The purpose of this study was to understand the reasons why frequent positive cloud-to-ground(+CG) flashes occur in severe thunderstorms. A three-dimensional dynamics-electrification coupled model was used to simulate...The purpose of this study was to understand the reasons why frequent positive cloud-to-ground(+CG) flashes occur in severe thunderstorms. A three-dimensional dynamics-electrification coupled model was used to simulate a severe thunderstorm to permit analysis of the conditions that might easily cause +CG flashes. The results showed that strong updrafts play an important role in the occurrence of intracloud flashes. However, frequent +CG flashes require not only strong updrafts but also strong downdrafts in the lower cloud region, conditions that correspond to the later phase of the mature stage and the period of the heaviest solid precipitation of a thunderstorm. During this stage, strong updrafts elevated each charge area in the updraft region to a higher level, which resulted in an inverted tripole charge structure. A wide mid-level region of strong positive charge caused largely by positively charged graupel, presented in the middle of the updraft region because of a non-inductive ice-ice collisional charging mechanism. The charge structure in the downdraft region was consistently more complex and revealed several vertically stacked charge regions, alternating in polarity. Much of the graupel/hail outside the updrafts was lowered to cloud-base by strong downdrafts. In this area, the graupel/hail was charged negatively because of the transportation of negatively charged graupel/hail from higher regions of negative charge in the updrafts, and via the inductive charging mechanism of collisions between graupel/hail and cloud droplets at the bottom of the cloud. Consequently, a large region of negative charge formed near the ground. This meant that +CG flashes were initiated more easily in the lower inverted dipole, i.e., the middle region of positive charge and lower region of negative charge. Frequent +CG flashes began almost synchronously with dramatic increases in the storm updrafts, hail volume, and total flash rate. Therefore, the occurrence of +CG flashes appears a good indicator of storm intensification and it could have some use as a predictor of severe weather in the form of hail.展开更多
Multiple methods were applied to study the deformation characteristics of hornblende in Archean plagioamphibolite mylonite from the Western Hills(Beijing),including optical microscopy(OM),electron backscatter diffract...Multiple methods were applied to study the deformation characteristics of hornblende in Archean plagioamphibolite mylonite from the Western Hills(Beijing),including optical microscopy(OM),electron backscatter diffraction(EBSD),transmission electron microscopy(TEM),and electron probe microanalysis(EPMA).The hornblendes are σ and δ type porphyroclasts with the new-born needle shaped grains as their tails.The analysis of lattice preferred orientation(LPO) of both the porphyroclasts and the new-born grains shows that the main slip system of the deformed hornblende is(100)<001>,suggesting that the fabric characteristics of new-born grains inherit that of porphyroclasts.Sub-microstructures show the porphyroclast core is dominated by dislocation tangle,little or no dislocations in the new-born grains,and the subgrains confined by dislocations in the transition zone between porphyroclasts and new-born grains.By using plagioclase-hornblende geothermometry and hornblende geobarometry,the estimated temperature and pressure of porphyroclasts are 675.3-702.9℃ and 0.29-0.41 GPa and those of new-born grains are 614.1-679.0℃ and 0.11-0.31 GPa.The bulging recrystallization is summarized as deformation mechanisms of hornblende by the discussions of the microstructures,EBSD fabric,sub-microstructures,and the deformed temperature and pressure.展开更多
基金supported by the National Key Basic Research Program of China (Grant No. 2014CB441403)the National Natural Science Foundation of China (Grant No. 41275008)+1 种基金R&D Special Fund for Public Welfare Industry (Grant No. GYHY201306069)Open Project of Key Laboratory of Meteorological Disaster of Ministry of Education (Grant No. KLME1004)
文摘The purpose of this study was to understand the reasons why frequent positive cloud-to-ground(+CG) flashes occur in severe thunderstorms. A three-dimensional dynamics-electrification coupled model was used to simulate a severe thunderstorm to permit analysis of the conditions that might easily cause +CG flashes. The results showed that strong updrafts play an important role in the occurrence of intracloud flashes. However, frequent +CG flashes require not only strong updrafts but also strong downdrafts in the lower cloud region, conditions that correspond to the later phase of the mature stage and the period of the heaviest solid precipitation of a thunderstorm. During this stage, strong updrafts elevated each charge area in the updraft region to a higher level, which resulted in an inverted tripole charge structure. A wide mid-level region of strong positive charge caused largely by positively charged graupel, presented in the middle of the updraft region because of a non-inductive ice-ice collisional charging mechanism. The charge structure in the downdraft region was consistently more complex and revealed several vertically stacked charge regions, alternating in polarity. Much of the graupel/hail outside the updrafts was lowered to cloud-base by strong downdrafts. In this area, the graupel/hail was charged negatively because of the transportation of negatively charged graupel/hail from higher regions of negative charge in the updrafts, and via the inductive charging mechanism of collisions between graupel/hail and cloud droplets at the bottom of the cloud. Consequently, a large region of negative charge formed near the ground. This meant that +CG flashes were initiated more easily in the lower inverted dipole, i.e., the middle region of positive charge and lower region of negative charge. Frequent +CG flashes began almost synchronously with dramatic increases in the storm updrafts, hail volume, and total flash rate. Therefore, the occurrence of +CG flashes appears a good indicator of storm intensification and it could have some use as a predictor of severe weather in the form of hail.
基金supported by the National Natural Science Foundation of China (Grant No.40772133)
文摘Multiple methods were applied to study the deformation characteristics of hornblende in Archean plagioamphibolite mylonite from the Western Hills(Beijing),including optical microscopy(OM),electron backscatter diffraction(EBSD),transmission electron microscopy(TEM),and electron probe microanalysis(EPMA).The hornblendes are σ and δ type porphyroclasts with the new-born needle shaped grains as their tails.The analysis of lattice preferred orientation(LPO) of both the porphyroclasts and the new-born grains shows that the main slip system of the deformed hornblende is(100)<001>,suggesting that the fabric characteristics of new-born grains inherit that of porphyroclasts.Sub-microstructures show the porphyroclast core is dominated by dislocation tangle,little or no dislocations in the new-born grains,and the subgrains confined by dislocations in the transition zone between porphyroclasts and new-born grains.By using plagioclase-hornblende geothermometry and hornblende geobarometry,the estimated temperature and pressure of porphyroclasts are 675.3-702.9℃ and 0.29-0.41 GPa and those of new-born grains are 614.1-679.0℃ and 0.11-0.31 GPa.The bulging recrystallization is summarized as deformation mechanisms of hornblende by the discussions of the microstructures,EBSD fabric,sub-microstructures,and the deformed temperature and pressure.
