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Raesly 1993.pdf
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Rain on Snow: Little Understood Killer in the North
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n October 2003, a severe rain-on-snow (ROS) event killed approximately 20,000 musk-oxen (Figure 1) on Banks Island, which is the westernmost of the Canadian Arctic islands (approximately 380 kilome- ters by 290 kilometers in size). The event reduced the isolated herd by 25% and sig- nificantly affected the people dependent on the herd’s well-being. Because of the sparsity of weather stations in the Arctic and the lack of routinely deployed weather equipment that was capable of accurately sensing the ROS event, its detection largely was based on reports from hunters who were in the affected areas at the time.Such events can significantly alter a fro- zen ecosystem—with changes that often persist for the remainder of a winter—by creating ice layers at the surface of, within, or below the snowpack. The water and ice layers are known to facilitate the growth of toxic fungi, significantly warm the soil surface under thick snowpack, and deter large grazing mammals.
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RAIN-ON-SNOW EVENTS IN THE WESTERN UNITED STATES
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Severity of rain on snow depends on a number of factors, and an overall decrease in these events appears to be driven, in part, by changes in El Niño–Southern Oscillation.
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rainfall preceded by air passage over forests
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Vegetation affects precipitation patterns by mediating moisture, energy and trace-gas fluxes between the surface and atmosphere1. When forests are replaced by pasture or crops, evapotranspiration of moisture from soil and vegetation is often diminished, leading to reduced atmospheric humidity and potentially suppressing precipitation2,3. Climate models predict that large-scale tropical deforestation causes reduced regional precipitation4–10, although the magnitude of the effect is model9,11 and resolution8 dependent. In contrast, observational studies have linked deforestation to increased precipitation locally12–14 but have been unable to explore the impact of large-scale deforestation. Here we use satellite remote-sensing data of tropical precipitation and vegetation, combined with simulated atmospheric transport patterns, to assess the pan-tropical effect of forests on tropical rainfall. We find that for more than 60 per cent of the tropical land surface (latitudes 30 degrees south to 30 degrees north), air that has passed over extens- ive vegetation in the preceding few days produces at least twice as much rain as air that has passed over little vegetation. We demonstrate that this empirical correlation is consistent with evapotranspiration maintaining atmospheric moisture in air that
passes over extensive vegetation. We combine these empirical rela- tionships with current trends of Amazonian deforestation to estimate reductions of 12 and 21 per cent in wet-season and dry- season precipitation respectively across the Amazon basin by 2050, due to less-efficient moisture recycling. Our observation-based results complement similar estimates from climate models4–10, in which the physical mechanisms and feedbacks at work could be explored in more detail.
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Raithel Hartenstine 2006.pdf
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Raleigh 1973.pdf
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Raley et al 2006.pdf
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Ramification of stream networks
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The geometric complexity of stream networks has been a source of fascination for centuries. However, a comprehensive understanding of ramification—the mechanism of branching by which such networks grow—remains elusive. Here we show that streams incised by groundwater seepage branch at a characteristic angle of 2π/5 = 72°. Our theory represents streams as a collection of paths growing and bifurcating in a diffusing field. Our observations of nearly 5,000 bifurcated streams growing in a 100 km2 groundwater field on the Florida Panhandle yield a mean bifurcation angle of 71.9° ± 0.8°. This good accord between theory and observation suggests that the network geometry is determined by the external flow field but not, as classical theories imply, by the flow within the streams themselves.
river networks | network growth | Laplacian growth
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Rand Wiles 1982.pdf
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Rapid deposition of oxidized biogenic compounds to a temperate forest
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We report fluxes and dry deposition velocities for 16 atmospheric compounds above a southeastern United States forest, including: hydrogen peroxide (H2O2), nitric acid (HNO3), hydrogen cyanide (HCN), hydroxymethyl hydroperoxide, peroxyacetic acid, organic hydroxy nitrates, and other multifunctional species derived from the oxidation of isoprene and monoterpenes. The data suggest that dry deposition is the dominant daytime sink for small, satu- rated oxygenates. Greater than 6 wt %C emitted as isoprene by the forest was returned by dry deposition of its oxidized products. Peroxides account for a large fraction of the oxidant flux, possibly eclipsing ozone in more pristine regions. The measured organic nitrates comprise a sizable portion (15%) of the oxidized nitrogen input into the canopy, with HNO3 making up the balance. We ob- serve that water-soluble compounds (e.g., strong acids and hydro- peroxides) deposit with low surface resistance whereas compounds with moderate solubility (e.g., organic nitrates and hydroxycarbon- yls) or poor solubility (e.g., HCN) exhibited reduced uptake at the surface of plants. To first order, the relative deposition velocities of water-soluble compounds are constrained by their molecular diffu- sivity. From resistance modeling, we infer a substantial emission flux of formic acid at the canopy level (∼1 nmol m−2·s−1). GEOS−Chem, a widely used atmospheric chemical transport model, currently under- estimates dry deposition for most molecules studied in this work. Reconciling GEOS−Chem deposition velocities with observations resulted in up to a 45% decrease in the simulated surface con- centration of trace gases.
biosphere−atmosphere exchange | isoprene | dry deposition | OVOCs | fluxes
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