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Item Prediction of Formation and Tracks of Bay of Bengal Cyclones(Dhaka University (DU), Dhaka, Bangladesh., 1998-12) Alam, Dr. Md. Mahbub; Shafee, Prof. SultanaRawindsonde data of 0000 UTC for standard isobaric surfaces at the surface, 850, 700, 500, 400, 300, 200, 150 and 100 hPa levels for the periods of different cyclones in the last decade were considered to study the different meteorological parameters. The vertical wind shear, the vertical variation of zonal wind speed and meridional wind speed and temperature were studied for different cyclones in the surroundings of the Bay of Bengal in relation to the movement of cyclones. The dry static energy, the latent heat energy, the moist static energy, the total energy, the meridional and zonal fluxes of moist static energy and their vertical distribution were studied in the surroundings of the Bay of Bengal in relation to the movement of cyclones and their ultimate landfall. A two dimensional model is developed for the prediction of cyclone tracks in the Bay of Bengal. The boundary value problem is then solved by using the complex potentials. The different cyclones are integrated in different time steps to find the relationship between the actual travel time and observed time by the models. The results obtained for cyclone tracks by this model are almost the same as the actual track. The cyclonic track data were collected from the Storm Warning Centre (SWC) of Bangladesh Meteorological Department (BMD). A simplified system of equations that can simulate the evolution and mature stages of tropical cyclones is presented. The study has been done using a three-dimensional numerical model constructed which produces circularly symmetric feature of the tropical cyclone. It has 5 layers in the vertical with the horizontal extent of 1200 km. The horizontal grid length of the model is 80 km. The model equations have been solved using the Leapfrog scheme. Numerical simulation shows that the model sensitivity to the vertical stability is similar to results obtained from other general models. Latent heat release associated with condensation is calculated from the moisture equation. The sensible heating parameter is added to the heating term. Initially the model atmosphere is assumed to be at rest and is then excited by a large scale heating to generate an initial vortex. The evolution of surface pressure drop, tangential wind, radial wind, heat released due to condensation, temperature anomaly, D-value, horizontal structure of vertical p-velocity, vertical structure of vertical p-velocity, vorticity and static stability parameter have been discussed based on the results obtained from the tropical cyclone modelling experiment.Item Temperature Dependence of Magnetization and Induced Magnetic Anisotropy of Some Fe, Co and Ni-Based Amorphous Ribbons(Bangladesh University of Engineering & Technology (BUET), Dhaka, Bangladesh, 1999-07) Sikder, Shibendra Shekher; Asgar, Prof. M. AliFerromagnetic amorphous ribbons of Fe-Si-B, Ni-Fe-B and Co-Fe-B-Si series have been prepared by rapid quenching technique and the amorphousity of the samples has been confirmed by X-ray diffraction technique. The kinetics of glass formation and crystallization as affected by a slight change in composition is studied by differential thermal analysis (DTA). It is observed that the thermodynamics of the amorphous ribbons in respect of the formation of glassy state and its stability is affected by the complexity of the composition. The magnetic ordering of Fe-based, Ni-based and Co-based amorphous ribbons is studied by measuring A.C. and D.C. magnetization as functions of temperature. D.C. measurements are made by using a vibrating sample magnetometer and A.C. initial permeability is measured, using a furnace in which the heating wire is wound in accordance with the bifiller technique. Coercivity, remanence, saturation magnetization and maximum permeability have been determined as static magnetic properties for Fe-based, Ni-based and Co-based specimens from the hysteresis loops obtained by Ballistic method. The dynamic characteristic such as the real and imaginary components of the complex permeability in A.C. conditions have been measured as a function of the instantaneous value of a sine wave core current density by means of an adapted LCR bridge method. Initial permeability, frequency dependence of complex permeability and relative quality factor have been measured in the frequency range 0.5KHz to 13MHz for all the different compositions and for annealing effect of initial permeability. The measurements of saturation specific magnetization of