How does terrain influence the upscale convective growth of orographic deep moist convection?

Satellite observations have revealed that some of the world’s most intense deep convective storms occur near the Sierras de Córdoba, Argentina, South America. A ground-based radar climatology during two austral spring and summer seasons (2015–2017) revealed that most of the storms were multicellular and initiated most frequently during the early afternoon and late evening hours just east of the Sierras de Córdoba. The peak occurrence of these storms was between December-February. Storm environments in Argentina tend to be characterized by larger convective available potential energy and weaker low-level vertical wind shear compared to the United States. One of the more intriguing results is the relatively fast transition, and close proximity to terrain, from first storms to larger mesoscale convective systems compared with locations in the United States. A canonical upscale convective growth case was simulated with the Weather Research and Forecasting model to understand the role of topography in this transition process. This case featured an orographic supercell that transitioned into a bowing mesoscale convective system over three-to-four hours. The simulation revealed enhanced low-level vertical wind shear along the eastern slopes of the Sierras de Córdoba that aided in the formation of a left moving supercell. Shortly thereafter, strong downdrafts and expansion of the cold pool resulted in a rapid transition to a bowing mesoscale convective system. Terrain height sensitivity experiments were conducted with only the control and higher terrain experiments resulting in a supercell-to-bowing mesoscale convective system transition. The control simulation, with the real terrain of the Sierras de Córdoba, resulted in the faster upscale convective growth owing to both terrain-driven environmental and storm-scale effects, such as variations to thermodynamic/kinematic profiles and terrain blocking of cold pools, respectively. Inspired by the aforementioned ground-based radar climatology and in-depth numerical modeling upscale convective growth case study in north central Argentina, a set of different initial terrain height idealized numerical modeling experiments were conducted. These experiments were devised to determine the relative roles of both direct and indirect influences of terrain on upscale convective growth of a supercell in a model configuration similar to those observed near the Sierras de Cόrdoba in Argentina. The experimental results indicated that when the terrain was systematically raised, convection initiation occurred earlier, supercells were wider and more intense, and upscale convective growth generally occurred faster. A direct influence of terrain was blocking of cold pools leading to a deepening of the cold pools that drove surging outflow and more rapid upscale convective growth. Indirect influences of terrain included modifications to the surrounding thermodynamic and kinematic profiles, with terrain-enhancements to the vertical wind shear profile prompting wider updrafts in higher terrain supercells. These wider supercell updrafts were accompanied by greater vertical mass flux, wider and stronger downdrafts, and deeper cold pools, promoting more rapid upscale convective growth

Effect of Mechanical Vibrations on the Negative Pressure of Pure Liquids

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Observations of misovortices within the 7 January 2014 Long Lake-Axis-Parallel lake-effect snow band during the Ontario Winter Lake-effect Systems Project

Recent lake-effect snow field projects in the Eastern Great Lakes region [e.g., the Long Lake-Axis-Parallel (LLAP; 2010-2011) Project and the Ontario Winter Lake-effect Systems (OWLeS; 2013-2014) Project] have revealed the presence of misovortices with diameters between 40 and 4000 m within LLAP bands in the vicinity of Lake Ontario. These misovortices usually develop along convergence boundaries associated with cyclonic horizontal shear zones that likely originate from solenoidally-forced secondary circulations within these warm-core bands. In most cases, the shear zone is co-located with a sharp horizontal gradient in radar reflectivity, which corresponds to the region of maximum vertical velocities. One particular band from the OWLeS Project, in which an abundance of misovortices developed, occurred on 7 January 2014. Steiger et al. (2013) postulated that the release of horizontal shearing instability (HSI) was the main mechanism for misovortexgenesis in two LLAP bands analyzed during the LLAP Project. With data from only a single mobile Doppler radar available, however, this hypothesis was not testable. In the present study, three-dimensional dual-Doppler (DD) wind syntheses reveal that two separate criteria for HSI are met along the low-level horizontal shear zone within this band. This strongly suggests that HSI was the likely cause of the misovortices within this band. Furthermore, the lack of anticyclonic-cyclonic vortex couplets throughout the event reveal that tilting of horizontal vorticity into the vertical is likely of less importance compared to the release of HSI and subsequent vortex strengthening via stretching of low-level vertical vorticity. This vortex stretching is maximized along the low-level convergence zone, where vertical velocities (generally between 1 - 3 m s-1) are greatest owing to the solenoidally-forced transverse secondary circulation of the band. A high-resolution Weather Research and Forecasting (WRF) simulation was conducted for this case study. The 333-m horizontal resolution domain depicts the presence of misovortices along the cyclonic horizontal shear zone within the band. The simulation also reveals that a linear vortex sheet originates in the vicinity of Lake Huron and Georgian Bay, and extends over Lake Ontario. This vortex sheet breaks into discrete vortices over Lake Ontario. The simulated vortices display remarkable similarities to the DD analyses in terms of intensity, depth, spacing, and size. The simulated vortices are persistent features over the length of the lake with lifetimes of well over 30 minutes. Once the vortices reach the eastern shore of Lake Ontario, however, they quickly dissipate. Competing hypotheses related to misovortexgenesis are presented herein and tested to reveal the key kinematic and dynamic properties of the misovortices within this snow band. Additionally, the origin of the vortex sheet is analyzed using the WRF simulation

