Quantifying the influence of land-use and surface characteristics on spatial variability in the urban heat island

The urban thermal environment varies not only from its rural surroundings but also within the urban area due to intra-urban differences in land-use and surface characteristics. Understanding the causes of this intra-urban variability is a first step in improving urban planning and development. Toward this end, a method for quantifying causes of spatial variability in the urban heat island has been developed. This paper presents the method as applied to a specific test case of Portland, Oregon. Vehicle temperature traverses were used to determine spatial differences in summertime ~2 m air temperature across the metropolitan area in the afternoon. A tree-structured regression model was used to quantify the land-use and surface characteristics that have the greatest influence on daytime UHI intensity. The most important urban characteristic separating warmer from cooler regions of the Portland metropolitan area was canopy cover. Roadway area density was also an important determinant of local UHI magnitudes. Specifically, the air above major arterial roads was found to be warmer on weekdays than weekends, possibly due to increased anthropogenic activity from the vehicle sector on weekdays. In general, warmer regions of the city were associated with industrial and commercial land-use. The downtown core, whilst warmer than the rural surroundings, was not the warmest part of the Portland metropolitan area. This is thought to be due in large part to local shading effects in the urban canyons.     

Helium isotopic variations in volcanic rocks from Loihi Seamount and the Island of Hawaii

Helium isotopic ratios ranging from 20 to 32 times the atmospheric ³He/⁴He(R_A) have been observed in a suite of 15 basaltic glasses from the Loihi Seamount. These ratios, which are up to four times higher than those of MORB glasses and more than twice those of nearby Kilauea, are strongly suggestive of a primitive source of volatiles supplying this volcanism. The Loihi glasses measured span a broad compositional range, and the ³He/⁴He ratios were found to be generally lower for the alkali basalts than for the tholeiites. The component with a lower ³He/⁴He ratio appears to be associated with olivine xenocrysts, within which fluid inclusions are probably the carrier of contaminant helium. One Loihi sample has a much lower isotopic ratio (<5 R_A), but a combination of low He concentration, high vesicularity, and presence of cracks lined with clay minerals suggests that the low ratio is due to gas loss and contamination by atmospheric helium.Crushing and melting experiments show that for modest vesicularities (<5% by volume) the Loihi glasses obey a MORB-type partitioning trend, but at higher vesicularities the data show considerably more scatter due to volatile mobilization. The high vesicularities, low extrusion pressure and generally low helium concentrations are consistent with a considerable degree of degassing. Analyses of dunites, plus a correlation between total helium concentrations with xenocryst abundances also suggest that xenocrysts are a significant carrier of contaminating (low ³He/⁴He) helium.³He/⁴He ratios from samples of other Hawaiian volcanoes (Kilauea, Mauna Loa, Hualalai, and Mauna Kea) show a smooth decrease in ³He/⁴He with increasing volcano age and volume. We interpret this to be a synoptic picture of the time evolution of a hot-spot diapir: the earliest stage is characterized by primitive (> 30 R_A) helium with some (variable) component of lithospheric contamination added during “breakthrough”, while the later stages are characterized by a relaxation toward lithospheric ³He/⁴He ratios (～ 8 R_A) due to isolation of the diapir from the mantle below (as the plate moves on), and subsequent mining of the inherited helium and contamination from the surrounding lithosphere. The abrupt contrast in ³He/⁴He ratios between Kilauea and Loihi, despite their close proximity, is indicative of the small lateral extent of the plume.     