Fe-based, Ni-based and Co-based amorphous ribbons have been measured by V.S.M. We have correlated the behaviour of magnetic moment in amorphous alloys containing 3d transition metals on the basis of rigid band model, assuming that the metalloid atoms contribute some of their S electrons and P electrons to fill the d band of the transition metal. This accounts for the reduction of magnetization of all the amorphous ribbons with increasing metalloid content. The temperature dependence of magnetization, of amorphous system decreases faster with increasing temperature as compared to crystalline materials. According to the mean field approximation, reduced magnetization versus reduced temperature graphs for different Fe-based, Ni-based and Co-based amorphous ribbons leads to fluctuation in the exchange interaction giving rise to a structural disorder. Induced magnetic anisotropy of amorphous ribbons having Fe-based, Ni-based and Co-based system have been measured for temperature and field dependence using torque magnetometer with proportional integrating differentiating compensation. The results of compositional dependence of initial permeability, coercive force and induced anisotropy are found to be mutually consistent for all the samples. The origin the uniaxial anisotropy in these amorphous materials is assumed to be the stress, induced during the preparation. The temperature dependence of uniaxial anisotropy is caused by stress, relieving and decreases with the increase of temperature. The corresponding curves of In [Ku(0)/ Ku(T)] versus In [σs(0)/ σs(T)] in amorphous ribbons with Fe-based, Ni-based and Co-based system follow straight lines arising from the disorder caused by randomly oriented easy axes. The values of the exponents are different for the different samples. All amorphous ribbons have been investigated using Mössbauer spectroscopy. The Mossbauer measurements were performed with a conventional constant acceleration spectrometer at room temperature in transmission geometry, using 57Co in a rhodium matrix. The velocity scale of a thin 57Fe sample and the isomer shifts are given relative to the centroid of the spectrum. The isomer shifts of Fe-atoms in all the Fe, Ni and Co-based samples behave as trivalent ions. The spectrum for the as prepared samples consists of six lines which are broadened as compered with the Mössbauer spectrum of a pure 57Fe sample in the form of a thin foil. The curve for the experimental spectrum is obtained by using the best fit of the distribution of hyperfine fields. The consistency between the average magnetization measured by V.S.M. and the magnetic moment of the Fe-atom as estimated from the hyperfine field distribution confirms the collinearity of the magnetic ordering of the sample.Item Electromagnetic Properties of Iron deficient Ni-Cu-Zn Ferrites and the influence of additives as sintering AID(Khulna University of Engineering & Technology (KUET), Khulna, Bangladesh, 2013-01) Khan, Md. Zakir Hossain; Sikder, Prof. Dr. Shibendra ShekherThe present work is focused on the influence of substitutions and sintering aids on electromagnetic properties of Fe-deficient Ni-Cu-Zn ferrite. Three series of ferrite samples of the compositions (Ni0.28Cu0.10Zn0.62O) (Fe2O3)1-x (where x=0.00, 0.02, 0.04, 0.06 & 0.08), Ni0.28Cu0.10+Zn0.62-x Fe1.98O4 (where x=0.00, 0.03, 0.06, 0.09 & 0.12) and Ni0.28Cu0.10Zn0.62Fe1.98O4 doped with V2O5 and Li2O were prepared by using the solid state reaction technique. The phase identification was carried out by using the X-ray diffraction. The X-ray diffraction analysis revealed that all the samples of the studied three series were found to crystallize in single-phase cubic spinel structure. A slight increase of Curie temperature, Tc and saturation magnetization, Ms of (Ni0.28Cu0.10Zn0.62O) (Fe2O3)1-x series with increasing iron deficiency were observed. Lattice parameter of Ni0.28Cu0.10Zn0.62O) (Fe2O3)1-x series slightly decreases with x content. Initial permeability was found to increase with increasing iron deficiency and also with increasing sintering temperature which may be attributed to enhanced density and reduced anisotropy energy as well as increase in grain size of the studied samples. The permeability spectrum with frequency follows the Snoek's limit. From hysteresis parameters it was revealed that optimum soft magnetic properties corresponds to the composition with x=0.08 sintered at Ts=1150°C having highest permeability of 358, maximum induction, minimum coercivity and hysteresis losses. From the detail study on the magnetic properties of Ni0.28Cu0.10+xZn0.62-xFe1.98O4 series, it appeared that optimum soft magnetic properties were found in the