Identifying Dynamic and Thermodynamic Patterns of Winter Heavy Rainfall in Iran (1960-2010), a Case Study: 1974/12/05 Heavy Rainfall

Introduction
Changes and fluctuations in shape and type of precipitation have positive and negative effects and have an important role in different aspects of human life. The change in the pattern produces negative and positive secondary feedback to other climatic parameters followed by changes in hydrological cycles, water resources, natural and artificial ecosystems, human and animal habitats, security and economics. Therefore, the first parameter that can be considered is the study of drought. The presence of many parts of Iran in the dry and semi-arid belt of the world, on the one hand, and the determinant role that play at the water supply in the country on the other hand, thus it is important to be aware of the trend of rainfall changes in Iran. The nature of the heavy rainfall and the consequences of this have caused this phenomenon to be of particular importance in environmental planning and water resource management. The heavy rainfall affects the planning, design, operation and management of water resources. Therefore, it is necessary to know the characteristics of the behavior of such rainfall in order to predict and better manage water resources of the country. Heavy rainfall is one of the natural hazards that being aware of their occurrence can help reduce potential damage .The dynamic state index of the atmosphere, which expresses the degree of deviation from non-static, drought and viscosity of the atmosphere, can be used as a new theory to determine the location and time of high-pressure and low-pressure systems and their intensity.
Iran has arid and semi-arid climate, with its annual rainfall averaging about one-third of the world's annual values. Parts of Iran's rainfall are also more widespread in south-east part of the country, with high rainfall intensities. Therefore, the analysis of the synoptic and dynamic conditions of atmospheric circulation patterns is very important in identifying the factors affecting the occurrence of heavy rainfall, in particular, when these types of rainfall can cause floods and catastrophic consequences by converting runoff. Thus, with respect to the importance of the subject, the present study aimed to identify the dynamic and thermodynamic patterns that govern the day of the event of heavy winter precipitation.  
Materials and methods
In order to study the dynamic and thermodynamic patterns of the studied day, the vorticity of dynamical quantities include relative vorticity, absolute vorticity, Rossby-Ertel vorticity at pressure level, convergence and divergence, vertical speed at high system and dynamical quantities of potential temperature written and definedTo analyze the temporal variations of the above quantities, their graphs were analyzed in the selected range of 0 to 80 degrees longitude and latitude 10 to 60 degrees north for the 0000 and 1200 Greenwich hours. It should be noted that the maps of the day before and after the day were also examined at different levels to further understand the patterns governing the day of heavy rainfall, but to reduce the amount of content, only the maps of the day of heavy rainfall occurred at 0000 and 1200 Greenwich are provided.
Results and discussion
Examination of the thermodynamic and dynamical quantities during heavy rainfall events of the case series of winter 1960-2010 shows that on the day of the heavy rainfall event, December 5, 1974, the intensities of these quantities at 1200 Greenwich hour were greater than other times. Therefore, it can be said that the peak of heavy rainfall at this day is 1200 hours where the dynamical and thermodynamic quantities changes are as follows:
-      Increasing the potential temperature vertical gradient across the country; this increases the velocity of the subtropical jet as well as the potential vorticity values in the upper atmosphere.
-      The formation of a strong convergence zone at the country level and its adaptation to the mid-level divergence zones of the atmosphere that has led to the dynamic rise of the air in most areas. The high volume of upward atmospheric movements at this time has occurred in the western half, especially in the windward slopes of the Zagros Mountains.
-      Increase in relative vorticity and absolute vorticity of 500 hp levels corresponding to southwest - northeast flows of eastern part of the trough.
-      Increasing the potential vorticity values of Rossby-Ertel pressure levels of 500 and 50 hp and the co-entropy level of 330 Kelvin in the western part of the country due to factors such as potential temperature gradient increase, caused absolute vorticity increase, as well as increased static stability in the upper atmosphere.
Conclusion
Changes and fluctuations in shape and type of precipitation have positive and negative effects and they play an important role in different aspects of human life. The change in them produces negative and positive secondary feedback in other climatic parameters in hydrological cycles, water resources, natural and artificial ecosystems, human and animal habitats, security and economics. Heavy rainfall is one of the natural hazards that being aware of their occurrence can help reduce potential damage. Therefore, the analysis of the synoptic and dynamic conditions of atmospheric circulation patterns is very important in identifying the factors affecting the occurrence of heavy rainfall, in particular, when these types of rainfall can cause floods and catastrophic consequences by converting runoff. Thus, with respect to the importance of the subject, the present study aimed to identify the dynamic and thermodynamic patterns prevailing on the day of the event of heavy winter precipitation. The results show that during the event of heavy rainfall, a strong convergence zone was formed at the country level, which caused the dynamic ascension of the atmosphere in accordance with the atmospheric divergence zone. Under such conditions, the relative vorticity values increases corresponding to the eastern part of the trough, which also results in an increase in absolute vorticity values. In addition, at this time, the potential temperature vertical gradient is also increasing throughout the country. The Rossby-Ertel potential vorticity values of 500 and 50 hp and 330 Kelvin co-entropy levels can also be increased due to potential temperature gradient increase, absolute vorticity, as well as increased static stability at upper atmospheric levels.