Origin and impact of initialisation shocks in coupled atmosphere-ocean forecasts

Current methods for initialising coupled atmosphere-ocean forecasts often rely on the use of separate atmosphere and ocean analyses, the combination of which can leave the coupled system imbalanced at the beginning of the forecast, potentially accelerating the development of errors. Using a series of experiments with the European Centre for Medium-range Weather Forecasts coupled system, the magnitude and extent of these so-called initialisation shocks is quantified, and their impact on forecast skill measured. It is found that forecasts initialised by separate ocean and atmospheric analyses do exhibit initialisation shocks in lower atmospheric temperature, when compared to forecasts initialised using a coupled data assimilation method. These shocks result in as much as a doubling of root-mean-square error on the first day of the forecast in some regions, and in increases that are sustained for the duration of the 10-day forecasts performed here. However, the impacts of this choice of initialisation on forecast skill, assessed using independent datasets, were found to be negligible, at least over the limited period studied. Larger initialisation shocks are found to follow a change in either the atmospheric or ocean model component between the analysis and forecast phases: changes in the ocean component can lead to sea surface temperature shocks of more than 0.5K in some equatorial regions during the first day of the forecast. Implications for the development of coupled forecast systems, particularly with respect to coupled data assimilation methods, are discussed

Food resources of stream macroinvertebrates determined by natural-abundance stable C and N isotopes and a 15N tracer addition

Trophic relationships were examined using natural-abundance 13C and 15N analyses and a 15N-tracer addition experiment in Walker Branch, a 1st-order forested stream in eastern Tennessee. In the 15N-tracer addition experiment, we added 15NH4, to stream water over a 6-wk period In early spring, and measured 15N:14N ratios in different taxa and biomass compartments over distance and time. Samples collected from a station upstream from the 15N addition provided data on natural-abundance 13C:12C and 15N:14N ratios. The natural-abundance 15N analysis proved to be of limited value in identifying food resources of macroinvertebrates because 15N values were not greatly different among food resources. In general, the natural-abundance stable isotope approach was most useful for determining whether epilithon or detritus were important food resources for organisms that may use both (e.g., the snail Elimia clavaeformis), and to provide corroborative evidence of food resources of taxa for which the 15N tracer results were not definitive. The 15N tracer results showed that the mayflies Stenonema spp. and Baetis spp. assimilated primarily epilithon, although Baetis appeared to assimilate a portion of the epilithon (e.g., algal cells) with more rapid N turnover than the bulk pool sampled. Although Elimia did not reach isotopic equilibrium during the tracer experiment, application of a N-turnover model to the field data suggested that it assimilated a combination of epilithon and detritus. The amphipod Gammarus minus appeared to depend mostly on fine benthic organic matter (FBOM), and the coleopteran Anchytarsus bicolor on epixylon. The caddisfly Diplectrona modesta appeared to assimilate primarily a fast N-turnover portion of the FBOM pool, and Simuliidae a fast N- turnover component of the suspended particulate organic matter pool rather than the bulk pool sampled. Together, the natural-abundance stable C and N isotope analyses and the experimental 15N tracer approach proved to be very useful tools for identifying food resources in this stream ecosystem

