- Ariel Cohen, The Hebrew University of JerusalemDr. Cohen is the Professor in the Institute of Earth Sciences at the H... moreAriel Cohen, The Hebrew University of JerusalemDr. Cohen is the Professor in the Institute of Earth Sciences at the Hebrew University of Jerusalem and Chairman of the Jerusalem Society of Science, Computers, and Software Engineering. In addition to his considerable experience teaching Earth and space sciences, he was among the founding scientists for introducing remote sensing research in Israel, including laser radar systems and satellite systems. E-Mail: cohariel@gmail.comedit
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The previously adopted values of the depolarization factor of clear atmospheric air are checked with the values obtained by lidar facilities, and with those obtained in laboratory measurements, using scattering of a He-Ne gas laser. These... more
The previously adopted values of the depolarization factor of clear atmospheric air are checked with the values obtained by lidar facilities, and with those obtained in laboratory measurements, using scattering of a He-Ne gas laser. These values and the importance of the depolarization factor in lidar measurements are discussed.
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The representation of turbulent mixing within the lower troposphere is needed to accurately portray the vertical thermodynamic and kinematic profiles of the atmosphere in mesoscale model forecasts. For mesoscale models, turbulence is... more
The representation of turbulent mixing within the lower troposphere is needed to accurately portray the vertical thermodynamic and kinematic profiles of the atmosphere in mesoscale model forecasts. For mesoscale models, turbulence is mostly a subgrid-scale process, but its presence in the planetary boundary layer (PBL) can directly modulate a simulation’s depiction of mass fields relevant for forecast problems. The primary goal of this work is to review the various parameterization schemes that the Weather Research and Forecasting Model employs in its depiction of turbulent mixing (PBL schemes) in general, and is followed by an application to a severe weather environment. Each scheme represents mixing on a local and/or nonlocal basis. Local schemes only consider immediately adjacent vertical levels in the model, whereas nonlocal schemes can consider a deeper layer covering multiple levels in representing the effects of vertical mixing through the PBL. As an application, a pair of co...
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ABSTRACT An analytical solution for calculation of double Raman-Mie scattering in the presence of a cloud is suggested. The model includes the Raman-Mie and Mie-Raman scattering processes occurring in the cloud volume as well as... more
ABSTRACT An analytical solution for calculation of double Raman-Mie scattering in the presence of a cloud is suggested. The model includes the Raman-Mie and Mie-Raman scattering processes occurring in the cloud volume as well as integrated Raman scattering signal from air molecules along the laser pulse from lidar to the cloud base. The developed algorithm allows for the comparison of relative contribution of these processes to the total Raman-shifted lidar signal. For typical lidar and cloud parameters, double scattering is not negligible and its contribution to the Raman signal is around 20%.
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Scattering matrices are calculated for hexagonal ice columns and plates with preferred orientation near the horizontal plane by means of an algorithm based on geometric optics. Distributions of scattered energy among the various arcs... more
Scattering matrices are calculated for hexagonal ice columns and plates with preferred orientation near the horizontal plane by means of an algorithm based on geometric optics. Distributions of scattered energy among the various arcs inherent to scattering by oriented crystals are obtained. Impact of small orientation deviations from the horizontal plane is discussed.
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A new method to retrieve sizes and flutter of ice crystals in the atmosphere when they reveal their preferably horizontal orientation is proposed and realized. The method consists of the measurement of angular width for the specular... more
A new method to retrieve sizes and flutter of ice crystals in the atmosphere when they reveal their preferably horizontal orientation is proposed and realized. The method consists of the measurement of angular width for the specular component of scattered light in the bistatic sounding scheme. The technique is realized with a floodlight beam and a CCD camera as a detector.
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ABSTRACT A method to obtain the size distribution function of aerosols in the single-scattering and the intermediate optical-depth regimes is presented. It is shown that the double-scattering inversion problem can be reduced to a... more
ABSTRACT A method to obtain the size distribution function of aerosols in the single-scattering and the intermediate optical-depth regimes is presented. It is shown that the double-scattering inversion problem can be reduced to a perturbation expansion in powers of the optical depth τ about the single-scattering solution. The perturbation expansion is summed for all the relevant values of the optical depth by using the Levin convergence acceleration method [J. Comp. Math. 3, 371 (1973)].
