African dust changes India's rainfall: Dust can affect planet's climate, research shows

A new analysis of satellite data reveals a link between dust in North Africa and West Asia and stronger monsoons in India. The study shows that dust in the air absorbs sunlight west of India, warming the air and strengthening the winds carrying moisture eastward. This results in more monsoon rainfall about a week later in India. The results explain one way that dust can affect the climate, filling in previously unknown details about Earth's system.

The study also shows that natural airborne particles can influence rainfall in unexpected ways, with changes in one location rapidly affecting weather thousands of miles away. The researchers analyzed satellite data and performed computer modeling of the region to tease out the role of dust on the Indian monsoon, they report March 16 in Nature Geoscience.

India relies heavily on its summer monsoon rains. "The difference between a monsoon flood year or a dry year is about 10 percent of the average summer rainfall in central India. Variations driven by dust may be strong enough to explain some of that year-to-year variation," said climate scientist Phil Rasch of the Department of Energy's Pacific Northwest National Laboratory.

Rasch, V. Vinoj of the Indian Institute of Technology Bhubaneswar, India, and their coauthors wanted to explore a correlation that appeared in satellite records: higher amounts of small particles called aerosols over North Africa, West Asia, and the Arabian Sea seemed to be connected to stronger rainfall over India around the same time. The team wanted to see if they could verify this and determine how those particles might affect rainfall.

To explore the connection, the team used a computer model called CAM5 and focused on the area. The model included humanmade aerosols from pollution, and natural sea salt and dust aerosols. First, the team ran the model and noted a similar connection: more aerosols in the west meant more rainfall in the east. Then they systematically turned off the contribution of each aerosol type and looked to see if the connection remained.

Dust turned out to be the necessary ingredient. The condition that re-created stronger rainfall in India was the rise of dust in North Africa and the Arabian peninsula.

To see how quickly dust worked, they ran short computer simulations with and without dust emissions. Without dust emissions, the atmospheric dust disappeared within a week compared to the simulation with dust emissions and rainfall declined in central India as well. This indicated the effect happens over a short period of time.

But there was one more mystery, how did dust do this to rainfall? To explore possibilities, the team zoomed in on the regional conditions such as air temperature and water transport through the air.

Their likeliest possibility focused on the fact that dust can absorb sunlight that would normally reach the surface, warming the air instead. This warmer dust-laden air draws moist air from the tropics northward, and strengthens the prevailing winds that move moisture from the Arabian Sea into India, where it falls as rain.

Although dust plays a role in strengthening monsoons, this natural phenomenon does not overpower many other processes that also influence monsoons, said Rasch. Other extremely important factors include the effect of temperature differences between land and ocean, land use changes, global warming, and local effects of pollution aerosols around India that can heat and cool the air, and also affect clouds, he said.

"The strength of monsoons have been declining for the last 50 years," he said. "The dust effect is unlikely to explain the systematic decline, but it may contribute."

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Microwave radar monitors sliding slopes: Geodesists research in the Alps

The "Steinlehnen" slope in Northern Tyrol (Austria) started to move in 2003. Rockfalls threatened people, streets and buildings. Meanwhile, peace has returned; although the slope is merely "creeping," Steinlehnen has become an interesting research object for scientists in recent years.

Professor Andreas Eichhorn of the Geodetic Measurement Systems and Sensors branch in the Department of Civil and Environmental Engineering at the Technical University of Darmstadt initiated the interdisciplinary project KASIP (Knowledge-based Alarm System with Identified Deformation Predictor) together with the Technical University of Vienna and the "alpS" research institute; the goal was to combine metrological observations of the slope with computer models.

"A slope is tremendously complex," says Eichhorn. It can be difficult to determine exactly how a mountain slope is composed and how a failure mechanism works in detail. Therefore, scientists will not be able to rely solely on computer-based models to predict mass movements in the future; they also need efficient and precise surveillance and monitoring systems that are as comprehensive as possible.

To do this, Eichhorn and his team tested different methods at Steinlehnen. "Installing sensors in highly active areas of the mountain is very dangerous," explains Eichhorn. "We were looking for a method that, among other things, makes non-contact observation possible." In the end, one method proved to be particularly suitable; although its basic physical principle has been used in geodesy for a long time, it was never used for the monitoring of slopes. This method uses a microwave radar of the Department of Physical Geodesy and Satellite Geodesy of the TU Darmstadt (Professor Matthias Becker), which was applied prototypically by Eichhorn's team of Darmstadt scientists.

Here, the entire surface of a slope is "shot" with microwaves that are reflected back from the surface and can then be analyzed. By comparing different measurements, the scientists can document changes of just a few millimeters. Accumulations or erosion of rock material, or even the beginning of a major landslide, can thus be recorded, Eichhorn says. In contrast to methods that scan the surface with laser light, for example, microwaves deliver much less disturbance. "A laser has too much noise," says Eichhorn. In her dissertation, doctoral candidate Sabine R?delsperger developed an evaluation strategy for interpreting the measured data; among other things, this also makes it possible to remove meteorological disturbances and to arrive at meaningful 3D images of the slope.

