Formation of intrathermocline eddies at ocean fronts by wind-driven destruction of potential vorticity

A mechanism for the generation of intrathermocline eddies (ITEs) at wind-forced fronts is examined using a high resolution numerical simulation. Favorable conditions for ITE formation result at fronts forced by “down-front” winds, i.e. winds blowing in the direction of the frontal jet. Down-front winds exert frictional forces that reduce the potential vorticity (PV) within the surface boundary in the frontal outcrop, providing a source for the low-PV water that is the materia prima of ITEs. Meandering of the front drives vertical motions that subduct the low-PV water into the pycnocline, pooling it into the coherent anticyclonic vortex of a submesoscale ITE. As the fluid is subducted along the outcropping frontal isopycnal, the low-PV water, which at the surface is associated with strongly baroclinic flow, re-expresses itself as water with nearly zero absolute vorticity. This generation of strong anticyclonic vorticity results from the tilting of the horizontal vorticity of the frontal jet, not from vortex squashing. During the formation of the ITE, high-PV water from the pycnocline is upwelled alongside the subducting low-PV surface water. The positive correlation between the ITE’s velocity and PV fields results in an upward, along-isopycnal eddy PV flux that scales with the surface frictional PV flux driven by the wind. The relationship between the eddy and wind-induced frictional PV flux is nonlocal in time, as the eddy PV flux persists long after the wind forcing is shut off. The ITE’s PV flux affects the large-scale flow by driving an eddy-induced transport or bolus velocity down the outcropping isopycnal layer with a magnitude that scales with the Ekman velocity.     

Summer temperature variability across four urban neighborhoods in Knoxville,Tennessee, USA

The urban heat island (UHI) is a well-documented effect of urbanization on local climate, identified by higher temperatures compared to surrounding areas, especially at night and during the warm season. The details of a UHI are city-specific, and microclimates may even exist within a given city. Thus, investigating the spatiotemporal variability of a city’s UHI is an ongoing and critical research need. We deploy ten weather stations across Knoxville, Tennessee, to analyze the city’s UHI and its differential impacts across urban neighborhoods: two each in four neighborhoods, one in more dense tree cover and one in less dense tree cover, and one each in downtown Knoxville and Ijams Nature Center that serve as control locations. Three months of temperature data (beginning 2 July 2014) are analyzed using paired-sample t tests and a three-way analysis of variance. Major findings include the following: (1) Within a given neighborhood, tree cover helps negate daytime heat (resulting in up to 1.19 ^°C lower maximum temperature), but does not have as large of an influence on minimum temperature; (2) largest temperature differences between neighborhoods occur during the day (0.38–1.16 ^°C difference), but larger differences between neighborhoods and the downtown control occur at night (1.04–1.88 ^°C difference); (3) presiding weather (i.e., air mass type) has a significant, consistent impact on the temperature in a given city, and lacks the differential impacts found at a larger-scale in previous studies; (4) distance from city center does not impact temperature as much as land use factors. This is a preliminary step towards informing local planning with a scientific understanding of how mitigation strategies may help minimize the UHI and reduce the effects of extreme weather on public health and well-being.     

Estimating Sea-Level Allowances for Atlantic Canada using the Fifth Assessment Report of the IPCC

Abstract

Sea-level allowances at 22 tide-gauge sites along the east coast of Canada are determined based on projections of regional sea-level rise for the Representative Concentration Pathway 8.5 (RCP8.5) from the Fifth Assessment Report of the Intergovernmental Panel on Climate Change (IPCC AR5) and on the statistics of historical tides and storm surges (storm tides). The allowances, which may be used for coastal infrastructure planning, increase with time during the twenty-first century through a combination of mean sea-level rise and the increased uncertainty of future projections with time. The allowances show significant spatial variation, mainly a consequence of strong regionally varying relative sea-level change as a result of glacial isostatic adjustment (GIA). A methodology is described for replacement of the GIA component of the AR5 projection with global positioning system (GPS) measurements of vertical crustal motion; this significantly decreases allowances in regions where the uncertainty of the GIA models is large. For RCP8.5 with GPS data incorporated and for the 1995–2100 period, the sea-level allowances range from about 0.5?m along the north shore of the Gulf of St. Lawrence to more than 1?m along the coast of Nova Scotia and southern Newfoundland.     

