27 September 2009

Dust and Bushfire: NSW-QLD

Another unusual weather day today across southern QLD and northern NSW, with a second large dust storm blowing across the region within the past few days. Bushfires flared across the state with near-extreme fire weather conditions across much of the area.

The image compares two views of the weather situation today. Both are from MODIS true colour images at 2km resolution; one from Terra (left) and one from Aqua (right), separated in time by about 2.5 hours. Both show the same region and come from the MODIS Rapid Response subset website. The QLD-NSW border is roughly in the centre of the image, a 1000 km by 1000 km square. Sydney is towards the bottom of the image; Brisbane about in the centre. The two images side by side provide nice examples for image interpretation, as well as simply awesome views of meteorological phenomenon.

One interesting, if obvious, feature is the dust storm itself. It extends (as seen in the image for at least 1400 km. News reports suggested that the dust would come strike Brisbane in two phases. These are clearly visible in the images; a more diffuse layer followed by a thicker cloud maybe 50-60 km behind the leading edge. Using landmarks on the image (and Google Maps!), I can estimate a speed of 20-30 km/h for the leading edge.

The second, and more relevant for this blog, item of interest is the evolution of the appearance of the fire between the two times, associated with the changing weather conditions as the day evolves. The fire activity across the region today was much the same as yesterday. Numerous fires continue to rage in QLD, particularly in the southeastern part of the state; a major fire in northern NSW's Clarence Valley broke containment but is not directly threatening property as this time. A small fire, quickly extinguished, was reported in northern Sydney overnight.

Consider the most obvious fires in the images, the Carnarvon National Park fire (smoke plume top left) and the N. Stradbroke Island fire (top right). In the Terra image (left), the plume are quite extensive but appear thin compared to the later Aqua image, where the smoke in those is quite dense and of thicker appearance. Careful comparison shows that there are more smoke plumes and widespread hotspots (apart from the main two) in the later image as well. Fire weather conditions in area worsened considerably between the time of the two images; the day got hotter (temps in low-30s...), wind speed picked up and relative humidity dropped to 10-20%. The conditions intensified the fires, producing the thicker, more frequent smoke apparent in the image.

Fire weather conditions are are predicted to remain in the very high range for much region tomorrow. Fire bans are in place for parts of SE QLD.

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