Objective Prediction of Warm Season Afternoon Thunderstorms in Northern Taiwan Using a Fuzzy Logic Approach

Abstract

In this study, a fuzzy logic algorithm is developed to provide objective guidance for the prediction of afternoon thunderstorms in northern Taiwan using preconvective predictors during the warm season (May-October) from 2005 to 2008. The predictors are derived from surface stations and sounding measurements. The study is limited to 277 days when synoptic forcing was weak and thermal instability produced by the solar heating is primarily responsible for thunderstorm initiation. The fuzzy algorithm contains 29 predictors and associated weights. The weights are based on the maximum of the critical success index (CSI) to forecast afternoon thunderstorms. The most important predictors illustrate that under relatively warm and moist synoptic conditions, sea-breeze transport of moisture into the Taipei Basin along with weak winds inland provide favorable conditions for the occurrence of afternoon convective storms. In addition, persistence of yesterday's convective storm activity contributed to improving today's forecast. Skill score comparison between the fuzzy algorithm and forecasters from the Taiwan Central Weather Bureau showed that for forecasting afternoon thunderstorms, the fuzzy logic algorithm outperformed the operational forecasters. This was the case for both the calibration and independent datasets. There was a tendency for the forecasters to overforecast the number of afternoon thunderstorm days. The fuzzy logic algorithm is able to integrate the preconvective predictors and provide probability guidance for the prediction of afternoon thunderstorms under weak synoptic-scale conditions, and could be implemented in real-time operations as a forecaster aid.

Summer Convective Afternoon Rainfall Simulation and Projection Using WRF Driven by Global Climate Model. Part I: Over Taiwan

“…The next question is what causes the projected decrease in CAR frequency over southwest Taiwan? Previous studies (e.g., Lin et al 2012;Huang and Wang 2014) examining the observational data suggested that the increase in daytime low-level wind convergence could provide more "dynamic" lift to facilitate CAR genesis. Using WRF-HiRAM and WRF-MRI as examples, the present-day simulations (Fig.…”

“…Because of its importance to local residents, the characteristics and causes of CAR have been examined in many studies (Chen et al 1991;Johnson and Bresch 1991;Kerns et al 2010;Lin et al 2011;Huang and Chan 2012;Wang et al 2013). Studies have noted that CAR generally occurs on a day associated with a stronger sea breeze circulation during the daytime (e.g., Lin et al 2012). Such a temporal sea breeze circulation evolution is found to be consistent with the local temperature change (Huang and Wang 2014).…”

Quasi-Biweekly Mode and Its Modulation on the Diurnal Rainfall in Taiwan Forecasted by the CFS

“…Past studies have found that the maximum hourly rainfall of diurnal convection occurs at about 1600-1700 local time (LT) along the western slopes of the mountains (e.g., Chen et al 1999;WC08;Lin et al 2011). In point of fact, afternoon thunderstorms in Taiwan frequently caused flash floods, traffic interruptions, and power shortages due to lightning strikes; these associated hazards have caused human casualties.…”

Tuning AutoNowcaster Automatically

“…Forecasters should examine the membership functions to ensure that they are reasonable for the dominant weather modes in their region. Lin et al (2012) suggest that the fuzzy membership functions themselves can be designed objectively using univariate conditional probabilities obtained from a longterm archive of data. Forecasters should also consider TABLE 6.…”