During the KASIP experiments, the geodesists from Darmstadt, together with their colleagues from the field of geophysics, achieved many important insights for the more accurate interpretation of observed geophysical phenomena and the correlation between the weather and the sliding behavior of the slope. But the research also has practical benefits, as Eichhorn explains: "Solely in terms of technology, it is possible to continuously monitor a large-scale critical slope in high-resolution. Accelerations -- early indicators of the possible slipping of large masses -- can be detected, and it can be determined when the slope stops moving."

Microwave radar devices are still very expensive, but the method already has potential as a good early warning system: "If you would observe critical slopes with them, you could reliably determine exactly where something is happening," says Eichhorn. "Then less expensive measurement systems and their sensors could be specifically applied there."

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First measurement flight: Research aircraft HALO explores trade wind clouds

Which climate effects do clouds have? Under what conditions do they warm or cool the atmosphere? Today, after more than five years of preparation, the specially equipped research aircraft HALO (High Altitude and Long Range Research Aircraft) takes off for its first measurement flight in atmospheric research. Prof. Bjorn Stevens and Dr. Lutz Hirsch from the Max Planck Institute for Meteorology (MPI-M) leave Oberpfaffenhofen in Germany for a ten-hour flight to Barbados.

They will operate numerous measuring instruments on board HALO on behalf of the German atmospheric research: "A day we have eagerly awaited," says Stevens. "It is the first major mission to exploit the novel capabilities of HALO to measure vertical profiles of all components of atmospheric water -- like vapor, liquid and ice, in both cloud and precipitation forms, as well as the aerosol particles upon which cloud droplets form -- from a high altitude. A new era of airborne atmospheric research." The aircraft, equipped with a large amount of advanced technology, is an initiative by German climate and environmental research institutions (see below) and is operated by the German Aerospace Center (DLR).

The flight is part of the NARVAL project (Next-generation Aircraft Remote-Sensing for Validation Studies) and will provide the scientists with more detailed information on the constitution of tropical clouds (Fig. 1). The transatlantic flights from Oberpfaffenhofen to Barbados will complement the stationary measurements of the cloud observatory on Barbados. The collected data will contribute to a better understanding of cloud and precipitation processes and will help to reduce uncertainties in climate models.

Remote sensing instruments, located in the "Belly Pod" underneath the aircraft?s hull, will detect vertical profiles of temperature and humidity and the distribution of droplets and aerosols (Fig. 2). Additionally, so-called dropsondes will be released during the flight. These radiosondes usually ascend from Earth with the help of a weather balloon and perform measurements on their way through the atmosphere. This time, they will be dropped by parachute and will glide back to the ground.

The first measurement flight is a joint project of the MPI-M with the Meteorological Institute of the University Hamburg, DLR, Universities of Cologne, Leipzig and Heidelberg and the Forschungszentrum J?lich. It will take the scientists on a long-haul flight to Barbados, where the MPI-M cloud observatory is located, and back. Ideally, comparison measurements with the satellite CloudSat will be performed during the flights. The satellite measures the Atlantic clouds in trajectories crosswise to the flight route. Short flights of HALO in parallel with these satellite trajectories make it possible to verify the satellite?s measurements (Fig. 3): the aircraft flies at a lower altitude than the satellite and can therefore detect the clouds much more accurate.

In total, the air route Oberpfaffenhofen -- Barbados and back should be flown three times in December 2013 ("NARVAL South"). During the second flight, a local flight from Barbados eastward through the trade wind clouds is planned. The aim is to detect clouds that are directly heading for the Barbados cloud observatory and to compare them to the land-based cloud observatory data.

The second part of the mission will be carried out under the direction of the University Hamburg in January ("NARVAL North"). HALO will be based on Iceland to examine the backsides of fronts over the North Atlantic. The amount of precipitation on the backsides of fronts is a controversial topic in science because satellite observations and model calculations provide different results. "Measured values are missing?? because ships do not sail in these typical storm zones" says principal investigator Prof. Felix Ament from the Center for Earth System Research and Sustainability (CEN), University of Hamburg. "A successful HALO mission could provide important facts and eliminate a 'blank spot' on the map of science."

The research aircraft HALO is an initiative by German climate and environmental research institutions. HALO is funded by: Federal Ministry of Education and Research (BMBF), German Research Foundation (DFG), Helmholtz Association, Max Planck Society, Leibniz Association, Free State of Bavaria, Karlsruhe Institute of Technology (KIT), GFZ German Research Centre for Geosciences, Forschungszentrum J?lich and German Aerospace Center (DLR)

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