Northwest cloudbands are tropical-extratropical feature that crosses the Australian continent originating from Australia’s northwest coast and develops in a NW-SE orientation. In paper, atmospheric and oceanic reanalysis data (NCEP) and Reynolds reconstructed sea surface temperature data were used to examine northwest cloudband activity across the Australian mainland. An index that reflected the monthly, seasonal, and interannual activity of northwest cloudbands between 1950 and 1999 was then created. Outgoing longwave radiation, total cloud cover, and latent heat flux data were used to determine the number of days when a mature northwest cloudband covered part of the Australian continent between April and October. Regional indices were created for site-specific investigations, especially of cloudband-related rainfall. High and low cloudband activity can affect the distribution of cloudbands and their related rainfall. In low cloudband activity seasons, cloudbands were mostly limited to the south and west Australian coasts. In high cloudband activity seasons, cloudbands penetrated farther inland, which increased the inland rainfall. A case study of the southwest Australian region demonstrated that, in a below average rainfall year, cloudband-related rainfall was limited to the coast. In an above average rainfall year, cloudband-related rainfall occurred further inland. 1. Introduction The northwest cloudband (NWCB) is one of three tropical-extratropical cloudbands that crosses the Australian continent [1]. It originates from Australia’s northwest coast (Figure 1(a)) and develops a NW-SE orientation, with no immediate connection to southern frontal systems [2]. Extratropical cloudbands are not unique to Australia; they also occur over Southern Africa [3], over South America, and in the Northern Hemisphere [4]. The Northern Hemisphere equivalent, the southwest (SW) cloudband, extends from Central America to the North American east coast and from Indochina to Japan and is most pronounced in winter [1]. Kuhnel [4] defined 14 tropical-extratropical cloudbands around the globe (seven in each hemisphere). The NWCB is the third most frequently occurring cloudband globally and the fourth most frequently occurring cloudband in the Southern Hemisphere. Figure 1: (a) Satellite image of a NWCB on 29 April 1996 (image source: Australian Bureau of Meteorology) and corresponding (b) OLR contours (140–250？W/m 2), (c) TCC contours (70–100%), and (d) latent heat flux contours (110–300？W/m 2). The NWCB is a large cloud structure, which can reach a length of 8000？km from
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