Drought and Associated Cloud Fields over the Canadian Prairie Provinces     

Heather Greene  Ronald E. Stewart 《大气与海洋》2013,51(4):356-365

Little is known about clouds during drought. From 1999 to 2005 the Canadian Prairies experienced one of the most severe and prolonged droughts in the historical record. This study characterizes clouds during drought in the Canadian Prairie provinces with a particular focus on this recent drought.

Drought severity was determined using the Standardized Precipitation Index (SPI) based on monthly precipitation on a 1° × 1° grid. Cloud fields from the National Aeronautics and Space Administration/Global Energy and Water Experiment's (NASA/GEWEX) Surface Radiation Budget database were used to examine overall cloud amount, optical thickness, and top-of-the-atmosphere albedo. Anomalies in monthly precipitation in the satellite record from 1984 to 2004, with an emphasis on the recent drought from 1999 to 2004, were related to anomalies in cloud fields.

During drought, a decrease in cloud amount was observed. During the spring and summer months of the 1999–2004 drought, for example, the observed cloud cover fraction decreased by approximately 7% between severely wet and severely dry conditions. There was, however, large month-to-month and spatial variability, and the correlation of cloud cover fraction anomaly with precipitation was weak. A higher correlation was found between the top-of-the-atmosphere albedo and precipitation. The occurrence of thick clouds and clouds of medium thickness did decrease with drought severity. These trends also applied to subregions within the overall domain. These observations further the understanding of the role of clouds in feedback mechanisms during drought.

R ésumé ?[Traduit par la rédaction] On sait peu de choses à propos des nuages durant les sécheresses. De 1999 à 2005, les Prairies canadiennes ont connu l'une des périodes de sécheresse les plus longues et les plus intenses depuis que l'on tient des relevés. La présente étude porte sur les nuages durant les périodes de sécheresse dans les Prairies canadiennes, en mettant l'accent sur les sécheresses récentes.

Nous avons déterminé l'intensité des sécheresses au moyen de l'indice de précipitations normalisé (SPI) fondé sur les précipitations mensuelles sur une grille de 1° × 1°. Nous avons étudié les champs de nuages obtenus de la base de données du bilan radiatif de la surface du GEWEX (Expérience mondiale sur les cycles de l'énergie et de l'eau) de la NASA pour examiner l'étendue générale des nuages, l'épaisseur optique et l'albédo du sommet de l'atmosphère. Nous avons mis en relation les anomalies dans les précipitations mensuelles d'après les données satellitaires de 1984 à 2004, en mettant l'accent sur la sécheresse récente de 1999 à 2004, avec les anomalies dans les champs de nuages.

Pendant les sécheresses, nous avons noté une diminution de l'étendue des nuages. Durant les mois de printemps et d'été de la période de sécheresse de 1999 à 2004, par exemple, la fraction de couverture du ciel observée a diminué d'environ 7 % entre les conditions très humides et très sèches. Cependant, la variabilité intermensuelle et spatiale est grande, et la corrélation de l'anomalie de la fraction de couverture nuageuse avec les précipitations est faible. Nous avons trouvé une meilleure corrélation entre l'albédo du sommet de l'atmosphère et les précipitations. La quantité de nuages épais et de nuages d'épaisseur moyenne diminuait quand l'intensité de la sécheresse augmentait. Ces tendances s'observent aussi dans les sous-régions à l'intérieur domaine général. Ces observations permettent de mieux comprendre le rôle des nuages dans les mécanismes de rétroaction au cours des sécheresses.  相似文献   

A stratigraphic study of southern Oceanus Procellarum using Clementine multispectral data     

D. J. Heather  S. K. Dunkin   《Planetary and Space Science》2002,50(14-15)

In this paper, we use Clementine multispectral data to demonstrate techniques from which spectrally distinct mare basalts can be mapped, and estimates of basalt thickness obtained. The region studied incorporates a portion of southern Oceanus Procellarum from 17.5°N to 20.5°S and 289°E to 317°E, including the Marius Hills, Damoiseau, Cavalerius and Flamsteed areas. Flamsteed provides a test area from which we validate the mapping techniques used in this work. Unit boundaries and basalt thickness details from these analyses are applied to the stratigraphy of the Oceanus Procellarum Group defined by Whitford-Stark and Head (J. Geophys. Res. 85 (1980) 6579). A total of 13 basalts are recognised in the region, 10 of which are spectrally distinct, and three of which represent previously unrecognised Members of the Oceanus Procellarum Group. The average thickness of the basalts is between 160 and 625 m, ranging from tens to hundreds of metres near the mare/highland boundaries and consistently greater than several hundred metres closer to the centre of the mare. These values provide a range of basalt volume in the region of 0.7–2.8×10⁵ km³. This represents between 8% and 32% of the total volume of basalts in Oceanus Procellarum.  相似文献   

