Potential use of the GLM for nowcasting and data assimilation

Abstract

Based on the relationship between lightning and thunderstorm microphysics, this paper aims to determine the averaged vertical profiles of polarimetric variables for different classes of lightning density according to the GLM grid and then evaluate the potential use of these profiles for data assimilation in models with high spatial and temporal resolutions. Polarimetric variables from an X-band radar located in Campinas-SP and data from the Brazilian Network were used to detect the microphysics properties and atmospheric discharges of clouds (GLM proxy). The main differences between the lightning density class-averaged profiles for the four variables of ZH, ZDR, KDP and ρHV were observed in the region above the melting layer. For the most intense lightning classes, the signatures associated with high concentrations of ice particles at high altitudes, the presence of supercooled drops above the freezing level and the occurrence of large and more oblate raindrops were observed. To analyze the possible use of reflectivity profiles as a way to indirectly assimilate GLM information into forecast models, two case studies were conducted using the Weather Research and Forecasting model. The analyses and forecasts obtained with the assimilation of radar data (reflectivity factor and Doppler winds) and with the indirect assimilation of GLM lightning density rates through mean reflectivity profiles were evaluated against a control run assimilating no data. Overall, the two assimilation experiments offered substantial improvements over the control run in terms of short-term forecasts of reflectivity patterns and storm motion. These encouraging results supports the ability of the GLM data to positively contribute to nowcasting and forecasting of convective-scale systems, especially over the vast regions of the South American continent currently suffering from limited and, even, an utter lack of observations.