Analysis of Systematic Error in Numerical Weather Prediction of Coastal Fronts in Japan's Kanto Plain

Abstract

 Local fronts formed near the coast of the Kanto Plain mainly in a cold season, so-called “coastal fronts”, tend to be forecast on the inland side of their actual positions by the operational mesoscale Numerical Weather Prediction (NWP with a horizontal grid spacing of 5 km) model at Japan Meteorological Agency (JMA). In this study, we confirm a systematic NWP error through statistical validations of coastal fronts that occurred with southerly onshore winds during 2015-2018. Using a nonhydrostatic numerical model (JMA-NHM), we explore the relevant physical mechanisms through sensitivity experiments involving different horizontal resolution, envelope orography, and physics parameterization schemes for three cases with typical errors. The operational NWP model is shown to have a systematic error, with local fronts being consistently shifted to the inland side of their actual positions when the forecast period exceeds 5 hours, regardless of precipitation. The sensitivity experiments suggest that the systematic error associated with coastal fronts may be primarily caused by an underestimation of the mountain barrier surrounding the Kanto Plain in the model. The northwestward distance error of coastal fronts, averaged over the three illustrative cases, can be reduced by 27 % and 37 % by increasing the horizontal resolution from 5 km to 2 km and 1 km, respectively, and can be almost entirely eliminated by using the envelope orography. Moreover, the evaporative cooling of precipitation shifts coastal fronts to the seaward.

 Most coastal fronts are thought to take the form of cold air trapped on the southeastern slope of the mountains surrounding the Kanto Plain, where the elevation angle of the frontal surface is roughly controlled dynamically. The local front shifts to the seaward when the ridgelines of the mountains become higher, and shifts to the seaward through the reduction of the elevation angle when the trapped air becomes colder.