July 8, 2019 | Published by | ,

The Australian Northwest Cloudband (NWCB) is a continental sized band of cloud that stretches from the East Indian Ocean to southern Australia and can cause widespread rainfall across the country. Previous records of NWCBs only extended to the end of the 20th Century and often relied on subjective methods like manually identifying NWCBs from images by eye, or models that are known to poorly simulate clouds. So, CLEX researchers created an automatic algorithm to objectively identify NWCBs from 31-years of satellite observations.

They then used this record to update the climatology of Australian NWCBs into the 21st Century. The researchers found a statistically significant positive trend (+0.87 per year) in the number of days with a NWCB from 1984-2014.

Moreover, they analysed the physical drivers associated with NWCBs to produce new conceptual models of summer and winter events. These showed the typical synoptic conditions are similar in summer and winter, but the key difference is the genesis mechanism. In winter, sea surface temperature gradients in the East Indian Ocean lead to an unstable atmosphere over northwest Australia, whereas in summer low pressure anomalies such as monsoon lows and tropical depressions trigger the cloudband.

Lastly, the researchers quantified the association between NWCBs and Australian mean and extreme precipitation. The key result is that NWCBs are associated with an increased probability of extreme rainfall over northwest, central and southern parts of Australia, but a decreased probability of extreme rainfall in northeast and southwest Australia.