400 metres. In general, rainfall was higher to the west (windward side) of the ranges, decreasing markedly to the east of the highest ground. This also is symptomatic of the orographic lifting mechanism, where rising moist air precipitates very efficiently, depositing the bulk of the rain onto windward slopes. Rainfall efficiency decreases rapidly as the air flows down the leeward slopes, warming and drying as it subsides. Figure 5d : Twenty-four hour rainfall to 0900 CDST, November, at selected stations superimposed over topography of the Greater Adelaide Area.