Institute of Geophysics, University of TehranJournal of the Earth and Space Physics2538-371X34120080320The effect of magnetic storms on ionospheric layersThe effect of magnetic storms on ionospheric layers1119443FAJournal Article20171031https://jesphys.ut.ac.ir/article_19443_b902f673d98e75439c9d89b3e016a979.pdfInstitute of Geophysics, University of TehranJournal of the Earth and Space Physics2538-371X34120080320The effects of the Alborz mountain range on the development of synoptic weather systemsThe effects of the Alborz mountain range on the development of synoptic weather systems1119444FAJournal Article20171031In this paper by using MM5 modeling system, the effects of the Alborz mountain range on the development of synoptic weather systems on the leeward side of the mountains have been studied. The primary aim was to see if the cyclones are produced and intensified (cyclogenesis), however the cyclogenesis was found not to be marked and hence the focus of the study was on the effects of the mountain on weather systems reaching this area. <br />The extent of the simulation domain is from 24 E to 74 E and 14.6 N to 53.6 N with horizontal grid resolution of 25 kilometer, and 23 half-sigma levels in the vertical. The effect of terrain elevation was considered by smoothing the elevation typically by 50 percent (near the peaks by 70 percent and about 10 percent near the topographic borders). We found that the north-westerly weather system(24th to 27th Dec.2005) crossing the Alborz mountain range showed a sharp difference between the case with topography and that without it. These changes were particularly observed in potential temperature field, Froude number, vertical speeds and precipitation. The vertical speeds as well as potential temperatures were found to be reduced markedly. Also the 48-hours accumulative precipitation was found to be reduced too. By removing the topography, the Froude number of the flow was substantially increased; indicating that wave activity over the area was reduced. Typical Froude numbers for the case with topography is about 0.9, which is a favorable condition for formation of Lee waves, but without topography it appears to be much larger than 1, which is not a favorable condition for wave formation. <br />We also found that the westerly and south-westerly weather systems moving to this area were hardly influenced by the Alborz mountain range. This may be due to the fact that these systems are greatly weakened by the smaller topographic features along their path as well as losing most of their moisture over the Zagros mountain range, although this needs further study.In this paper by using MM5 modeling system, the effects of the Alborz mountain range on the development of synoptic weather systems on the leeward side of the mountains have been studied. The primary aim was to see if the cyclones are produced and intensified (cyclogenesis), however the cyclogenesis was found not to be marked and hence the focus of the study was on the effects of the mountain on weather systems reaching this area. <br />The extent of the simulation domain is from 24 E to 74 E and 14.6 N to 53.6 N with horizontal grid resolution of 25 kilometer, and 23 half-sigma levels in the vertical. The effect of terrain elevation was considered by smoothing the elevation typically by 50 percent (near the peaks by 70 percent and about 10 percent near the topographic borders). We found that the north-westerly weather system(24th to 27th Dec.2005) crossing the Alborz mountain range showed a sharp difference between the case with topography and that without it. These changes were particularly observed in potential temperature field, Froude number, vertical speeds and precipitation. The vertical speeds as well as potential temperatures were found to be reduced markedly. Also the 48-hours accumulative precipitation was found to be reduced too. By removing the topography, the Froude number of the flow was substantially increased; indicating that wave activity over the area was reduced. Typical Froude numbers for the case with topography is about 0.9, which is a favorable condition for formation of Lee waves, but without topography it appears to be much larger than 1, which is not a favorable condition for wave formation. <br />We also found that the westerly and south-westerly weather systems moving to this area were hardly influenced by the Alborz mountain range. This may be due to the fact that these systems are greatly weakened by the smaller topographic features along their path as well as losing most of their moisture over the Zagros mountain range, although this needs further study.https://jesphys.ut.ac.ir/article_19444_35ef6f2e52d796e2b2cd3a2ce2167aa7.pdfInstitute of Geophysics, University of TehranJournal of the Earth and Space Physics2538-371X34120080320Numerical simulation of the impact of orography on active synoptic weather systems over IranNumerical simulation of the impact of orography on active synoptic weather systems over Iran1119445FAJournal Article20171031Orography plays a substantial role in the formation and evolution of many atmospheric phenomena. Observations indicate that two mountain ranges to the southwest of the Red Sea and west of Iran (Zagross mountain range) play a crucial role in the formation, evolution and activities of synoptic weather systems over Iran. Observations also, indicate that there are considerable differences between the amount of precipitation over the west and east of the Zagross mountain range. <br />In this paper, the effects of those