Lightning Initiation and Intensity Nowcasting based on Isothermal Radar Reflectivity—A Conceptual Model

Based on a simple conceptual model of charge production and separation in a convective cloud, a scheme based on radar reflectivity for the nowcasting of lightning initiation and intensity was developed. Precursors for lightning initiation were identified with the following radar signatures: (a) high reflectivities at the freezing and sub-freezing levels indicating the presence of graupels; (b) high echo top indicating strong updraft; (c) high values of vertically integrated liquid (VIL) water indicating an abundant supply of cloud water for ice and graupel formation. To retrieve radar reflectivity on isothermal layers, temperature structure of the atmosphere at the retrieval times are required. For this, the 3-dimensional temperature analysis information from an hourly-updated Local Analysis and Prediction System was used. Lightning initiation was treated as an on-off process when the crucial radar parameters exceeding some threshold values. By examining the time evolution of 14 lightning-producing storms, a total of 86 sets of candidate threshold values were derived. Two optimal sets of threshold values were selected by: (1) maximizing the forecast skill in terms of Critical Success Index (CSI); (2) maximizing the lead time, over an independent set of test data comprising 21 storm cells with and without lightning initiation.

Furthermore, taking the logarithm of lightning flash rate as a measure of lightning intensity, it is found that lightning intensity has an approximate linear relationship with the crucial radar parameters. From the training data set, a multiple linear regression formulae of the crucial radar parameters was determined for lightning intensity prediction. Preliminary test results indicated that in the vicinity of Hong Kong, echo top above 7 km, VIL water in excess of 5 mm, 0°C reflectivity over 42 dBZ and –10°C reflectivity higher than 17 dBZ are good indicators of the onset of cloud-to-ground lightning in the next, say, 10-20 minutes. The conceptual model is considered potentially useful for operational use and will be put under real-time testing in the warm seasons of 2007 for possible inclusion into the automated nowcasting system operated by the Hong Kong Observatory.