عنوان مقاله [English]
Due to the challenges coming from numerical modeling, measurements, observations, lack of data in some regions, not enough investigations, many aspects of the mechanism for the occurrence of precipitation are not well understood. These challenges are shown clearly in mountainous studies. This is particularly the case for the occurrence of precipitation in the warm season in mid-latitudes, which is controlled by convective processes at small spatial scales while cloud belts are associated with cyclones and atmospheric fronts at larger scales. Iran, and especially North-West of it, is a mountainous area. Due to mountainous conditions, the temporal and spatial variations of precipitation in this region are large. In this region, the significant of spring precipitation is the most important characteristic of the precipitation regime. Therefore, on average, over 40% of the total annual precipitation is falling in the spring season. According to studies and evaluation of available data, most of the total annual precipitation in the North West of Iran (NWI) is occurring by convective systems (thunderstorms). To study the mechanism for the occurrence of spring precipitation in the NWI, the main consideration of researchers focused on synoptic patterns, especially on mid-tropospheric trough. Thus, there is still a poor understanding regarding the dynamical, thermo-dynamical and mechanical processes which occur in mountainous area in mesoscale over the NWI.
In this study, the occurrence of spring precipitation in the NWI is investigated using a combination of data to clarify the dynamical and thermo-dynamical processes which are governing the springtime precipitation events. Both synoptic patterns and mesoscale phenomena are considered along with a numerical simulation. A regional climate model (RegCM4) is used to evaluate the role of local topography and meso to regional scale processes on the occurrence of spring convective precipitation. A combination of data including station data, reanalysis and model outputs are used to clarify the role of the mountains, as well as local and regional thermal and mechanical forcing for the occurrence of convective precipitation in the NWI. The results indicate that, in the spring, western waves do not completely leave the NWI. The results also demonstrate that the large-scale positive vorticity advection in the mid-troposphere is associated with a local forcing from surface heating and surface fluxes (i.e. moisture convergence, latent and sensible heat fluxes) that are the main factors for formation and development of these convective systems. Therefore, unlike previous studies, in the absence of large scale dynamical forcing, such as the mid-tropospheric trough, convective precipitation will not have occurred in the NWI. In fact, local heating alone, due to lack of sufficient moisture in the area and inadequacy to provide buoyancy, cannot to create the required instability for the occurrence of precipitation. Calculating diabatic heating for the NWI is demonstrating that the horizontal advection of heat from the mountain slopes is playing an important role to formation and development of meso-scale convective systems. While the western and southern slopes of the mountains are playing as an elevated heat source, the eastern and northern slopes are playing as a sink. Therefore, the heating on southern and western slopes of the mountains as a local forcing plays an important role in providing the available heat and energy for the development of convection and convective precipitation. As a result, the responsible mechanism for the occurrence of convective precipitation in the NWI is due to an interaction between the mesoscale and synoptic/large scale processes over the region. In other words, none of the above-mentioned factors, alone or in the absence of other factors, cannot be able to create springtime heavy convective precipitation in the NWI. In fact, in the springtime, we face certain rain generating systems that their instability, ascent and humidity advection, on the one hand, are due to large-scale/synoptic-scale forcing and on the other hand, due to surface local heating. Thus, they are not entirely similar to rain generating systems in the monsoon region of India, Western Africa or Brazil which fully utilizing thermodynamic conditions and internal heating and not similar to mid-latitudes rain generating systems which are merely a result of the large scale advection of humidity and heat and the general ascent of the westerly waves.