I. V. Mingalev et al. absorption / emission of infrared radiation, as well as due to phase transitions of water vapor to micro drops of water and ice particles, which play an important role. The finite-difference method and explicit scheme are applied for solving the system of governing equations. The calculated parameters are determined on a uniform grid. The latitude step and longitude step are equal to 0.08°, and height step is equal to 200 m. More complete details of the applied regional mathematical model maybe found in the studies of Belotserkovskii et al. (2006, 2009). Simulation results It may be recalled that an intratropical convergence zone is similar to a band, in which zonal westward flow of air predominates, with the air velocity being abnormally high. The width of the intratropical convergence zone can achieve a value of some hundreds of kilometers. As pointed out previously, it was established in the study of Mingalev et al. (2011) that the origin of a convexity in the configuration of the intratropical convergence zone, having the dimension of 800-1000 km, can lead to the formation of a cyclone. However, in the study of Mingalev et al. (2011), calculations were made for three cases in which the initial forms of the intratropical convergence zone were different and contained convexities with distinct shapes. The common feature of these shapes is that the left crook of the convexity is sharp while the right crook of the convexity is gently sloping, with the deviation of the convexity in the north direction achieving a value of a few hundreds of kilometers. The distinction of their shapes is that the right end of the convexity may be both at the same latitude as the left end of the convexity and at more northern or at more southern latitudes than the left end of the convexity. The results of modeling have indicated that the origin of a convexity of the intratropical convergence zone, having three considered shapes, can lead to the formation of a cyclone during the period for about one day. Its center is close to the southern edge of the initial intratropical convergence zone. The horizontal wind velocity in this cyclone can achieve a value of 20 m/s during the period of 27 hours. The cyclone has a horizontal extent of about 600 km. In the present study, we disposed the south boundary of the simulation domain in the vicinity of the equator. It was supposed that, at the initial moment, distributions of zonal, meridional and vertical components of the wind were consistent with the situation when the intratropical convergence zone intersects the simulation domain in the west-east direction. Calculations were made for various cases in which the initial forms of the intratropical convergence zone were different and contained convexities with distinct shapes. Initially, let us consider the first case when, at the initial moment, the intratropical convergence zone contains a convexity in the north direction. The initial form of the intratropical convergence zone may be easy seen from the top panel of the Fig.l, where it is like a light curved band. It can be noticed that, in the first case, the left crook of the convexity is sharp and the right crook of the convexity is sharp too, with the left and right ends of the convexity being at the same latitudes. The time evolution of model parameters was numerically simulated using the mathematical model during the period for about one day. The results of time-dependent modeling are partly shown in Fig.l. As can be seen from this figure, in the course of time, the initial distribution of horizontal component of the air velocity was considerably transformed. A pair of cyclonic and anticyclonic vortexes arose in the vicinity of the initial intratropical convergence zone. A cyclonic vortex arose whose center is close to the southern edge of the initial intratropical convergence zone and an anticyclonic vortex arose whose center is close to the northern edge of the initial intratropical convergence zone. The horizontal wind velocity in these vortexes achieved a value of 20 m/s during the period of twenty seven hours. The radii of these large-scale vortexes are about 400 km. It should be emphasized that the results of satellite monitoring of the Earth’s atmosphere often indicated a simultaneous origin of a cyclone-anticyclone pair. Naturally, a formation of a single cyclone or a single anticyclone was observed by satellites repeatedly. It can be noticed that, according to observations, an initially originated single cyclonic or anticyclonic vortex as well as one of vortexes, belonging to a cyclone-anticyclone pair, as well as both vortexes, belonging to a cyclone-anticyclone pair, sometimes can be attenuated in the course of time and will not achieve a status of the long-live large-scale atmospheric vortexes. Let us consider the second case when, at the initial moment, the intratropical convergence zone contains a convexity, analogous to the convexity of the first case, but deviated in the south direction. The initial form of the intratropical convergence zone may be easy seen from the top panel of the Fig.2, where it is like a light curved band. The results of time-dependent modeling for second case of the initial configurations of the intratropical convergence zone are partly shown in Fig.2. As can be seen from this figure, in the course of time, a pair of cyclonic and anticyclonic vortexes arose in the vicinity of the initial intratropical convergence zone. These vortices are analogous to those obtained for the first case. The results of simulation indicate that physical reason of the formation of the calculated pair of cyclonic and anticyclonic vortexes is the origin of a convexity in the configuration of the intratropical convergence zone, having the specific forms. As a rule, such convexities are observed during the periods of rebuilding of the global circulation of the atmosphere. The origin of a convexity of the intratropical convergence zone leads to beginning of an instability of air flow. As a consequence, a pair of cyclonic and anticyclonic vortexes arise in the lower atmosphere. In the course of time, the horizontal wind velocity in the vortexes increases due to a transformation of energy, 190

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