An investigation of a cold-season heavy rainfall event during YESRain2021 field experiment in Taiwan: The mechanisms revealed by a 4DVar data assimilation system

Abstract

This research explores in detail the mechanisms leading to heavy rainfall during the cold seasons in the northeastern part of the island of Taiwan, whereas severe rainfall in the other part of the island often comes from mesoscale convection systems (MCSs) during the warm seasons. The mechanisms of precipitation intensification and maintenance are investigated using the analyses obtained by assimilating observations into a high-resolution numerical model through the four-dimensional variational (4DVar) technique. The results reveal that, initially, the rainfall was triggered due to the convergence of the northeasterly prevailing wind and the so-called return flow, which was induced by the local concave-shape terrain. This finding is consistent with previous studies conducted several decades ago. However, this research also provides further evidence that at the later stage of the event, it was the outflow with very shallow depth from the cold pool generated by the pre-existing rainfall systems that helped sustain the precipitation. It is also found that the thermodynamic condition that affects the precipitation at the plain, coastal, and mountainous regions is a result of the interaction among the northeasterly wind, outflow, and the return flow. While the outflow helped provide a favorable environment to maintain the rainfall near the coastal area, the rainfall in the inland and valley regions dissipated earlier because of the cold air accumulation. These findings provide a valuable basis for forecasting this unique phenomenon during the cold seasons in Taiwan.