A mid-winter, tropical extreme flood-producing storm in southern Israel: Synoptic scale analysis

Summary An exceptional rainstorm affected the Negev Desert, the southern part of Israel, during 20–23 December 1993 as a result of an active Red Sea trough (ARST). The latter refers to a low pressure trough that extends from eastern Africa northward, along the Red Sea, accompanied by an upper-level cyclonic system. Rainfall totals in certain parts exceeded December averages by a factor of 5. Major precipitating systems had a typical Meso-scale Convective System structure accompanied by showers, with peak intensities of 60–80mmh^–1, and by hail and flash floods, resulting in loss of life and heavy damage.The origin of rainfall systems and of the moisture at lower latitudes, as well as the type of precipitation involved, indicate its tropical nature. The timing of the storm around the winter solstice is unusual, since these tropical-like storms are most frequent in the region in October–November.Unusual synoptic circumstances explain the development and intensity of this storm and this exceptional timing. In contrast to the normal situation in December the mid-level subtropical high over the Sahara was replaced by a cyclone, while the persistent high over the Arabian Peninsula intensified. As a result, a southerly flow persisted over the Red Sea several days prior to the storm initiation and transported tropical moisture and heat northward. The ascending air motion observed along the Red Sea enhanced convection there, which transported lower-level moisture from the water surface and transported it to the surrounding regions. Finally, a moving upper-trough that approached the region from the west transported the tropical moisture further to the Negev and supplied the lifting that transformed this moisture and instability into heavy showers.     

A Genetic Algorithm for Stochastic Inversion in Contaminant Subsurface Hydrology

Identifying the spatial distribution of hydrological properties of aquifers is a key problem in subsurface hydrology. The aquifer structure plays an important role in contaminant transport. Identifying the properties (primarily the hydraulic conductivity) is essentially an inversion problem that is ill-posed, non-unique and computationally intensive by definition. In this work, the non-uniqueness of the inverse problem is tackled via a novel Genetic Algorithm approach combined with a geostatistical method (Sequential Indicator Simulations) for construction of realizations of properties spatial distributions, which are modeled as random. The Genetic Algorithm cross-over operator is based on a novel concept of pilot-planes: daughter realizations adopt pilot-planes from one of their parents. In addition, each aquifer realization is conditioned on the geological hard data and is constructed by sampling the facies distribution, evaluated by indicator variograms. The approach is illustrated in two test cases: a synthetic two-dimensional (2D) case and an actual three-dimensional (3D) case. The results have shown the ability of the proposed approach to generate a set of realizations, where each individual exhibits minor deviations from the measurements. Further, a comparison between the proposed approach and direct (Monte Carlo) approach shows that the Genetic Algorithm was able to generate an ensemble of solutions with a better fitting of the measurements than the direct approach by a significantly reduced computational effort.     

Indication for circular organization of column sprite elements associated with Eastern Mediterranean winter thunderstorms

We report synchronized optical observations of sprites in Israel during the winters of 2006/7–2007/8. Based on several events, we suggest that the elements of columniform sprites are organized in spaced intervals on the circumference of a circle centered directly above, or a little offset, to the vertical direction from the parent lightning. In 2D images most of the cases show columns to be arranged in highly eccentric elliptical forms or in straight rows. The analysis of the optical images provided the geometrical dimensions of the columns and their spatial organization. We used an electrostatic model of the QE field with reasonable assumptions on the location and magnitude of the cloud charge center, constrained by ELF evaluation of the Charge Moment Change in the parent flash, to show that the observed diameter of the columnar arrangement closely matches the conventional breakdown field line contour at the same altitude.     

Prediction of Remediation of a Heterogeneous Aquifer: A Case Study

Contaminant plumes whose characteristic length is smaller than the horizontal integral scale of the hydraulic conductivity, K, are abundant in shallow, phreatic aquifers. In such cases, the aquifer can be regarded as layered, with K being only a function of the vertical coordinate. The heterogeneity of K has a critical role upon the efficiency of remediation of such sites, for example, by Pump and Treat schemes. The expected efficiency is a random variable, with uncertainty. Quantifying this uncertainty can be of great importance to decision making. In this study, we focus on a case study in the coastal aquifer of Israel and compare two different approaches for constructing realizations of K: continuous and indicator. We observe a significant difference between the constructed realizations, which results in a considerable difference in the predicted remediation efficiency and its uncertainty. Furthermore, we study the effect of conditioning the realizations by a rather limited number of K data points. We find that the conditioning results in a major reduction of the uncertainty. In addition, we compare the results of the transport model to a simplified semi‐analytical solution that is based on assuming radial flow. We find a good agreement with the three‐dimensional numerical model. This result illustrates that the simplified solution can be used for prediction of the remediation efficiency when the flow at the plume vicinity can be regarded as radial.     

Source signature and elastic waves in a half‐space under a sustainable line‐concentrated impulsive normal force

First, the response of an ideal elastic half‐space to a line‐concentrated impulsive normal load applied to its surface is obtained by a computational method based on the theory of characteristics in conjunction with kinematical relations derived across surfaces of strong discontinuities. Then, the geometry is determined of the obtained waves and the source signature—the latter is the imprint of the spatiotemporal configuration of the excitation source in the resultant response. Behind the dilatational precursor wave, there exists a pencil of three plane waves extending from the vertex at the impingement point of the precursor wave on the stress‐free surface of the half‐space to three points located on the other two boundaries of the solution domain. These four wave‐arresting points (end points) of the three plane waves constitute the source signature. One wave is an inhibitor front in the behaviour of the normal stress components and the particle velocity, while in the behaviour of the shear stress component, it is a surface‐axis wave. The second is a surface wave in the behaviour of the horizontal components of the dependent variables, while the third is an inhibitor wave in the behaviour of the shear stress component. An inhibitor wave is so named, since beyond it, the material motion is dying or becomes uniform. A surface‐axis wave is so named, since upon its arrival, like a surface wave, the dependent variable in question features an extreme value, but unlike a surface wave, it exists in the entire depth of the solution domain. It is evident from this work that Saint‐Venant's principle for wave propagation problems cannot be formulated; therefore, the above results are a consequence of the particular model proposed here for the line‐concentrated normal load. Copyright © 2002 John Wiley & Sons, Ltd.