Pinhas Alpert

The paradoxical increase of Mediterranean extreme daily rainfall in spite of decrease in total values

] Earlier reports indicated some specific isolated regionsexhibiting a paradoxical increase of extreme rainfall in spite ofdecrease in the totals. Here, we conduct a coherent study of thefull-scale of daily rainfall categories over a relatively largesubtropical region- the Mediterranean- in order to assess whetherthis paradoxical behavior is real and its extent. We show that thetorrential rainfall in Italy exceeding 128 mm/d has increasedpercentage-wise by a factor of 4 during 1951–1995 with strongpeaks in El-Nino years. In Spain, extreme categories at both tails ofthe distribution (light: 0-4 mm/d and heavy/torrential: 64 mm/d andup) increased significantly. No significant trends were found inIsrael and Cyprus. The consequent redistribution of the dailyrainfall categories -torrential/heavy against the moderate/lightintensities - is of utmost interest particularly in the semi-aridsub-tropical regions for purposes of water management, soilerosion and flash floods impacts. I

Factor Separation in Numerical Simulations

Abstract A simple method is developed for computing the interactions among various factors influencing the atmospheric circulations. It is shown how numerical simulations can be utilized to obtain the pure contribution of any factor to any predicted field, as well as the contributions due to the mutual interactions among two or more factors. The mathematical basis for n factors is developed, and it is shown that 2n simulations are required for the separation of the contributions and their possible interactions. The method is demonstrated with two central factors, the topography and surface fluxes, and their effect on the rainfall distribution for a cyclone evolution in the Mediterranean.

HYMEX , a 10-year Multidisciplinary Program on the mediterranean water cycle.

The Mediterranean countries are experiencing important challenges related to the water cycle, including water shortages and floods, extreme winds, and ice/snow storms, that impact critically the socioeconomic vitality in the area (causing damage to property, threatening lives, affecting the energy and transportation sectors, etc.). There are gaps in our understanding of the Mediterranean water cycle and its dynamics that include the variability of the Mediterranean Sea water budget and its feedback on the variability of the continental precipitation through air–sea interactions, the impact of precipitation variability on aquifer recharge, river discharge, and soil water content and vegetation characteristics specific to the Mediterranean basin and the mechanisms that control the location and intensity of heavy precipitating systems that often produce floods. The Hydrological Cycle in Mediterranean Experiment (HyMeX) program is a 10-yr concerted experimental effort at the international level that aims to advance the scientific knowledge of the water cycle variability in all compartments (land, sea, and atmosphere) and at various time and spatial scales. It also aims to improve the processes-based models needed for forecasting hydrometeorological extremes and the models of the regional climate system for predicting regional climate variability and evolution. Finally, it aims to assess the social and economic vulnerability to hydrometeorological natural hazards in the Mediterranean and the adaptation capacity of the territories and populations therein to provide support to policy makers to cope with water-related problems under the influence of climate change, by linking scientific outcomes with related policy requirements.

Quantification of dust-forced heating of the lower troposphere

Aerosols may affect climate through the absorption and scattering of solar radiation and, in the case of large dust particles, by interacting with thermal radiation. But whether atmospheric temperature responds significantly to such forcing has not been determined; feedback mechanisms could increase or decrease the effects of the aerosol forcing. Here we present an indirect measure of the tropospheric temperature response by explaining the ‘errors’ in the NASA/Goddard model/data-assimilation system. These errors, which provide information about physical processes missing from the predictive model, have monthly mean patterns that bear a striking similarity to observed patterns of dust over the eastern tropical North Atlantic Ocean. This similarity, together with the high correlations between latitudinal location of inferred maximum atmospheric heating rates and that of the number of dusty days, suggests that dust aerosols are an important source of inaccuracies in numerical weather-prediction models in this region. For the average dust event, dust is estimated to heat the lower atmosphere (1.5–3.5 km altitude) by ∼0.2 K per day. At about 30 dusty days per year, the presence of the dust leads to a regional heating rate of ∼6 K per year.

