Simulation Of The Inverted Charge Structure Formation In Clouds

The inverted charge structure appears in the special developing stage of severe storms, resulting in the substantial positive cloud-to-ground lightning. Moreover, it is always associated with disaster weather. The formation of the inverted charge structure is puzzling. The inverted charge structure is also considered appearing in the tropical cyclone (TC). In the meantime, the characteristics of TC lightning could be further examined by analyses of the electrification and the charge structure of TC. The inverted charge structure formation of the severe storm and TC was investigated using the Advanced Weather Research and Forecasting model coupled with electrification and discharge schemes (WRF-Electric). The main conclusions are summarized as follows:(1) The WRF-Electric model was improved. The non-inductive charging schemes in the model were enriched. Four different charge separation schemes are introduced into two microphysics schemes. Different charge separation schemes are examined in an idealized supercell case. The solution of the Poisson equation is also improved. The result of the electric potential is more accurate by using a computational scheme suitable for the non-uniform model grid.(2) A dynamical-derived mechanism of inverted charge structure formation was confirmed by the numerical model:the inverted structure was formed by strong updraft and downdraft under normal-polarity charging conditions such that the graupel charged negatively in the main charging region in the middle-upper level of the cloud. The inverted charge structure formation of a hailstorm in North China was investigated using the WRF-Elec. Simulation results showed the storm presenting a normal charge structure before and after hail-fall; while during the hail-fall stage, it showed an inverted charge structure-negative charge region in the upper level of the cloud and a positive charge region in the middle level of the cloud- appearing at the front edge near the strong updraft in the hailstorm. The charging processes between the two particles mainly occurred at the top of the cloud where the graupel charged negatively and ice crystals positively due to the strong updraft. When the updraft air reached the top of the storm, it would spread to the rear and front. The light ice crystals were transported backward and forward more easily. Meanwhile, the positively charged ice crystals were transported downward by the frontal subsidence, and then a positive charge region formed between the -25℃ and -10℃ levels. Subsequently, a negative charge region materialized in the upper level of the cloud, and the inverted charge structure formed.(3) The inverted charge structure in the convective region of severe storm could be formed by the inverted charging process. The snow plays an important role in the formation of the inverted diploe structure in the stratiform region. The charge structure of a multicellular storm from STEPS was simulated by using two different non-inductive charge separation schemes. Results indicate that the convective and stratiform regions display different charge structures. The stratiform region exhibits an inverted diploe structure with a negative charge region above a positive charge region. The main reason is that positive charge ice mostly transforms into snow, resulting in a positive charge region. The positive charge snow and the negative charge graupel composed into an inverted charge region. The "’normal-inverted-normal" evolution of the charge structure was simulated successfully by the SP98 non-inductive scheme, which is consistent with the observation. Strong updrafts (> 16 m s-1) and high LWC (> 2 g m-3) existing in the convective region lead to the inverted charging process in the cloud top, which means the graupel charged positively and the ice charged negatively in the colder region, and then the inverted charge structure formed. Different processes may be responsible for inverted charge structure in different storms and regions.(4) This study makes an attempt to illustrate the evolution of the charge structure of TCs. Evolution of the electrification of an idealized TC is simulated by using the WRF-Electric model. The findings thus obtained are able to unify most of the previous inconsistent observational and simulation studies. Results indicate that the TC eyewall generally exhibits an inverted dipole charge structure with negative charge above the positive. In the intensification stage, however, the extremely tall towers of the eyewall may exhibit a normal tripole structure with a main negative region between two regions of positive charge. The outer spiral rainband cells display a simple normal dipole structure during all the stages. It is further found that the differences in the charge structure are associated with different updrafts and particle distributions. Weak updrafts, together with a coexistence region of different particles at lower levels in the eyewall, result in charging processes that occur mainly in the positive graupel charging zone (PGCZ). In the intensification stage, the occurrence of charging processes in both positive and negative graupel charging zones is associated with strong updraft in the extremely tall towers. In addition, the coexistence region of graupel and ice crystals is mainly situated at upper levels in the outer rainband, so the charging processes mainly occur in the negative graupel charging zone (NGCZ).(5) The influence of landfall on the evolution of the charge structure in Typhoon Molave (2009) was investigated. The charge structure and formation in Typhoon Molave (2009) before and after landfall and during the decaying stage are investigated using satellite, lightning detection data and a mesoscale simulation. Results show that Molave was prior to landfall, with a well-defined eye and relative high-frequency lightning activity in the eyewall. Convection near the eyewall exhibited positive tripole charge structure, with negative charge located between the levels of -25℃ and -10℃ sandwiched by two positive charge regions. However, the charge structure of convection becomes negative bipole, along with negative charge in the middle and positive charge at the bottom of convection clouds after Molave reaches its maximum intensity. The charge structure of eyewall convection is closely associated with typhoon intensity, but not in a direct correlation to landfall. The outer spiral rainband cells display a positive tripole charge structure before landfall, while it features a positive bipole after landfall. Previous studies suggested that outer rainband only features a positive bipole charge structure. It is formed with different mechanisms:one resembles that in the eyewall, and the other has a positive charge region composed by hail and a positive bipole region composed by graupel and ice crystals in the upper levels, thereby forming a positive tripole charge structure. During the decaying stage of typhoon, weak convection is mainly featured by a negative bipole, similar to the terrestrial thunderstorms in the dissipative stage. In addition, different charge structures and corresponding convection intensity are also discussed.