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A method is presented for obtaining optical parameters that are relevant to problems of radiative transfer in such fields as polyester fiber insulation and the passage of radiation through aerosol clouds. The concept of the asymmetry... more
A method is presented for obtaining optical parameters that are relevant to problems of radiative transfer in such fields as polyester fiber insulation and the passage of radiation through aerosol clouds. The concept of the asymmetry factor is generalized to include nonspherical particles in order to calculate the ratio of the power of radiated light into the forward direction to the power of backscattering light. The geometry for scattering from an infinite cylinder randomly oriented is discussed and related to the problem of identifying the forward and backward directions. This geometry is used to calculate the asymmetry factor versus the angle which the cylinder axis makes with the direction of incidence. The asymmetry factor is also plotted as a function of the size parameter of the cylinder for random orientations of the cylinder.
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The Shifrin perturbation theory is applied to the scattering of light from finite cylinders and spheroids, and a procedure is described for averaging over the possible orientations of the symmetry axes of these various target shapes. The... more
The Shifrin perturbation theory is applied to the scattering of light from finite cylinders and spheroids, and a procedure is described for averaging over the possible orientations of the symmetry axes of these various target shapes. The possible axis orientations of very long cylinders is limited by the conical shape of the scattering patterns, and the manner of taking this into account in the averaging process is detailed. It is found that the scattering intensities for the scattering of light from randomly oriented long cylinders, with aspect ratios (length/diameter) less than about 200, show differences compared with infinite cylinders of the same radius and dielectric constant (m=1.5). This is especially true for high and low scattering angles and when the plane of linear polarization is changed by the scattering. Moreover, the scattering intensities for the scattering of light from randomly oriented short cylinders and spheroids, with aspect ratios ranging from 0.1 to 10 and t...
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Double scattering lidar events involving long cylindrical particles are discussed. The scatterers are allowed to be randomly oriented and the geometry required in the scattering solution is described in detail. The geometry is general,... more
Double scattering lidar events involving long cylindrical particles are discussed. The scatterers are allowed to be randomly oriented and the geometry required in the scattering solution is described in detail. The geometry is general, made to be applicable to any type of particle for which the single scattering matrix is provided, relative to the main axis of the scatterer. Results of the double scattering calculations are presented for the penetration of the lidar light into clouds of different optical depth τ (τ≤0.5).
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ABSTRACT Lidar detection of atmospheric molecules using a Raman scattering technique being usually limited by low signals is enhanced by integration of the forward Raman scattered light over a large atmospheric volume. This integral can... more
ABSTRACT Lidar detection of atmospheric molecules using a Raman scattering technique being usually limited by low signals is enhanced by integration of the forward Raman scattered light over a large atmospheric volume. This integral can be measured instantaneously in the presence of a reflector at one edge of the optical path, increasing the SNR by a factor of 100 in the case of a perfect reflector and a beam path of ~2 km. Natural reflectors such as clouds leading to the same effect are also discussed.
Research Interests: Mechanical Engineering, Physics, Optics, Medical Imaging, Fiber Optics, and 15 moreLiDAR, Photonics, Medicine, Optical, Light, Applied Optics, Optical physics, Raman Scattering, Raman Spectra, Atmospheric Optics, Mie Scattering, Electrical And Electronic Engineering, Backscattering, Atmospheric Scattering Model, and Path Integral
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An iterative approach to the scattering of light from a finite dielectric cylinder first developed by Shifrin and extended by Acquista is applied to cases where the phase shift is <2, and the cylinder is arbitrarily oriented. It is... more
An iterative approach to the scattering of light from a finite dielectric cylinder first developed by Shifrin and extended by Acquista is applied to cases where the phase shift is <2, and the cylinder is arbitrarily oriented. It is found that the first 2 orders of the iteration converge to within 1% when the aspect ratio (length/diameter) of the cylinder is as small as 20. The results are compared to the exact theory for infinite cylinders, and the effects of finite size are calculated and discussed.
