Wednesday, December 30, 2009

POLLUTION ALTERS ISOLATED THUNDERSTORMS

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New climate research reveals how wind shear — the same atmospheric conditions that cause bumpy airplane rides — affects how pollution contributes to isolated thunderstorm clouds. Under strong wind shear conditions, pollution hampers thunderhead formation. But with weak wind shear, pollution does the opposite and makes storms stronger.

The work improves climate scientists' understanding of how aerosols — tiny unseen particles that make up pollution — contribute to isolated thunderstorms and the climate cycle. How aerosols and clouds interact is one of the least understood aspects of climate, and this work allows researchers to better model clouds and precipitation.

"This finding may provide some guidelines on how man-made aerosols affect the local climate and precipitation, especially for the places where 'afternoon showers' happen frequently and affect the weather system and hydrological cycle," said atmospheric scientist Jiwen Fan of the Department of Energy's Pacific Northwest National Laboratory. "Aerosols in the air change the cloud properties, but the changes vary from case to case. With detailed cloud modeling, we found an important factor regulating how aerosols change storms and precipitation."

Fan discussed her results Thursday, December 17 at the 2009 American Geophysical Union meeting. Her study uses data from skies over Australia and China.

The results provide insight into how to incorporate these types of clouds and conditions into computational climate models to improve their accuracy.

Deep convective clouds reflect a lot of the sun's energy back into space and return water that has evaporated back to the surface as rain, making them an important part of the climate cycle. The clouds form as lower air rises upwards in a process called convection. The updrafts carry aerosols that can seed cloud droplets, building a storm.

Previous studies produced conflicting results in how aerosols from pollution affect storm development. For example, in some cases, more pollution leads to stronger storms, while in others, less pollution does. Fan and her colleagues used computer simulations to tease out what was going on. Of concern was a weather phenomenon known as wind shear, where horizontal wind speed and direction vary at different heights. Wind shear can be found near weather fronts and is known to influence storms.

The team ran a computer model with atmospheric data collected in northern Australia and eastern China. They simulated the development of eight deep convective clouds by varying the concentration of aerosols, wind shear, and humidity. Then they examined updraft speed and precipitation.

In the first simulations, the team found that in scenarios containing strong wind shear, more pollution curbed convection. When wind shear was weak, more pollution produced a stronger storm. But convection also changed depending on humidity, so the team wanted to see which effect — wind shear or humidity — was more important.

The team took a closer look at two cloud-forming scenarios: one that ended up with the strongest enhancement in updraft speed and one with the weakest. For each scenario, they created a humid and a dry condition, as well as a strong and weak wind shear condition. The trend in the different conditions indicated that wind shear had a much greater effect on updraft strength than humidity.

When the team measured the expected rainfall, they found that the pattern of rainfall followed the pattern of updraft speed. That is, with strong wind shear, more pollution led to less rainfall. When wind shear was weak, more pollution created stronger storms and more rain — up to a certain point. Beyond a peak level in weak wind shear conditions, pollution led to decreased storm development.

Additional analyses described the physics underlying these results. Water condensing onto aerosol particles releases heat, which contributes to convection and increases updraft speed. The evaporation of water from the cloud droplets cools the air, which reduces the updrafts. In strong wind shear conditions, the cooling effect is always larger than the heating effect, leading to a reduction in updraft speed.

(Photo: UCAR/Carlyle Calvin)

Pacific Northwest National Laboratory

BLACK HOLES IN STAR CLUSTERS STIR UP TIME AND SPACE

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Within a decade scientists could be able to detect the merger of tens of pairs of black holes every year, according to a team of astronomers at the University of Bonn’s Argelander-Institut fuer Astronomie, who publish their findings in a paper in Monthly Notices of the Royal Astronomical Society. By modelling the behaviour of stars in clusters, the Bonn team find that they are ideal environments for black holes to coalesce. These merger events produce ripples in time and space (gravitational waves) that could be detected by instruments from as early as 2015.

Clusters of stars are found throughout our own and other galaxies and most stars are thought to have formed in them. The smallest looser ‘open clusters’ have only a few stellar members, whilst the largest tightly bound ‘globular clusters’ have as many as several million stars. The highest mass stars in clusters use up their hydrogen fuel relatively quickly (in just a few million years). The cores of these stars collapse, leading to a violent supernova explosion where the outer layers of the star are expelled into space. The explosion leaves behind a stellar remnant with gravitational field so strong that not even light can escape – a black hole.

When stars are as close together as they are in clusters, then although still rare events, the likelihood of collisions and mergers between stars of all types, including black holes, is much higher. The black holes sink to the centre of the cluster, where a core that is completely made of up of black holes forms. In the core, the black holes experience a range of interactions, sometimes forming binary pairs and sometimes being ejected from the cluster completely.

Now Dr Sambaran Banerjee, Alexander von Humboldt postdoctoral fellow, has worked with his University of Bonn colleagues Dr Holger Baumgardt and Professor Pavel Kroupa to develop the first self-consistent simulation of the movement of black holes in star clusters.