Collective resources in the repopulation of New Orleans after Hurricane Katrina     

Weil  Frederick D.  Rackin  Heather M.  Maddox  David 《Natural Hazards》2018,92(2):927-942

This study explores the influence of three factors on a person’s decision to drive in winter weather: destination, affected area, and caution level. Participants (n = 555) completed an online survey that included scenarios with text of a simulated radio message involving a character named Mike. After the scenario, participants answered Likert-scaled questions related to their intention to drive (what would you do) and their recommended behavior for others (what should Mike do). There was a significant effect of destination and caution level on the decision to drive. Participants were more likely to respond that they would drive if the destination was work rather than dinner, and if the caution statement was “exercise caution” rather than “do not drive.” There were similar significant effects of destination and caution level on what the scenario character should do. It is recommended that a clear directive be included in warning messages to encourage drivers to stay off the roadways during hazardous weather.  相似文献   

Investigating the influence of aquifer heterogeneity on the potential for thermal free convection in the Yarragadee Aquifer,Western Australia     

Dylan J. Irvine  Heather A. Sheldon  Craig T. Simmons  Adrian D. Werner  Cedric M. Griffiths 《Hydrogeology Journal》2015,23(1):161-173

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Characterization of dissolved organic matter fluorescence in the South Atlantic Bight with use of PARAFAC model: Relationships between fluorescence and its components,absorption coefficients and organic carbon concentrations     

Piotr Kowalczuk  William J. Cooper  Michael J. Durako  Amanda E. Kahn  Michael Gonsior  Heather Young 《Marine Chemistry》2010,118(1-2):22-36

In this study, the CDOM absorption coefficient at 350 nm [a_CDOM(350)] and CDOM excitation emission matrix (EEM) fluorescence were used to estimate annual fluxes of dissolved organic carbon (DOC) from the Cape Fear River to Long Bay in the South Atlantic Bight. Water samples were collected during a 3.5 year period, from October 2001 through March 2005, in the vicinity of the Cape Fear River (CFR) outlet and adjacent Onslow Bay (OB). Parallel factor analysis (PARAFAC) of CDOM EEM spectra identified six components: three terrestrial humic-like, one marine humic-like and two protein-like. Empirical relationships were derived from the PARAFAC model between DOC concentration and a_CDOM(350), total fluorescence intensity and the intensities of respective EEM components. DOC concentration and CDOM optical parameters were very well correlated and R² values ranged from 0.77 to 0.90. Regression analyses revealed that the non-absorbing DOC fraction, in DOC concentration estimated from CDOM optical parameters, varied with the qualitative composition of the CDOM. DOC concentration and intensity of the humic-like CDOM components characterized by excitation maxima at longer wavelengths have significantly higher estimated non-absorbing DOC compared to the analogous relationships between DOC and intensity of the humic-like CDOM components characterized by excitation maxima at shorter wavelengths. The relationships between DOC concentration and intensity of one of the protein-like components resulted in significantly reduced non-absorbing DOC fraction in DOC concentration estimation. Results of regression analyses between fluorescence intensities of specific EEM components and CDOM-specific absorption coefficients suggest that the relative proportion of humic-like CDOM components (characterized by excitation maximum at longer wavelengths) and the main protein-like component have the most impact on the values of a?_CDOM(350). Based on the relationships between a_CDOM(350), Cape Fear River flow, and DOC concentrations, DOC fluxes were estimated for 2002, 2003 and 2004. DOC fluxes varied from 1.5 to 6.2 × 10¹⁰ g C yr^? 1, depending on river flow.  相似文献   

Defending the Universe     

Nigel Henbest  Heather Couper 《Astronomy& Geophysics》2000,41(2):2.9-2.9

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Paleoclimate and growth rates of speleothems in the northwestern Iberian Peninsula over the last two glacial cycles     

Heather M. Stoll  Ana Moreno  Ana Mendez-Vicente  Saul Gonzalez-Lemos  Montserrat Jimenez-Sanchez  Maria Jose Dominguez-Cuesta  R. Lawrence Edwards  Hai Cheng  Xianfeng Wang 《Quaternary Research》2013

Speleothem growth requires humid climates sufficiently warm to stimulate soil CO₂ production by plants. We compile 283 U/Th dates on 21 stalagmites from six cave systems in the NW coast of Spain to evaluate if there are patterns in stalagmite growth that are evidence of climatic forcing. In the oldest stalagmites, from marine oxygen isotope stage (MIS) 7–5, growth persists through the glacial period. Hiatuses and major reductions in growth rate occur during extreme minima in summer insolation. Stalagmites active during the last interglaciation cease growth at the MIS 5–4 boundary (74 ka), when regional sea-surface temperature cooled significantly. During MIS 3, only two stalagmites grew; rates were highest between 50 and 60 ka during the maximum in summer insolation. One stalagmite grew briefly at 41 ka, 36.5 and 28.6 ka, all during warm phases of the Dansgaard–Oeschger cycles. A pronounced Holocene optimum in stalagmite growth occurs from 9 to 6 ka. The cessation of most growth by 4.1 ka, coincident with broad increases in aridity over the Mediterranean and areas influenced by the North African Monsoon, suggest that regions such as NW Spain, with dominant Atlantic moisture sources, also experienced increased aridity at this time.  相似文献   

