Constraining the Diurnal Cycle of Tropical Ice Clouds Using Satellite Observations and Reanalysis

Tropical ice clouds influence Earth’s radiation budget and hydrological cycle by reflecting incoming solar radiation and trapping outgoing longwave radiation. However, their diurnal variability remains poorly quantified across observational and reanalysis products. This study evaluates the diurnal and seasonal behavior of tropical Ice Water Path (IWP) using three complementary datasets: European Centre for Medium-Range Weather Forecasts Reanalysis v5 (ERA5) reanalysis, NOAA Climate Prediction Center Infrared (IR) (CPCIR)–based Ice Water Path, and Cloud Profiling Radar and Cloud–Aerosol Lidar and Infrared Pathfinder Satellite Observations (CloudSat–CALIPSO radar-lidar retrievals for 2007-2010).

ERA5 provides hourly global estimates of total column ice (TCIW) and snow water (TCSW), CPCIR offers hourly total ice water path (TIWP) derived from combined infrared and microwave observations, and CloudSat-CALIPSO provides vertically resolved measurements at fixed local times (01:30 AM/PM) that serve as an observational reference for day and night contrasts. Spatially, all datasets exhibit consistent latitudinal IWP distributions within 30° S-30° N, with maxima along the Intertropical Convergence Zone (ITCZ) and minima in the subtropical dry zones. We find IWP peaks between 6-8° N with values of 0.350 kg m⁻² (CALIPSO), 0.269 kg m⁻² (CPCIR), and 0.115 kg m⁻² (ERA5), indicating that ERA5 underestimates IWP magnitude despite capturing the correct spatial structure. Seasonal variability reflects the meridional migration of the ITCZ, with maxima shifting northward during boreal summer (JJA) and southward during boreal winter (DJF). Both longitudinal and latitudinal analyses confirm that the three datasets reproduce similar large-scale IWP patterns across tropical regions. 

The diurnal cycle derived from ERA5 and CPCIR reveals comparable phase behavior, with IWP local peaks both occurring at 4 LST, and global peaks on 15 and 16 LST. This alignment shows consistent timing of convective development across datasets, although CPCIR shows a larger diurnal amplitude and a peak that occurs approximately one hour later in the afternoon. Comparison with CALIPSO day-night retrievals supports that tropical IWP peaks in the afternoon. Comparison between land and sea diurnal cycle reveals that land areas dominate the diurnal signal, supporting that the structure of the diurnal cycle is associated with continental convection, whereas oceanic regions display weaker and flatter cycles. Overall, the results demonstrate that while reanalysis and satellite datasets differ in IWP magnitude, they exhibit consistent spatial, seasonal, and diurnal patterns. The strong land-ocean contrast highlights the key role of continental convection. These findings provide a benchmark for ice cloud diurnal cycle analysis in climate models.