Under the cover of darkness: Australia's 'missing' dust storms

The passage of mid-latitude cold fronts and propagation of their northern sector into the subtropics is the principal synoptic scale meteorological forcing of dust entrainment and transport in Australia. During these events dust may be entrained by prefrontal northerlies, frontal westerlies and prost-frontal anticyclonic southerly winds. Direct observations of dust entrainment by these cold fronts has been limited to date to daylight hours, relying heavily on observer reports and analysis of corresponding daytime satellite products such as MODIS true colour visible images, and corresponding surface reflectance and thermal IR dependent products such as Deep Blue. However, intense daytime surface heating of inland Australia during spring and early summer causes fronts to weaken due to turbulent mixing of the boundary layer leading to frontolytic conditions. As a result, a front may stall or even dissipate. Under these conditions dust transport would cease with only remnant dust haze remaining. Following sunset and onset of radiative surface cooling boundary layer turbulence moderates and frontogenesis occurs as baroclinic conditions re-establish. The front then surges forward again with the potential to entrain and transport dust, but is unseen by many observers and is not detected by analysis of visible image satellite products. Here we present a case study analysis of a dust event associated with the nocturnal passage of a cold front and trough through central northern Queensland, Australia. Using different remote sensing satellite products (Himawari and EUMETSAT) in conjunction with Doppler weather radar we are able to track the dust storm including its passage over the city of Mt Isa where it was not recorded by local air quality monitoring stations. The most likely explanation for this is that the dust overrode a very stable nocturnal surface layer in which vertical mixing was suppressed. As a result, no significant increase in PM10 concentrations was monitored at the surface. We concluded that nocturnal frontogenesis in the arid subtropics of central and northern Australia may be a very significant dust transport mechanism which has to date, been under reported in records of Australian dust storms.