Subgrid variability of snow water equivalent at operational snow stations in the western USA

The spatial distribution of snow water equivalent (SWE) is a key variable in many regional‐scale land surface models. Currently, the assimilation of point‐scale snow sensor data into these models is commonly performed without consideration of the spatial representativeness of the point data with respect to the model grid‐scale SWE. To improve the understanding of the relationship between point‐scale snow measurements and surrounding areas, we characterized the spatial distribution of snow depth and SWE within 1‐, 4‐ and 16‐km² grids surrounding 15 snow stations (snowpack telemetry and California snow sensors) in California, Colorado, Wyoming, Idaho and Oregon during the 2008 and 2009 snow seasons. More than 30 000 field observations of snowpack properties were used with binary regression tree models to relate SWE at the sensor site to the surrounding area SWE to evaluate the sensor representativeness of larger‐scale conditions. Unlike previous research, we did not find consistent high biases in snow sensor depth values as biases over all sites ranged from 74% overestimates to 77% underestimates. Of the 53 assessments, 27 surveys indicated snow station biases of less than 10% of the surrounding mean observed snow depth. Depth biases were largely dictated by the physiographic relationship between the snow sensor locations and the mean characteristics of the surrounding grid, in particular, elevation, solar radiation index and vegetation density. These scaling relationships may improve snow sensor data assimilation; an example application is illustrated for the National Operational Hydrologic Remote Sensing Center National Snow Analysis SWE product. The snow sensor bias information indicated that the assimilation of point data into the National Operational Hydrologic Remote Sensing Center model was often unnecessary and reduced model accuracy. Copyright © 2012 John Wiley & Sons, Ltd.     

Vegetation and cloud cover shape semi-arid carbonate landform development

We study the relationships among precipitation, vegetation, and morphological characteristics of watersheds draining either side of the Dhofar Mountains in southeastern Oman to understand the geomorphic signature of water availability in a semi-arid carbonate landscape. Water availability is expressed in terms of vegetation and cloud cover. The integral and the statistical moments of the hypsometric curve were used to determine whether hyper-arid, inland-draining watersheds are significantly different from seasonally wet watersheds on the coast side of the mountain range. We demonstrate that the vegetation and cloud cover are correlated, with locations with longer cloud periods also having a longer period with a vegetation canopy. The analysis shows that the hypsometric curve and its statistical moments capture the morphological difference between wet watersheds shaped by groundwater sapping and dry watersheds with fluvial morphology. Specifically, the curves exhibit two shapes: watersheds with more vegetation and cloud cover are characterized by higher convexity, and those with less vegetation and cloud cover are characterized by higher concavity. A variance analysis of cloud cover, vegetation, and hypsometric integral shows that they are significantly different between the wet and dry watersheds. The link between hydrology and morphology is not strong at the scale of a single watershed, but it is significant when the watersheds are aggregated in zones. The statistical moments of the hypsometric curve in the range of values of the integral and skewness show good separation between watersheds dominated by sapping and fluvial erosion processes. We can separate the watersheds draining the mountain range in two distinct groups on the basis of their bimodal hydrological and morphological characteristics. Our findings support other studies that hypothesize a trade-off from chemical- to mechanical-dominated denudation in carbonate terranes as precipitation decreases.     

Ground-based calibration of the TAUVEX flight model

We describe in detail the procedures used for the ground testing and calibration of the flight model of the TAUVEX space telescope. We present the measurements done for this purpose, the various assumptions adopted during the ground calibration process, the results obtained, and provide a first approximation to the real performance of the experiment. The results of the ground-based calibration are that the system complies with the predicted performance in all aspects except for the total throughput, which is a factor of ∼5 smaller than expected. However, the instrument sensitivity measurement is highly uncertain due to known limitations of the available equipment and a more accurate calibration will be performed in orbit.     

Exotic UV astronomy

After considering a number of historical but somewhat “forgotten” UV astronomy experiments, I discuss a number of ways of non-conventional astronomy in the ultraviolet that, on first considerations, could be viable alternatives and valuable complements to classical space observations. These are (a) UV astronomy from the Antarctic or the Arctic regions that take advantage of the “ozone hole”, (b) the use of high-altitude stratospheric balloon-borne telescopes, and (c) the operation of UV telescopes on the Moon. The advantages of these options are discussed and evaluated against the costs of each option and, one by one, are mostly rejected as not fully justifying the specific alternative. The possibility to achieve valuable (but limited) UV science, such as imaging at ～2000 Å, using long-duration stratospheric balloons is described. The option of lunar UV observatories is retained to be implemented for the case of a UV interferometer, where the stability of the lunar regolith is seen as a significant advantage in comparison to free-flying interferometers. A location beyond the main asteroid belt, where the background due zodiacal light may be negligible, is advocated as an ideal location for a UV observatory in the Solar System.     