two mountain ranges on the formation, evolution and activities of a synoptic system over Iran between 23rd and 30th of December 2004 have been studied. To do this fifth-generation of PSU (Pennsylvania State University) /NCAR (National Center for Atmospheric Research) Meso- and Micro-scale Modeling system (MM5) was used. To provide a clear picture of the development and evolution of the system an area bounded to 10 to 50 degrees north and 20 to 70 degrees east was selected. A rectangle grid by 101 zonal grid points and 81 meridional grid points with a distance of 55.5km between the grid points was implemented for simulation and modeling purposes and the study of dynamical and thermodynamical processes involved in the development and evolution of the system. Betts-Miller, Grell, Blackadar, and MRF schemes were used to model different physical, thermodynamical and dynamical processes. <br />Model results show that the elimination of the mountain range to the southwest of the Red Sea leads to widen the existing low pressure over the southwest area of the Red Sea towards the east. It also leads to a decrease in the small high pressure cell that is believed to play a substantial role in the formation of the so-called Red Sea trough towards the north of the Red Sea and southeast of the Mediterranean Sea, where many of the weather systems that pass over Iran during the winter form. Therefore, it can be concluded that elimination of that mountain range will destroy part of the mechanism essential for the formation of some weather systems that are important for Iran. <br />Model results also state that the elimination of Zagross mountain range could lead to the enhancement of precipitation over the central part of the country, east of the range.Orography plays a substantial role in the formation and evolution of many atmospheric phenomena. Observations indicate that two mountain ranges to the southwest of the Red Sea and west of Iran (Zagross mountain range) play a crucial role in the formation, evolution and activities of synoptic weather systems over Iran. Observations also, indicate that there are considerable differences between the amount of precipitation over the west and east of the Zagross mountain range. <br />In this paper, the effects of those two mountain ranges on the formation, evolution and activities of a synoptic system over Iran between 23rd and 30th of December 2004 have been studied. To do this fifth-generation of PSU (Pennsylvania State University) /NCAR (National Center for Atmospheric Research) Meso- and Micro-scale Modeling system (MM5) was used. To provide a clear picture of the development and evolution of the system an area bounded to 10 to 50 degrees north and 20 to 70 degrees east was selected. A rectangle grid by 101 zonal grid points and 81 meridional grid points with a distance of 55.5km between the grid points was implemented for simulation and modeling purposes and the study of dynamical and thermodynamical processes involved in the development and evolution of the system. Betts-Miller, Grell, Blackadar, and MRF schemes were used to model different physical, thermodynamical and dynamical processes. <br />Model results show that the elimination of the mountain range to the southwest of the Red Sea leads to widen the existing low pressure over the southwest area of the Red Sea towards the east. It also leads to a decrease in the small high pressure cell that is believed to play a substantial role in the formation of the so-called Red Sea trough towards the north of the Red Sea and southeast of the Mediterranean Sea, where many of the weather systems that pass over Iran during the winter form. Therefore, it can be concluded that elimination of that mountain range will destroy part of the mechanism essential for the formation of some weather systems that are important for Iran. <br />Model results also state that the elimination of Zagross mountain range could lead to the enhancement of precipitation over the central part of the country, east of the range.https://jesphys.ut.ac.ir/article_19445_d55c1b79b4a6ab64122db394dadede3c.pdfInstitute of Geophysics, University of TehranJournal of the Earth and Space Physics2538-371X34120080320Post processing of MM5 forecasts for minimum and maximum temperature using a Kalman filterPost processing of MM5 forecasts for minimum and maximum temperature using a Kalman filter1119446FAJournal Article20171031https://jesphys.ut.ac.ir/article_19446_115f3024a5e23cd034dc1c7b79cd79c2.pdfInstitute of Geophysics, University of TehranJournal of the Earth and Space Physics2538-371X34120080320Application of cross-spectral analysis to computations of sea surface topography case study: Persian GulfApplication of cross-spectral analysis to computations of sea surface topography case study: Persian Gulf1119447FAJournal Article20171031https://jesphys.ut.ac.ir/article_19447_140fcdba7e0764a294a40ab43c2fc438.pdfInstitute of Geophysics, University of TehranJournal of the Earth and Space Physics2538-371X34120080320Exploration of groundwater-bearing fracture zones integrating VLF and resistivity methodsExploration of groundwater-bearing fracture zones integrating VLF and resistivity methods1119448FAJournal Article20171031Because of urgent needs of groundwater resources in the Tochal region of north Tehran, an integration of electric and electromagnetic methods was applied to investigate the location and the direction of water bearing fractures. First, the VLF electromagnetic profiling was performed on suitable places and two locations were