Trend variations in large-scale atmospheric systems like subtropical high pressure play a significant role in climate change. In this study, to achieve the objectives, mid-level atmospheric Geopotential altitude data were employed based on the European Center Database of Atmospheric Medium-Term Forecasting. The data that have a spatial resolution of 1*1 degree of curveand are collected on a daily average. The statistical period of the research ranges from 1980 to 2018 for Iran and included 155 cells. Mann-Kendall trend test was used to explore the Geopotential altitude trend on Iran. The results showed that the atmospheric Geopotential altitude on Iran in June, July, and August has an increasing (positive) trend which is at the significant level of 1.96. The decreasing trend of Geopotential altitude in the eastern and southeastern regions of Iran is remrkable. Moreover, in all the investigated months, Iranian atmosphere altitude in the central, western and northwestern regions had an upward trend, which is generally influenced by the high-pressure subtropical level. These changes cause abnormalities in climatic patterns of the regions. The study also showed that continuing subtropical pressure stack on top of ever-increasing trend in the region is significant.

Topographic masses above the geoid are considered as a major obstacle in geoid determination by using Global Gravitational Models (GGMs). GGMs provide the possibility of the Earth's potential field modeling as the expansion of the external-type series of spherical harmonics. Applying the external expansion to obtain disturbing potential on the geoid within the topographic masses will cause a bias called „ topographic bias‟ . This study deals with calculating geoidal height using Earth Gravitational Model 2008 (EGM08). In order to do so, two methods of Direct Analytical Continuation one and Rapp's Indirect one are utilized. The Analytical Continuation Approach is based on using EGM08 within the topographic masses and applying topographic bias. Alternatively, Rapp‟ s Approach is based on calculating height anomaly and its downward continuation on the geoid. The success of these two methods to geoid simulation on 490 GPS-Levelling stations in mountainous region of Colorado in the USA were evaluated. The results are an indicator of the fact that two methods are compatible with each other with centimetric accuracy compared to GPS-Levelling points. Also, it suggests an improvement in the relative and absolute accuracy of the geoidal height resulting from EGM08 about 60% in both methods. The numerical investigation revealed that taking advantage of height harmonic models instead of point actual height can bring a bias in the matter of a few centimeters on the geoid. Moreover, the absolute accuracy of Rapp's Approach is higher than Analytical Continuation Approach in geoid determination in comparison GPS-Levelling points.

Global Geopotential models (GGMs) are mainly used in the remove-compute-restore (RCR) technique applied to gravity field modeling such as geoid determination and height datum unification. The increase in the number and quality of gravity data has led the developers of GGMs to produce models with higher resolution and accuracy. Basically, the long-wavelength coefficients of the gravity field are computed based on satellite data, while the medium- and short-wavelength coefficients are calculated based on terrestrial (land and sea) data. One of the main challenges regarding the evaluation of high-degree GGMs is to compute the associated Legendre functions of the first kind based on the usual recursive formulas. Since most computational softwares use the double-precision format by default, an important question is whether this level of precision is sufficient to numerically evaluate the associated Legendre functions of the first kind? To answer this question, the computation of the associated Legendre functions of the first kind in different degrees and latitudes is studied based on MATLAB software, which uses the double-precision format by default. From the numerical results, we find that the calculation of associated Legendre functions of the first kind up to degree of 2190 (the highest degree of existing GGMs), does not have sufficient accuracy at latitudes between 56°20׳ and 78°33׳, where the most critical state occurs at the latitude 60°. We also find that the accuracy of the calculation of associated Legendre functions at the latitude 60° (the most critical state) significantly decreases for the degrees higher than 2029. These results imply that the usual computational softwares based on the double-precision format are not suitable for calculating the associated Legendre functions in all degrees and latitudes. This is due to the fact that if we consider the associated Legendre functions of the first kind in the form of a matrix with the dimensions corresponding to the degree and order of the functions, as the degree increases, the numbers on the main diagonal approach to the number 10-308 and thus they are considered zero. In the recursive method, the entries below the main diagonal are calculated from the entries on the main diagonal. Since the entries below the main diagonal become very large as they move away from the main diameter, any error in computing the main diagonal entries leads to a large error in computing the entries below the main diagonal. In this paper, we also study the challenges of using the associated Legendre functions of the first kind in the production of gravity field functionals based on a GGM, utilizing MATLAB software. The results show that the gravity potential computation up to degree of 2190 suffers from very large computational errors at latitudes between 57°32׳ and 60°13׳. We observe that the safe degrees for the gravity potential computation in all latitudes are degrees less than 2065. The critical latitudes and degrees for the gravity calculation are somewhat different. The results indicate that the gravity computation up to degree of 2190 leads to very large errors at latitudes between 57°41׳ and 60°13׳. In addition, the maximum degree of expansion that grants sufficient accuracy for the calculation of gravity for all latitudes is estimated to be 2071. Therefore, since the usual computational software based on the double-precision format is not suitable for evaluating the current high-degree GGMs, in this research, a new proposal based on the use of the “long double-precision” format is presented and evaluated. Based on our evaluations, the use of the long double-precision format throughout the computational procedure provides sufficient accuracy to compute the gravity field functionals based on the current high-degree GGMs.

