The Red Sky: investigating the hurricane Ophelia Saharan dust and biomass burning aerosol event

On 16th October 2017 ex-hurricane Ophelia passed over the UK, bringing with it a unique aerosol loading which caused the Sun and sky to turn a dramatic red colour. Here we use an ensemble of modelling and remote sensing techniques in a ‘top-down, bottom-up’ approach comprising instruments onboard orbital platforms and a ground-based lidar, to interrogate the aerosol-cloud matrix in order to determine its composition and origins. Satellite observations were successfully employed to provide a detailed, holistic ‘snapshot’ (from the ‘top-down’) of the Ophelia weather system with high spatial coverage but low temporal resolution and the ground-based lidar was used to provide complementary, near real-time information (from the ‘bottom-up’) on the types of particles entrained in the air mass as it slowed and passed over the UK. By using top-down measurements from space it was possible to analyse the effect of the embedded aerosol on the light that was being scattered and subsequently the reddening of the Sun. Satellite measurements showed that the aerosol loading of the air mass was high, with an aerosol index >5, and contained significant levels of CO, up to almost 300 ppbV. Multi-spectral imaging of the system allowed the differentiation of cloud types and identification of regions of aerosol. By using bottom-up lidar measurements it was also possible to observe the temporal evolution, composition and the vertical extent of the aerosol-laden air mass. Combined with back-trajectory modelling, the satellite and ground-based measurements allowed the sources of the aerosol to be determined as Saharan dust uplift and wildfires across Portugal and Spain. The lidar measurements showed that the red sky event comprised two distinct phases. The initial phase was dominated by Saharan dust (volume depolarisation ratio; δ = 0.15 – 0.25) and the second by an optically dense layer starting at ~0.7 km comprising a mixture of Saharan dust and biomass burning particles (δ = 0.08 – 0.18) from the intense wildfires in the Iberian Peninsula. It was this unique loading of aerosol types, rarely observed in this combination and magnitude over the UK, that resulted in enhanced scattering of the short/medium visible wavelengths of incoming solar radiation, and it was this that led to the Sun and sky taking on such a dramatic appearance.