The bias of integrated assessment models that ignore climate catastrophes

Climate scientists currently predict there is a small but real possibility that climate change will lead to civilization threatening catastrophic events. Martin Weitzman has used this evidence along with his controversial “Dismal Theorem” to argue that integrated assessment models of climate change cannot be used to determine an optimal price for carbon dioxide. In this paper, I provide additional support for Weitzman’s conclusions by running numerical simulations to estimate risk premiums toward climate catastrophes. Compared to the assumptions found in most integrated assessment models, I incorporate into the model a more realistic range of uncertainty for both climate catastrophes and societal risk aversion. The resulting range of risk premiums indicates that the conclusions drawn from integrated assessment models that do not incorporate the potential for climate catastrophes are too imprecise to support any particular policy recommendation. The analysis of this paper is more straightforward and less technical than Weitzman’s, and therefore the conclusions should be accessible to a wider audience.     

Evaluation of high-resolution simulations of daily-scale temperature and precipitation over the United States

Extreme climate events have been increasing over much of the world, and dynamical models predict further increases in response to enhanced greenhouse forcing. We examine the ability of a high-resolution nested climate model, RegCM3, to capture the statistics of daily-scale temperature and precipitation events over the conterminous United States, using observational and reanalysis data for comparison. Our analyses reveal that RegCM3 captures the pattern of mean, interannual variability, and trend in the tails of the daily temperature and precipitation distributions. However, consistent biases do exist, including wet biases in the topographically-complex regions of the western United States and hot biases in the southern and central United States. The biases in heavy precipitation in the western United States are associated with excessively strong surface and low-level winds. The biases in daily-scale temperature and precipitation in the southcentral United States are at least partially driven by biases in circulation and moisture fields. Further, the areas of agreement and disagreement with the observational data are not intuitive from analyzing the simulated mean seasonal temperature and precipitation fields alone. Our evaluation should enable more informed application and improvement of high-resolution climate models for the study of future changes in socially- and economically-relevant temperature and precipitation events.     

High-redshift objects as probes of nearby cosmic voids

We show that present day observational techniques make it possible to detect diffuse gas masses down to ～10¹⁰ M _⊙ in cosmic voids with linear sizes of ～1 Mpc or larger. The observations consist of searches for absorption lines, similar in character to those produced in galactic interstellar clouds, in the spectra of high-redshift objects located beyond previously identified voids adjoining nearby superclusters.     

Estimating the timescale of fluvial response to anthropogenic disturbance using two generations of dams on the South River,Massachusetts, USA

Centuries-long intensive land-use change in the north-eastern United States provides the opportunity to study the timescale of geomorphic response to anthropogenic disturbances. In this region, forest-clearing and agricultural practices following EuroAmerican settlement led to deposition of legacy sediment along valley bottoms, including behind mill dams. The South River in western Massachusetts experienced two generations of damming, beginning with mill dams up to 6-m high in the eighteenth–nineteenth century, and followed by construction of the Conway Electric Dam (CED), a 17-m-tall hydroelectric dam near the watershed outlet in 1906. We use the mercury (Hg) concentration in upstream deposits along the South River to constrain the magnitude, source, and timing of inputs to the CED impoundment. Based on cesium-137 (¹³⁷Cs) chronology and results from a sediment mixing model, remobilized legacy sediment comprised % of the sediment load in the South River prior to 1954; thereafter, from 1954 to 1980s, erosion from glacial deposits likely dominated (63 ± 14%), but with legacy sediments still a substantial source (37 ± 14%). We also use the CED reservoir deposits to estimate sediment yield through time, and find it decreased after 1952. These results are consistent with high rates of mobilization of legacy sediment as historic dams breached in the early twentieth century, and suggest rapid initial response to channel incision, followed by a long decay in the second half of the century, that is likely dependent on large flood events to access legacy sediment stored in banks. Identifying sources of sediment in a watershed and quantifying erosion rates can help to guide river restoration practices. Our findings suggest a short fluvial recovery time from the eighteenth–nineteenth century to perturbation during the first half of the twentieth century, with subsequent return to a dominant long-term signal from erosion of glacial deposits, with anthropogenic sediment persisting as a secondary source. © 2020 John Wiley & Sons, Ltd.