This study showed that despite the existence of the same meteorological systems throughout the studied area, the main factors that cause differences in the spectral densities in the stations are the topographic conditions and roughness of the earth [15]. By investigating the periodic behavior of the wind regime, it was revealed that the first harmonic is an indication of winds related to pressure variations in masses and air systems on a synoptic scale. These masses, formed beyond the geographical boundaries, enter the area in the winter with the arrival of the westerlies. This is the reason that the severe winds, clouds growth and precipitation are usually accompanied by low pressure masses. In summers, due to the direct sunlight on the desert areas of the central Iran and the warming up of the adjacent air in the surface of the earth, the low-pressure system is dominant in these areas and because of the instability resulting from the establishment of this low-pressure system, usually strong winds and dust are prevailed. In the southern/central parts of the area, the first harmonic is more effective than the other parts. Its effect is larger than the overseas systems and is lower than roughness. Although, in some eastern/northwest parts of the studied area (Alvand Mountains), the magnitude of the second harmonic was greater compared with the other parts (which expresses the impact of topography and local conditions on wind), the magnitude of the variance of the second harmonic in these areas was ≥ 25%. Local winds occurrence are affected by local phenomena like the mountainous/topographic effects of the region. As the study area is located in the Zagros mountain range, which has high summits, it is exposed to the plain-mountain winds. The proximity of the plains to the highlands/mountainous areas increases the second harmonic of the wind in this region and creates local winds. By comparing harmonic changes in the first, second and third decades, it seems that the second harmonic has an increasing contribution in the region, which means that the wind regime in these areas is mostly affected by local factors and features of the earth’s surface and the effects of the systemic winds (synoptic pressure difference) have been reduced by an average of 3–7% in the areas.
Trend analysis of the harmonics indicated that topography and altitude are important factors in establishment of wind and the maximum wind-speed happens in east/northeast regions (largerest amplitudes). Our findings, also, showed that the time of the maximum wind-speed starts (finishes) from early (late) autumn. Deacreasing (increasing) the variance of the first (second) harmonic in the second decade compared with the first decade indicates an increase in local winds and in the contribution of geographic factors in wind formation. Moreover, in the second decade, the time of the maximum wind-speed for large-scale systems occurs 15 days later than the first decade. This can be attributed to the climatic/atmospheric changes in a macro scale. The time of the occurrence of the maximum wind-speed in the second harmonic is in late November which is due to the cooling of the altitudes and establishing thermal high-pressures at altitudes. The magnitude of the amplitudes of the second harmonic was greater in the East/Northeast, i.e. the maximum wind-speed happens in these areas and it tends to decrease from the northeast towards the southwest.
Assessing the periodic behavior of the wind indicated the type of the wind regime and its influencibility by the altitudes. Comparing the results of the harmonic analysis over a thirty-year period indicates that in the study area, altitudes play a major role in establishment of wind blowing and its maximum wind-speed especially in the northern and eastern parts of the area.
By evaluating the trends for the magnitude of the amplitudes (maximum daily wind-speed), it was found that the trend is slightly decreasing in the first harmonic and it is increasing with the same amount in the second harmonic. This can be interpreted as follows: as the systemic winds blows, that is a function of large-scale systems (synoptic), topography and high mountains play a significant role in accelerating the wind-speed.
By investigating wind regime changes using harmonic analysis in the present study, it was indicated that by increasing the contribution of the variance of the second harmonic, the altitudes and geographic factors contribute more to the wind establishment. For winds with a systemic origin and large-scale systems, the occurrence time of the maximum wind-speed has changed from early November to mid-November. This delay is due to the delays in the entrance of westerlies and extratropical systems to the region, and it might be related to the climate change and global warming. Nevertheless time has not changed in the second harmonic. The results of this research can be applied in the management of energy resources and environmental planning. It is suggested to investigate the changes and displacement in the wind regime in different regions of Iran.
Since the values of the first harmonic are affected due to the arrival of exterior (synoptic) systems in the region and it is variable (although its values are larger than the second harmonic), planning based on this harmonic is associated with high risk. On the contrary, because the values of the second harmonic are affected by the ripples and the topography of the region and this harmonic is a constant geographic factor, therefore, planning based on this harmonic is associated with lower risk. Accordingly, in areas where the second harmonic values are higher in the wind regime, the development and installation of wind turbines and long-term planning for the use of wind energy are more logical. Thus, it can be concluded that the eastern and northwest regions of the study area are more suitable for the use of wind energy as well as the development and installation of wind turbines and it can play a significant role in the economic and social prosperity of the region.