Evaluation of the CORDEX-Africa multi-RCM hindcast: systematic model errors

Monthly-mean precipitation, mean (T_AVG), maximum (T_MAX) and minimum (T_MIN) surface air temperatures, and cloudiness from the CORDEX-Africa regional climate model (RCM) hindcast experiment are evaluated for model skill and systematic biases. All RCMs simulate basic climatological features of these variables reasonably, but systematic biases also occur across these models. All RCMs show higher fidelity in simulating precipitation for the west part of Africa than for the east part, and for the tropics than for northern Sahara. Interannual variation in the wet season rainfall is better simulated for the western Sahel than for the Ethiopian Highlands. RCM skill is higher for T_AVG and T_MAX than for T_MIN, and regionally, for the subtropics than for the tropics. RCM skill in simulating cloudiness is generally lower than for precipitation or temperatures. For all variables, multi-model ensemble (ENS) generally outperforms individual models included in ENS. An overarching conclusion in this study is that some model biases vary systematically for regions, variables, and metrics, posing difficulties in defining a single representative index to measure model fidelity, especially for constructing ENS. This is an important concern in climate change impact assessment studies because most assessment models are run for specific regions/sectors with forcing data derived from model outputs. Thus, model evaluation and ENS construction must be performed separately for regions, variables, and metrics as required by specific analysis and/or assessments. Evaluations using multiple reference datasets reveal that cross-examination, quality control, and uncertainty estimates of reference data are crucial in model evaluations.     

Possible modes of coral-reef development at Molokai, Hawaii, inferred from seismic-reflection profiling

High-resolution, seismic-reflection data elucidate the late Quaternary development of the largest coral-reef complex in the main Hawaiian Islands. Six acoustic facies were identified from reflection characteristics and lithosome geometry. An extensive, buried platform with uniformly low relief was traced beneath fore-reef and marginal shelf environments. This highly reflective surface dips gently seaward to ~130 m depth and locally crops out on the seafloor. It probably represents a wave-cut platform or ancient reef flat. We propose alternative evolutionary models, in which sea-level changes have modulated the development of reef systems, to explain the observed stratigraphic relationships. The primary difference between the models is the origin of the underlying antecedent surface, which arguably could have formed during either regression/lowstand or subsequent transgression.     

An ion microprobe study of the partitioning of trace elements between clinopyroxene and liquid in the system diopside-albite-anorthite

The partitioning of trace elements (Sc, Ti, Sr and Sm) between diopsidic clinopyroxene and liquid was studied experimentally in the system diopside-albite-anorthite at 1250°C, 1300°C and 1345°C at 1 atm. Twelve different bulk compositions were selected to study the effects of temperature and chemical composition. A Cameca ion microprobe was used to determine trace element concentrations in both clinopyroxene and liquid. Experiments of different run duration $\text{1}\text{4}\text{8}\text{?}$ days) showed that equilibrium was approached in less than 4 days at 1275°C. Equilibrium was also evaluated by a reversal run. A series of runs of constant bulk composition but with variable trace element contents showed that Henry's Law was obeyed over concentration ranges of the trace elements similar to those encountered in natural systems. The partition coefficients show significant ranges: Sc, 0.345~2.61; Ti, 0.084~0.214; Sr, 0.075~0.136; Sm, 0.054~0.328; the values are comparable with those obtained experimentally by other investigators. The partition coefficients vary as a function of both temperature and chemical composition. The experimental results are discussed in terms of exchange equilibria using the Bottinga-Weill silicate melt model. It is demonstrated that analytical uncertainties of both major and trace elements play an important role in understanding trace element exchange equilibria; propagation of analytical errors in the thermodynamic treatment is equally important.     

Benthic foraminifera show some resilience to ocean acidification in the northern Gulf of California,Mexico

Extensive CO₂ vents have been discovered in the Wagner Basin, northern Gulf of California, where they create large areas with lowered seawater pH. Such areas are suitable for investigations of long-term biological effects of ocean acidification and effects of CO₂ leakage from subsea carbon capture storage. Here, we show responses of benthic foraminifera to seawater pH gradients at 74–207 m water depth. Living (rose Bengal stained) benthic foraminifera included Nonionella basispinata, Epistominella bradyana and Bulimina marginata. Studies on foraminifera at CO₂ vents in the Mediterranean and off Papua New Guinea have shown dramatic long-term effects of acidified seawater. We found living calcareous benthic foraminifera in low pH conditions in the northern Gulf of California, although there was an impoverished species assemblage and evidence of post-mortem test dissolution.