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In recent years, the astronomy teaching community has been called upon to include more cultural aspects of the influence of astronomy across the world in university courses. As an important component connecting the science of predictable... more
In recent years, the astronomy teaching community has been called upon to include more cultural aspects of the influence of astronomy across the world in university courses. As an important component connecting the science of predictable sky alignments with historical human events, students studying the history of science and astronomy often find it cognitively challenging to recognize the fact that till the 16th century AD the most fundamental picture of the planetary system had been distorted with the sun considered to be the fourth planet of the earth rather than the center of our modern planetary system. Similarly, students are often amazed in realizing that the vast majority of all professional and knowledgeable astronomers had also believed that planets control the destiny of all human beings,and, in particular, in predicting extreme events. In this presentation, we concentrate on such impacts of astronomers who stood behind the determination of the chronology of the Bible. Ha...
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Solar light scattering from atmospheric layers is perceived by an observer as the background key. The shape of this background is built in the observer's eyes by the visibility distances in all slant directions. In an average day the... more
Solar light scattering from atmospheric layers is perceived by an observer as the background key. The shape of this background is built in the observer's eyes by the visibility distances in all slant directions. In an average day the atmosphere has a turbidity value in the vicinity of 2 implying that the average contribution to the scattered light by the air molecules is approximately equal to the scattering by the spherical and non-spherical particles in the atmosphere. Since the atmospheric air density and the particulate number density in the horizontal direction are both assumed to be constant whereas their values decreases in a slant direction, the resulting varying visibility distances in the various directions are shown to form a flat sky shape. The multiple scattering calculations are based on scattering functions for spherical and non-spherical particles. The basic approach which is used to handle the scattering of non-spherical particles, is discussed in part II of this presentation.
Research Interests: Engineering and Physics
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The reflection from large absorbing scatterers having a complex refractive index m=n1−iκ (κ>0), is determined by the Fresnel reflection coefficients. Lasing materials with inverted population or having a complex refractive index... more
The reflection from large absorbing scatterers having a complex refractive index m=n1−iκ (κ>0), is determined by the Fresnel reflection coefficients. Lasing materials with inverted population or having a complex refractive index m=n1+iκ ‘‘reflect’’ the incident light with different coefficients, since internal reflections must be taken into account. The reflection coefficients for long cylindrical active scatterers are derived as asymptotic results of the theory of light scattering by large tilted cylinders and explained by polarization considerations. The resulting coefficients can be applied to active scatterers with very large reflection coefficients (negative extinctions).
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Research Interests: Mechanical Engineering, Materials Science, Optics, Medical Imaging, Fiber Optics, and 14 moreTelecommunications, Light Scattering, Photonics, Medicine, Society, Optical, Light, Applied Optics, Optical physics, Dye Lasers, Optical Thickness, Frequency Response, Electrical And Electronic Engineering, and Atmospheric Scattering Model
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When teaching an introductory science survey course to college students learning astronomy at the Hebrew University of Jerusalem, Israel, we have devoted four hours to teaching the history of astronomy as a fruitful strategy to introduce... more
When teaching an introductory science survey course to college students learning astronomy at the Hebrew University of Jerusalem, Israel, we have devoted four hours to teaching the history of astronomy as a fruitful strategy to introduce important concepts surrounding the development of general scientific knowledge throughout history. In order to illustrate the impact of improved accuracy of astronomical measurements, we propose using the example the development of the calendars and, in particular, the widespread Hebrew calendars used throughout the adjacent Millennia of B.C. and A.C. The changes in the several determinations of the Hebrew calendar are demonstrated based on Babylonian and Jewish documents as well as works by al-Khwarizmi from the 9th century AD, found in the Khuda Bakhsh Oriental Library, in Patna India. Our experience suggests that the teaching of calendar development and evolutions demonstrates the interconnectedness between scientific endeavors and social-religi...