detected for supplementary resistivity surveys. Then, a number of vertical electrical soundings using Schlumberger array were carried out on suitable locations. With the presumption of a layered earth, three soundings in every two locations were merged and the 2D electrical resistivity cross-sections were made and the best location for groundwater- bearing fracture zone was detected. <br />The VLF method has been applied successfully to map the resistivity contrast at boundaries of fractured zones having a high degree of connectivity (Parasnis, 1973). Further, the VLF method yields a higher depth of penetration in hard rock areas because of their high resistivity. Therefore, a combined study of VLF and DC resistivity has the potential to be successful (Bernard and valla, 1991; Benson et al., 1997). <br />For tilt angle measurements, magnetic field coupling with the fracture zone is important. Therefore, the VLF-transmitter should be located along the strike of the target. The depth of investigation is dependent on the frequency used and the resistivity of the host medium. Sharma and Baranwal (2005) suggested a method using real component curve of VLF measurements to locate water bearing fractures. In this method, every significant leap in the data curve indicates a conductive anomaly referring to a fracture in a hard rock medium. <br />Integrated geophysical studies were performed on the Tochal telecabin region, north of Tehran, Iran. Tochal telecabin is a cultural-sporting complex that begins from the end of Velenjak Street and continues to the vertex of Tochal Mountain. This study was accomplished by a VLF survey followed by a resistivity survey using Schlumberger array to find a suitable location for drilling a well in the Tochal telecabin region. <br />In order to find the locations that contain water-bearing fractures, nine VLF profiles were performed to study along the main road of the Tochal telecabin. First a suitable transmitter with sufficient power and admissible direction was found and then the measurements with 5m station spacing were done. It is important to note that almost all of the measurements were taken before sunrise when the city wireless transmitters had not begun to work. Because city wireless transmitters generate noise, the instrument cannot get a suitable signal. High noise level is a serious problem on a cold and snowy day. Moreover, the active installations and high voltage transporter cables affect the signal and increase the noise level. <br />VLF data were collected using an ABEM-WADI instrument. With regard to the pseudo current-density section of first VLF profile, we focused on two major zones along the road. For zone 1, since the probable fault direction in the formation was approximately in the E–W direction, the HWU transmitter in this direction with a frequency of 18.3 kHz located in Le Blance, France was used. For zone 2, since the probable fault direction in the formation was approximately in the N–S direction, the RCV transmitter in this direction with a frequency of 27.1 kHz located in Russia, was used. Next, the other profiles were surveyed along hydrogeologically suitable locations with probable fractures and fault. <br />Six Schlumberger resistivity soundings were performed in two zones using a DC resistivity meter. Sounding locations were selected by detailed study of the area with a VLF survey as well as by their hydrogeological suitability. <br />After gathering the measured data, for further detailed information of the subsurface, the measured real anomaly was filtered using the approach of Karous and Hjelt (1983). This process yields pseudo-section of relative current density variationBecause of urgent needs of groundwater resources in the Tochal region of north Tehran, an integration of electric and electromagnetic methods was applied to investigate the location and the direction of water bearing fractures. First, the VLF electromagnetic profiling was performed on suitable places and two locations were detected for supplementary resistivity surveys. Then, a number of vertical electrical soundings using Schlumberger array were carried out on suitable locations. With the presumption of a layered earth, three soundings in every two locations were merged and the 2D electrical resistivity cross-sections were made and the best location for groundwater- bearing fracture zone was detected. <br />The VLF method has been applied successfully to map the resistivity contrast at boundaries of fractured zones having a high degree of connectivity (Parasnis, 1973). Further, the VLF method yields a higher depth of penetration in hard rock areas because of their high resistivity. Therefore, a combined study of VLF and DC resistivity has the potential to be successful (Bernard and valla, 1991; Benson et al., 1997). <br />For tilt angle measurements, magnetic field coupling with the fracture zone is important. Therefore, the VLF-transmitter should be located along the strike of the target. The depth of investigation is dependent on the frequency used and the resistivity of the host medium. Sharma and Baranwal (2005) suggested a method using real component curve of VLF measurements to locate water bearing fractures. In this method, every significant leap in the data curve indicates a conductive anomaly referring to a fracture in a hard rock medium. <br />Integrated geophysical studies were performed on the Tochal telecabin region, north of Tehran, Iran. Tochal telecabin is a cultural-sporting complex that begins from the end of Velenjak Street