COMET ‘highlight’ article April 2014 – March 2015
Satellite data show alteration of cloud droplets downwind of degassing volcanoes in pristine oceanic regions
Aerosols affect how reflective the Earth is by either absorbing and scattering solar radiation directly, or by modifying the properties of clouds. However, we are still uncertain of exactly how aerosols change cloud properties, how they affect global climate, and therefore how they impact on climate change.
It is more difficult to make satellite measurements of the effects of aerosols from passively degassing volcanoes deep in the lower atmosphere than from explosive eruptions that inject gas and aerosol up into the stratosphere. However, the impact of such ‘background’, volcanic activity is increasingly thought to be important to atmospheric processes. Prior to our study, measurements of a volcanic impact on cloud properties had been made only during a few episodes of elevated degassing.
We use data from three independent satellite sensors (MODIS, AATSR, CERES) to examine differences in cloud and aerosol properties upwind and downwind of isolated volcanic islands. By comparing this information with that from islands without active volcanoes, we can see how volcanic emissions are affecting the clouds.
Figure 1. Multiannual mean Aerosol Optical Depth from NASA’s MODIS Aqua Collection 6 dataset. Darker red indicates higher aerosol burden. The volcanoes and islands in our study are all in regions of low aerosol optical depth and may therefore be representative of the pre-industrial atmosphere.
By analysing a decade of satellite measurements of aerosol and cloud properties, we demonstrate that these volcanoes have a long-term net impact on cloud properties. Downwind of the volcanoes, the concentration of aerosol is higher and the cloud droplet size is lower than upwind. Top of atmosphere shortwave radiation flux is also higher downwind of the volcanoes, as smaller droplets tend to be more effective at reflecting solar radiation.
This was the case for a range of eruptive styles including high flux degassing (Kilauea), Strombolian eruptions (Yasur) and minor explosions (Piton de la Fournaise). Measurements of aerosol effects at isolated volcanic islands may now be the closest analogue to the pre-industrial atmosphere, and offer a rare chance to observe atmospheric processes as they would have been before the industrial revolution.
Figure 2. Aerosol optical depth (a & b) and cloud effective droplet radius (c & d) plots for Piton de la Fournaise between 2002 and 2008. Data are rotated according to wind direction at the height of emission, so that for each panel the upper quadrant is downwind of the volcano and the bottom panel is upwind (i.e. the arrow shows wind direction). Panels a & c show aerosol and cloud properties during quiescence, while b & d show elevated aerosol and smaller droplets during minor explosive activity.
You can see a short video describing the research on the NASA Goddard Space Flight Center YouTube channel.
Ebmeier, S. K., A. M. Sayer, R. G. Grainger, T. A. Mather, and E. Carboni (2014). Systematic satellite observations of the impact of aerosols from passive volcanic degassing on local cloud properties. Atmospheric Chemistry and Physics, 14, 10601-10618, doi:10.5194/acp-14-10601-2014.