Showing papers by "Pinhas Alpert published in 2009"

Frontal Rainfall Observation by a Commercial Microwave Communication Network

Abstract: A novel approach for reconstruction of rainfall spatial–temporal dynamics from a wireless microwave network is presented. It employs a stochastic space–time model based on a rainfall advection model, assimilated using a Kalman filter. The technique aggregates the data in time and space along the direction of motion of the rainfall field, which is recovered from the simultaneous observation of a multitude of microwave links. The technique is applied on a standard microwave communication network used by a cellular communication system, comprising 23 microwave links, and it allows for observation of near-surface rainfall at the temporal resolutions of 1 min. The accuracy of the method is demonstrated by comparing instantaneous rainfall estimates with measurements from five rain gauges, reaching correlations of up to 0.85 at the 1-min time interval with a bias and RMSE of −0.2 and 4.2 mm h−1, respectively, and up to 0.96 with RMSE of 1.6 mm h−1 at the 10-min time interval for a 22-h intensive rainsto...

Technical Note: Novel method for water vapour monitoring using wireless communication networks measurements

Abstract: . We propose a new technique that overcomes the obstacles of the existing methods for monitoring near-surface water vapour, by estimating humidity from data collected through existing wireless communication networks. Weather conditions and atmospheric phenomena affect the electromagnetic channel, causing attenuations to the radio signals. Thus, wireless communication networks are in effect built-in environmental monitoring facilities. The wireless microwave links, used in these networks, are widely deployed by cellular providers for backhaul communication between base stations, a few tens of meters above ground level. As a result, if all available measurements are used, the proposed method can provide moisture observations with high spatial resolution and potentially high temporal resolution. Further, the implementation cost is minimal, since the data used are already collected and saved by the cellular operators. In addition – many of these links are installed in areas where access is difficult such as orographic terrain and complex topography. As such, our method enables measurements in places that have been hard to measure in the past, or have never been measured before. The technique is restricted to weather conditions which exclude rain, fog or clouds along the propagation path. Strong winds that may cause movement of the link transmitter or receiver (or both) may also interfere with the ability to conduct accurate measurements. We present results from real-data measurements taken from two microwave links used in a backhaul cellular network that show convincing correlation to surface station humidity measurements. The measurements were taken daily in two sites, one in northern Israel (28 measurements), the other in central Israel (29 measurements). The correlation between the microwave link measurements and the humidity gauges were 0.9 and 0.82 for the north and central sites, respectively. The Root Mean Square Differences (RMSD) were 1.8 g/m3 and 3.4 g/m3 for the northern and central site measurements, respectively.

Temporal rainfall fluctuations in Israel and their possible link to urban and air pollution effects

Abstract: In this paper we analyze spatial variations of the annual rainfall that have taken place in the non-arid regions of Israel (annual rainfall>200 mm) during the years 1952–2006, incorporating all available data. The results of the present study over the research area as a whole indicate that no significant temporal change of the annual rainfall occurred in any region of the study area. However, focusing on spatial rainfall fluctuations between sub-regions in the study area, a significant increase was observed between the stations located downwind and those upwind of the Greater Tel Aviv region. This increase supports previous reports showing that rainfall enhancement is observed downwind (and close) to urban centers. In contrast to a few previous reports, no decrease in the ratio between the mountain precipitation to that over the coastal region was found. Over the period of the present study, the rainfall ratio between the upwind slopes and the seashore remained unchanged, with a slight increase in the central part of the country. The only hilly place where a slight decrease in annual rainfall was observed is the lee side (eastern slopes) of the Galilee Mountains. This result is important because the eastern slopes of the Galilee Mountains have for years been part of the target area for Israeli artificial cloud seeding for rain enhancement. The results therefore suggest that unless there was a pronounced change in the synoptic conditions during rain spells, seeding in Israel had no positive effect on rainfall amounts.

Trends in daily rainfall intensity over Israel 1950/1-2003/4.

