The summer diurnal cycle of precipitation and its relationship with near surface variables over North America in ISD station data

In summer, the diurnal cycle of precipitation over the continental United States is primarily shaped by afternoon convection and mesoscale convective systems (MCSs). Using hourly station data across US, we investigate how the diurnal cycle of precipitation depends on near-surface conditions using a multistep methodology that involves 1) clustering stations based on total precipitation and the characteristics of the diurnal cycle of precipitation, 2) categorizing days based on the amount and the timing of precipitation and 3) analyzing how precipitation characteristics vary with 2-m virtual temperature and relative humidity. The resulting clusters are geographically coherent and distinguish regions with distinct strengths and timings of the diurnal cycle, suggesting dominance by different underlying mechanisms. Categorizing days enables the identification of precipitation associated with dominant diurnal mechanisms -such as afternoon convection or MCSs- and allows for a better characterization of their corresponding surface conditions. Analyzing precipitation characteristics as a function of near-surface conditions reveals distinct regional behaviors: in the MCS-dominated regions (eastern Great Plains), precipitation shows little dependence on near-surface conditions, consistent with non-local. In contrast, precipitation at stations located in afternoon convection-dominated regions (Florida and the eastern US) is strongly modulated by the diurnal cycle of near-surface temperature and humidity, with warm and moist conditions favoring the triggering of convective events. For afternoon convection dominated regions, diurnal cycle differences are largely attributable to variations in these surface conditions. This framework offers a process-based diagnostic to evaluate the representations of the diurnal precipitation cycle in climate and weather models.