GeoLog

precipitation modelling

GeoTalk: Yagmur Derin on posters and precipitation

This week in GeoTalk, we’re talking to Yagmur Derin, a masters student from Middle East Technical University, Turkey. She tells us about the intersecting fields of hydrology, climate science and remote sensing, and what it’s like to take the plunge and present your first poster at an international conference.

Firstly, can you introduce yourself and what you’ve been investigating as part of your MSc course?

I’m Yagmur Derin, MSc student, currently working as a research fellow in Department of Geological Engineering of Middle East Technical University (Ankara, Turkey). I graduated from this same department with the mind that I should get involved in academia. I took courses from different areas of undergraduate study to see what makes me feel most excited. At the end of the graduation I was sure that I should study hydrology and remote sensing in more detail. I was lucky that my current advisor had an open position in one of his projects when I graduated and was able to start working on that project immediately. The project’s focus is to see how satellite rainfall measurements can be applied to hydrologic modelling, and flood monitoring in particular.

Yagmur Derin out in the field. (Credit: Yagmur Derin)

Yagmur Derin out in the field. (Credit: Yagmur Derin)

How can we use satellite data to improve hydrological models?

The accuracy of any hydrologic study depends on the availability of good quality precipitation estimates. Precipitation estimation can be obtained from rain gauges, radar networks and satellites. The most direct physical measurement is conducted by rain gauges, but they are susceptible to certain errors due to location, spatial scale, wind, and density. Our ability to measure precipitation is limited in remote parts of the world and developing countries, where rain gauge and radar networks are either sparse or non-existent (mainly due to the high cost of establishing and maintaining the infrastructure). This situation is further worsened in regions with complex topography where rain gauges are generally located in lowland. This means that highland precipitation, which is the main interest in hydrologic studies, is underrepresented. Fully distributed hydrological models require high resolution information about the precipitation field and its variability. However, interpolating rain gauge measurements in regions with complex topography – especially over data sparse regions – gives erroneous results. Satellite-based precipitation products are perhaps the only data source to fill this important gap. 

Recent improvements in satellite-based precipitation retrieval algorithms allowed us to better represent the high variability in precipitation over space and time with near global coverage. This makes satellite data attractive for hydrologic modelling studies in data sparse regions. Satellite-based precipitation algorithms estimate precipitation rate based on remotely-sensed characteristics of clouds, such as reflectivity of clouds (visible), cloud-top temperature (infrared, IR) and scattering effects of raindrops or ice particles (passive-microwave, PMW), and these products have certain limitations too. Ongoing improvements and multiple satellite missions planned for the future make them potentially useful for hydrologic modelling studies. 

Earlier this year, you were awarded the Outstanding Student Poster Award for your poster presentation at EGU 2013, what inspired you to attend the conference and present your work?

EGU brings together geoscientists from all over the world, providing a great opportunity for scientists to present and discuss their ideas. Since my career goals lie in academia, my advisor encouraged me to attend the EGU General Assembly back 2012 so that I could meet other scientists and discuss my research.

The award-winning poster presented at EGU 2013: “Evaluation and Bias Adjustment of Multiple Satellite-based Precipitation Products over Complex Terrain” (see the credited link for a larger image). (Credit: Yagmur Derin and Koray K. Yilmaz, 2013)

The award-winning poster presented at EGU 2013: “Evaluation and Bias Adjustment of Multiple Satellite-based Precipitation Products over Complex Terrain” (see here for a larger image). (Credit: Yagmur Derin and Koray K. Yilmaz, 2013)

EGU 2012 was my first international conference with a presentation and it definitely helped broaden my perspective. It was a great experience which made me realise that I should study much more if I want to succeed with my MSc studies. Joining talks, poster sessions and communicating with fellow scientists helped me identify my shortcomings and work out how I could overcome them. After one hardworking year, I wanted to attend EGU 2013 too – the whole conference inspired me to study and participate in academia much more. This year, I benefitted from all hard work and I feel much more confident in my studies.

I suggest everyone attends as many sessions as they can during the conference, and that they meet as many scientists as they can too. Explaining present projects and getting feedback on them helps improve your research significantly. 

What do you plan to do after you have completed your masters course?

I am planning to defend my MSc thesis in June, 2014 and after that I will continue with a PhD. My area of interest has become more focused throughout my masters and I would like to continue my research in the fields of surface water hydrology, land-atmosphere interaction, rainfall-runoff modelling, hydrometeorology, remote sensing of precipitation and GIS. Currently I am applying several universities for PhDs that are related to my research interests and look forward to the possibility of starting a PhD next year.

If you’d like to suggest a scientist for a Geotalk interview, please contact Sara Mynott.

Geosciences Column: Rainfall and Climate – a Dynamic Problem

“Rain is grace; rain is the sky descending to the earth; without rain, there would be no life.” – John Updike

Rain quenches the thirst of soils and vegetation, fuelling ecosystems and much of the world’s agriculture. Whether it ruins a day on the beach or destroys a season’s harvest, it makes humans deeply aware of their vulnerability to the vagaries of the atmosphere. It’s important to understand how rainfall changes in a changing climate. Here, I will describe the issues in understanding precipitation changes and how two recent papers help to solve the puzzle.

