Improving probabilistic ensemble forecasts of convection through the application of QPF-POP relationships

Quantitative precipitation forecasts provide an accumulated precipitation amount for a given time period, and accurate forecasts depend on the correct prediction of areal coverage, timing, and intensity of precipitation. These forecasts are important to a variety of people for many different purposes, so expressing a likelihood of precipitation is also useful. Most simply, probabilities of precipitation are determined by considering the percentage of ensemble members forecasting precipitation greater than a specified threshold amount. Probabilities of precipitation can also be formed from quantitative precipitation forecasts through statistical post-processing. Past research has shown that there are many ways to post-process precipitation data, such as by binning the precipitation amounts, applying statistical calibration, and/or considering the percentage of an area receiving precipitation. The main goal of this study was to expand upon relationships between quantitative precipitation forecasts and probabilities of precipitation by developing new approaches that yield more accurate probabilities of precipitation than methods that are currently more commonly used. Ensemble forecasts from the 2007 and 2008 NOAA Hazardous Weather Testbed Spring Experiments were used to provide quantitative precipitation forecasts for various days. In the study, four main approaches were developed and tested extensively using Brier scores and other statistics. Brier scores for different approaches were compared to traditional methods of calculating probabilities of precipitation. It was shown at both 20 km and 4 km grid spacings that new approaches were able to produce statistically significantly better forecasts than a traditional method that relies upon calibration of POP forecasts derived using equal-weighting of ensemble members. A deterministic approach developed during the study was also able to produce forecasts comparable to those of the calibrated traditional method