Agricultural Meteorology
H. Asakereh; N. Varnaseri
Abstract
Introduction: Precipitation is one of the most important climatic variables playing a decisive role for different purposes. Temporal changes in precipitation affect many climatic and environmental phenomena (such as runoff, floods, air temperature, and humidity) as well as many human activities (such ...
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Introduction: Precipitation is one of the most important climatic variables playing a decisive role for different purposes. Temporal changes in precipitation affect many climatic and environmental phenomena (such as runoff, floods, air temperature, and humidity) as well as many human activities (such as agriculture and housing). The precipitation regime includes all characteristics and statistics of precipitation in relation to its distribution throughout the year, and the temporal distribution of precipitation according to the months or seasons of the year is called the "Precipitation Regime". Materials and Methods: The daily data of 385 stations were obtained from Iran Meteorological Organization and the Ministry of Energy for the period of 2016-1966 (51 years). The hidden aspects of precipitation and precipitation regime of the Iranian coast of Caspian Sea were studied. At first, these stations were used in order to create maps with a spatial resolution of 3 × 3 km, and the general specifications of the monthly and annual precipitation were presented. Sinusoidal behaviors of monthly precipitation in each pixels were then investigated. Accordingly, first to sixth harmonics were extracted. Finally, the cluster analysis method was used based on the Euclidean distance and the "Ward" method of linkage to identify the spatial patterns of precipitation based on the contribution of different periodic and its zoning. Then, the homogeneity and seasonal index of precipitation was estimated. Results and Discussion: The results show that the mean annual precipitation is higher on the coastline, especially in the southwest of the Caspian Sea, and decreases as it passes from the coast. In the southwest parts of the Caspian Sea, maximum precipitation occurs in the autumn. At the Alborz highlands, the maximum and minimum precipitation fall during winter and summer, respectively. The monthly precipitation coefficient of variation indicates that with seasonal changes from winter to spring, precipitation changes in the Caspian region are declining, and with changes in seasonal precipitation from summer to autumn, precipitation changes are in the ascendant. The largest variability coefficient of the month to month of the precipitation (60 to 70 percent) was calculated at the coastline of the Caspian Sea. This shows notable month to month precipitation changes and seasonal instability in these areas. The coefficient of variation is gradually reduced by distance from the coastline. The lowest coefficient of variation was obtained in the southern parts of the Caspian Sea (the Alborz altitudes) between 15% and 30%. This suggests a small difference in rainfall over the course of the months. In other words, they indicate the activity of various rainy systems, or at least the continuity of rainy systems in these areas, and the stability of the precipitation season. The homogeneity index indicates that the precipitation distribution is more concentrated in the coastal areas of the Caspian Sea, and it becomes more uniform with the advance towards the southern parts of the area (part of the Alborz heights). The seasonal precipitation index of the Caspian Sea region indicates three types of precipitation regime. The lowest spatial extent (6.28%) is related to the uniform precipitation regime, found in small parts of the Alborz heights. The most abundant regime has a wetter season. This precipitation regime, which includes 76.13% of the Iranian coast of Caspian Sea, is observed in the eastern and western regions of the Caspian Sea. The third regime (13.25% of the study area), which is mainly seasonal with a short dry season, covers the Caspian Sea coastline, parts of the Talesh heights, and a small part of the eastern region. Conclusion: The results revealed that the precipitation classes obtained based on the seasonal index were closer to the reality due to the similarity of these classes with the average monthly and annual precipitation. Therefore, this index seems to be the most optimal tool for determining precipitation regimes in the Caspian region. According to this precipitation regime classification, there are three classes of precipitation regime in the Caspian region. The existence of these three classes indicates the presence and activity of different synoptic and local systems in the Caspian region.
H. Asakereh; S.A. Masoodian; M. Darand; S. Zandkarimi
Abstract
Introduction: Studies of the atmosphere over the last hundred years have shown that human activities have caused changes in the atmosphere. The tropopause is one of the layers of the atmosphere whose changes have recently been introduced as a sign of a human impact on climate change. The height ...
