Analysis of structure evolution and development mechanism of a Central-Asian cyclone moving northward

Central-Asian cyclone is one of the major synoptic systems that causes disastrous weather such as heavy rain, snowstorm, strong wind, and sand dust in Xinjiang. In order to deeply explore the structure and the formation of Central-Asian cyclones, taking a Central-Asian cyclone with typical northward path from August 11 to 12 in 2017 as an example, we analyze its activity characteristics and circulation situation, using conventional meteorological observations, FY satellite water vapor images, and NCEP reanalysis data. We reveal its vertical thermodynamic structural characteristics, with the focus on its mechanism of occurrence and development based on the quasi-geostrophic theory and the potential vorticity conservation. The main results are as follow. (1) The Central-Asian cyclone in front of the high trough at 500 hPa is due to the low vortex and shear line at 850 hPa and the divergence in upper atmosphere. Such condition and background is favorable to the occurrence of Central-Asian cyclone. (2) It can be seen from the water vaper images that the Central-Asian cyclone is in various stages from initial, development, and weakening, and the corresponding cloud systems in the three stages experience a transition from a baroclinic leafy shaped to a comma shaped, and then to a spiral shaped. (3) Vertical cross sections of physical fields in the center of surface cyclone show that there is an obvious cold front and a positive relative vorticity column at the beginning of the cyclone, and the upward motion is only located above the frontal zone. During the development of cyclone, the front intensified and steepened with strong upward motion both above and below. The positive relative vorticity at the lower level above the cyclone center increases, and the high positive relative vorticity at the upper level on the west side moves eastward towards the cyclone center. At the weakening stage of the cyclone, the front zone on its west side and the upward motion over and below the cyclone weakens, with the positive relative vorticity at lower level decreasing and the whole positive relative vorticity column becoming steeper. (4) The warm advection at 700 hPa increases significantly, the difference of vorticity advection between high and low levels increases, and the divergence at the high level strengthens, which leads to the occurrence and development of the cyclone. In addition, the subsidence of the high potential vorticity zone of dry air at the high level and the eastward moving of positive gradient center of potential vorticity contribute to the development of cyclone.