and continues to the vertex of Tochal Mountain. This study was accomplished by a VLF survey followed by a resistivity survey using Schlumberger array to find a suitable location for drilling a well in the Tochal telecabin region. <br />In order to find the locations that contain water-bearing fractures, nine VLF profiles were performed to study along the main road of the Tochal telecabin. First a suitable transmitter with sufficient power and admissible direction was found and then the measurements with 5m station spacing were done. It is important to note that almost all of the measurements were taken before sunrise when the city wireless transmitters had not begun to work. Because city wireless transmitters generate noise, the instrument cannot get a suitable signal. High noise level is a serious problem on a cold and snowy day. Moreover, the active installations and high voltage transporter cables affect the signal and increase the noise level. <br />VLF data were collected using an ABEM-WADI instrument. With regard to the pseudo current-density section of first VLF profile, we focused on two major zones along the road. For zone 1, since the probable fault direction in the formation was approximately in the E–W direction, the HWU transmitter in this direction with a frequency of 18.3 kHz located in Le Blance, France was used. For zone 2, since the probable fault direction in the formation was approximately in the N–S direction, the RCV transmitter in this direction with a frequency of 27.1 kHz located in Russia, was used. Next, the other profiles were surveyed along hydrogeologically suitable locations with probable fractures and fault. <br />Six Schlumberger resistivity soundings were performed in two zones using a DC resistivity meter. Sounding locations were selected by detailed study of the area with a VLF survey as well as by their hydrogeological suitability. <br />After gathering the measured data, for further detailed information of the subsurface, the measured real anomaly was filtered using the approach of Karous and Hjelt (1983). This process yields pseudo-section of relative current density variationhttps://jesphys.ut.ac.ir/article_19448_9f8ce961240d40e247f5f5418b0223e1.pdfInstitute of Geophysics, University of TehranJournal of the Earth and Space Physics2538-371X34120080320A Comparison of direct and indirect regularization methods for downward continuation problem of geoid computations without applying Stokes formulaA Comparison of direct and indirect regularization methods for downward continuation problem of geoid computations without applying Stokes formula1119449FAJournal Article20171031The problem of downward continuation of the gravity field from the Earth’s surface to the reference ellipsoid arises from the fact that the solution to the boundary value problem for geoid determination without applying Stokes formula is sought in terms of the disturbing potential on the ellipsoid but the disturbing gravity observations are only available on the Earth’s surface. Downward continuation is achieved via Abel-Poisson integral and its derivatives. Using discrete observations, the Abel-Poisson integral has to be transformed into a summation form: <br /> (1) <br />Where the matrix is the design matrix and stands for the disturbing <br />gravity observations vector. The downward continuation problem is an inverse <br />problem. Inverse problems are ill-posed, like any ill-posed problem it must be regularized. The objective of this paper is the comparison between direct and <br />iterative methods for solving downward continuation of the gravity field from the Earth’s surface to the reference ellipsoid for geoid determination without applying Stokes formula. <br />Direct regularization methods are methods where the solution is directly derived. In this contribution truncated method, standard Tikhonov method and generalized Tikhonov method using discretized norms at Sobolov subspaces , and Sobolov semi norms and are implemented. Based on SVD, in truncated methods, the solution can be obtained as: <br /> (2) <br />Where and are the right and the left singular vectors, respectively. is rank of matrix that is a L2 norm approximation for matrix . In the case of TGSVD the solution is obtained as <br /> (3) <br />In standard Tikhonov method, the minimizing function can be written as: <br /> (4) <br />In this method, filter coefficients and solution become: <br /> (5) <br />In standard Tikhonov method, the matrix was . In generalized Tikhonov method, we select the matrix as follows <br /> (6) <br />Where the is obtained from discretization of derivative operators up to order s and coefficients are weight coefficients. <br />In contrast to direct methods, in iterative methods, normal equations are solved via construction of a sequence of the solutions that converge to the pseudo-inverse solution of the equations. In this contribution classical iterative method, Landweber-Fridman method, Tikhonov iterative method, Algebraic Reconstruction Technique (ART), conjugate gradient method and LSQR method are implemented. <br />Classical iterative methods are based on construction of sequences of solutions . For the matrix equation , The following relationship holds between solution and solution : <br /> (7) <br />In Landweber-Fridman method the matrix is equal to diagonal matrix . Ergo, in this method, iterative relation between the solutions is defined as: <br /> (8) <br />In Tikhonov iterative method, iterative relation between the solutions is defined as: <br /> (9) <br />The