عنوان مقاله [English]
Atmospheric blocking is an important large-scale weather phenomenon at mid-high latitudes in the atmosphere which has a profound effect on local and regional climates in the immediate blocking domain as well as in regions upstream and/or downstream of the blocking event.
Commonly used definitions can be divided into four categories of methods to identify blocking. All of the definitions of blocking do not wholly address dipole-type blocking, so new criteria for identifying dipole-type blocking have to be established. In this research, diagnostic case of dipole-type blocking life in the Middle East region for the period time from 13 to 20 November 2019 was studied. Several basic features of the dipole-type blocking, along with the calculation of the thermal wind, are introduced to identify and distinguish the dipole-type blocking from the other blockings.
The results showed that the dipole-type blocking event usually occurs at lower-tropospheric pressure levels compared to the upper-tropospheric pressure as they have weaker westerly winds. The life time of the blocking event considered was made shorter by increasing of the westerly winds on higher pressure levels. For the occurrence of a blocking event, the wind must reach the minimum at the mid-tropospheric pressure levels of the atmosphere and the flow must be split into two branches. However, with the reduction of the thermal wind, the geostrophic wind as well as the total are weakened, resulting in the formation of split flow and blocking event. It was observed that the magnitude of the thermal wind at the time of the formation of the blocking tends to zero. The thermal wind and its westerly and easterly parts over the dipole were determined at the 500 hPa pressure level. At the time of the event, a dipole-type blocking of at least one closed contour with 4 geopotential decameters interval, simultaneously occurred for both high and low centers. Moreover, examining the westerly winds over the blocking entrance showed that during the time of this event, the speed of westerly winds decreased to a minimum amount. For the dipole-type blocking, unlike the cut-off low single-pole type, there is no sign of a fairly intense wind at the entrance of blocking. During the life cycle, the movement of blocking is less than 10 degrees per day. Furthermore, the high of blocking rests in a latitude above . The computed blocking index is also no lower than 20 m/s. The use of some features blocking and the calculation of thermal wind during the life time of a typical case of dipole-type blocking illustrates the utility of this method for identifying dipole-type blocking. The analysis, however, needs to be extended to several other cases for a more definite conclusion.