Time–frequency analysis to profile hydrological regimes: application to Haiti

Abstract

Comprehensive characterization of its flow rates is prerequisite to a proper understanding and water management of a given hydrological region. Several studies question the soundness of stationarity in time series and suggest the need for a quantification of the events and non-stationary features in flow rate time series. In this study, we combine statistical and time–frequency (TF) analyses to characterize and classify the flow rates of an understudied region, namely Haiti. Wavelet transforms and cyclostationarity analyses were combined with principal component analysis and hierarchical clustering to identify three groups of hydrological regimes in the country, suggesting similar management: (1) relatively stable flow rates with TF behaviour; (2) periodic and strongly seasonal flow rates; and (3) unstable flow rates. We argue that the TF methodology can yield additional information in regard to flow events and multiscale behaviour, even for short records. Flow rate characterization would benefit from the exhaustive approach described here.

EDITOR Z.W. Kundzewicz ASSOCIATE EDITOR E. Toth     

Hydrological and meteorological extremes derived from taxation records: the estates of Brtnice,Třebíč and Velké Meziříčí, 1706–1849

Abstract

This paper addresses the hydrological and meteorological extremes that may be deduced from the taxation records of the estates of Brtnice, T?ebí? and Velké Mezi?í?í, all in the Moravian-Bohemian Highlands of the Czech Republic, for the years 1706–1849. At that time, damage to agricultural crops constituted grounds for tax remission for individual farmers and landowners. Where it survives, the relevant administrative documentation generally includes a statement from the applicant, a report by the official commission tasked with checking the contents of it, and any decisions made by taxation authorities at regional and “land” level (the Jihlava regional office and the Moravian Land Administration (“Gubernium”) respectively). Data extracted may include the type of event, dating, places of occurrence and damage done. The chronology of hydrological and meteorological extremes (torrential rain, flash flood, flood, hailstorm, lightning, frost) covers the period 1706–1849, but only four events are evident before 1748 and there is a gap in records between 1757 and 1789. Extremes are analysed from a spatio-temporal point of view. A total of 97 extreme events (171 extremes of particular type) were identified for the region studied. Torrential rain, hailstorm and flash flood were the major devastating phenomena, and occurred mainly from May to August. Torrential rain and hailstorm are clearly attributable to thunderstorms with very intense convection. Five outstanding events and their impacts upon individual farmers are described in detail. The results are discussed with respect to uncertainties in the basic data and in the context of the Czech Lands, because only some of the extremes disclosed are known and confirmed by other documentary data.

Editor Z.W. Kundzewicz

Citation Dolák, L., Brázdil, R., and Valá?ek, H., 2013. Hydrological and meteorological extremes derived from taxation records: the estates of Brtnice, T?ebí? and Velké Mezi?í?í, 1706–1849. Hydrological Sciences Journal, 58 (8), 1620–1634.     

Influence of the orographic roughness of glacier valleys across the Transantarctic Mountains in an atmospheric regional model

Glacier valleys across the Transantarctic Mountains are not properly taken into account in climate models, because of their coarse resolution. Nonetheless, glacier valleys control katabatic winds in this region, and the latter are thought to affect the climate of the Ross Sea sector, frsater formation to snow mass balance. The purpose of this paper is to investigate the role of the production of turbulent kinetic energy by the subgrid-scale orography in the Transantarctic Mountains using a 20-km atmospheric regional model. A classical orographic roughness length parametrization is modified to produce either smooth or rough valleys. A one-year simulation shows that katabatic winds in the Transantarctic Mountains are strongly improved using smooth valleys rather than rough valleys. Pressure and temperature fields are affected by the representation of the orographic roughness, specifically in the Transantarctic Mountains and over the Ross Ice Shelf. A smooth representation of escarpment regions shows better agreement with automatic weather station observations than a rough representation. This work stresses the need to improve the representation of subgrid-scale orography to simulate realistic katabatic flows. This paper also provides a way of improving surface winds in an atmospheric model without increasing its resolution.     

Influence of coupling on atmosphere, sea ice and ocean regional models in the Ross Sea sector, Antarctica

Air–sea ice–ocean interactions in the Ross Sea sector form dense waters that feed the global thermohaline circulation. In this paper, we develop the new limited-area ocean–sea ice–atmosphere coupled model TANGO to simulate the Ross Sea sector. TANGO is built up by coupling the atmospheric limited-area model MAR to a regional configuration of the ocean–sea ice model NEMO. A method is then developed to identify the mechanisms by which local coupling affects the simulations. TANGO is shown to simulate realistic sea ice properties and atmospheric surface temperatures. These skills are mostly related to the skills of the stand alone atmospheric and oceanic models used to build TANGO. Nonetheless, air temperatures over ocean and winter sea ice thickness are found to be slightly improved in coupled simulations as compared to standard stand alone ones. Local atmosphere ocean feedbacks over the open ocean are found to significantly influence ocean temperature and salinity. In a stand alone ocean configuration, the dry and cold air produces an ocean cooling through sensible and latent heat loss. In a coupled configuration, the atmosphere is in turn moistened and warmed by the ocean; sensible and latent heat loss is therefore reduced as compared to the stand alone simulations. The atmosphere is found to be less sensitive to local feedbacks than the ocean. Effects of local feedbacks are increased in the coastal area because of the presence of sea ice. It is suggested that slow heat conduction within sea ice could amplify the feedbacks. These local feedbacks result in less sea ice production in polynyas in coupled mode, with a subsequent reduction in deep water formation.     