composition with x=0.09 with the manifestation of highest permeability of 586 with low coercivity of 0.75 Oe and hysteresis loss of 15.5 W/kg. Saturation magnetization, M increases with x content with high value of 61.8 emu/g for x0.09. Lattice parameter decreases linearly with increasing Cu content while Curie temperature increases linearly with Cu content. The decrease of Curie temperature may be attributed to the strengthening of A-B (JAB) exchange interaction. Initial permeability increases gradually with increasing Cu content and sintering temperature, Ts attaining a maximum value for x0.09 at Ts=1200°C. This enhancement of permeability may be correlated with improved microstructural features. The effect of dopants such as V2O5 (0.4 wt %) and Li2O (0.4 wt %) on the composition Ni0.28Cu0.10Zn0.62Fe1.98O4 with Ts=1150°C revealed that a substantial increase of initial permeability from 284 to 476 for V2O5 and 380 for Li2O is observed. This may be accounted for due to increased density and grain size. A noticeable decrease of coercivity and increase of magnetization of the doped samples have been observed resulting in high permeability.Item Study of Additive Effects on the Structural, Magnetic and Transport Properties of Cobalt Ferrites(Khulna University of Engineering & Technology (KUET), Khulna, Bangladesh, 2011-04) Noor, Saroaut; Sikder, Prof. Dr. Shibendra ShekherA detailed study was carried out on four different series of spinel Co-based ferrites, with composition Co1-xCdxFe2O4 and Co1-xZnxFe2O4 (where x = 0.0 - 1.0 in steps of 0.1), rare-earth (RE) substituted diluted system Co0.2Cd0.8 Fe2-xRExO4 (RE = Ho and Sm; x = 0.05, 0.1) and Co0.2Zn0.8Fe2RExO4 (RE = Gd and Eu: x = 0.05, 0.1). The samples were prepared by conventional double sintering ceramic technique and were found to be single-phase cubic spinel structure of Co-Cd and Co-Zn series by X-ray diffraction technique, while all rare-earth (RE) doped samples showed additional peaks other than spinet. The lattice constant of Co1-xCdxFe2O4 and Co1-xZnxFe2O4 ferrites increase linearly with the increase of Cd and Zn content, followed Vegard's law. Bulk density and X-ray density increases significantly with the increase of Cd and Zn content. Curie temperature, Tc decreases almost linearly with increasing x content up to x =0.7. Samples with x > 0.8 show paramagnetic characteristic at T = 5K with no spontaneous magnetization that revealed through Arrot-Belov-Kouvel plots. The continuous decrease of Tc with the substitution of non-magnetic Cd and Zn in Co1-xCdxFe2O4 and Co1-x ZnxFe2O4 system is attributed to progressive weakening of the strength of A-B intersublattice exchange interaction, JAB. Small thermal Hysteresis of initial permeability, μ' is observed below Te. Saturation magnetization, Ms and magnetic moment are observed to increase upto x = 0.4 - 0.5 and decrease thereafter due to the spin canting in B-sites. The change of saturation magnetization with the increase of Cd/Zn substitution has been explained on the basis of Ned's collinear two-sublattice magnetization model and Yafet-Kittel's non-collinear magnetization model. The initial permeability is found to increase with the increase of Cd2 and Zn2 ions upto x = 0.6. Temperature dependence of low field DC magnetization in the field-cooled and zero-field cooled conditions are performed for the dilute Co1-x CdxFe2O4 and Co1-x ZnxFe2O4 (x = 0.7, 0.8, 0.9, 1.0) spinet ferrites. Samples with x = 0.7 and x = 0.8 show re-entrant spin-glass behavior while the samples, x = 0.9 and x = I show the spin-glass behavior. Frequency dependent complex AC susceptibility measurement as a function of temperature as the samples CdFe2O4 and ZnFe2O4 indicates a spin-glass behavior with the manifestation of shift of spin freezing temperature. The samples also show a typical spin-glass behavior with a manifestation of non-equilibrium dynamics, such as aging, rejuvenation, and memory effects. Large magnetic hysteresis effect has been observed at low temperature for the diluted ferrite compositions. Low field B-H loops, at room temperature, were measured at constant frequency. The hysteresis behavior of Co1-x CdxFe2O4 and Co1-x ZnxFe2O4 reveals the softer ferromagnetic nature of the studied materials with the increase of Cd and Zn content. DC electrical resistivity increases significantly with the increase of Cd and Zn content. The AC resistivity and dielectric constant, ε`, of the samples are found to decrease with increasing frequency, exhibiting normal ferrimagnetic behavior. Dielectric constant, c, decreases rapidly with the increase in frequency at lower frequencies and slowly at higher frequencies, which may be due to