Sensitivity of the summertime diurnal precipitation cycle over three representative regions of China to convective trigger and closure treatments in a single-column model

Abstract

Using a single-column model, we investigate how trigger and closure treatments in ZM, TDK, and UNICON convection schemes affect the diurnal cycle of precipitation over China. For late-afternoon rainfall, TDK and ZM schemes with standard CAPE closure simulate precipitation peaks around noon, substantially earlier than observed. The UNICON scheme, with cold-pool feedback deactivated, also produces a noontime peak. For nocturnal rainfall, all schemes spuriously produce a noontime peak, in stark contrast to the observed nocturnal maximum. These behaviors are closely related to closure assumptions and the resulting cloud-base mass flux, which is primarily governed by CAPE in ZM and TDK, and responds more directly to surface fluxes in UNICON. By accounting for the imbalance between deep convection and boundary-layer production through specific modifications: adopting the dCAPE trigger in ZM, activating cold-pool feedback in UNICON, and applying the non-equilibrium closure in TDK, the simulated afternoon precipitation is delayed and in better agreement with observations. In addition, allowing elevated convection in ZM and UNICON improves nocturnal rainfall. The improved diurnal cycle is also evident in the diabatic heating fields. All modifications lead to a delayed and more pronounced upward-tilting heating structure as a result of the suppression of early-afternoon convection, with ZM and TDK extending to higher altitudes, while both intensified and extended in UNICON. These changes alleviate the upper-level cold bias in TDK and ZM. In UNICON, however, the overly strong convection reverses the upper-tropospheric bias from cold to warm with stronger magnitude, thereby worsening the bias.