Improving Diurnal Precipitation Forecasts Through Coherent Coupling of Cumulus and Planetary Boundary Layer Parameterizations

This study explores the impact of coupling cumulus and planetary boundary layer (PBL) parameterizations on diurnal precipitation forecasting during the plum rainy season in Jiangsu Province, China, using a double grid-nesting approach. Results show that coherent coupling of cumulus (only in the 15 km grid outer domain [O]) and PBL parameterizations leads to improved forecasting of diurnal variations in the morning, afternoon, and the evening. Increasing the frequency of the Kain-Fritsch (KF) cumulus scheme in [O] enhances subgrid precipitation while reducing grid-scale precipitation, resulting in a more accurate representation of daytime convective activities and a reduction in over-forecasting of evening valley and early-morning precipitation. Additionally, coupling a suitable PBL scheme mitigates the overpredicted afternoon peak by facilitating turbulent mixing to penetrate higher altitudes with a thicker layer, thereby reducing instability energy accumulation. A higher KF frequency in [O] retains less low tropospheric moisture, reducing moisture convergence into the 1 km grid inner domain [I] and decreasing overpredicted daytime precipitation in [I]. Various PBL schemes produce distinct vertical distributions of turbulent moisture and heat transport, impacting convection and precipitation in [I] resolved by cloud microphysics processes. The coherent coupling of these parameterizations maintains a balanced supply of convective energy and water vapor, significantly improving diurnal precipitation forecasts in [I]. Isolating these parameterizations between nested grids may undermine this improvement. Accurate diurnal precipitation forecasting remains a challenge. This study aims to effectively couple cumulus and planetary boundary layer (PBL) parameterizations to enhance diurnal precipitation forecasts using a double grid-nesting approach. We showed that coherent coupling of cumulus and PBL parameterizations improves forecasting of diurnal variations during the plum rainy season in Jiangsu Province, China. Increasing the frequency of the KF cumulus scheme more accurately represents daytime convective activities and reduces over-forecasting of evening valley and early-morning precipitation. Coupling an appropriate PBL scheme that extends turbulent mixing into deeper layers mitigates the accumulation of instability energy, thereby reducing overpredicting of the afternoon peak. A higher KF frequency in the 15 km grid outer domain [O] retains less low tropospheric moisture, reducing moisture convergence into the 1 km grid inner domain [I] and decreasing overpredicted daytime precipitation in [I]. Diverse PBL schemes result in varying vertical distributions of turbulent moisture and heat transport, affecting explicit convection and precipitation in [I]. Therefore, the cohesive integration of cumulus and PBL parameterizations across nested grids significantly enhances diurnal precipitation forecasts. Coherent coupling between cumulus and planetary boundary layer (PBL) parameterizations improves the nested rainfall diurnal cycle forecast Increasing cumulus parameterization frequency reduces the nested morning and outer-domain evening valley rainfall overforecast Adjusting PBL parameterization weakens the rainfall afternoon peak and daytime total amount overprediction