A Parameterization Scheme For Upward Lightning In The Cloud Model And A Discussion

The upward lightning (UL) initiated from the top of tall buildings (at least above 100 m) is a type of atmospheric discharge. Currently, we understand the nature of the UL from ground observations, and the corresponding theoretical research is lacking. Based on an existing bidirectional leader stochastic model, a stochastic parameterization scheme for the UL has been built and embedded in an existing two-dimensional thundercloud charge/discharge model. The ULs simulated from the experiments with two-dimensional high resolution agree generally with the Observation results. By analyzing the charge structure of thunderstorm clouds, we determined the in-cloud environmental characteristics that favor the initiation of conventional ULs and cloud-to-ground (CG) flashes, and analyzed the differences and similarities of some characteristics of the positive and the negative UL. The qualitative analysis of the parameters of the UL is also carried out and the UL and CG is compared. The main conclusions are as follows:(1) The positive ULs are typically other-lightning-triggered ULs (OLTUL) and are usually a discharge phenomenon between the ground and the lower positive charge region appearing below the main middle negative charge region. The effect of the previous in-cloud lightning (IC) process of space electrical field provides favorable conditions for the initiation of a positive UL. Its entire discharge process is limited, and the branches of the leader are fewer in number as its discharge is not sufficient. A negative UL is generally a discharge phenomenon of the dipole charge structure between the ground and the main negative charge region. The lower temperature stratification and the sinking of the hydrometeors typically initiate a negative UL. Negative ULs develop strongly and have more branches. The OLTUL is initiated mainly during the development stage of a thunderstorm, while the self-triggered UL (STUL) is initiated mainly during the dissipation stage of a thunderstorm.(2) The height of the bottom charge region plays an important role in the initial of the UL, and the amount of charge plays an auxiliary role. The lower height is easy to trigger the UL. Meanwhile, the more charge is more likely to trigger the UL. The ULs triggered by the tri-polar charge structure of thunderstorm clouds occurred mostly in the lower bottom charge region, while The ULs triggered by the dipolar charge structure of thunderstorm clouds occurred mostly in the higher bottom charge region. The main positive and negative charges destroyed by the IC are easy to trigger the UL.(3) The initial condition of the UL is quite different from that of the CG. The positive CG is mainly triggered in the dipole charge structure, when the upper main positive charge region is dominant. The positive UL is either the OLTUL of the tri-polar charge structure or a STUL of the special inverse dipole charge structure, when the positive charge stack at the bottom is sufficient. The negative CG is triggered between the middle negative charge region and the lower positive charge region at the bottom of the tri-polar charge structure. The magnitude of the lower positive charge region at the bottom is the precondition for the initiation of the negative UL. The negative UL is mainly initiated from the dipole structure in the dissipating stage of a thunderstorm cloud, whose main negative charge region is closer to the ground and whose magnitude is larger. The lower positive charge region at the bottom hinders the initiation of the negative UL.