كليدواژه :
سواحل جنوبي خزر , بارش سنگين و فراگير , فصل پاييز , اختلاف درجه حرارت دريا و هوا
چكيده لاتين :
Introduction
With the onset of fall season, the temperature of air and Caspian Sea diminishes, but due to the difference in the specific heat capacity of air and water, the rate of reduction of temperature in air is far greater than that of sea. Therefore, the surface of Caspian Sea water remains warmer than air. Establishment of dynamic high-pressure centers or diffusion of Siberian high pressure system over Caspian Sea especially during fall results in development of southward air stream over Caspian Sea. Caspian Sea as the first supplier of humidity resources for heavy precipitation and especially ultra-heavy precipitation of its southern coasts, plays a principal role in explanation of occurrence of precipitations in these coasts. The changes in Caspian Sea water temperature can result in variations in the precipitation in southern coasts of Caspian Sea. However, as stated by Alijani, only the seawater temperature is not sufficient for the precipitations of the southern coasts of Caspian Sea and the air temperature over the sea should also be taken into consideration. The sea-air temperature difference (?T) has attracted the attention of researchers as humidity intake index, to such an extent that it has become the most important thermodynamic factor in development of abundant and heavy precipitations in the southern coasts of Caspian Sea. Gradual elevation of the sea surface temperature in its proper arrangement from the north to the south and intake of humidity from the sea (?T index) have been mentioned as necessary for development of heavy and ultra-heavy precipitations in the southern coasts of Caspian Sea.
Materials and methods
To do this research, the daily precipitation data of seven synoptic stations of Northern provinces of the country have been collected from 1968 to 2013. The threshold of heavy precipitations was defined as the day in which its precipitation level is greater than the percentile 95 of that year. Next, all of the days in which at least five stations have recorded heavy precipitation simultaneously were extracted and considered as regional heavy precipitation days. By extracting the air temperature at 2 meter above the sea level (Air2m) and the sea surface temperature (SST), the ?T value was calculated as follows:
Benefiting from Ward linkage method and calculation of the correlation between the data, ?T values were clustered in days in which regional heavy precipitation had occurred in the northern stations of the country. In each of the clusters, the day that had the greatest correlation with other days was chosen as the sample. For the sample days, SLP, VWnd, Uwnd, and HGT data in 1000 HPa for plotting and analysing synopsis conditions leading to regional heavy precipitation were extracted from http://www.esrl.noaa.gov. The relationship between regional heavy precipitation in the stations and ?T was analysed through which the extracted clusters were interpreted and analysed.
Results and discussion
With the onset of fall season, the air temperature declines more rapidly than seawater temperature. Therefore in this season, the sea will be warmer than air. The long-term mean (1986-2013) of SST and Air2m and ?T during fall has been shown in Fig. 1.
Fig.1: Long-term mean (1986-2013) of SST and Air2m and ?T during fall.
The relationship between regional heavy precipitation and ?T was investigated in the studied stations. Unexpectedly, direct relationship between ?T in the south of Caspian Sea and regional heavy precipitation does not exist in all of the stations and this relationship only applies to the stations located in the East, whereas in the stations situated in the West, this relationship was seen to be inverse.
Benefiting from Ward linkage method and calculating the correlation between the data, three clusters were recognized and in each of the clusters, the day that had the greatest correlation with other days was chosen as the representative of the cluster (Fig. 2). The first cluster is representative of days with regional heavy precipitation in conditions where ?T gradient reaches its maximum from North West to the south east. The second cluster represents incidence of regional heavy precipitation in conditions where the maximum ?T gradient is from the north to the south. Eventually, the third cluster can be considered as the greatest ?T gradient is observed from the West to the East (also Southwest to Northeast).
Fig.2: ?T in 13 Sep 1977 (third cluster), 22 Oct 1988 (second cluster) and 29 Sep 1986 (first cluster) respectively from left.
Investigation of synopsis maps resulting in regional heavy precipitation in the selected days suggests their similar synoptic conditions such that establishment of high-pressure center in the northwest of Caspian Sea leads to influx of cold-air from more northern latitudes and over Caspian Sea towards southern latitudes. Passage of cold air over a warmer sea results in injection of humidity and modification of its temperature, such that by reaching the Caspian southern coasts, it becomes instable, eventually causing heavy precipitation in these coasts. The stations located in the east of the studied region, will receive greater precipitation when ?T reaches its maximum level in the southern coasts of Caspian Sea (the second cluster). In contrast, the stations located in the West, will receive greater precipitation when ?T gradient increases from the east to the west (the third cluster).
Conclusion
With the increase in ?T in the southern parts of Caspian Sea, regional heavy precipitation grow significantly only in the eastern part of the studied region. The results obtained from ?T clustering unveiled three clusters, where the first, second, and third clusters represented the maximum ?T gradient from the Northwest to the southeast, north to the south, and west to the east, respectively. The stations located in the east of the studied region will have greater precipitation when ?T reaches its maximum in the southern coasts of Caspian Sea (the second cluster). In contrast, the stations located in the West will receive the maximum precipitation when ?T gradient increases from the east to the west (the third cluster). These results were in line with the results obtained from investigating the correlation between ?T and regional heavy precipitation. The analysis of synoptic maps indicated that in all of the three clusters, establishment of high-pressure center in the northwest of Caspian Sea results in infusion of cold air from more northern latitudes and over Caspian Sea towards southern latitudes.
