Science is in the Air

Chrissy’s Tutorial Group
Student lead discussion session 2, led by Antonia

Land-atmosphere interactions are a dynamic and exciting area of research – starting from the 1970s with papers such as the one by Idso et al., 1974 discussing the most basic of physical processes, up to now, when we have gained an in-depth knowledge on these same processes and many more.

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Figure 1. Remnants of the Tokage Tropical Cyclone visible by AQUA MODIS. Source: NASA

Because of the above-mentioned reasons, for my tutorial this week (15/11/16), I chose two papers which framed the topic – one from the early days of the field by Charney et al., 1975, and another by the team of modellers of Wang et al., published in 2015.

 

The first paper proposed a theory that changes in surface albedo due to human intervention were the sole reason for the Sahelian drought in the 1970s. Charney et al., 1975 suggested that the system was locked in a positive feedback loop. An increase in surface albedo, due to a reduced plant cover, decreased net incoming radiation. This led to an increased cooling of the air and consequential rainfall reduction. This whole process is suggested to have been initiated by overgrazing of cattle. In an earlier work, the same author briefly mentions the same theory, giving as a reason for vegetation loss the livestock trampling of vegetation while they are taken to the local water wells. The only parameter which was manipulated in the study, was the surface albedo.

Our tutorial group agreed that the simulations were very simplistic. Although the feedback loop mentioned above does sound straightforward and is an established physical pathway, the simulations did not take neither external influences nor global atmosphere circulation into account. The group agreed that a greater system complexity should be added to account for the overall micrometeorology changes. Another shortcoming of the paper was that the net effect was not considered. In order to do that, both biogeophysical and biogeochemical effects should be taken into account. A model representing the biosphere’s characteristics should also be incorporated within the simulations before any conclusions can be drawn.

Although the paper has many flaws, as seen from the above, it was one of the pioneers in the area, and the first one to tackle the effect albedo has on the microclimate of an area. If examined in that light, it is indeed a valuable paper because it provoked a lot of thought. In fact, it was disproved the very next year (1976) by Ripley et al., who discovered that the draught has less to do with anthropogenic influences prompting a change in albedo and more to do with Atlantic circulation.

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Figure 2. Figure taken from Wang et al., 2015 showcasing the fine scale that we are able to track atmospheric changes over. Displayed here are the observed temperature differences between 1980s and 1950s.

 

This leads us to the second paper by Wang et al. (2015) which demonstrates a far more sophisticated understanding of the matter. It includes several different models assessing the impact of land use land cover changes on the Sahelian microclimate. Some of the group members pointed out that the paper was very technical, and we were in agreement that some of the model descriptions were hard to understand without a solid understanding of modelling, which none of us have (yet!). This paper showed how much our understanding has changed since 1975. The main drivers of the drought were pointed out as the state of the boundary layer conditions, and evapotranspiration (ET). ET is shown to be a key element, which was not previously considered. The group also though that the paper dealt better with external influences such as the African Easterly Jet.

Although the two papers were not part of our expertise, we thought that this particular area of research has come a long way, as showcased above. Future research is heavily dependent on satellite observations, and more sophisticated models, incorporating local, regional and global conditions altogether. At the end of the tutorial, I briefly mentioned the challenge that the role of clouds presents in the area, as there has not yet been developed an approach for their incorporation into models. The 2.6 billion dollar U.S. Global Change Research Program for Fiscal Year 2016 puts “the role of clouds” as their most important and costly area of research within their Climate and Hydrological Systems research.

I personally find climate tracking and model predictions extremely interesting, so if you share that curiosity, take a peek at the following websites:

http://cola.gmu.edu/ – The Center for Ocean-Land-Atmosphere Studies

http://www.noaa.gov/ – National Oceanic and Atmospheric Association

http://www.metoffice.gov.uk/public/weather/storm-tracker/#?tab=map – Met Office storm tracker

https://www.nasa.gov/mission_pages/hurricanes/main/ – NASA Hurricanes and Tropical Storms

 

 

References

Charney, J., Stone, P. H., Quirk, W. J. (1975) Drought in the Sahara: A Biogeophysical Feedback Mechanism. Science 187: 434-435

 

Idso, S. B., R. D. Jackson, R. J. Reginato, B. A. Kimball, and F. S. Nakayama (1975) The dependence of bare soil albedo on soil water content. Journal of Applied Meteorology 14, 109–113.

Wang, G., Yu, M., Xue, Y. (2015) Modelling the potential contribution of land cover changes to the late twentieth century Sahel drought using a regional climate model: impact of lateral boundary conditions. Climate Dynamics 382: 1-21

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