A New Tethered Balloon-Borne Payload for Fine-Scale Observations in the Cloudy Boundary Layer

A new scientific payload is introduced for fine-scale measurements of meteorological (wind vector, static air temperature, humidity, and air pressure) and microphysical (aerosol particles and cloud droplets) properties, suspended below a tethered balloon. The high resolution sensors and the tethered balloon are described. Measurements in a lifted fog layer from a first field campaign are presented.The detailed investigation of the fog/haze and the temperature inversion layer demonstrates the damping influence of the fog on temperature fluctuations, while thewind fluctuations are significantly decreased by theevolving temperature inversion, whichwas about 30 m above the fog layer.From spectral analysis the noise floors of the high-resolution sensors are determined to10^-6 kg m^-3 for the LWC (liquid water content) and 4 mK for the fast temperature sensor (UFT-B). The correlation betweentemperature and LWC structures in shallow haze layers is investigated. The release of latent heat and the corresponding warming in the haze of about 0.1 K could be quantified.     

Multi-Source Emission Determination Using an Inverse-Dispersion Technique

Inverse-dispersion calculations can be used to infer atmospheric emission rates through a combination of downwind gas concentrations and dispersion model predictions. With multiple concentration sensors downwind of a compound source (whose component positions are known) it is possible to calculate the component emissions. With this in mind, a field experiment was conducted to examine the feasibility of such multi-source inferences, using four synthetic area sources and eight concentration sensors arranged in different configurations. Multi-source problems tend to be mathematically ill-conditioned, as expressed by the condition number κ. In our most successful configuration (average κ = 4.2) the total emissions from all sources were deduced to within 10% on average, while component emissions were deduced to within 50%. In our least successful configuration (average κ = 91) the total emissions were calculated to within only 50%, and component calculations were highly inaccurate. Our study indicates that the most accurate multi-source inferences will occur if each sensor is influenced by only a single source. A “progressive” layout is the next best: one sensor is positioned to “see” only one source, the next sensor is placed to see the first source and another, a third sensor is placed to see the previous two plus a third, and so on. When it is not possible to isolate any sources κ is large and the accuracy of a multi-source inference is doubtful.     

Magmatic-to-hydrothermal crystallization in the W–Sn mineralized Mole Granite (NSW, Australia): Part I: Crystallization of zircon and REE-phosphates over three million years—a geochemical and U–Pb geochronological study

Zircon, monazite and xenotime crystallized over a temperature interval of several hundred degrees at the magmatic to hydrothermal transition of the Sn and W mineralized Mole Granite. Magmatic zircon and monazite, thought to have crystallized from hydrous silicate melt, were dated by conventional U–Pb techniques at an age of 247.6 ± 0.4 and 247.7 ± 0.5 Ma, respectively. Xenotime occurring in hydrothermal quartz is found to be significantly younger at 246.2 ± 0.5 Ma and is interpreted to represent hydrothermal growth. From associated fluid inclusions it is concluded that it precipitated from a hydrothermal brine ≤ 600 °C, which is below the accepted closure temperature for U–Pb in this mineral. These data are compatible with a two-stage crystallization process: precipitation of zircon and monazite as magmatic liquidus phases in deep crustal magma followed by complete crystallization and intimately associated Sn–W mineralization after intrusion of the shallow, sill-like body of the Mole Granite. Later hydrothermal formation of monazite in a biotite–fluorite–topaz reaction rim around a mineralized vein was dated at 244.4 ± 1.4 Ma, which distinctly postdates the Mole Granite and is possibly related to a younger hidden intrusion and its hydrothermal fluid system.

