Competing Timescales in Deep Convection: A Framework for Assessing Warm-Rain vs. Mixed-Phase Pathways over the Arabian Peninsula

This study addresses the persistent challenge of understanding precipitation inefficiency in the arid and semi-arid environment of the Arabian Peninsula. Despite frequent deep convective development, surface rainfall is often limited by sub-cloud evaporation and virga. To explain this disconnect, we propose a framework based on competing physical timescales, where precipitation formation is viewed as a race between warm-rain growth, mixed-phase (ice) growth, and evaporation/dilution "clocks."

We analyze thermodynamic and microphysical data by integrating in-situ radiosonde observations from the Integrated Global Radiosonde Archive (IGRA) with MERRA-2 reanalysis cloud diagnostics. Our analysis focuses on two climatically distinct regions: the elevated, orographic terrain of Abha and the continental interior of Hail.

Our results demonstrate that the warm-rain clock is systematically truncated across the region due to high lifting condensation levels (LCLs) and shallow warm-cloud layers (typically <1 km). We find a notable "CAPE paradox": high instability, which usually favors storm intensity, actually suppresses the warm-rain pathway by lofting droplets into the freezing zone before they can grow via collision-coalescence. Consequently, the mixed-phase clock dominates precipitation production. In the elevated Abha region, lower cloud bases and higher moisture availability allow growth processes to complete, whereas in Hail, the evaporation clock consistently wins the race, leading to significant rainfall loss. By framing these processes through competing timescales, we provide a physically consistent mechanism to explain the regional and seasonal variations in precipitation efficiency across the peninsula.