Evaluating modifications to Tiedtke cumulus parameterization for improving summer precipitation forecasts in the nested grid of Taiwan Global Forecast System (TGFS v1.1)

Abstract

The Taiwan Global Forecast System (TGFS), which is a global numerical weather prediction system equipped with a nested regional domain, has become operational at Taiwan's Central Weather Administration since September 2023. To improve summertime precipitation forecasts in the 4.8 km-resolution nested simulations of TGFS, numerous modifications to the Tiedtke cumulus parameterization are made. First, scale-aware treatments to the convective mass flux and trigger condition are applied in the Tiedtke scheme to enhance the role of grid-scale microphysics processes, which effectively improves the pattern and location of afternoon thunderstorm precipitation. Based on the scale-aware Tiedtke version, further changes are made to reduce the precipitation biases. Constraints on convective updrafts are modified to reduce the occurrence and intensity of convection. The definition of convective cloud tops is revised to reduce their height to a more reasonable level. Additionally, the convective trigger function, entrainment, and detrainment rates are assumed to be more sensitive to environmental relative humidity, resulting in a further reduction in precipitation. The combination of these modifications successfully improves the precipitation forecast skill for summer rainfall events.