Abstract: The study focuses on long-term trends of daily rainfall in Israel as a function of their intensity in order to identify potential trends in rainfall extremity. The study period is the rainy season, October-May between 1950/1 and 2003/4. For the total rainfall, an increased trend is shown across Israel, especially for the central and southern regions, though non-significant. Daily rainfall intensity showed non-significant trends of increase in the heavy rainfall at the center and south and decrease at the north. The light to moderate rainfall trends increased in the north while they decreased at the center and south. Trends are significantly correlated with known teleconnection patterns, especially the East Atlantic- Western Russia and the North Sea-Caspian Sea patterns. Positive trends toward heavier rainfall are noted in Israel, which are significant in several specific locations. This finding has to be carefully followed since the region is a climatic border subjected to severe water shortage and is predicted to dry-up in most global warming scenarios. During the last decades there is an increasing evidence for more extreme rainfall at different places around the globe. Global warming was suggested to be linked with an increase in heavy rainfall due to an increase in atmospheric vapor and the warmer air. This trend is shown in the 4 th Assessment Report of the IPCC (1). In general, the Mediterranean Basin shows decreasing trends in the precipitation amount (2-4). The EM shows mixed rainfall trends but almost all stations show a decreasing trend in the precipitation which is especially large and significant during winter in the Greek, Cypriot and Turkish stations (5). Precipitation reduction over most Greek stations, combined with enhanced anticyclonic activity over almost the whole Mediterranean was found during the late 1980s and the early 1990s, a period characterized by high North Atlantic oscillation index values (6). Time series of extreme precipitation cases in Greece for the period 1970-2002, did not reveal significant positive or negative trend (7). Alpert et al. (8) showed the paradoxical increase of Mediterranean extreme daily rainfall in spite of the decrease in the total rainfall. They showed an increase in the torrential rainfall in Italy and Spain, heavy rainfall in Israel and heavy- torrential rainfall over Cyprus for the period 1951-1995. These trends were significant only in Italy and Spain. Kostopoulou and Jones (5) analyzed seasonal and annual trends over the central and Eastern Mediterranean (EM, 1958-2000). They showed statistically significant positive trends toward intense rainfall events and greater amount of precipitation for stations around the Italian Peninsula. For the EM, i.e., the Balkan Peninsula, western Turkey and

Synoptic classification of Moderate Resolution Imaging Spectroradiometer aerosols over Israel

Abstract: [1] The origins and variety of aerosols in Israel are strongly influenced by synoptic conditions as well as other variables. Days affected by a weak Persian trough system are characterized by coarse aerosols from North Africa, which contain mainly mineral dust aerosols, whereas days affected by the Red Sea trough with an eastern axis are characterized by fine aerosols from northern Europe. Some synoptic systems contain both groups of aerosols as well as mixed aerosols, e.g., the Red Sea trough with a central axis. This study utilizes the Moderate Resolution Imaging Spectroradiometer (MODIS) Terra Satellite to obtain a synoptic classification of aerosols over Israel for 6 years. MODIS synoptic classification revealed a number of different aerosol types. For weak Persian trough, a high from the west and a medium Persian trough, the MODIS Terra aerosol optical thickness (AOT) fine mode fractions are 0.51, 0.54, and 0.51, respectively. While for Red Sea trough with an eastern or central axis AOT fine mode fraction (‘f’) is higher and is equal to 0.63 and 0.59, respectively. This is the first study in the region that investigates average back trajectory with regards to both synoptic systems and aerosol type. Investigation of back trajectories for the Persian trough synoptic system indicates that days with low ‘f’ at 700 hPa are associated with flows from North Africa, while days with high ‘f’ are linked to flow from Europe. Most of our findings are not unexpected, but provide the first 6 years quantified aerosol classification which is based on the synoptic systems.

