If lake ice is assumed to deform and fail as a linear viscoelastic material under the action of wind stress, then a simple ice concentration scaling law can be constructed suitable for ...one‐dimensional lake models embedded within environmental prediction systems. Most 1‐D lake models assume no ice mechanics at all, while others adapt the viscous‐plastic rheology common in ice‐ocean models for the purpose of estimating ice fraction. Elastic buckling is generally disregarded as a significant failure mechanism in ice under low stress conditions at geophysical scales. However, by adding viscosity to the constitutive equation, the conditions for viscoelastic buckling seem quite plausible over a wide range of lake size and ice thickness. An ice concentration scaling law based on this process is evaluated here in multiannual simulations over North America and found to produce superior ice phenology statistics compared with simulations based on plastic failure or no ice mechanics.
Plain Language Summary
Most mid‐ and high‐latitude lakes experience periods of partial ice cover (i.e., ice concentration < 100%) during early winter. While very small lakes might freeze solid in a single night under calm conditions, larger lakes may take days or weeks to completely freeze because wind stress continually breaks the ice cover resulting in patches of open water. The extent of wintertime open water is very important for both lake ecology and for regional weather conditions (e.g., lake‐effect snowstorms). Many weather and climate models employ one‐dimensional lake models that do not represent fractional ice cover at all, or parameterize it based on mechanical ideas from sea ice models, resulting in poor timing and duration of simulated ice cover. Here we propose a new scheme based on different mechanics that improves these simulated features.
Key Points
Most one‐dimensional lake models neglect ice mechanics or assume plastic failure, and frequently perform poorly regarding ice phenology
Assuming lake ice fails as a linear viscoelastic material at geophysical scales leads to a simple parameterization of ice concentration
The new scheme outperforms plastic failure or the absence of mechanics with respect to ice—on and ice duration
From 2011 to 2013, Texas experienced its worst drought in recorded history. This event provided a unique natural experiment to assess species‐specific responses to extreme drought and mortality of ...four co‐occurring woody species: Quercus fusiformis, Diospyros texana, Prosopis glandulosa, and Juniperus ashei. We examined hypothesized mechanisms that could promote these species' diverse mortality patterns using postdrought measurements on surviving trees coupled to retrospective process modelling. The species exhibited a wide range of gas exchange responses, hydraulic strategies, and mortality rates. Multiple proposed indices of mortality mechanisms were inconsistent with the observed mortality patterns across species, including measures of the degree of iso/anisohydry, photosynthesis, carbohydrate depletion, and hydraulic safety margins. Large losses of spring and summer whole‐tree conductance (driven by belowground losses of conductance) and shallower rooting depths were associated with species that exhibited greater mortality. Based on this retrospective analysis, we suggest that species more vulnerable to drought were more likely to have succumbed to hydraulic failure belowground.
The 2011–2013 drought in Texas was possibly the most severe drought in that area in over 1,000 years. We found, counterintuitively, that species that had the most embolism‐resistant xylem had the greatest mortality. We also found that belowground hydraulic failure was the most likely cause of mortality. Several previously used metrics for predicting mortality were not consistent with mortality in our study. This work has strong implications for previous work that has used branch resistance to hydraulic dysfunction as a proxy for the drought tolerance of entire plants.
Stomatal regulation presumably evolved to optimize CO2 for H2O exchange in response to changing conditions. If the optimization criterion can be readily measured or calculated, then stomatal ...responses can be efficiently modelled without recourse to empirical models or underlying mechanism. Previous efforts have been challenged by the lack of a transparent index for the cost of losing water. Yet it is accepted that stomata control water loss to avoid excessive loss of hydraulic conductance from cavitation and soil drying. Proximity to hydraulic failure and desiccation can represent the cost of water loss. If at any given instant, the stomatal aperture adjusts to maximize the instantaneous difference between photosynthetic gain and hydraulic cost, then a model can predict the trajectory of stomatal responses to changes in environment across time. Results of this optimization model are consistent with the widely used Ball–Berry–Leuning empirical model (r2 > 0.99) across a wide range of vapour pressure deficits and ambient CO2 concentrations for wet soil. The advantage of the optimization approach is the absence of empirical coefficients, applicability to dry as well as wet soil and prediction of plant hydraulic status along with gas exchange.
Current land surface models struggle to represent the complex and species‐specific manner by which stomata respond to environmental cues, especially soil drought. This paper offers a solution to this problem by assuming that the goal of stomatal regulation is to maximize photosynthetic gain minus hydraulic cost. A trait‐ and process‐based ‘profit‐maximizing’ algorithm predicts realistic stomatal behaviour in response to the gamut of environmental stimuli. This new approach to stomatal optimization theory may prove useful in large‐scale modelling of responses to climate change.
Observations and models of solar prominences are reviewed. We focus on non-eruptive prominences, and describe recent progress in four areas of prominence research: (1) magnetic structure deduced from ...observations and models, (2) the dynamics of prominence plasmas (formation and flows), (3) Magneto-hydrodynamic (MHD) waves in prominences and (4) the formation and large-scale patterns of the filament channels in which prominences are located. Finally, several outstanding issues in prominence research are discussed, along with observations and models required to resolve them.