the Maxwell-Wagner interlacial polarization. The variation of electrical and dielectric properties is explained on the basis of Fe2+ /Fe3+ ionic concentration as well as the electronic hopping frequency between Fe2+ and Fe3+ ions. Rare-earth (RE) doping of Ho3+ and Sm3+ in Co0.2Cd0.8Fe2-xRExO4 ferrite shows some anomalous behavior. A minority second ferromagnetic phase having well defined Tc has been detected Ho3+ doped sample, where Tc is found to increase with Ho3+ content. This ferromagnetic phase may be assigned as ferromagnetic cluster containing Ho3+. Magnetization is found to increase with Ho3+ addition having higher magnetization for higher Ho content while Sm3+ addition reduces magnetization. The decrease of magnetization with Sm3 substitution for Fe3+ may be explained as due to lower free ion magnetic moment of Sm3+. Samples Co0.2Zn0.8Fe2RExO4 (RE = Gd and Eu) show substantial increase of magnetization with Gd3+ which attains higher value with higher content of Gd3+ while decreasing with Eu3+ doped sample. The decrease of magnetization for Eu3+ doping is due to nonmagnetic nature of Eu which has no magnetic moment of the free Eu3+ ion even at low temperature.Item Prediction of High Impact Precipitation Events over Bangladesh and its surrounding areas using NWP Models(Khulna University of Engineering & Technology (KUET), Khulna, Bangladesh, 2012-03) Akhter, Md. Abdullah Elias; Alam, Prof. Dr. Md. MahbubBangladesh is a disaster prone country. Almost every year, the country experience disasters of one kind or another - such as tropical cyclones, storm surges, coastal erosion, hoods due to heavy rainfall and droughts - causing heavy loss of life and property and jeopardizing the development activity. Because of south-westerly wind, high amount of moisture come to the Bangladesh region from the warm surface of the Bay of Bengal and heavy rainfall occurs by convection in our region. Tropical cyclone also forms over the warm oceans and ravage life and property especially over the coastal belt due to extremely strong winds and associate storm surges at the time of landfall. The Bay of Bengal is highly vulnerable to tropical cyclogenesis. To save the life and minimize the damage it is necessary to make advance warning and prediction of both the heavy rainfall and tropical cyclone quite ahead of time. Therefore a study has been conducted to investigate the formation and vertical structure of the heavy rainfall events and structure, intensity and track with landfall time of the tropical cyclone events. In the present study, two state-of-the-art mesoscale models MM5 and WRF-ARW have been used to evaluate their performances in the simulation of heavy precipitation events and tropical cyclone (TC) and its impact on Bangladesh and its surrounding areas. Three test cases for the heavy rainfall events are considered and the cases are during 1-3 May 2009, 1-3 July 2008 and 9-14 June 2007. First one is the pre-monsoon seasonal case and last two are the monsoon seasonal test cases. On the other hand, three test cases for the TCs are considered and the cases are TC Aila, 23-27 May 2009; TC Sidr, 11 - I 7 November 2007 and TC Rashmi, 24-28 October 2008. For TC prediction, one nested domain is used inside the mother domain with horizontal resolution mother and nested domain are 90 and 30 km respectively. On the other hand, for heavy rainfall events, two nested domains are used inside the mother domain with the resolution of mother domain is 90 km that for two nested domains are 30 and 10 km respectively. For MM5 model, in this present study Medium Range Forecast (MRF) PBL scheme, Kain - Fritisch (KF) cumulus parameterization (CP) scheme, Dudhia Simple Ice microphysical Scheme for moisture anticipation, Cloud Radiation Schemes for radiation calculation and 5- Layer Soil model to predict soil temperature is used as model physics. Model equations in the surface flux form and solved on Arakawa B grid. Leapfrog time integration scheme with time splitting technique is used in model integration. For WRF model, the modified Kain - Fritisch cumulus parameterization scheme, WRF Single-Moment (WSM) 3-class simple ice scheme, Translèr Model (RRTM) scheme, Dudhia scheme short wave scheme, Yonsci University Scheme (YSU) Planetary Boundary Layer (PBL) parameterization which is the next generation MRF—PBL is used as model physics. For the heavy rainfall events Mean Seal Level Pressure (MSLP), wind with rain, wind with humidity, rainfall, vertical structure of vertical velocity, divergence, relative vorticity, relative humidity and mixing ratio have been simulated and analyzed to understand the convective activity of the precipitation system