Variations of meridional aerosol distribution and solar dimming

Abstract: Meridional distribution of aerosol optical thickness (AOT) over the ocean was analyzed by using the eight-year MISR and MODIS-Terra data sets, from March 2000 to February 2008, as well as the five-year MODIS-Aqua data set, from July 2002 to June 2007. The three independent sensors show that there was a pronounced meridional aerosol asymmetry. It was found that there were strong seasonal variations in the hemispheric aerosol asymmetry: it was pronounced during the half-year period, from March to August (the most pronounced asymmetry was observed from April to July). There was no noticeable asymmetry during the season from September to December. Not only has the Northern hemisphere, where the main sources of aerosols are located, but also the Southern hemisphere contributed to the formation of noticeable aerosol asymmetry. The increase in AOT, averaged over the Northern hemisphere during the season of pronounced hemispheric aerosol asymmetry, was accompanied by a decrease in AOT, averaged over the Southern hemisphere. In both hemispheres, amplitudes of seasonal AOT variations decreases from mid-latitudes (60N - 30N and 30S - 60S) to low latitudes (30N - 0 and 0 - 30S) respectively, indicating that the contribution of AOT averaged over mid-latitudes to the formation of pronouncedmore » meridional hemispheric asymmetry and its seasonal variations is more significant than the contribution of AOT averaged over low latitudes. For the season of prominent hemispheric aerosol asymmetry, from April to July, during the eight-year period under consideration, a declining long-term tendency of AOT prevailed at latitudes between 30oN and 60oN, suggesting brightening over the cloud-free ocean.« less

Variations of meridional aerosol distribution and solar dimming

Abstract: Meridional distribution of aerosol optical thickness (AOT) over the ocean was analyzed by using the eight-year MISR and MODIS-Terra data sets, from March 2000 to February 2008, as well as the five-year MODIS-Aqua data set, from July 2002 to June 2007. The three independent sensors show that there was a pronounced meridional aerosol asymmetry. It was found that there were strong seasonal variations in the hemispheric aerosol asymmetry: it was pronounced during the half-year period, from March to August (the most pronounced asymmetry was observed from April to July). There was no noticeable asymmetry during the season from September to December. Not only has the Northern hemisphere, where the main sources of aerosols are located, but also the Southern hemisphere contributed to the formation of noticeable aerosol asymmetry. The increase in AOT, averaged over the Northern hemisphere during the season of pronounced hemispheric aerosol asymmetry, was accompanied by a decrease in AOT, averaged over the Southern hemisphere. In both hemispheres, amplitudes of seasonal AOT variations decreases from mid-latitudes (60N - 30N and 30S - 60S) to low latitudes (30N - 0 and 0 - 30S) respectively, indicating that the contribution of AOT averaged over mid-latitudes to the formation of pronouncedmore » meridional hemispheric asymmetry and its seasonal variations is more significant than the contribution of AOT averaged over low latitudes. For the season of prominent hemispheric aerosol asymmetry, from April to July, during the eight-year period under consideration, a declining long-term tendency of AOT prevailed at latitudes between 30oN and 60oN, suggesting brightening over the cloud-free ocean.« less

Projections of Climate Change over Non-boreal East Europe During First Half of Twenty-First Century According to Results of a Transient RCM Experiment

Abstract: Climate change trends over the southern east-Europe are evaluated according to results of a climate simulation experiment with the ICTP RegCM3 regional climate model driven from the lateral boundaries by results of ECHAM5/MPI-OM1 transient climate simulation from 1960 to 2060 (SRES A1B emission scenario after 2001). The trends projected include – precipitation: winter and spring – rise over the central east-Europe and drop over the eastern Mediterranean region, summer-autumn – drop over east-Europe and northern eastern-Mediterranean, rise over the Middle East (especially in autumn); 2-m air temperature: winter and spring – rise over the whole region with a maximum over its eastern and north-eastern (especially) and south-eastern parts, summer – rise with a maximum over the Middle East and minimum over north-eastern part, autumn – rise with maximum over the Caspian, Black Seas and northern areas of the European Territory of Russia.