Probabilistic Baseline Diurnal Cycle at Indonesian Surface Stations

Previous studies highlight strong influences of large‐scale synoptic, intraseasonal, and seasonal perturbations on the diurnal cycle of the Indo‐Pacific Maritime Continent (MC). Isolating local diurnal forcing of insolation from other effects can be difficult. To address this difficulty, we introduce two new concepts: the probabilistic diurnal cycle and the coastal baseline diurnal cycle. The probabilistic approach is applicable to non‐Gaussian variables, such as rainfall and wind speed. Using seven years of observations from 31 surface stations across Indonesia, we establish the baseline diurnal cycle by identifying patterns of high probabilities across daily hours. In this baseline diurnal cycle, surface temperature rises from an early‐morning minimum to a midday peak. Weak land breezes transition to strong sea breezes in the morning, with wind speeds peaking shortly after noon before land breezes return in the late afternoon. Specific humidity plateaus near midday after its early‐morning minimum. Surface pressure exhibits a clear semidiurnal cycle driven by atmospheric tides. Rainfall typically peaks in the afternoon and reaches a minimum in the morning. This baseline diurnal cycle, primarily driven by local diurnal forcing (insolation), exists at 24 of the 31 surface stations. A significant deviation from the baseline occurs at a few stations where rainfall peaks in the early morning. This deviation appears randomly distributed in space, and its controlling mechanisms likely reside outside the atmospheric boundary layer. Processes that determine the location of the persistent early‐morning peak in rainfall warrant further investigation.