idea of Algebraic Reconstruction Technique iteration to solve the matrix equation is to partition the system row wise, either into single rows or into blocks of rows. Each of these rows defines a hyper plane of dimension . The idea of the <br />ART iteration is to project the current approximate solution successively onto each one these hyper planes. It turns out that such a procedure converges to the solution of the system. <br />A best known method for solving large scale equations system is conjugate gradient. Conjugate gradient is a type of Krylov subspace method. Conjugate gradient method is suitable for positive definite operators. <br />In LSQR method, solution vector is defThe problem of downward continuation of the gravity field from the Earth’s surface to the reference ellipsoid arises from the fact that the solution to the boundary value problem for geoid determination without applying Stokes formula is sought in terms of the disturbing potential on the ellipsoid but the disturbing gravity observations are only available on the Earth’s surface. Downward continuation is achieved via Abel-Poisson integral and its derivatives. Using discrete observations, the Abel-Poisson integral has to be transformed into a summation form: <br /> (1) <br />Where the matrix is the design matrix and stands for the disturbing <br />gravity observations vector. The downward continuation problem is an inverse <br />problem. Inverse problems are ill-posed, like any ill-posed problem it must be regularized. The objective of this paper is the comparison between direct and <br />iterative methods for solving downward continuation of the gravity field from the Earth’s surface to the reference ellipsoid for geoid determination without applying Stokes formula. <br />Direct regularization methods are methods where the solution is directly derived. In this contribution truncated method, standard Tikhonov method and generalized Tikhonov method using discretized norms at Sobolov subspaces , and Sobolov semi norms and are implemented. Based on SVD, in truncated methods, the solution can be obtained as: <br /> (2) <br />Where and are the right and the left singular vectors, respectively. is rank of matrix that is a L2 norm approximation for matrix . In the case of TGSVD the solution is obtained as <br /> (3) <br />In standard Tikhonov method, the minimizing function can be written as: <br /> (4) <br />In this method, filter coefficients and solution become: <br /> (5) <br />In standard Tikhonov method, the matrix was . In generalized Tikhonov method, we select the matrix as follows <br /> (6) <br />Where the is obtained from discretization of derivative operators up to order s and coefficients are weight coefficients. <br />In contrast to direct methods, in iterative methods, normal equations are solved via construction of a sequence of the solutions that converge to the pseudo-inverse solution of the equations. In this contribution classical iterative method, Landweber-Fridman method, Tikhonov iterative method, Algebraic Reconstruction Technique (ART), conjugate gradient method and LSQR method are implemented. <br />Classical iterative methods are based on construction of sequences of solutions . For the matrix equation , The following relationship holds between solution and solution : <br /> (7) <br />In Landweber-Fridman method the matrix is equal to diagonal matrix . Ergo, in this method, iterative relation between the solutions is defined as: <br /> (8) <br />In Tikhonov iterative method, iterative relation between the solutions is defined as: <br /> (9) <br />The idea of Algebraic Reconstruction Technique iteration to solve the matrix equation is to partition the system row wise, either into single rows or into blocks of rows. Each of these rows defines a hyper plane of dimension . The idea of the <br />ART iteration is to project the current approximate solution successively onto each one these hyper planes. It turns out that such a procedure converges to the solution of the system. <br />A best known method for solving large scale equations system is conjugate gradient. Conjugate gradient is a type of Krylov subspace method. Conjugate gradient method is suitable for positive definite operators. <br />In LSQR method, solution vector is defhttps://jesphys.ut.ac.ir/article_19449_eda2fc64cb0b3900013f218c803adc6c.pdfInstitute of Geophysics, University of TehranJournal of the Earth and Space Physics2538-371X34120080320Time-varying deconvolution operator in Gabor domainTime-varying deconvolution operator in Gabor domain1119450FAJournal Article20171031https://jesphys.ut.ac.ir/article_19450_edd601970ed70eee39daa7f0301fba8e.pdfInstitute of Geophysics, University of TehranJournal of the Earth and Space Physics2538-371X34120080320Investigations of seismic-wave arrival time biases in the stations of Tehran Seismic NetworkInvestigations of seismic-wave arrival time biases in the stations of Tehran Seismic Network1119451FAJournal Article20171031https://jesphys.ut.ac.ir/article_19451_eb1db3ac45287df6a5efd6ab40cbc528.pdfInstitute of Geophysics, University of TehranJournal of the Earth and Space Physics2538-371X34120080320Comparison of ANN and PCA based multivariate linear regression applied to predict the daily average concentration of CO: a case study of TehranComparison of ANN and PCA based multivariate linear regression applied to predict the daily average concentration of CO: a case study of Tehran1119452FAJournal Article20171031https://jesphys.ut.ac.ir/article_19452_46f52721b042c036fee869f8a70f3a3b.pdf