Global dimming or local dimming?: Effect of urbanization on sunlight availability

[1] From the 1950s to the 1980s, a significant decrease of surface solar radiation has been observed at different locations throughout the world. Here we show that this phenomenon, widely termed global dimming, is dominated by the large urban sites. The global-scale analysis of year-to-year variations of solar radiation fluxes shows a decline of 0.41 W/m 2 /yr for highly populated sites compared to only 0.16 W/m 2 /yr for sparsely populated sites (<0.1 million). Since most of the globe has sparse population, this suggests that solar dimming is of local or regional nature. The dimming is sharpest for the sites at 10°N to 40°N with great industrial activity. In the equatorial regions even the opposite trend to dimming is observed for sparsely populated sites.

A possible link between El Niño and precipitation in Israel

Various data sets related to precipitation in Israel have been analysed to see whether any statistical relationships exist between the El Nino phenomenon and weather in Israel. An understanding of patterns of precipitation in Israel is extremely important given the limited water resources in the Middle East. The authors have analysed seasonal rainfall, streamflow, snowfall and lake level data, and all of these data sets indicate enhanced precipitation during the winter seasons associated with El Nino years. One intriguing finding is that the statistically significant correlations appear only in the last 25 years.

Intermonthly Variability of Cyclone Tracks in the Mediterranean

Abstract Objective calculated monthly cyclonic tracks in the Mediterranean are presented. The intermonthly variations within each season are shown to be quite significant and an particularly notable for December-January, April-May and June-July. We suggest that composite seasonal pictures are not faithful representations of the cyclonic routes over a complex terrain region like the Mediterranean.

Chapter 3 Relations between variability in the Mediterranean region and mid-latitude variability

Publisher Summary The Mediterranean climate is under the influence of both tropical and mid-latitude climate dynamics, being directly affected by continental and maritime air masses with significant origin differences. The peak of the winter season occurs between December and February, when the mid-latitude cyclone belt has usually reached its southernmost position. However, spring and autumn also contribute to a significant amount of precipitation. Being located at the southern limit of the North Atlantic storm tracks; the Mediterranean region is particularly sensitive to interannual shifts in the trajectories of mid-latitude cyclones that can lead to the remarkable anomalies of precipitation and, to a lesser extent, of temperature. Storm-track variability impacts primarily the western Mediterranean, but it hasa signature clearly detected in the eastern Mediterranean as well. The complex orography that characterizes most regions surrounding the Mediterranean basin can modulate and even distort climate anomaly patterns that otherwise would be geographically much more homogenous. Lack of water in winter and spring reflects in the crop yield. However, too much water in winter is harmful by drowning the seeds and retarding root development. The variability of precipitation plays a crucial role in the management of regional agriculture, in environment, in water resources and ecosystems, as well as social development and behavior.

A Model Simulation of the Summer Circulation from the Eastern Mediterranean past Lake Kinneret in the Jordan Valley

Abstract A model is described for the representation and study of air flow from the eastern Mediterranean (on the west side of the models domain) past Lake Kinneret in the Jordan Valley (about 210 m below MSL) and beyond to the east (on the east side of the model) in the summer months. The primary purpose of the model is to improve our understanding of two striking features of the meteorology of the lake area in summer: 1) the almost daily development of strong winds and an associated storm on the lake in the afternoon, and 2) a nearly 50% drop in wind speed across the lake, from the western to the eastern shore, over a distance of only 10 km, again in the afternoon. The model is two-dimensional. It uses the sigma coordinate system and is thus hydrostatic. The horizontal grid distance is 4 km. It has 10 levels in the vertical, between the surface and the top at 750 mb. The first level in the vertical is 10 m above the surface and each of the higher levels is at an attitude that is approximately twice the...

Long term changes in diurnal temperature range in Cyprus

Abstract Long term temperature data from two stations on the island of Cyprus have been analysed. Increasing trends of approximately 1°C/100 years in the annual mean temperatures are found at both stations. However, the minimum daily temperatures have generally increased at a larger rate than the maximum daily temperatures, resulting in a decrease in the long-term diurnal temperature range. This decrease ranges from −0.5°C/100 years to −3.5°C/100 years, depending on the location. The reduction in the diurnal temperature range is consistent with observations from other parts of the globe, and may indicate that the climate in this region of the globe is part of a larger global climate change that has been occurring over the last century. It is possible that long term changes in greenhouse gas concentrations in the atmosphere are responsible for the long-term annual mean temperature increase. Furthermore, the changes in the diurnal temperature range can possibly be explained by increases in cloud cover and/or tropospheric aerosols. It is possible that part of these changes is caused by local land-use changes, primarily by the increasing urbanization of Cyprus.