Study On Thunderstorm Development And Evolution, And Its Mechanisms In The Yangtze River Delta Region

The main purposes of the present study are:1) to investigate some basic features of thunderstorms over the Yangtze River Delta (YRD) region, including the spatial and temporal distribution, motion characteristics, evolution, and life time of the thunderstorms;2) to analyze the vertical structure of storm cell and lightning activity during the evolutionary stages of several types of thunderstorms, and to build up a basic conceptual model of thunderstorm evolution;3) to investigate some other "features" which may relate to changes in thunderstorm development and evolution, such as meso-cyclone, gust front, and thunderstorm merger; and4) to reveal the mechanism of evolution and the key environmental factors of a long-lived squall line in a moist warm synoptic situation.Using storm structure data derived from the WSR-88D Doppler Weather radar in Shanghai, lightning location data, and other meteorological data in the years of2004to2012, the storm motion path length and length of life cycle are calculated according to the intensity of the thunderstorms. The spatial and temporal distribution of storm cells and storm intensity change distribution are statistically analyzed. A conceptual model of storm vertical structure and lightning activity change with thunderstorm evolution is also given. Some of the key impact elements, such as gust front, meso-cyclone and storm merger phenomena, and their impact mechanisms on thunderstorms are investigated. Using a mesoscale model’s idealized experiment and real-case simulation, two long-lived squall lines are compared. YRD region thunderstorms have the following main characteristics:1) Most of thunderstorms, especially those local storms, are close to some medium-sized cities, some mountains or hills (especially those isolated), and some water-land borders. This implies solar radiation and difference of underlying surface features are major causes of this type of thunderstorm. The medium path thunderstorms tend to be close to the borders of land and water, while the long path thunderstorms concentrate in several major corridors under certain synoptic situations. Thunderstorms peak in the afternoon in midsummer while thunderstorms show a bimodal diurnal distribution during monsoon season. Midsummer thunderstorms show the tallest and strongest vertical structure, followed by Meiyu season thunderstorms, and those with tropical weather systems the lowest. However, severe thunderstorms show less difference among the various seasons. Thunderstorm initiation is often associated with an urban heat island, valley circulation, and land-water (sea or lake) circulation, mostly at12-18(BJT) for the short-and medium-path thunderstorms.2) The motion of weaker thunderstorms had no correlation to the steering flow. The higher the storm top (ET) is, the stronger of intensity and the higher the core center is. However, the greater the deviation of the motion, the more right-moving the storms are. Life time shows no connection with its motion deviation. Storm motion speed decreases as its height, intensity, and core height increase. More right-moving storms are found under southwest flow, while a higher proportion of left-moving storms than right-moving are observed under northwest flow. Long life cycle and long path severe thunderstorms are found to have a tendency to move right.3) Local storms tend to intensify and weaken in the same areas, such as cities, isolated mountains or hills, and water-land borders. A city center is a storm weakening location, but a storm intensifying center is found about10-30km downwind of the city center. The strengthening and weakening mechanisms are related to the underlying surface features. Significantly, the strengthening area of medium-path storms also shows a downwind effect with a distance of20-40km from medium-sized cities and centers of larger cities, while significant weakening areas are located over water body downwind of land.. Long-path thunderstorms usually intensified in two kinds of areas:in the land areas downwind of large water bodies, and over terrain with a windward slope. The statistical results show a significant change in the vertical structure and lightning activity during the life cycle of thunderstorms.1) In the initiation stage, its core center is first observed in the mid-level, and the top and base of the storm are close to the core. Then the top and core climb as the updraft intensifies, while the base descends as precipitation forms in the mid-level.2) When the thunderstorm develops to a certain extent, the core center can not be lifted by the updraft even if the updraft is still strong. The core reaches its maximum height, while the top and base of the storm continue to rise and descend respectively. At the same time, the strengthening of the updrafts also causes a burst of intra-cloud (IC) lightning activity and a relatively small amount of cloud-to-ground (CG) lightning.3) With the mass of hydrometeors (water droplets, hail, graupel, etc.) increasing, the core center begins a downward trend. But the storm top may still continue to rise with the upward motion, and thunderstorm height increases. Precipitation falls to ground. When the thunderstorm reaches its maximum height, VIL is also close to its highest stage.4) The thunderstorm enters the outbreak stage, the IC lightning peaks, and the core center with big rain drops and hail declines rapidly. Heavy rain, sometimes hail (even large hail), and strong winds touch ground, but storm top does not decline obviously. At this stage, updrafts weaken and downdrafts intensify. IC lightning begin to weaken after reaching its peak and CG lightning increases significantly since large amounts of charged water drops and hail descend.5) The decaying storm shows a fast descending top, and core center very close to ground. Heavy rain and downdrafts dominate the storm in this stage. CG flashes peak and a higher positive CG ratio follows. At last, the thunderstorm dissipates. Therefore, some vertical features of storms and lightning activity can be indicators of thunderstorm evolution.Studies on mesocyclones show that storms experience constant intensification before the first mesocyclone and weakening after the last mesocyclone. The strengthening phase is longer, and a drop of VIL and ET will be a signal of the decaying stage. Some mesocyclones occur in a thunderstorm’s decaying stage due to an intensifying rotated updraft by the uplifting of warm moist air by gust fronts, similar to an occluded front. The collision of gust fronts from different storms may trigger a new convective cell. The low level vorticity between two gust fronts may be absorbed into an updraft and a mesocyclone can be detected at middle levels. Dual wind profiler observation comparisons show that the ambient wind field of the supercell might have produced more low level vertical wind shear and storm relative helicity. On the other hand, the mesoscale convective system (MCS) might have changed the ambient wind structure. As a result of the interaction of the MCS and the environment, a positive feedback effect made the MCS a long-lived system.Comparisons are made for two types of long-lived squall lines.1) Under a dry cold airflow aloft, a strong cold advection aloft is the main mechanism for potential instability. A rapid increase in low-level moisture is conducive to severe thunderstorm initiation. Precipitation evaporative cooling in a dry environment strengthens sinking motion to form a cold pool, and the rear inflow compensation airflow to strengthen the gust front. The vortex pairs due to gust front and vertical wind shear stimulate new convection at the leading edge of squall line, and make the system long-lived.2) Under a warm and humid environment, one of the key mechanisms for a long-lived squall line is the high-level entrainment of the dry (warm) air, which provides more precipitation evaporation and strengthens the downward motion within the storm. As a result, this dry air with downdraft lowers the humidity from high levels to low levels and finally a cold pool forms in a storm in a moist warm environment. The cold pool and vertical wind shear also form a pair of low-level vortices at the front of the squall line, which make the system long-lived. Therefore, the vortex pair made by strong outflow (gust front) and strong environmental vertical wind shear is a key mechanism for squall line maintenance.