There are some γ-scale thunder storms moving from either the west or north mountain areas to the city of Beijing under the weak circulation systems of synoptic scale within 15-17 June 2014.It is interesting that the strength of each down-to-hill thunder storm is changing differently.For example, the first and the last moving storms become stronger while closing to the city of Beijing, but the others become weaker, which increases the difficulties in the prediction for the local forecasters.In order to find the clues to understand the various changing, several observation data, such as surface auto weather station data, sounding data, wind profiles and Doppler Radar data, are used to analysis these cases.According to the study, except for the strength of moving storms themselves is an obvious factor to influence the changes after leaving the hills, an important reason been found is that the thermal and dynamic energy would be consumed, the cold pool and the downward flow are created by the moving of the thunder storms from mountain to the plain at first, so there is not enough energy and advantages available for the next thunder storms' to development.Then with the temperature of the plain increasing because of the sunshine and south wind, the thermal and dynamical condition become advantageous available again to maintain and strengthen the convective cell.Moreover, it should be noted that if the thunder storms' strength can be intensified in the mountain area, then the strength will maintain and even become stronger easily when leaving away from the hill.In conclusion, it is necessary to observe the change of the thermal and dynamic conditions in city areas, which is key to successfully forecast the strength change of down-to-hill thunder storms.
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