The scientists assembled their own star clusters on a high-performance supercomputer, and then calculated how they would evolve by tracing the motion of each and every star and black hole within them.

According to a key prediction of Einstein’s General Theory of Relativity, black hole binaries stir the space-time around them, generating waves that propagate away like ripples on the surface of a lake. These waves of curvature in space-time are known as gravitational waves and will temporarily distort any object they pass through. But to date no-one has succeeded in detecting them.

In the cores of stars clusters, black hole binaries are sufficiently tightly bound to be significant sources of gravitational waves. If the black holes in a binary system merge, then an even stronger pulse of gravitational waves radiates away from the system.

Based on the new results, the next generation of gravitational wave observatories like the Advanced Laser Interferometer Gravitational-wave Observatory (Advanced LIGO) could detect tens of these events each year, out to a distance of almost 5000 million light years (for comparison the well known Andromeda Galaxy is just 2.5 million light years away).

Advanced LIGO will be up and running by 2015 and if the Bonn team are right, from then on we can look forward to a new era of gravitational wave astronomy.

Sambaran comments, “Physicists have looked for gravitational waves for more than half a century. But up to now they have proved elusive. If we are right then not only will gravitational waves be found so that General Relativity passes a key test but astronomers will soon have a completely new way to study the Universe. It seems fitting that almost exactly 100 years after Einstein published his theory, scientists should be able to use this exotic phenomenon to watch some of the most exotic events in the cosmos.”

(Photo: LIGO Scientific Collaboration (LSC) / NASA)

Royal Astronomical Society

PRE-ERUPTION EARTHQUAKES OFFER CLUES TO VOLCANO FORECASTERS

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Like an angry dog, a volcano growls before it bites, shaking the ground and getting "noisy" before erupting. This activity gives scientists an opportunity to study the tumult beneath a volcano and may help them improve the accuracy of eruption forecasts, according to Emily Brodsky, an associate professor of Earth and planetary sciences at the University of California, Santa Cruz.

Brodsky presented recent findings on pre-eruption earthquakes on Wednesday, December 16, at the fall meeting of the American Geophysical Union in San Francisco.

Each volcano has its own personality. Some rumble consistently, while others stop and start. Some rumble and erupt the same day, while others take months, and some never do erupt. Brodsky is trying to find the rules behind these personalities.

"Volcanoes almost always make some noise before they erupt, but they don't erupt every time they make noise," she said. "One of the big challenges of a volcano observatory is how to handle all the false alarms."

Brodsky and Luigi Passarelli, a visiting graduate student from the University of Bologna, compiled data on the length of pre-eruption earthquakes, time between eruptions, and the silica content of lava from 54 volcanic eruptions over a 60-year span. They found that the length of a volcano's "run-up"--the time between the onset of earthquakes and an eruption--increases the longer a volcano has been dormant or "in repose." Furthermore, the underlying magma is more viscous or gummy in volcanoes with long run-up and repose times.

Scientists can use these relationships to estimate how soon a rumbling volcano might erupt. A volcano with frequent eruptions over time, for instance, provides little warning before it blows. The findings can also help scientists decide how long they should stay on alert after a volcano starts rumbling.

"You can say, 'My volcano is acting up today, so I'd better issue an alert and keep that alert open for 100 days or 10 days, based on what I think the chemistry of the system is,' " Brodsky said.

Volcano observers are well-versed in the peculiarities of their systems and often issue alerts to match, according to Brodsky. But this study is the first to take those observations and stretch them across all volcanoes, she said.

"The innovation of this study is trying to stitch together those empirical rules with the underlying physics and find some sort of generality," Brodsky said.

The underlying physics all lead back to magma, she said. When the pressure in a chamber builds high enough, the magma pushes its way to the volcano's mouth and erupts. The speed of this ascent depends on how viscous the magma is, which depends in turn on the amount of silica in the magma. The less silica, the runnier the magma. The runnier the magma, the quicker the volcanic chamber fills and the quicker it will spew, according to Brodsky.

The path from chamber to surface isn't easy for magma as it forces its way up through the crust. The jostling of subsurface rock causes pre-eruption tremors, which oscillate in length and severity based on how freely the magma can move.

"If the magma's very sticky, then it takes a long time both to recharge the chamber and to push its way to the surface," Brodsky said. "It extends the length of precursory activity."

Thick magma is the culprit behind the world's most explosive eruptions, because it traps gas and builds pressure like a keg, she said. Mount St. Helens is an example of a volcano fed by viscous magma.

Brodsky and Passarelli diagrammed the dynamics of magma flow using a simple analytical model of fluids moving through channels. The next step, Brodsky said, is to test the accuracy of their predictions on future eruptions.

Volcanoes are messy systems, however, with wildly varying structures and mineral ingredients. Observatories will likely have to tweak their predictions based on the unique characteristics of each system, she said.

(Photo: Cyrus Read, AVO/USGS)

University of California, Santa Cruz

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