by Staff Writers
Los Angeles CA (SPX) Aug 09, 2016
To address the recent drought in California, policymakers have created incentives for homeowners to replace existing lawns with drought tolerant vegetation.
However, new research from George Ban-Weiss, an assistant professor in the Astani Department of Civil and Environmental Engineering at the USC Viterbi School of Engineering, has found that these efforts might have some hidden consequences on local climate.
Ban-Weiss and post-doctoral scholar Pouya Vahmani used a model of the Los Angeles basin to investigate the climate impacts of widespread adoption of drought tolerant vegetation.
Their findings, put forth in the article "Climatic Consequences of Adopting Drought Tolerant Vegetation over Los Angeles as a Response to the California Drought" in the journal Geophysical Research Letters, indicate that in fact, if all lawns were replaced with drought tolerant vegetation, that Angelenos could expect an average daytime warming of 1.3 degrees Fahrenheit due largely to decreased evaporative cooling, as irrigation is stopped.
For the hottest regions of the Los Angeles basin, such as the inland empire and San Fernando valley, the researchers predict a daytime increase in temperature of 3.4 degrees Fahrenheit. Such temperature increases could exacerbate heatwaves, increase photochemical smog production, and increase air conditioning energy use.
However, one effect of widespread planting of drought tolerant vegetation-- which the researchers believe could counteract these higher daytime temperatures-- is an even greater decrease in nighttime temperatures. The researchers forecast that the average nighttime temperature decrease could be as much as 6 degrees Fahrenheit.
Lower nighttime temperatures are important for preventing adverse human health consequences like heat stroke or even death during heat waves, says Ban-Weiss. People, especially vulnerable populations like the elderly, need temperatures to reduce sufficiently at night to allow their bodies to recover from high daytime temperatures and prevent heat-related illness.
"Our interest in this topic was initially piqued because we hypothesized that the reductions in irrigation associated with adopting drought-tolerant vegetation would cause temperature increases," says Ban-Weiss.
"We were surprised to find the reduced temperature signal at nighttime. But this actually has a simple physical explanation, since reducing soil moisture decreases upward heat fluxes from the sub-surface to the surface at night, subsequently reducing surface temperatures."
"Our research highlights how water and climate are intimately coupled," says Ban-Weiss. "You can't change one without effecting the other."
Clarifying the fusion plasma confinement improvement mechanism
Clarifying the fusion plasma confinement improvement mechanism
The right side of the figure where the electric field's absolute value is small corresponds to the L-mode plasma and the left side having large electric field to the H-mode. The black line indicates the experimental value of the electric current and the red line the theoretical value used in the model based upon differences in the trajectories. Image courtesy Tatsuya Kobayashi. For a larger version of this image please go here.
National Institutes of Natural Sciences
The key to solving this problem was accidentally discovered on a German experimental device in 1982. There, turbulence in the edge region was suppressed, and the plasma state called the "H-mode"*2), in which the temperature of the entire plasma was raised, was realized. In contrast to this, plasma in which turbulence is great and the temperature is low is called "L-mode" plasma. Subsequently, H-mode plasma was reproduced in devices around the world. H-mode is used as the standard operation mode in the ITER.
In research to date, many researchers have attempted to clarify the H-mode mechanism. In the normal condition in which there is a balance of ions and electrons, there cannot be a strong electric field in a plasma. This condition corresponds to the L-mode. According to theory, in the H-mode, due to a small deviation in the distribution of ions and electrons, in the plasma's edge region a strong electric field is generated, and suppression of the turbulence was predicted.
Subsequently, using the most advanced diagnostics at that time, the structure of the electric field that theoretical researchers had predicted was shown to actually exist. How does this structure of the electric field form was the remaining riddle. Regarding this riddle, research has been advancing since the discovery of the H-mode, and this issue was not solved for more than thirty years, until now.
Here, the research group of professors Tatsuya Kobayashi, Kimitaka Itoh, and Takeshi Ido of NIFS engaged in collaborative research with the National Institutes for Quantum and Radiological Science and Technology (QST) and Kyushu University. Using the "Heavy Ion Beam Probe" developed at NIFS they measured the plasma potential of the QST's JFT-2M tokamak. Resulting from their analysis of experimental data they discovered the electric field generation mechanism which had been a riddle for the past thirty years.
The "deviation" in the distribution of ions and electrons producing the strong electric field is made by the electric current that flows in the radial direction. The mechanism that produces this electric current has been proposed numerous times, but which effect was especially important had not yet been clarified.
Through their measurements and analysis, they found that the effects born from the differences in the trajectories of electrons and ions play a particularly important role in the generation of the electric current.
Although the experimental data used in these experiments were collected in 1999, they produced cutting edge results from recent theoretical physics and developments in analytical methods. That results from seventeen years earlier have contributed to advances in plasma physics today indicates the high quality of this experimental data.
These research results are widely available in Scientific Reports (online edition), an academic science journal published in Great Britain by the Nature Publishing Group. The article is dated August 4, 2016, and is widely available.
Figure 1 shows the dependence of the electric current in the radial direction (Jr) on the electric field (Er). The right side of the figure where the electric field's absolute value is small corresponds to the L-mode plasma and the left side having large electric field to the H-mode.
The black line indicates the experimental value of the electric current and the red line the theoretical value used in the model based upon differences in the trajectories. In this experiment, when the plasma changes from L-mode to H-mode the theoretical value well matches the experimental value. From this, the mechanism by which the electric current is generated becomes clear. However, the electric current value of the L-mode indicates a large difference between the theoretical value and the experimental value. In order to clarify the cause of this, further research is necessary.
Figure 2 shows the temperature profiles of the L-mode plasma and the H-mode plasma. In the H-mode, the temperature increased in the edge region, and in the whole plasma a high temperature was achieved.
Explanation of Terminology:
*2) H-mode is the high-confinement mode that means "a good confinement condition," and was discovered in the German experimental device ASDEX. When a plasma is not in H-mode (L-mode stands for low confinement mode), even when power for heating the plasma is increased, it is difficult to raise the temperature because of the existence of turbulence, and conditions for achieving fusion cannot be met.
On the other hand, when a plasma is in H-mode because of suppressing turbulence in the plasma's edge region by the electric field the temperature throughout the plasma can be raised. Thus, the H-mode, as a key for achieving success with the future fusion reactor, these principles and applications are being researched around the world.
University of Southern California
Climate Science News - Modeling, Mitigation Adaptation
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