Poster's note : paper and article both below. http://environmentalresearchweb.org/cws/article/news/59579
Volcanoes alter rainfall more than models predict Designers of geoengineering schemes should be aware that climate models underestimate changes in precipitation after volcanic eruptions, according to researchers in the UK. Volcanic eruptions inject sulphur dioxide into the stratosphere, where it is converted to sulphate aerosols. These aerosols spread out worldwide and reflect incoming solar radiation. This causes widespread surface and tropospheric cooling that lasts a few years, along with changes in atmospheric circulation and precipitation. The mechanism for the resulting decrease in global mean precipitation is relatively well known – less short-wave radiation reaches the surface, reducing evaporation, stabilizing the atmosphere and cutting the air's saturation mixing ratio, as well as the cooler atmosphere allowing less condensation – but climate models seem consistently to underestimate the effect of volcanoes on precipitation. Carley Iles and Gabriele Hegerl from the University of Edinburgh have built on previous work that used the climate model HadCM3 to investigate the precipitation response to volcanic eruptions. Their new research uses the latest models – CMIP5 – many of which have higher horizontal and vertical resolutions and extend higher into the stratosphere than HadCM3. Iles and Hegerl were keen to find out if the new models would give the same results as the previous study. The main features of the precipitation response to volcanic eruptions in the CMIP5 models are consistent with those found in HadCM3, the pair found. "Both showed a significant global decrease in precipitation after large volcanic eruptions and also underestimated the size of the precipitation response when compared with observed values," Iles told environmentalresearchweb. "They also agreed on the fact that wet tropical regions became drier while drier regions got wetter after an eruption." Iles and Hegerl extended their analysis by comparing the modelled response to a satellite-gauge dataset that includes ocean coverage, allowing them to test whether the long-lasting ocean-precipitation response found in HadCM3, and the wettening response in the dry tropical ocean regions, are supported by observations. They found that the ocean response was longer-lived than that over land in all models with more than one ensemble member. "We were surprised to find that over land, precipitation responds faster than air temperature," said Iles. "Instead it matches the timescale of a decrease in land–ocean temperature contrast, which will lead to weakened monsoon winds and less rainfall in monsoon regions. Land precipitation also matches the timescale of the aerosol forcing, suggesting a directly forced effect independent of temperature." The dataset used to examine the ocean-precipitation response, the Global Precipitation Climatology Project (GPCP) combined satellite-gauge dataset, began in 1979. Since then, there have been only two major eruptions. In 1987, a microwave-based sensor was introduced, which improved retrieval accuracy. There has been one major eruption since this development. "Based on historical records, it is likely that there will be a large volcanic eruption in the next few decades," said Iles. "The satellite records will then become extremely valuable to further constrain the observed response, particularly over the oceans." Iles and Hegerl would like designers of geoengineering schemes to take their findings into consideration. "We also hope that our research helps people plan ahead and mitigate the effects of future volcanic eruptions," said Iles. "It could also help researchers determine how eruptions may be linked with certain climate events such as droughts." http://m.iopscience.iop.org/1748-9326/9/10/104012/article The global precipitation response to volcanic eruptions in the CMIP5 models OPEN ACCESS Carley E Iles and Gabriele C Hegerl 2014 Environ. Res. Lett. 9 104012 doi:10.1088/1748-9326/9/10/104012 Published 14 October 2014 Abstract We examine the precipitation response to volcanic eruptions in the Coupled Model Intercomparison Project Phase 5 (CMIP5) historical simulations compared to three observational datasets, including one with ocean coverage. Global precipitation decreases significantly following eruptions in CMIP5 models, with the largest decrease in wet tropical regions. This also occurs in observational land data, and ocean data in the boreal cold season. Monsoon rainfall decreases following eruptions in both models and observations. In response to individual eruptions, the ITCZ shifts away from the hemisphere with the greater concentration of aerosols in CMIP5. Models undergo a longer-lasting ocean precipitation response than over land, but the response in the short satellite record is too noisy to confirm this. We detect the influence of volcanism on precipitation in all three datasets in the cold season, although the models underestimate the size of the response. In the warm season the volcanic influence is only marginally detectable. -- You received this message because you are subscribed to the Google Groups "geoengineering" group. To unsubscribe from this group and stop receiving emails from it, send an email to [email protected]. To post to this group, send email to [email protected]. Visit this group at http://groups.google.com/group/geoengineering. For more options, visit https://groups.google.com/d/optout.
