资源环境科技发展态势分析平台

This study describes a new mechanism governing the diurnal variation of vertical motion in tropical oceanic heavy rainfall zones, such as the intertropical convergence zone. In such regions, the diurnal heating of widespread anvil clouds due to shortwave radiative absorption enhances upward motion in these upper layers in the afternoon. This radiatively driven ascent promotes an afternoon maximum of anvil clouds, indicating a diurnal cloud-radiative feedback. The opposite occurs at nighttime: While rainfall exhibits a dominant peak at night-early morning, the boundary layer rooted upward motion and latent heating tied to this peak are forced to be more bottom heavy by the nighttime anomalous radiative cooling at upper levels. This mechanism therefore favors the stratiform top-heavy heating mode during daytime and suppresses it nocturnally. These diurnal circulation signatures arise from microphysical-radiative feedbacks that manifest on the scales of organized deep convection, which may ultimately impact the daily mean radiation budget.

Plain Language Summary Observations show that the rainfall in tropical thunderstorms over the oceans peaks at night-early morning. This is because solar radiation leads to warming in the upper troposphere in rainy and cloudy regions, which suppresses the convective overturning motions that cause the rainfall. Given the nocturnal peak in rainfall, we intuitively expect the upper-level outward spreading anvil clouds of thunderstorms to maximize at night, when thunderstorms are strongest. While anvil clouds indeed peak at night, they additionally peak in the midafternoon, which cannot be explained by this nocturnal convection. In this study we show through global model simulations that the afternoon peak of anvil clouds over tropical oceans is caused by the upper-cloud daytime maximum of radiative warming. Although this radiative warming suppresses convective motions (and hence rainfall), it causes a circulation response in the upper troposphere that manifests in locally increased upward motion. This upward motion enhances the production and spreading of anvil clouds in the afternoon.