عنوان مقاله [English]
The cyclone Chapala was the second strongest tropical cyclone among the cyclones that has been formed and recorded over the Arabian Sea. On October 28, 2015, the cyclone Chapala developed over western India from the monsoon trough. After reaching its peak intensity on October 30, 2015, it started to move toward the Yemeni island of Socotra. Then, on November 2, 2015, the cyclone entered the Gulf of Aden and became the strongest cyclone ever developed in that water area. The cyclone Chapala was finally decayed on November 4, 2015.
The present work is devoted to the study of the extratropical transition of the cyclone Chapala and its impact on the development of mid-latitude disturbances and, in particular, the jet stream over the western part of Iran. In fact, the main objective of the current work is to find out whether there is any link between the extreme rainfall over western Iran and the cyclone Chapala via the extratropical transition of the cyclone and its impact on the development of mid-latitude weather systems including the jet stream. To this end, the Weather Research and Forecasting (WRF) model is used to simulate the cyclone Chapala during its lifetime from the development stage to the decay stage. The advanced research WRF model is a fully compressible, non-hydrostatic mesoscale numerical weather prediction model. This model has been developed at National Center for Atmospheric Research (NCAR). For the ARW dynamical core, an Arakawa-C horizontal grid is used, and for temporal integration of governing equations, a Runge–Kutta scheme with a smaller time step for fast waves (such as sound waves) is used. The WRF model simulations are performed for the period 1 to 11 November 2015. To perform the WRF model simulations, the NCEP FNL (Final) Operational Global Analysis data, which are available operationally every six hours, are used to prepare the initial and lateral boundary conditions. In this study, the ARW dynamical core of the WRF model is used. The WRF model is configured with one nest and 45 km horizontal grid resolution in a Lambert projection. The computational domain of the WRF model covers Iran, the Persian Gulf, the Oman Sea and the Arabian Sea. In addition, the following physical parametrizations are used: the WSM3 scheme for the microphysics, the RRTM scheme for the longwave radiation, the Dudhia scheme for the shortwave radiation, the MM5 method for the surface layer, the Noah method for the land surface, the YSU scheme for the planetary boundary layer, and the Kain–Fritsch scheme for the cumulus convection. Further, to simulate the air parcel trajectories, the Hybrid Single Particle Lagrangian Integrated Trajectory Model (HYSPLIT) is used. The HYSPLIT model can be used for numerical simulation of air parcel trajectories as well as the complex transport, dispersion, chemical transformation, and deposition simulations. Here, the HYSPLIT is coupled with the WRF model to carry out forward and backward simulations of air parcel trajectories over Iran during the period of the activity of the cyclone Chapala.
The diagnostics like potential vorticity, as computed and presented based on the WRF numerical model results and the air parcel trajectory simulations by the HYSPLIT model, point to a clear transfer of mass and energy from the tropical lower troposphere to the upper troposphere in midlatitudes during the extratropical transition of the cyclone Chapala. The marked effect of the cyclone on the weather systems leading to the extreme precipitation in the southwest of Iran is confirmed.