This research aims to study the impact of temperature and wind in the southern low-pressure system and its associated precipitation in the southern regions of Iran. As The southern low pressure system moves eastward, it crosses the southern regions of Iran, causing medium and heavy rainfall in these areas. In this study, two southern low-pressure systems that caused heavy rainfall on March 11, 2015 and January 17, 2000 in southern Iran were selected, analyzed and simulated using the numerical Weather Research and Forecasting (WRF) model. Since the wind and temperature fields play a significant role in the southern low-pressure systems, four experiments were performed for investigating the effects of temperature and wind on the intensification and weakening of the southern system. The simulation results showed that the simulation for the increased (decreased) temperature caused the weakened (intensified) the southern low pressure in the studied area. This result showed that the vertical structure of the southern low-pressure and its physical characteristics are similar to the mid-latitudes cyclones, and these systems were different from the thermal low pressures. The results of wind speed changes showed that the increased (decreased) wind speed simulation caused an increase (decrease) in relative vorticity, thus the southern low pressure was intensified (weakened). In both cases, the rainfall was decreased by the increased temperature simulation, and decreased temperature caused an increase in rainfall. It was also seen that the increase in wind speed caused the special humidity advection to be increased and then the rainfall increased. Also the amount of rainfall decreased when conditions did not provide for the advection of specific humidity or the wind speed reduced.

In this study, we tried to identify the sources of moisture and its direction of heavy rainfall in south and southwest of Iran by using a new algorithm based on atmospheric rivers. For this purpose, daily rainfall of 17 synoptic stations in the period 1986 to 2015 in south and southwestern Iran that have a common time span and fully cover the study area is used.Also from the data set of the National Center for Environmental Prediction / National Center for Atmospheric Research (NCEP / NCAR) European Mid-Term Forecast Center (ECMWF) and ERA-interim data with spatial resolution of 0.75 It was used at 0.75 latitude and longitude with 6 hour resolution. The variables used are integrated water vapor (IWV), specific humidity (q), and orbital and meridional components (u, v). In this research, an algorithm based on the calculation of Vertical Horizontal Vapor Transfer Integral (IVT) is used to identify and navigate atmospheric rivers. The results show that the main source of rainfall moisture is in south and southwestern Iran, south of Red Sea and Gulf of Aden. Of course, the maps show that the Arabian Sea was not affected by the humidity.The Arabian Peninsula also, due to the high moisture transfer rate, as a transitional route, transmits a large amount of moisture to the study area.Finally, the path of moisture to the study area was mapped and identified, and thus considering the three main conditions for the atmospheric river, it can be said that the path obtained is the same as the atmospheric river.