عنوان مقاله [English]
When a fluid is forced by buoyancy and Coriolis forces, it undergoes oscillations. The frequency of ageostrophic oscillations resulting from these two forces is between buoyancy and inertial frequencies; they are thus called inertia–gravity waves (IGWs). These waves play important roles in propagation of energy and momentum in the atmosphere and exert influence on many atmospheric phenomena. The necessary condition for horizontal propagation of IGWs over a large horizontal distance in the atmosphere is the existence of a statically stable layer in the lower troposphere, which is called a wave duct. Actually a wave duct acts as a waveguide and traps wave energy. The underlying mechanism for the formation of a wave duct is provided by reflection of waves propagating from a layer near the surface of the earth and the constructive interference of the reflected waves with the primary propagating waves. The wave duct provides suitable environment for the maintenance of waves through over-reflection and amplification mechanisms. There are two types of wave duct in atmosphere: thermal wave duct and wind wave duct.
Considering that in Iran only a few studies have been carried out on IGWs and their ducts, this study is devoted to the detection and study of the wave ducts happened over Iran in the three-month period from January to March 2016. For this purpose, the data from the Global Forecast System (GFS) with 0.25° × 0.25° resolution are used to identify the spatio–temporal characteristics of the ducts. To identify the wave duct, two conditions are considered that permit wave propagation in the duct: duct should be statically stable and have a minimum thickness of a quarter of the vertical wavelength of the waves observed. By applying these conditions to the data in the domain of study, the number of candidate cases for wave ducts is reduced. Most cases occurred in the Caspian Sea, the Persian Gulf, the north and northeast of Iran. So 10 cases of wave duct were identified; in this research, results for two cases are presented. Also by considering the condition of the existence of an unstable or the reflective layer on top of the wave duct for the two cases, the wave ducts obtained using the first two conditions were further screened. Considering only the points obeying the foregoing three conditions, the wave duct characteristics were then estimated.
Given that the lifetime of IGWs propagation in the atmosphere is short, the GFS data used to detect wave ducts are not suitable for identification of IGWs and determination of their propagation mechanisms. In order to detect IGWs propagating in the wave duct, we simulated one of the cases with the WRF mesoscale model. Then, for estimating IGWs properties, the horizontal velocity divergence in different pressure levels was used in the internal domain. The results of led to the identification of two wave packets at different times. Also the cross section of horizontal velocity divergence was used to estimate the properties of the two wave packets. Results showed that for both wave packets, a quarter of the dominant wavelength was less than the average thickness of the ducting layer, so they were consistent with the thickness criterion required for the wave duct formation.