NITROGEN CYCLING IN A FOREST STREAM DETERMINED BY A 15N TRACER ADDITION

Nitrogen uptake and cycling was examined using a six‐week tracer addition of 15N‐labeled ammonium in early spring in Walker Branch, a first‐order deciduous forest stream in eastern Tennessee. Prior to the 15N addition, standing stocks of N were determined for the major biomass compartments. During and after the addition, 15N was measured in water and in dominant biomass compartments upstream and at several locations downstream. Residence time of ammonium in stream water (5–6 min) and ammonium uptake lengths (23–27 m) were short and relatively constant during the addition. Uptake rates of NH4 were more variable, ranging from 22 to 37 μg N·m−2·min−1 and varying directly with changes in streamwater ammonium concentration (2.7–6.7 μg/L). The highest rates of ammonium uptake per unit area were by the liverwort Porella pinnata, decomposing leaves, and fine benthic organic matter (FBOM), although epilithon had the highest N uptake per unit biomass N. Nitrification rates and nitrate uptake lengths and rates were determined by fitting a nitrification/nitrate uptake model to the longitudinal profiles of 15N‐NO3 flux. Nitrification was an important sink for ammonium in stream water, accounting for 19% of the total ammonium uptake rate. Nitrate production via coupled regeneration/nitrification of organic N was about one‐half as large as nitrification of streamwater ammonium. Nitrate uptake lengths were longer and more variable than those for ammonium, ranging from 101 m to infinity. Nitrate uptake rate varied from 0 to 29 μg·m−2·min−1 and was ∼1.6 times greater than assimilatory ammonium uptake rate early in the tracer addition. A sixfold decline in instream gross primary production rate resulting from a sharp decline in light level with leaf emergence had little effect on ammonium uptake rate but reduced nitrate uptake rate by nearly 70%. At the end of the addition, 64–79% of added 15N was accounted for, either in biomass within the 125‐m stream reach (33–48%) or as export of 15N‐NH4 (4%), 15N‐NO3 (23%), and fine particulate organic matter (4%) from the reach. Much of the 15N not accounted for was probably lost downstream as transport of particulate organic N during a storm midway through the experiment or as dissolved organic N produced within the reach. Turnover rates of a large portion of the 15N taken up by biomass compartments were high (0.04–0.08 per day), although a substantial portion of the 15N in Porella (34%), FBOM (21%), and decomposing wood (17%) at the end of the addition was retained 75 d later, indicating relatively long‐term retention of some N taken up from water. In total, our results showed that ammonium retention and nitrification rates were high in Walker Branch, and that the downstream loss of N was primarily as nitrate and was controlled largely by nitrification, assimilatory demand for N, and availability of ammonium to meet that demand. Our results are consistent with recent 15N tracer experiments in N‐deficient forest soils that showed high rates of nitrification and the importance of nitrate uptake in regulating losses of N. Together these studies demonstrate the importance of 15N tracer experiments for improving our understanding of the complex processes controlling N cycling and loss in ecosystems

Homer2 deletion alters dendritic spine morphology but not alcohol-associated adaptations in GluN2B-containing N-methyl-D-aspartate receptors in the nucleus accumbens

Repeated exposure to ethanol followed by withdrawal leads to the alterations in glutamatergic signaling and impaired synaptic plasticity in the nucleus accumbens (NAc) in both clinical and preclinical models of ethanol exposure. Homer2 is a member of a family of postsynaptic density (PSD) scaffolding proteins that functions in part to cluster NMDA signaling complexes in the PSD, and has been shown to be critically important for plasticity in multiple models of drug and alcohol abuse. Here we used Homer2 KO mice and a chronic intermittent intraperitoneal (IP) ethanol injection model to investigate a potential role for the protein in ethanol-induced adaptations in dendritic spine morphology and PSD protein expression. While deletion of Homer2 was associated with increased density of long spines on medium spiny neurons of the NAc core of saline treated mice, ethanol exposure had no effect on dendritic spine morphology in either wild-type (WT) or Homer2 KO mice. Western blot analysis of tissue samples from the NAc enriched for PSD proteins revealed a main effect of ethanol treatment on the expression of GluN2B, but there was no effect of genotype or treatment on the expression other glutamate receptor subunits or PSD95. These data indicate that the global deletion of Homer2 leads to aberrant regulation of dendritic spine morphology in the NAc core that is associated with an increased density of long, thin spines. Unexpectedly, intermittent IP ethanol did not affect spine morphology in either WT or KO mice. Together these data implicate Homer2 in the formation of long, thin spines and further supports its role in neuronal structure

Nutrient dynamics in streams and the role of J-NABS.

Abstract. Nutrient dynamics in streams has been an important topic of research since the 1960s. Here we review this topic and the significant role played by J-NABS in its development. We limit this review almost exclusively to studies of N and P because these elements have been shown to limit productivity in streams. We use the expression nutrient dynamics for studies that included some measures of biological processes occurring within streams. Prior to the 1970s, instream biological processes were little studied, but through 1985 conceptual advances were made, and 4 types of studies made important contributions to our understanding of instream processes: 1) evidence of increased plant production and decomposition in response to nutrient addition, 2) studies showing a downstream decrease in nutrient concentrations, 3) studies using radioisotopes, and 4) budget studies. Beginning with the first paper printed in its first issue, J-NABS has been the outlet for key papers advancing our understanding of rates and controls of nutrient dynamics in streams. In the first few years, an important review and a conceptual model for conducting experiments to study nutrient dynamics in streams were published in J-NABS. In the 1990s, J-NABS published a number of papers on nutrient recycling within algal communities, the role of the hyporheic zone, the role of spawning fish, and the coupling of data from field 15 N additions and a N-cycling model to provide a synoptic view of N dynamics in streams. Since 2000, J-NABS has published influential studies on nutrient criteria for streams, rates of and controls on nitrification and denitrification, uptake of stream nutrients by riparian vegetation, and nutrient dynamics in urban streams. Nutrient dynamics will certainly continue to be an important topic in J-NABS. Topics needing further study include techniques for studying nutrient dynamics, nutrient dynamics in larger streams and rivers, the ultimate fate of nutrients taken up by plants and microbes in streams, ecological stoichiometry, the effects of climate change, and the role of streams and rivers in nutrient transformation and retention at the landscape scale