In this paper, the standard deviation of the spheroid up to degree and order 20 in Iran, when using various satellite-derived Geopotential models, has been studied. For the comparison, the standard deviations of potential coefficients in various Geopotential models are examined. According to the results, the ElGEM-GL04C Geopotential model up to degree and order 20 showed small standard deviation over the territory of Iran.

The Food and Agriculture Organization (FAO) has proposed the FAO 56 Penman-Monteith model as a standard method for estimating reference evapotranspiration (ETo). However, using this model for the high-altitude regions mostly faces uncertainty due to the lack of meteorological stations, observation problems, and incomplete meteorological data. Therefore, in these regions models should be used that require fewer meteorological variables and can also be considered as a function of altitude. In the present study, the data from 28 synoptic stations in Iran with an altitude of more than 2000 meters above sea level in the period of 1989-2019 were used to modify the Hargreaves-Samani model based on the altitude and also correct the temperature factor coefficients. The comparison of the results of the FAO 56 Penman-Monteith model as a standard method, the original Hargreaves-Samani, and the modified Hargreaves-Samani models showed that the modified model offers better results than the original model and is able to estimate the ETo more accurately in the highaltitude regions of Iran. Based on the results of MBE, MAE, RMSE, PVC, LVC, d, r, and PI statistical indices, the highest agreement in estimating ETo with modified Hargreaves-Samani and FAO 56 Penman-Monteith models was observed at Firuzkuh, Baft, Baladeh, Sisakht, Avaj, Khansar, Abali, Daran, Fereidunshahr, Borujen, Samirom, Aligudarz, Farokhshahr, Damavand, Sepidan and Saman stations, which had an average error reduction of 14% compared to the values obtained from the original Hargreaves-Samani model.

In this paper the influence of solid tide on the Earth gravity field is considered. In this consideration the Earth can be regarded as either an elastic or inelastic body. Each one of these elastic and inelastic bodies has two main tidal components, Frequency dependent and trequency independent components. In this article how to compute the effect of these components in the Earth's gravity field is presented. In this investigation, an attempt is made to find out whether the Earth should be regarded as an elastic or inelastic body in practical applications. Computations show equivalent effects on the gravity field due to the elastic and inelastic Earth model. The effect of the Frequency dependent component of solid tide due to the elastic and inelastic Earth is much smaller than the Frequency independent components. It depends on time and tidal constituents and it should be considered in precise applications. Comparisons between the solid tides due to the elastic and inelastic Earth model show that the inelastic Earth is contracted at poles about 3mm and expanded at equator about 2.5mm more than the elastic case.

Iran country because of great spreading with a view to geographical longitude and latitude‚ existence the contortion of unevenness configuration and locating in exposed of air masses attacking has special circumstances in terms of temporal. The overall structure influenced by latitude‚ altitude and air masses. So that with changing each of these factors the temperature will change. In other words the general condition of temperature is a function of latitude and altitude and other factors such as aquatic area and land forms has a role in creating temperature structure that is referred as local factors...