Assessment of observing time-variable gravity from GOCE GPS and accelerometer observations   总被引：2，自引：1，他引：1  

P. N. A. M. Visser  W. van der Wal  E. J. O. Schrama  J. van den IJssel  J. Bouman 《Journal of Geodesy》2014,88(11):1029-1046

An assessment has been made of the possibility to estimate time-variable gravity from GPS-derived orbit perturbations and common-mode accelerometer observations of ESA’s GOCE Earth Explorer. A number of 20-day time series of Earth’s global long-wavelength gravity field have been derived for the period November 2009 to November 2012 using different parameter setups and estimation techniques. These techniques include a conventional approach where for each period, one set of gravity coefficients is estimated, either excluding or including empirical accelerations, and the so-called Wiese approach where higher frequency coefficients are estimated for the very long wavelengths. A principal component analysis of especially the time series of gravity field coefficients obtained by the Wiese approach and the conventional approach with empirical accelerations reveals an annual signal. When fitting this annual signal directly through the time series, the sine component (maximum in spring) displays features that are similar to well-known continental hydrological mass changes for the low latitude areas, such as mass variations in the Amazon basin, Africa and Australia for spatial scales down to 1,500 km. The cosine component (maximum in winter), however, displays large signals that can not be attributed to actual mass variations in the Earth system. The estimated gravity field changes from GOCE orbit perturbations are likely affected by missing GPS observations in case of high ionospheric perturbations during periods of increased solar activity, which is minimal in Summer and maximal towards the end of autumn.  相似文献   

The impact of stress history on bed structure     

Anne‐Marie Ockelford  Heather Haynes 《地球表面变化过程与地形》2013,38(7):717-727

Recent research has started to focus on how prolonged periods of sub‐threshold flows may be capable of imparting structural changes that contribute to increased bed stability. To date, this effect (termed ‘stress history’) has been found to be significant in acting to increase a bed's critical shear stress at entrainment threshold. However, it is supported by only limited, qualitative and often speculative information on the mechanisms of this stabilization process in grade‐specific studies. As such, this paper uses high resolution laser scanning to quantitatively ascertain the granular mechanics underpinning the relationship between stress history and entrainment threshold for beds of a range of grain size distributions. Employing a bed slope of 1/200, three grain size distributions with median grain sizes (D₅₀) of 4·8 mm [uniform (σ_g = (D₈₄/D₁₆)^0.5 = 1·13; bimodal (σ_g = 2·08); and, unimodal (σ_g = 1·63)] were exposed to antecedent stress histories of 60 and 960 minutes duration. Antecedent shear stress magnitude was set at 50% of the critical shear stress for the D₅₀ when no stress history period was employed. Two laser displacement scans of the bed surface (approximate area 100 mm × 117 mm) were taken, one prior to the antecedent period and one after this period, so that changes to surface topography could be quantified (resolution of x = 0·10 mm, y = 0·13 mm and z = 0·24 mm). Rearrangement of bed surface structure is described using statistical analysis and two‐dimensional (2D) semi‐variograms to analyse scaling behaviour. Results reveal vertical settlement, changes to bed roughness and particle repositioning. However, the bed grain size distribution influences the relative importance of each mechanism in determining stress history induced bed stability; this is the focus of discussion in this paper. Copyright © 2012 John Wiley & Sons, Ltd.  相似文献   

GIS in local government: some findings from Great Britain     

Heather Campbell  Ian Masser 《International journal of geographical information science》2013,27(6):529-546

Abstract

Local government is one of the most important groups of users of geographical information systems (GIS). The significant interest amongst users and the varied demands to be made of GIS have stimulated discussion in many countries. This article presents some of the findings of a research project which is currently exploring the impact of GIS on British local government. The discussion examines the results of a comprehensive survey of all 514 local authorities in Great Britain. These findings provide an overview of GIS adoption in relation to type of local authority and geographical location. Particular attention is focused on the type of systems which have been adopted, the take-up of GIS over time, the software and hardware being utilized and the perceived benefits and problems associated with GIS. The results demonstrate that take-up has been most pronounced in the larger authorities, particularly in the southern part of Great Britain, with the most popular combination of equipment being Arc/Info software and workstations. For those authorities presently implementing a GIS, the technology is expected to improve existing capabilities for information processing rather than enhance decision-making or help achieve savings. In seeking these benefits authorities are encountering considerable problems which include basic technical difficulties in getting the equipment to work, data-related issues surrounding the collection, standardization and maintenance of geographical information, and profound organizational considerations associated with the introduction of information technology into complex environments. There is an urgent need for in-depth case studies which evaluate the experiences of users in relation to the vital process of implementation.  相似文献