Bias correction to improve the skill of summer precipitation forecasts over the contiguous United States by the North American multi-model ensemble system

Abstract

Improvements in skill of summer forecasted precipitation as produced by the North American multi-model ensemble (NMME) system over the contiguous United States (CONUS) are examined by applying a new bias correction method. The uncorrected precipitation produced by NMME hindcasts exhibits good prediction skill in fall and winter, while the spring and summer forecasts are marked with extremely poor skill. We propose a new method to correct the forecasted precipitation distribution based on skillfully predicted 2-m air temperature (T2m) forecasts to fully exploit the stronger co-variability that exists between precipitation and T2m in nature. The occurrence of enhanced recycled precipitation over CONUS provides an ideal situation to hone precipitation forecast skills using the T2m forecasts. The proposed bias correction is shown to successfully reduce the root mean square error in precipitation hindcasts in summer and can easily be extended to real-time forecasts, thus providing a framework to dynamically link precipitation with other predictors besides T2m. Process understanding of the observed T2m-precipitation relation will offer a framework for diagnosing poor model skill.