Identification of the Global Cloud‐Clear Sky Transition Zone and Its Shortwave Radiation Effects

Atmospheric conditions are typically categorized as either clear‐sky or cloudy‐sky scenarios, yet a distinct Cloud‐Clear Sky Transition Zone (CCTZ) inherently exists. Current studies on CCTZ identification and its shortwave radiative effects remain constrained by limited applicability over land and insufficient consideration of cloud‐type differentiation. To bridge this gap, we integrated Moderate Resolution Imaging Spectroradiometer (MODIS) satellite observations (1 km spatial resolution) with radiative transfer modeling to develop a globally consistent CCTZ detection method applicable to both land and ocean. We subsequently analyzed its effects on direct and diffuse shortwave radiation across seven major cloud types. Key findings include: (a) The CCTZ exhibits a globally coherent but cloud‐type‐dependent spatial scale, with a statistical boundary located approximately 5 km from clouds. The frequency of CCTZ occurrence reaches ∼34% in “no‐cloud” regions over land, significantly higher than over ocean (∼21%). (b) Analysis of shortwave radiation responses, incorporating cloud classification, revealed three distinct mechanisms corresponding to distance from the nearest cloud: Increasing‐then‐decreasing (Stratocumulus‐Stratus, Cirrus), Fluctuating (Altostratus‐Altocumulus, Cumulus, Cumulonimbus), and Stable (Nimbostratus, Cirrus‐Altocumulus‐Altostratus). (c) Compared to pure clear‐sky conditions, the CCTZ enhances global mean diffuse shortwave radiation by ∼16.3%, while reducing direct shortwave radiation by ∼0.8%, with magnitudes modulated by cloud type. This study clarifies the globally coherent yet cloud‐type‐modulated characteristics of the CCTZ and its distinct radiative mechanisms. These findings underscore the need for advanced cloud detection and more refined modeling of transition zones to better constrain their representation in climate systems, thereby helping to reduce uncertainties in global climate predictions.