As continued COVID-19 disruption looks likely across the world, perhaps until 2021, contingency plans are evolving in case of further disruption in the 2020-2021 academic year. This includes ...delivering face-to-face programs fully online for at least part of the upcoming academic year for new and continuing cohorts. This temporary pivot will necessitate distance teaching and learning across almost every conceivable pedagogy, from fundamental degrees to professionally accredited ones. Each institution, program, and course will have its own myriad of individualized needs; however, there is a common question that unites us all: how do we provide teaching and assessment to students in a manner that is accessible, fair, equitable, and provides the best learning whilst acknowledging the temporary nature of the pivot? No "one size fits all" solution exists, and many of the choices that need to be made will be far from simple; however, this paper provides a starting point and basic principles to facilitate discussions taking place around the globe by balancing what we know from the pedagogy of online learning with the practicalities imposed by this crisis and any future crises.
Celotno besedilo
Dostopno za:
DOBA, IZUM, KILJ, NUK, PILJ, PNG, SAZU, SIK, UILJ, UKNU, UL, UM, UPUK
Smoking is an important risk factor for peripheral arterial disease (PAD).
To undertake a meta-analysis of the association between cigarette smoking and PAD.
Medline, Embase, PubMed and Web of ...Science databases.
Relevant studies published up to 30 April 2012 were identified.
Performed by two researchers. Overall and stratified random effects meta-analyses, cumulative meta-analyses and metaregression analyses were conducted. Heterogeneity was tested using the I(2) test, and publication and small study bias using funnel plots and Egger's test.
Fifty-five eligible studies were identified: 43 cross-sectional, 10 cohort and 2 case-control. Of the 68 results for current smokers, 59 (86.8%) were statistically significant and the pooled OR was 2.71 (95% CI 2.28 to 3.21). There was a high level of heterogeneity (I(2) 94.9%, p<0.001) and Egger's test was significant (p=0.023). The association with smoking was significant among both general (OR 3.08, 95% CI 2.56 to 3.69) and disease populations (OR 1.54, 95% CI 1.31 to 1.83). Of the 40 results for ex-smokers, 29 (72.5%) were statistically significant and the pooled OR was 1.67 (95% CI 1.54 to 1.81). There was moderate heterogeneity (I(2) 54.7%, p<0.001) and Egger's test was significant (p<0.001).
There is now substantial evidence of an association between active smoking and PAD. The magnitude of the association is greater than that reported for coronary heart disease. The risk is lower among ex-smokers but, nonetheless, significantly increased compared with never smokers. The results highlight the need for interventions both to encourage quitting among existing smokers and discourage commencement among never smokers.
Ecosystem models have difficulty predicting plant drought responses, partially from uncertainty in the stomatal response to water deficits in soil and atmosphere. We evaluate a ‘supply–demand’ theory ...for water-limited stomatal behavior that avoids the typical scaffold of empirical response functions. The premise is that canopy water demand is regulated in proportion to threat to supply posed by xylem cavitation and soil drying.
The theory was implemented in a trait-based soil–plant–atmosphere model. The model predicted canopy transpiration (E), canopy diffusive conductance (G), and canopy xylem pressure (P
canopy) from soil water potential (P
soil) and vapor pressure deficit (D).
Modeled responses to D and P
soil were consistent with empirical response functions, but controlling parameters were hydraulic traits rather than coefficients. Maximum hydraulic and diffusive conductances and vulnerability to loss in hydraulic conductance dictated stomatal sensitivity and hence the iso- to anisohydric spectrum of regulation. The model matched wide fluctuations in G and P
canopy across nine data sets from seasonally dry tropical forest and piñon–juniper woodland with < 26% mean error.
Promising initial performance suggests the theory could be useful in improving ecosystem models. Better understanding of the variation in hydraulic properties along the root–stem–leaf continuum will simplify parameterization.
Context.
Direct magnetic field measurements in solar prominences occur infrequently and are difficult to make and interpret. As a consequence, alternative methods are needed to derive the main ...properties of the magnetic field that supports the prominence mass. This is important for our understanding of solar prominences, but also for understanding how eruptive prominences may affect space weather.
Aims.
We present the first direct comparison of the magnetic field strength derived from spectro-polarimetric observations of a solar prominence, with corresponding results from a theoretical flux rope model constructed from on-disc normal component magnetograms.
Methods.
We first used spectro-polarimetric observations of a prominence obtained with the magnetograph THEMIS operating in the Canary Islands to derive the magnetic field of the observed prominence by inverting the Stokes parameters measured in the He D3 line. Next, we constructed two data-constrained non-linear force-free field (NLFFF) models of the same prominence. In one model we assumed a strongly twisted flux rope solution, and in the other a weakly twisted flux rope solution.
Results.
The physical extent of the prominence at the limb (height and length) is best reproduced with the strongly twisted flux rope solution. The line-of-sight average of the magnetic field for the strongly twisted solution results in a magnetic field that has a magnitude of within a factor of 1−2 of the observed magnetic field strength. For the peak field strength along the line of sight, an agreement to within 20% of the observations is obtained for the strongly twisted solution. The weakly twisted solution produces significantly lower magnetic field strengths and gives a poor agreement with the observations.
Conclusions.
The results of this first comparison are promising. We found that the flux rope insertion method of producing a NLFFF is able to deduce the overall properties of the magnetic field in an observed prominence.
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