Application of Q vector and suitable pressure level of its analysis for the diagnosis of vertical motion in two rainfall case studies over Iran

Calculating the Q vector and preparing its output products can be very effective for meteorology and weather forecasting. The vector and its convergence determine the upward motions of the atmosphere, so maps involving the vector and its convergence at different pressure levels, along with other meteorological maps, for weather forecasts and related warnings, increases the accuracy of this process. In order for the omega equation to have a more appropriate description and analysis, and also to combine sentences in the equation that neutralize each other to some extent, the Q vector has been defined and replaced in the equation. Thus, convergence and divergence for the Q vector show the ascending and descending motions of the atmosphere, respectively. In this study, Q vector values and their convergence were calculated at different pressure levels and the accuracy of the results was studied for two incoming meteorological systems with heavy rainfall. A period of time includes February 25 and 26, 2020, during which regions in the west of the Iran and the southern slopes of the Alborz mountain range received significant rainfall. The second period is July 13 and 14, 2020, with relatively heavy rainfall in parts of the north of the Iran.To perform the calculation, global GFS model output data with a horizontal resolution of 0.5 degrees and 3-hour forecast time step, temperature values and geopotential height at grid points were extracted. the Q vector and its convergence were calculated using the central second-order finite difference method and the numerical noise arising from high spatial resolution was reduced using a discrete spatial filter. Convergence maps of the Q vector representing the upward motion were analyzed by the accumulated precipitation, ground surface information, satellite images, and synoptic maps. The results of vector calculation and its convergence had considerable accuracy and consistency with the time and place of heavy rainfall leading to flooding in the two cases studied. In the first case, the precipitation system was imported from the west of the Iran with a dynamic active trough, and there was a good humidity flux from lower altitudes, which caused heavy rainfall, especially in Ilam province and areas of the southern slopes of the Alborz mountain range, including Tehran province. Studying of synoptic maps, cumulative precipitation and analyzes performed and their comparison with the results of Q vector calculation and its convergence in different forecasting hours, shows the appropriate accuracy of the Q vector calculation. The convergence of Q vector, corresponds to the hours and areas of heavy rainfall in the west of the country and the southern slopes of the Alborz mountain range. In the second case, the existing conditions from the analysis of synoptic maps indicate that the influence of the high pressure system from the north to the shores of the Caspian Sea, which is a common cause of mechanically ascending due to precipitation in this region, does not have a strong presence and being located at the cold output of the jet, the presence of the trough in the middle of the atmosphere in the northwest of the Iran and the flux of moisture from the eastern Mediterranean have caused instability and precipitation. These rains caused floods and damage in areas of the northern slopes of the Alborz mountain range, especially in the western and eastern heights of Gilan province. Despite the differences in precipitation forecasts by GFS and ARPEGE models for this time period, the studies show a significant temporal and spatial agreement between the results of Q vector calculation and its convergence with the analysis of synoptic maps and land surface information.The results showed that the Q vector results for the western regions and along the Zagros mountain range at the level of 500 hPa, for the southern areas of the Alborz mountain range at the level of 700 hPa and for Gilan province and the southern shores of the Caspian Sea at the level of 850 hPa, was more consistent with the cumulative precipitation measurements and synoptic analyzes. Considering more cases continuously and operationally, can provide the better estimate the conditions of forecasting by Q vector and its accuracy, according to the specific conditions of each region, such as altitude, moisture resources, location of roughness and mountain ranges and other factors.