Objective verification of manual and automated forecasts of cumulonimbus clouds from World Area Forecast Centres

The two World Area Forecast Centres (WAFCs) are responsible for providing meteorological hazard forecasts to aviation customers around the world. The forecasts are presented in graphical format as Significant Weather (SIGWX) charts.

The desire from the international aviation community for a gridded product, more suitable for ingestion into flight planning systems, led to the development of automated gridded forecasts produced in gridded binary (GRIB) format. These have been distributed on a trial basis by the two WAFCs for several years.

Until now little objective verification has been available for the forecaster produced WAFC SIGWX or automated GRIB WAFC products. Although a number of data sources are available which could be used for verification, the one chosen for initial study was the UK Met Office's lightning detection system. The system detects the vertical component of electromagnetic radiation generated by a lightning discharge at around 13 kHz (VLF) received at a number of `sensor' sites. The system provides good coverage for an area comprising approximately 40% of the earth's surface.

This paper proposes an objective verification scheme using Relative Operating Characteristic (ROC) analysis to investigate the skill in both the operational SIGWX and new GRIB Cb forecasts from both WAFC London and WAFC Washington. Global verification results using Sferics data are presented from two periods, one during winter 2008-9 and one during summer 2009.