عنوان مقاله [English]
Inertia–gravity waves (IGWs) play an important role in transporting energy and momentum, in making and shaping turbulence and mixing, and in influencing the mean circulation and thermal structure of the atmosphere. They cause perturbations in the main dynamical fields, such as pressure, temperature and wind. Therefore, knowledge of their sources and analysis of their characteristics are very important. Given that there has been no study to identify and simulate IGWs that may have occurred over Tehran during the last few decades, the current research aims to investigate IGWs in the lower troposphere based on the long-term data of Mehr-Abad meteorological station during the period from 1960 to 2015.
One method for detecting IGWs is checking the large amplitude hourly surface pressure changes. At first, hourly surface pressure observations for Tehran in a 55-yr period have been used to find the distribution of large hourly pressure changes defined as falls or rises in excess of 3 hPa per hour. After making careful checks, 101 cases were identified as potential candidates of IGWs occurrences. In the second part of the paper, we constructed composite maps of mean sea level pressure, 1000-500 hPa thickness, 500 hPa geopotential height, and 850 hPa, 500 hPa and 200 hPa wind. The cyclone-related IGW composite showed that the IGW events clustered poleward of the estimated position of the surface warm front, northeastward of the surface cyclone center, downstream and within the poleward exit region of a jet streak that was upstream of a ridge. By analyzing the synoptic-scale environments in which IGWs evolve, the probable candidates for IGWs were reduced to 17, among which 13 events were associated with convection and 4 events were related to cyclone development.
In order to evaluate our analysis and to obtain the IGWs characteristics, the Weather Research and Forecasting (WRF) model was used. We determined the position of wave packets and their propagation during the time evolution by drawing the horizontal velocity divergence. The horizontal and vertical wavelength, intrinsic frequency, the period, horizontal and vertical velocity and the group velocity were determined by drawing the cross sections of horizontal velocity divergence and making use of the dispersion relation for hydrostatic waves, as well as the relations for the horizontal and vertical components of group velocity and the intrinsic phase speed. Then we drew time series of the horizontal velocity divergence and sea level pressure, and evaluated the correlation between these two quantities. By checking the horizontal velocity divergence at 800 hPa which is near the surface of the earth, it was observed that negative/positive divergence is associated with pressure rise/fall. On the other hand, the strength or weakness of wave’s amplitude was investigated by using the time series of horizontal divergence at different pressure levels. The numerical simulation results not only confirmed the validity of the method employed to analyze and identify IGWs, but also determined the connection between the IGWs sources, their propagation as well as their effects on meteorological fields such as pressure, temperature and wind over Tehran, especially at Mehr-Abad meteorological station.