عنوان مقاله [English]
This research is aimed to study the global distribution of tropopause folding frequency and its seasonal changes, emphasizing the ones over the Southwest Asia, for a 3-year period from Jan. 2013 up to Dec. 2015. For this purpose, the European Centre for Medium-Range Weather Forecasts (ECMWF) (ERA- Interim) reanalysis data set including wind, temperature and geopotential height were used. The horizontal resolution of the initial fields is 1×1 degrees in longitudinal and latitudinal directions prepared operationally every six hours at 60 levels. Applying the initial fields, the secondary fields, such as potential vorticity and potential temperature were calculated. From the 60 vertical levels, about 19 levels extending from 600 to 100 hPa cover the depth of all tropopause folding events studied here. In this research, we define the 2PVU potential vorticity surface as the dynamical tropopause (1PVU corresponds to 10-6 m2s-1Kkg-1). Identification of tropopause folding is based on the algorithm developed by Sprenger et al. (2003) and Gray (2003) and refined by Škerlak et al. (2014) using pseudosoundings in each of the grid points. A 3-D labeling algorithm is used to distinguish between stratospheric and tropospheric air masses and labeling them according the PV values. After labeling, the tropopause folds are identified at every grid points from the vertical profiles of the label field as areas of multiple crossings of dynamical tropopause. The frequency of folds at each grid point over a chosen period is calculated from the number of folding divided by the total 6-hourly instances corresponding to the season, and finally expressed as a percentage. According to this algorithm, tropopause folds are classified into three categories as shallow, medium and deep.
The analysis of spatio–temporal distributions of tropopause folds shows that the frequency of folding events over subtropical and mid latitude regions (between 20° to 40° north and south latitudes) is higher than the other latitudes in both the Northern and Southern Hemispheres and their frequency is increased remarkably in the winter season. Tropopause foldings in the Northern Hemisphere winter are seen as a relatively narrow band located in the subtropical latitude that surrounds zonally the whole Hemisphere, while in the summer season, foldings are concentrated in the subtropical region of the Eastern Hemisphere. Also, tropopause foldings occur mainly as shallow type in the subtropical region but as medium or deep ones in higher latitudes. Foldings in high latitudes are attributed to large-scale deformation fields, as noted by Holton and Hakim (2013), that are confirmed with water vapor satellite images, while the ageostrophic frontal circulations affect the tropopause deformation in mid latitudes.
The other noticeable point is that the Southwest Asia region has positive anomalous values of tropopause folding frequency annually, relative to the Northern Hemisphere mean. This can be partly due to the Rossby wave breaking as pointed out by Martius et al. (2007) and Gabriel and Peters (2008). These anomalous values of folding frequency change in different seasons and obtain their maximum amounts in the summer time. Two regions with the maximum value of the folding frequency more than 5 times the Northern Hemisphere mean, seen over Iran–Afghanistan and the eastern of the Mediterranean Sea that occurred in June. The increase of folding frequency in the Southwest Asia during the summer season can be related mainly to the formation and existence of the monsoon anticyclone over the subtropical region of the Indian Ocean (Tyrlis et al., 2013) and partly to the baroclinic instability events. Results of the case study relevant to tropopause foldings in June 2015 show the existence of two strong jet streams in the aforementioned regions. Also, in the meridional cross-sections of wind and PV fields two principal areas of tropopause folding are seen in the west and downward of the jet streams locations. As expected, the potential temperature maps indicate the existence of marked baroclinic regions associated with the tropopause foldings.