Application of Lightning Data Assimilation to Numerical Forecast of Super Typhoon Haiyan (2013)

Previous observations from World Wide Lightning Location Network (WWLLN) and satellites have shown that typhoon-related lightning data have a potential to improve the forecast of typhoon intensity. The current study was aimed at investigating whether assimilating TC lightning data in numerical models can play such a role. For the case of Super Typhoon Haiyan in 2013, the lightning data assimilation (LDA) was realized in the Weather Research and Forecasting (WRF) model, and the impact of LDA on numerical prediction of Haiyan’s intensity was evaluated. Lightning data from WWLLN were used to adjust the model’s relative humidity (RH) based on the method developed by Dixon et al. (2016). The adjusted RH was output as a pseudo sounding observation, which was then assimilated into the WRF system by using the three-dimensional variational (3DVAR) method in the cycling mode at 1-h intervals. Sensitivity experiments showed that, for Super Typhoon Haiyan (2013), which was characterized by a high proportion of the inner-core (within 100 km from the typhoon center) lightning, assimilation of the inner-core lightning data significantly improved its intensity forecast, while assimilation of the lightning data in the rainbands (100-500 km from the typhoon center) led to no obvious improvement. The improvement became more evident with the increase in LDA cycles, and at least three or four LDA cycles were needed to achieve obvious intensity forecast improvement. Overall, the improvement in the intensity forecast by assimilation of the inner-core lightning data could be maintained for about 48 h. However, it should be noted that the LDA method in this study may have a negative effect when the simulated typhoon is stronger than the observed, since the LDA method cannot suppress the spurious convection.