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Introduction: Studies of the atmosphere over the last hundred years have shown that human activities have caused changes in the atmosphere. The tropopause is one of the layers of the atmosphere whose changes have recently been introduced as a sign of a human impact on climate change. The height of the tropopause is affected by its upper and lower layers (the stratosphere and troposphere). The results of the studies conducted by various researchers have shown that different factors affect the height of tropopause and its changes, which can be divided into two groups. The first group of natural factors (such as changes in solar radiation and weather due to volcanoes, etc.) and the second one is human factors (including changes in greenhouse gases, human-induced changes affecting the ozone of the stratosphere and the production of air vents from human resources, etc.). Thus, altitude tropopause is naturally influenced by spatial characteristics (e.g. latitude and altitude), time (such as the time of year and hours of the day) as well as the frequency of atmospheric actions that determine climatic conditions. Materials and Methods: Compared to the studies performed globally, a limited number of studies concerning the tropopause have been conducted in Iran. Moreover, the applied methods and the length of the dataset were often inadequate. Therefore, in the present study, the daily data of temperature, and geopotential height from the European Centre for Medium-Range Weather Forecasts (ECMWF) for 700 to 50 hpa with a spatial resolution of 0.25 × 0.25 longitude/latitude were applied from 1979 to 2018 for the detection of tropopause. Accordingly, 2491 cells covered across Iran. The LRT was used to detect tropopause. The tropopause is defined as ‘‘the lowest level at which the lapse-rate decreases to 2 ºC/km or less, provided that the average lapse-rate between this level and all higher levels within 2 km does not exceed 2 ºC /km”. In the present study, in addition to changing the position, changing the scale (variance) as well as the shape of the frequency distribution (skewness and elongation) of the tropopause pressure level in each of the pixels on Iran was investigated. To calculate skewness, and kurtosis, daily tropopause height data were used. For each of the months studied, diffraction, skewness, and elongation were extracted using daily data and finally using data during the 40 years. The extracted trends of variance, skewness, and kurtosis were examined for each month. To track the synchronicity and conformity of changes in altitude and trend of tropopause pressure level with the trend of changes in mean monthly temperature in the lower and upper levels of the tropopause and the trend of the temperature difference between the two layers around tropopause was also evaluated over 40 years. In order to evaluate the long-term trend of each of the studied indices (mean, variance, skewness, and kurtosis) in relation to the height and pressure level of the tropopause, linear regression method with least-squares error method was used. Results and Discussion: The results of the study of altitude trend and tropopause pressure level showed that in most of the months studied and in most parts of the country, the trend of changes in tropopause pressure level was not significant at the level of 95% confidence. According to the results obtained for the winter months, it was found that the trend of a tropopause pressure level in December had no statistical significance over Iran at a 95% confidence level. In January and February, the obtained trend was not statistically significant except for southeastern areas. In the summer months, unlike the winter months, the trend of tropopause pressure levels was significant in most regions. During the summer months, in areas where the trend was significant, the trend of tropopause pressure levels was positive. Examination of the trend of tropopause height in terms of meters showed different results with pressure level. During the winter months, the trend was positive in all regions, and in January and February, this trend was significant in many areas, while the summer months did not exhibit a significant tropopause. The results of examining the trend of the low temperature of the tropopause in summer and winter months showed that the observed trend was not statistically significant in December, but in other months, a positive and significant trend was detected. Examination of the temperature trend in the high level of tropopause also showed that the temperature trend in this part of the atmosphere, like the low level of the tropopause in large parts of the country in the studied seasons, lacked statistical significance. Examination of the trend of the temperature difference between high and low levels also showed that the trend of the temperature difference between these two levels was statistically insignificant at the majority of cases. The temperature difference trend of the two levels studied in the summer months was negative and significant at most regions. In other words, the decrease in the temperature difference between low and high tropopause in these two seasons and in some areas indicates a strong decrease in tropopause. Examination of the trend of variance, kurtosis and skewness also showed that the observed trend lacked statistical significance in the two studied chapters at most areas. There was also no relationship between the surface temperature trend and changes in tropopause height. Conclusion: The results of this study showed that tropopause had no statistically significant trend in most areas and months. Moreover, the significant trend was not related to the two temperatures around tropopause and surface temperatures.
H. Asakereh; R. Khoshraftar; F. Sotudeh
Abstract
Rainfall and debit of rivers are two tempo-spatial non-linear and changeable factors. One way to study and analysis these parameters is investigate appearance and latent oscillations. Spectral Analysis is a useful technique to reveal these oscillations in time series. In this paper it has been attempted ...
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Rainfall and debit of rivers are two tempo-spatial non-linear and changeable factors. One way to study and analysis these parameters is investigate appearance and latent oscillations. Spectral Analysis is a useful technique to reveal these oscillations in time series. In this paper it has been attempted to detect cycles in rainfall and debit time series at Mashinkhaneh station in Talesh (Garakanrood) catchment’s during Mehr 1354 to Shahrivar 1386 in the three time scales (annual, seasonal and monthly). Accordingly, the discharge and precipitation data at Mashinkhaneh station in Talesh (Garakanrood) catchment from Mehr 1354 to Shahrivar 1386 have been used. The results of applying the spectral analysis procedures to discharge and rainfall time series in each three category of time scales, suggested the absence of significant non-sinusoidal (trend) in the 95% confidence level. However, significantly sinusoidal cycles various in the two time series were extracted. The 2-4 year cycle, and 4-5.3 years have the most occurrences in the both time series. In the annual scale, 6.4 years cycle, 2-5.3 years, 7.7 years seasonal and 2-4, 4- 5.3, 6.4, 8, 10.7 and 16 year in the monthly scale cycles has been extracted. Studies carried out by many researchers indicate that the mentioned cycles are in relation with oscillation periods of ENSO, NAO and QBO in other parts of the world.