Flow dynamics at the origin of thin clayey sand lacustrine turbidites: Examples from Lake Hazar,Turkey

Turbidity currents and their deposits can be investigated using several methods, i.e. direct monitoring, physical and numerical modelling, sediment cores and outcrops. The present study focused on thin clayey sand turbidites found in Lake Hazar (Turkey) occurring in eleven clusters of closely spaced thin beds. Depositional processes and sources for three of those eleven clusters are studied at three coring sites. Bathymetrical data and seismic reflection profiles are used to understand the specific geomorphology of each site. X‐ray, thin sections and CT scan imagery combined with grain‐size, geochemical and mineralogical measurements on the cores allow characterization of the turbidites. Turbidites included in each cluster were produced by remobilization of surficial slope sediment, a process identified in very few studies worldwide. Three types of turbidites are distinguished and compared with deposits obtained in flume studies published in the literature. Type 1 is made of an ungraded clayey silt layer issued from a cohesive flow. Type 2 is composed of a partially graded clayey sand layer overlain by a mud cap, attributed to a transitional flow. Type 3 corresponds to a graded clayey sand layer overlain by a mud cap issued from a turbulence‐dominated flow. While the published experimental studies show that turbulence is damped by cohesion for low clay content, type 3 deposits of this study show evidence for a turbulence‐dominated mechanism despite their high clay content. This divergence may in part relate to input variables, such as water chemistry and clay mineralogy, that are not routinely considered in experimental studies. Furthermore, the large sedimentological variety observed in the turbidites from one coring site to another is related to the evolution of a sediment flow within a field‐scale basin made of a complex physiography that cannot be tackled by flume experiments.     

Evaluation of a high-resolution regional climate simulation over Greenland

A simulation of the 1991 summer has been performed over south Greenland with a coupled atmosphere–snow regional climate model (RCM) forced by the ECMWF re-analysis. The simulation is evaluated with in-situ coastal and ice-sheet atmospheric and glaciological observations. Modelled air temperature, specific humidity, wind speed and radiative fluxes are in good agreement with the available observations, although uncertainties in the radiative transfer scheme need further investigation to improve the model’s performance. In the sub-surface snow-ice model, surface albedo is calculated from the simulated snow grain shape and size, snow depth, meltwater accumulation, cloudiness and ice albedo. The use of snow metamorphism processes allows a realistic modelling of the temporal variations in the surface albedo during both melting periods and accumulation events. Concerning the surface albedo, the main finding is that an accurate albedo simulation during the melting season strongly depends on a proper initialization of the surface conditions which mainly result from winter accumulation processes. Furthermore, in a sensitivity experiment with a constant 0.8 albedo over the whole ice sheet, the average amount of melt decreased by more than 60%, which highlights the importance of a correctly simulated surface albedo. The use of this coupled atmosphere–snow RCM offers new perspectives in the study of the Greenland surface mass balance due to the represented feedback between the surface climate and the surface albedo, which is the most sensitive parameter in energy-balance-based ablation calculations.     

Laue diffraction lenses for astrophysics: Theoretical concepts

Beyond the present technologies, Laue diffraction lenses are very promising tools in the field of gamma-ray astrophysics. The theoretical concepts of this kind of instruments are based on the Laue diffraction in crystals, discovered almost 100 years ago. Though they are commonly used in crystallography, their application to γ-ray focusing in astrophysics requires some specific developments, e.g. in terms of energy and imaging responses. The present article describes the physics of X-ray diffraction in crystals. In the context of the Darwin model of mosaic crystals, some peculiar aspects, relevant to the astrophysical observation, are discussed. The evaluation and optimization of diffraction efficiency are discussed, especially with rigards to the crystal’s mosaicity and thickness, its spatial extent and deviations to the “ideally imperfect” Darwin model. PACS 95.55.Ka, 61.50.Ah, 61.10.−i, 41.50.+h     

Windstorm of the eighteenth century in the Czech Lands: course,extent, impacts

Theoretical and Applied Climatology - This paper addresses the course, extent, and impacts of a windstorm that occurred on 20–21 December 1740, in the Czech Lands. The analysis is based on...     

Uncertainty Modeling in Buffer Operations Applied to Connectivity Analysis

In this paper we will study the potential connectivity of red squirrels in a fragmented landscape, using a buffer operation that takes into account the difficulty of moving through the landscape. The outcome of such an analysis is greatly influenced by the various sources of uncertainty that are introduced in the model. Two main sources of uncertainty can be identified: source layer uncertainty and model uncertainty. In this paper the propagation of source layer uncertainty resulting from a multivariate statistical classification of remotely sensed data is studied using Monte Carlo simulation, taking the spatial structure of uncertainty into account. Model uncertainty results from the adoption of deterministic model parameters regarding the dispersal capacity and the landscape effect, and is examined using fuzzy set theory. Comparing the outcome of error sensitized models to the observed dispersal activity of squirrels, demonstrates how modeling of uncertainty can help to explain the dispersal activity of red squirrels.