Quantifying Cloud‐Fog‐Induced Reductions in Near‐Surface Solar Radiation Using the Reduction Ratio Method

Abstract

Cloud‐fog formation substantially reduces near‐surface solar radiation and modulates land‐atmosphere interactions, but quantifying such reduction is limited by sparse mountain observations. Using a 1‐km geostationary satellite‐retrieved product, we apply the Reduction Ratio of Solar Radiation (RRSR), an empirical normalized metric that quantifies near‐surface solar radiation deficits relative to a local maximum solar radiation baseline. Applying RRSR over Taiwan reveals a strong seasonal contrast. Summer RRSR exhibits a distinct land‐type gradient (montane cloud forests > other forests > non‐forests), peaking within the mid‐elevations (500–2,500 m a.s.l.). In winter, prevailing northeasterly flow overrides local land‐type influences. Strong northeasterly events enhance RRSR > 1.5‐fold in windward lowlands (<1,000 m a.s.l.), producing light limitation comparable to, or exceeding, that of typical montane cloud forests. Overall, RRSR provides a practical diagnostic for ecohydrology and surface energy budgets, although its uncertainties are larger under cloudy or foggy conditions in complex terrain.