Analysis of Moisture Sources and Spatial-Temporal Patterns Affecting Spring Snowfall in Chaharmahal and Bakhtiari Province of Iran

The climatic history of Chaharmahal and Bakhtiari province in the Central Zagros region shows a huge amount of snowfall in the cold season. In recent years, the tendency of precipitation from snow to rain has increased in autumns and winters and wintertime snowfall has decreased compared to the long-term average of the province, while sometimes springtime snowfall can be seen in the province. In spring, dynamic systems that stimulate atmospheric instability are still present in the region, and sometimes the combination of dynamic-thermodynamic conditions causes heavy rainfalls. In the present study, the dynamic and thermodynamic conditions for three springtime snowfalls were analyzed and the effective moisture sources in springtime snowfall were obtained.
 

Precipitation and temperature values in synoptic stations of Chaharmahal and Bakhtiari province in the period of 2000 to 2018 were provided by the Meteorological Organization. To select abnormal springtime precipitation, the anomaly of temperature, precipitation, and geopotential height at the level of 500-hPa were analyzed. The ERA5 analysis of the data from ECMWF with a horizontal resolution of 0.25° was used to calculate moisture flux, perceptible water and vorticity advection, the monthly anomaly of precipitation, height and temperature of 500-hPa, and instability indices. To analyze the causes of snowfall in spring and also to investigate the sources of moisture, the monthly anomalies of rainfall, the temperature at a height of 2 meters above the ground and height, and temperature of 500-hPa level were compared to the 30-year average (1981-2010). In a dynamic study, vorticity advection at the level of 500-hPa was measured. To identify the trajectory of atmospheric moisture, moisture flux at the level of 850-hPa and perceptible water were calculated. To investigate the thermodynamic conditions and atmospheric instability, Skew-T diagram and atmospheric instability indices including KI, TT, PW, and CAPE were used at Shahrekord station at 00 UTC on the day of snowfall.

Snowfall is a climatic feature of Chaharmahal and Bakhtiari province that also occurs in spring, but in recent years, due to warmer weather and reduced snowfall in winter, snowfall in spring seems somewhat unexpected. Precipitation is one of the quantities whose prediction of location and intensity is associated with uncertainty. Therefore, in this study, for more accurate prediction, the moisture sources, and the dynamic and thermodynamic conditions of spring snowfall in the province were investigated. To select unusual springtime precipitation, anomalies of temperature, precipitation, and geopotential height at 500-hPa were examined. As a result, snowfalls were selected in the spring of 2004, 2009, and 2016, which were different from normal compared to the long-term 30-year average.Examination of the dynamic conditions of the mentioned systems showed that at the level of 500-hPa with the formation of a deep trough in the eastern Mediterranean to the Red Sea, the location of the study area in the east of this trough has caused instability and upward movements. In addition, there is a positive vorticity at the level of 500-hPa. Given that these conditions have occurred for all three systems, it can be concluded that the occurrence of snowfall in spring is due to a dynamic process. It is noteworthy that in April 2016, when the amount of snow was more than the other two cases, the trough formed in the area was much deeper than the other two ones, and the vorticity advection was higher. Analysis of the quantities of moisture flux and perceptible water showed that these systems supplied their moisture from the Arabian Sea, the southern Red Sea, the Sea of Oman, and the northern Indian Ocean. The sources of moisture for precipitation in the region are mainly the Arabian Sea, the Red Sea, the Sea of Oman, and the North Indian Ocean are located more than 2000 km far from the southwest of Iran.Moisture flux continues from a few days before the operation of the system with south and the southwest winds from the Arabian Sea and south of the Red Sea to the southwest of Iran. In addition, the amount of perceptible water on the day of the event increases sharply. Temperature analysis showed that the decrease in temperature on the days of the phenomenon was more severe than the previous days and compared to the climatic average, and the coldness of the entire air column illustrates snowfall in spring. Examination of climatic conditions including anomaly analysis of precipitation showed that their values in all cases were much higher than the long-term and the normal average of the region, and is consistent with prominent temperature and height anomalies at the level of 500-hPa. 15 to 30 decameter drop in height and more than 1° drop in temperature were observed at this level compared to the long term. Cooling of the atmospheric column due to the process of evaporation or melting along the path and especially in the adjacent layers of the earth's surface has an important role in precipitation in snow form. Examination of the values of instability indices in Shahrekord station also showed that these indices were prominent in the hours before the event and intensified the activity and convective cooling of the system.

The results showed that the sources of moisture for precipitation in the region are mainly the Red Sea, Arabian Sea, Oman Sea, and the North Indian Ocean, which were associated with a positive vorticity advection. Examination of thermodynamic conditions also showed that the instability indices in the hours before the onset of precipitation were favorable and intensified the convective activity of the systems. Convection cooling along with a severe decrease in temperature has shifted rain to snow. A significant decrease in temperature compared to the long-term average and the atmospheric cold column justifies the snowfall occurrence in spring.