A mechanism analysis of the thermo-dynamical field of a suddenly intensifying storm from mountains in the Beijing area with the radar data 4DVar

With a three-dimensional cloud-scale numerical model and the rapid update cycling 4DVar assimilation technique applied to the 4 new-generation radars, a preliminary thermo-dynamical mechanism analysis is made on a short-duration case which suddenly intensified during the period from the mountains to the plains. The analysis indicates that the precipitation episode is under the weak weather background and stratification in the low levels, the cooperation of the cool pool and environment wind fields is a trigger and strengthening mechanism for the storm during the period from the mountains to the plains, and the short duration of the storm is due to the weak low-level vertical wind shear. At the stage of the initiation of storm over mountains, cool pool generates due to the uneven distribution of thermo-dynamical field, and blocks wind propagating at the southern brim of cool pool. This mechanism results in relative high convergence, relatively large shear and helicity, which are conducive to storm propagating from the mountains to the plains. At the stage when storms reached at the foothill of the mountains, the original long-duration cool pool still provides relatively high convergence, large shear and helicity for storm propagating from the mountains to the plains, and new storms trigger some cool pools. Squeezes between new-born and original cool pools lead the northern weakening storm to re-intensifying and trigger new storms, and while these storms are drifting toward each other, it will eventually lead to organized line-echoes. At the stage of line-echoes spreading over the plains, the perturbed temperature field shows that the cold pool further intensifies and expands. The wind field shows that gust-front intensifies, becomes tilting forward, and begins to be away from the storm. The thermo-dynamical field of line-echoes shows some characters of squall line. But there is so weak wind shear in the path of propagating of the storms that it is not in equilibrium with cool pool with the result that the gust front departs gradually away from the original storm and it becomes weaker and weaker. The gust front collides with the south-east wind to generate a new storm. The helicity and shear are computed based on the simulated data. The results indicate that shear (specially the 0-3 km shear) and helicity have good correlation with the storm. The coincident conclusion with other investigations implies the simulated helicity/shear are significant of the development of the storm.