by both the models. Both of the models would simulate the convective activity of the precipitation events fairly well. Amount of precipitation are more than those of the observed by TRMM in monsoon and than pre-monsoon. On the other hand, for the TC events MSLP, wind (vector, radial, tangential, vertical wind), vorticity, temperature anomaly, relative humidity, mixing ratio, rainfall and tracks have been analyzed to understand the structure and behavior of the cyclones by the models. Both the models would simulate the cyclonic nature at the lower and anticyclonic nature at the upper levels. Simulations of intensity are more or less than the observed but realistic to observed except for TC Sidr. Amount of precipitation are more than those of the observed by TRMM. To understand the knowledge about the sensitivity of various physics options of the MM5 model, model was run for various sensitivity cases: sensitivity study on PBL with CP is both for heavy precipitation events and TC events, sensitivity study on microphysics, radiation are only for TC Aila. In case of sensitivity on PBL with CP for heavy precipitation events, it is observed that some schemes have overestimated the rainfall and someone has underestimated that. The options for simulations of heavy rainfall events with MM5 have been found dependable on resolution and location of the area. According to our study, no single option may be considered as the most suitable among the 10 options for the assessment of rainfall over Bangladesh but it can be understood that AK CP is better for daily rainfall prediction and KF2 is better for total rainfall prediction. More case studies are necessary to make final commend for our region. The PBL option with CP play very vital role on the track and intensity of TC. No one combination plays the best in the three TC cases but PBL MRF with KF CP plays better among the 10 combination in the three TC cases. Sensitivity test on microphysical parameterization option and radiation parameters options is done only for TC Aila. It is seen that they have their own impact on the simulation of track and intensity of TC Aila. The present study has investigated only one cyclone, and more cases should be examined to supplement these results. It is suggested that it would be desirable to make sensitivity experiments with all possible combinations of the schemes of the physical processes. Other sensitivity tests like horizontal resolution, bogussing vortex may be done to improve the performance of the two models. Then the two models would be used as an operational purpose for the simulation of all kind of heavy precipitation events.Item Study of Nor'westers and Development of Prediction Techniques in Bangladesh during the Pre-Monsoon Season(Khulna University of Engineering & Technology (KUET), Khulna, Bangladesh, 2005-10) Karmakar, Samarendra; Alam, Prof. Dr. Md. MahbubThis dissertation is an attempt to study the nor'westers/thunderstorms over Bangladesh and their associated meteorological conditions with a view to finding the possible causes of formation together with the determination of the critical criteria of a number of meteorological parameters. The study includes the spatial and temporal distribution of mean monthly and seasonal nor'westers/thunderstorms and thunderstorm days over Bangladesh. This statistical analytic overview will be helpful to identify the climatological risk area for the nor'westers/thunderstorms and thunderstorm days over the country. The inter-annual variability and trends of the frequency of thunderstorms have also been studied, which reveals that there exists 3-7 years cycles of variation in the monthly and seasonal frequency of thunderstorms over Bangladesh. The country-averaged monthly and seasonal frequency of thunderstorms during the pre-monsoon season has increasing trends. The rate of increase in the frequency of thunderstorms are 0.1912, 0.1059 and 0.2365/year in March, April and May respectively and 0.4566/year during the pre-monsoon season. The spatial distribution of the trends in the frequency of thunderstorms has been studied too. The surface and upper air synoptic conditions, minimum temperature and surface humidity on the dates of occurrence of nor'westers in Bangladesh have been studied and the favourable conditions for the occurrence of nor'westers have been determined. Nor'westers have been found to occur over Bangladesh near the trough line at the surface. The causes of early nor'westers in Bangladesh have been found out. The study reveals that two air masses of different temperatures are found to be present over Bangladesh and adjoining West l3engal specially at 0000 UTC on the dates