Predicting rainfall is difficult. It is a small-scale phenomenon, especially in the towers of convective cloud in the Tropics. Weather forecasting models are just beginning to capture them properly at scales of a kilometre or so, but climate models, which have to be run for decades rather than days, calculate atmospheric conditions on scales of hundreds of kilometres. Rainfall has to be simplified in these models, since we cannot calculate the physical properties of individual clouds. These simplified representations are called parameterisations. A precipitation parameterisation relates the average rainfall over a large area to the average amount of water in the air. Different models do this in different ways and, because it’s a simplification, there is no definitive ‘right’ way. This means there is some disagreement among climate models about how rainfall will change in the future, especially in the Tropics (areas on the figure which are not stippled).

Climate model projections of precipitation change in a future with high greenhouse gas emissions. Left: current generation of models, Right: previous generation of models (around 2005). Top: December-February, Bottom: June-August. Stippling shows areas where models largely agree. White areas show complete disagreement among models (source: Knutti & Sedlacek, 2013).

If we think about precipitation in general theoretical terms, we can find laws which must be followed and use them to make predictions, as Issac Held & Brian Soden did in their study of how the hydrological cycle responds to global warming. Rain is caused by the upward transport of water vapour from the surface into the atmosphere, where it condenses, forms clouds and rains out. The amount of moisture going up must, of course, balance the amount coming back down as rain.

As the climate warms, the amount of water vapour a fixed mass of air can hold increases. This means that, as long as the circulations transporting water upwards remain the same, the total amount of water vapour going upwards must increase – which means the amount of rain coming down must also increase. This is called the ‘rich get richer’ mechanism, because it increases rainfall in regions where there is already a lot of rain driven by upward moisture transport. It’s a fundamental mechanism driven by thermodynamic laws…but that doesn’t mean it’s the only thing going on.

Convective raincloud in tropical Africa (photo credit: Jeff Attaway).

If climate model projections followed the ‘rich get richer’ mechanism, precipitation would increase most in the regions with the most precipitation currently. In fact it is more complicated than that. Robin Chadwick and his colleagues explored the effect of weaker vertical motions in a warmer climate. We can understand this by thinking about what carbon dioxide does to the vertical temperature profile. It warms the mid-troposphere (about 5 km up) more than the surface. To get convective upward motion, the air at the surface must be less dense (i.e. warmer) than the air above. Warming the air aloft suppresses this motion. The Chadwick decomposition calculates the part of the precipitation changes caused by changes in moisture (which goes at about 7% per K) and the part caused by the reduction in upward transport. They find the two tend to roughly cancel each other out, which means the spatial shifts in precipitation are determined by changing patterns of surface temperature (since warm surfaces produce upward motion).

Sandrine Bony and her team decompose precipitation changes into two main components rather than three: one is the ‘dynamical’ component, associated with changing upward motions, and the other is the ‘thermodynamical’ component, including changes in atmospheric moisture content. Unlike the Chadwick method, the thermodynamical component is not designed solely to represent the ‘rich get richer’ mechanism. This means the thermodynamical component isn’t just a 7% per K increase; it includes things like the spatial changes in surface temperature. The dynamical component isolates the change in precipitation caused by changes in upward motion.

Monsoon raincloud over a lake in the Tibetan Plateau (photo credit: Janneke Ijmker).

The ‘rich get richer’ rule of thumb becomes increasingly irrelevant at smaller scales. This is frustrating, because these are the scales we really care about! It’s not particularly useful knowing what will happen in a general sense over the whole Tropical region. Farmers want to know what will happen to the seasonal rains on their small piece of land.

Bony also points out that geoengineering schemes which aim to reduce incoming solar radiation to cool the planet’s surface would leave the dynamical component of precipitation change untouched. This is because the dynamical component is caused by the warming of the mid-troposphere by carbon dioxide, and this remains even if we cool the surface. It is an example of the inexact nature of the cancellation between carbon dioxide increases and geoengineering schemes to decrease the amount of carbon dioxide in the atmosphere, and demonstrates that the only way to stop carbon dioxide-driven climate change properly is to stop emitting carbon dioxide.

Bony and Chadwick’s decompositions show how one can glean a lot more information from climate model projections than one would expect from first glance. We have established some general facts about climate change related to the Earth’s energy budget. In that sense we understand quite well what will happen in a warming climate. However, there is still a lot of diversity between model projections, most of which comes from differences in the dynamical response. Local changes in rainfall are related to changes in circulation, and this is the area in which a lot more work needs to be done.

By Angus Ferraro, PhD student at Reading University

References:

Bony, Sandrine, Gilles Bellon, Daniel Klocke, Steven Sherwood, Solange Fermepin & Sébastien Denvil, 2013: Robust direct effect of carbon dioxide on tropical circulation and regional precipitation, Nat. Geosci., doi:10.1038/ngeo1799

Chadwick, Robin, Ian Boutle & Gill Martin, 2013: Spatial Patterns of Precipitation Change in CMIP5: Why the Rich don’t get Richer in the Tropics. J. Climate, doi: 10.1175/JCLI-D-12-00543.1

Held, Isaac M., Brian J. Soden, 2006: Robust Responses of the Hydrological Cycle to Global Warming. J. Climate, 19, 5686–5699. doi: 10.1175/JCLI3990.1

Knutti, Reto & Jan Sedláček, 2013: Robustness and uncertainties in the new CMIP5 climate model projections, Nat. Clim. Change, doi: 10.1038/nclimate1716