We suggest that the teaching of Astronomy should be partly used as a unique platform to illustrate the advancement of science from ancient times along with the strong interrelation between science and religion. Here we chose to describe... more
We suggest that the teaching of Astronomy should be partly used as a unique platform to illustrate the advancement of science from ancient times along with the strong interrelation between science and religion. Here we chose to describe the influence of astronomical measurements that led to the determination of calendars with emphasis on the Islamic epoch:
During the second century the Ethiopian Church placed the world’s year of creation (YOC) at exactly 5500 years before the Incarnation, thus expressing the view that it is related to the first day of the second half of the sixth millennium since their believed to be the YOC. The Ethiopian Church also believed that the astronomical visualization of the sky in the YOC which placed the vernal equinox and the newmoon in the same day, repeated itself in the year 5500.
We showed that the notion of such an "astronomical coincidence" originated from the Jews who believed that the YOC, Exodus, and the building of the Temple were mystically connected by similar rare newmoon events relative to the vernal equinox.
Here we show that the founders of Islam believed in a similar mystical coincidence, explicitly that the 16th of July, 622 AD,- the epoch of the Islamic calendar-, is exactly the day in which the 6000th lunar year started after the biblical creation based on the number of solar years from creation as determined by Eusebius.
We show that our astronomical calculations are in accordance with a tradition mentioned by Abu al-Faḍl and
During the second century the Ethiopian Church placed the world’s year of creation (YOC) at exactly 5500 years before the Incarnation, thus expressing the view that it is related to the first day of the second half of the sixth millennium since their believed to be the YOC. The Ethiopian Church also believed that the astronomical visualization of the sky in the YOC which placed the vernal equinox and the newmoon in the same day, repeated itself in the year 5500.
We showed that the notion of such an "astronomical coincidence" originated from the Jews who believed that the YOC, Exodus, and the building of the Temple were mystically connected by similar rare newmoon events relative to the vernal equinox.
Here we show that the founders of Islam believed in a similar mystical coincidence, explicitly that the 16th of July, 622 AD,- the epoch of the Islamic calendar-, is exactly the day in which the 6000th lunar year started after the biblical creation based on the number of solar years from creation as determined by Eusebius.
We show that our astronomical calculations are in accordance with a tradition mentioned by Abu al-Faḍl and
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The connections between astronomy and timekeeping are longstanding. One approach to the teaching of Astronomy is it can serve as a unique platform to illustrate the advancement of science from ancient times along with the strong... more
The connections between astronomy and timekeeping are longstanding. One approach to the teaching of Astronomy is it can serve as a unique platform to illustrate the advancement of science from ancient times along with the strong interrelation between science and religion. Here we chose to describe the influence of astronomical measurements that led to the determination of calendars with emphasis on the Islamic epoch: During the second century the Ethiopian Church placed the world's year of creation (YOC) at exactly 5500 years before the Incarnation, thus expressing the view that it is related to the first day of the second half of the sixth millennium since their believed to be the YOC. The Ethiopian Church also believed that the astronomical visualization of the sky in the YOC which placed the vernal equinox and the newmoon in the same day, repeated itself in the year 5500. In a previous work we showed that "Astronomical coincidence" is a notion originated from Jews who believed that the YOC, Exodus, and the building of the Temple were mystically connected by similar rare newmoon events relative to the vernal equinox. Here we show that the founders of Islam believed in a similar mystical coincidence, explicitly that the 16th of July, 622 AD,-the epoch of the Islamic calendar-, is exactly the day in which the 6000th lunar year started after the biblical creation based on the number of solar years from creation as determined by Eusebius. We show that our astronomical calculations are in accordance with a tradition mentioned by Abu al-Fadl and Badauni. Downloads Download data is not yet available. Abstract 0 | PDF Downloads 0 PDF
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The previously adopted values of the depolarization factor of clear atmospheric air are checked with the values obtained by lidar facilities, and with those obtained in laboratory measurements, using scattering of a He-Ne gas laser. These... more
The previously adopted values of the depolarization factor of clear atmospheric air are checked with the values obtained by lidar facilities, and with those obtained in laboratory measurements, using scattering of a He-Ne gas laser. These values and the importance of the depolarization factor in lidar measurements are discussed.