Obtaining precise age data for magmatic and hydrothermal minerals of the Mole Granite is hampered by uncertainties introduced by different corrections required for multiple highly radiogenic minerals crystallising from evolved hydrous granites, including ²³⁰Th disequilibrium due to Th/U fractionation during monazite and possibly xenotime crystallization, variable Th/U ratios of the fluids from which xenotime was precipitating, elevated contents of common lead, and post-crystallization lead loss in zircon, enhanced by the fluid-saturated environment. The data imply that monazite can also survive as a liquidus phase in protracted magmatic systems over periods of 10⁶ years. The outlined model is in agreement with prominent chemical core-rim variation of the zircon.     

Dissolution of calcitic marble and dolomitic rock in high iron concentrated acid mine drainage: application to anoxic limestone drains

Oxidation of sulphide mining waste can generate acid mine drainage (AMD) that has the potential to seriously affect the ecosystems. Acid mine drainage is characterised by a high acidity, high concentrations of sulphates and metals. To reduce the environmental impacts due to AMD, neutralisation using limestone drains is an option proposed in the literature and used around the world. The present study focuses on the influence of the carbonate rock mineralogy and their particle size on the neutralising capacity. The tests were performed in two different anoxic conditions: in batch reactors, and in columns having a hydraulic retention time of 15?h. The results showed that the neutralisation capacity of calcite was more important than for dolomitic rock, and smaller particle size gave higher alkalinity production (fine calcite dissolved faster in contact with AMD). A characterization of metal precipitate in sludge and in limestone coating was performed and demonstrated that gypsum, lepidocrocite and goethite were the predominant secondary minerals to be formed. Finally, this study underlines that anoxic limestone drain cannot be used alone to treat high iron concentrated AMD.     

Oxygen isotopic composition of Central Sierra Nevada precipitation, I. Identification of ice-phase water capture regions in winter storms

Measurements of the stable isotopic ratios of oxygen and simultaneously observed ice crystal structure in freshly fallen snow, have been used to estimate the weighted mean altitudes of ice-phase precipitation formation in winter clouds over the Central Sierra Nevada.Observations of dominant, diffusionally grown ice-crystal habits were used to estimate relatively narrow ranges of temperatures of initial formation of the precipitation particles using the Nakaya (1954) ice-crystal classification techniques. The mean oxygen isotope ratio ¹⁸O/¹⁶O for each snow sample, together with local upper air soundings, were used to estimate ranges of temperature-altitude within the clouds where the precipitating particles had captured their ice-phase water. For this initial study, snow samples were collected each five (5) to ten (10) minutes during three snowfall periods on 27 January, 26 February and 28 February, 1983.For the 27 January case, the ice formation mechanism was predominantly vapor deposition and hence the narrow range of temperatures determined by the ice-crystal habits was directly relatable to the mean δ¹⁸O value.The 26 and 28 February cases were more complicated because the solid-phase precipitation was formed by vapor deposition and by freezing of drops and droplets (which occurs without isotopic fractionation). In these cases, the oxygen isotopic composition of the snow reaching the ground was representative of solid phase precipitation which had formed at warmer temperatures than those corresponding to the primary ice-crystal habits alone. There was no apparent relationship between δ¹⁸O values and surface temperature at the sampling site.If relationships between ¹⁸O/¹⁶O and temperature-altitude are established for this geograpic region for winter snowfall conditions, this crystal habit—isotopic composition technique can provide knowledge about the regions of the clouds in which the ice-phase precipitation is forming over the Central Sierra Nevada.     

The Delphi technique and judgmental forecasting

The Delphi technique for judgmental forecasting by expert groups is described and the controversy surrounding its use is summarized. The technique clearly does not eliminate all unwanted psychological effects on group judgment. Furthermore, the design of most Delphi studies makes it impossible to separate the signal from the noise in expert judgment. A methodological standard for evaluating judgmental forecasts is proposed.