of occurrence of nor'westers and they form a discontinuous line. Sometimes more than one discontinuous line was present over the region. This indicates that 'pseudo-frontal' activity takes place over Bangladesh on the dates of occurrence of nor'westers. When this pseudo-frontal activity is found in the morning, severe thunderstorms/nor'westers occur in the evening or early night. Nor'westers have been found to occur near the point of intersection of axes of moist and thy areas. The meteorological conditions associated with two tornadoes over Saturia and Tangail on 26 April 1989 and 13 May 1996 have been studied extensively. The tornadoes occurred near the point of intersection of the axes of moist and dry zones. The positive area on Tephigrarn on the dates of occurrence was bigger than that on the previous day, indicating more instability of the troposphere on the dates of occurrence of the tornadoes. The available instability energy of the troposphere increased significantly on the dates of occurrence of tornadoes and these increases were 90.931003x 106 Joule m-2 and 44.75421 x 105 Jm-2 in case of Saturia and Tangail tornadoes respectively as compared to that on the previous days. This tremendous quantity of instability energy was responsible for the occurrence of the tornadoes over Saturia and Kalihati on 26 April 1989 and 13 May 1996. The rawinsonde observations at 0000 UTC over Dhaka, Chittagong and some stations of India have been utilized to compute different instability indices, water vapour and its fluxes, precipitable water content and different energy components of the atmosphere and their fluxes prior to the occurrence of nor'westers. The criteria of different instability for the occurrence of nor'westers have been determined. Some of the instability indices have been modified in order to have better criteria of these instability indices. For severe to very severe nor'westers, the critical values of MCT, MVT, MTT, MSWI, MKI and MET at 0000 UTC over Dhaka are: MCT ≥ 22°C, MVT ≥ 28°C, MTT ≥ 54°C, MSWI ≥300, MKI ≥ 48°C and MET< -9 Joule gm-1 respectively. For 5 cases of tornadoes, the critical values of the indices are: MCT ≥ 24°C, MVT > 30°C, MTT ≥ 55.0°C, MSWI ≥ 500, MET < -9 and MKI >58°C. Satellite and radar cloud imageries have been used to study the movement of nor'westers over Bangladesh and to estimate vertical velocity associated with nor'westers from radar imageries. The available instability of the troposphere gives a better indication for the occurrence of nor'westers. The higher the available instability energy of the troposphere, the greater is the possibility of occurrence of nor'westers, provided the other conditions especially the synoptic conditions are favourable. The vertical wind shear on the dates of occurrence of nor'westers has also been studied. Simulated average surface wind fields and the rainfall fields for 24 hrs over the region bounded by Lat. 08-32° N and 70-110° E have been obtained by using the NCEP data. The wind fields indicate the surface circulation patterns favourable for the occurrence of nor'westers in Bangladesh and the rainfall fields for 24 hrs over Bangladesh and adjoining area give a good signature for the rainfall occurrence due to nor’westers in Bangladesh. The forecasting techniques for the prediction of nor'westers and associated winds and rainfall have been developed. Four linear multiple regression equations for computation of maximum gusty wind speed associated with nor'westers have been developed. The R2 values for the regressions are 0.2771, 0.3498, 0.3493 and 0.2433 which correspond to the correlation co-efficient of 0.52640, 0.59 144, 0.59102 and 0.49325 respectively, which are not statistically significant. The computed values of maximum gusty wind speed have statistically significant correlation with the actual maximum gusty wind speed. The regression equations have been verified with data of current year (2005) and will be useful for the operational meteorologists for the computations of maximum gusty wind speed associated with nor'westers. Simple linear regression equations and linear multiple regression equations have also been developed for the statistical prediction of 24 hrs rainfall over Dhaka, country averaged 24 hrs rainfall over Bangladesh and 24 hrs maximum rainfall over Bangladesh due to nor'westers. The correlation co-efficients corresponding to these regression equations are statistically significant. These equations have also been verified with the data of current year. The synoptic and statistical techniques developed for the forecasting of nor'westers and associated gusty wind and rainfall will be useful in operational purposes.