شناسایی الگوهای فرارفت دمایی ایران در سال‌های سرد

Introduction
While it seems that the temperature variations follow a general pattern, that is dependent to the variations in the sun's changes angle and how the heat energy is distributed during the months of the year, but the temperature is not fixed over time and some losses or increases are seen. For example, during the cold seasons of the year it is seen that an almost warm or mild weather enters a region; or during the warm seasons, a cool weather overcomes for some days. This kind of charging cool or warm weather is called temperature advection. Temperature advections that are in particular frequency happening during the cold seasons of the year are not under the control of local factors but are rather controlled by pressure elements and/or factors. Accordingly, the wind advection and it does determine the temperature advections and in other words, they are characteristics of the air masses they bring with. Also during the warm seasons that an almost the same and fixed pressure pattern (subtropical high pressure) covers a large part of the country, these temperature advections don't occur frequently and mostly a clear, calm, sunny and same weather appears, in such a way that most of the days are same, climatically. But in cold seasons, by spreading the west wind over the country and the appearance of variations pressure patterns, more temperature advections happen.

Materials and Methods
In order to obtain the temperature advection patterns during the cold years, first, based on the time series, the average annual temperature of the country in a 30- year course of 1970- 2000 was selected and then the years of 1976, 1982, 1992, 1974 and 1975, were determined as the cold years. After that, three of U wind, V wind, and the Air temperature in the level of 1000 HP were determined for the hour of 12GMT in the aforementioned course in the geographical limit of 20? to 50? North latitude and 35 to 70 east longitude, with the spatial resolution of 2.5*2.5? and from the NCEP/NCAR climatic database. The temperature advection of each year calculated using programming in the Grads software environment. Finally, the data matrix is arranged with the dimension of 1827*56 in an S- mode. In the next step, a principal component analysis is done on the data matrix of the temperature advection. This analysis showed that with 15 components more than 93% of the variance in data can be explicated. Based on this method, two matrices with the dimensions of 56*15 for the spatial pattern, and 1827* 15 for the tempo pattern were provided. The component loadings matrix (56*15) was converted to the map, and the components score matrix, with the dimension of 1827*15, that resemble the temporal pattern, were converted to 15 charts. In the next stage a cluster analysis with the integration ward method on the components scores matrix (1827*15) was data. This review showed that according to the scores of the 15 main components in 1827 days (5 years), 12 advection patterns can be determined.

Results and Discussion
After determining the representative days, the temperature advection maps of the levels of 1000, 700 and 500 HP 12 hour GMT of these days were collected from the climatic data base of NECP/NCAR and were measured and plotted with grads software. In these maps, the solid line with positive scores and is the warm advection and the dotted lines the negative numbers represent the cold advection. This study reveals that low temperature in the cold years has an external origin and is due to the spread of cold temperature advection in the style and form of atmospheric pressure systems. The reviews showed that the temperature advections, devastate the country in two forms of eastern advection (from the east and north east) and the western ones (from the north west, west and south west). The eastern advections are related to how Siberian high pressure happens, while the western advections are due to the spread of the western winds across Iran.

Conclusion
This study revealed that eastern advections are stronger, while the western advections are a little mild. On the other hand, the eastern advection in the maps of 1000 level, has a wider view and spread, while in the maps of 700 level, their intensity is reduced and it represents that the eastern advections are moved in lower levels of the atmosphere.
Evaluating the map of representative days shows that the patterns No. 1, 5, 6, 8, 11 and 12 have an eastern origin and No. 2, 4 and 7, have a western origin and the patterns No. 3, 9 and 10, have a western- eastern origin. Accordingly, it can be concluded that the cold advection, has a more eastern origin.
And also the reviews demonstrated that during the study of cold advections, a more abundance available than the warm advection. It means that the coldness of the air during the cold period, is not only because of the local elements, but also is due to the cold advections, while the warmness of the air during the warm period is due to the local factors and the pressure of an almost stable air that is due to the subtropical high pressure.
Evaluating the frequency of the temperature advection in the months of the year showed that the patterns of 9, 11 and 12, are in the summer and in 32% cases are in the second group. Therefore, it can be said that the temperature advections happen more during the cold period of the year, as they are dependent to the pressure patterns and these patterns are stronger in the cold time and they are more abundant, too, and moreover, they expanded to the lower latitudes.