The Sensitivity of Falling Hail to Different Melting and Terminal Velocity Parameterizations and Environmental Conditions

Abstract The processes that dictate hail melt and hail terminal velocity are complex, poorly understood, and important to the characteristics of the hail that ultimately reaches the ground. Multiple parameterizations describing hail melt and terminal velocity have been developed since the 1960s. However, there has never been a systematic comparison of these parameterizations and how they respond to different environments. The current study combines unique in-cloud hail size distribution data with a melt-only, one-dimensional version of the HAILCAST hail trajectory model, termed HAILMELT, to explore how different hail melt (HMP) and terminal velocity (TVP) parameterizations simulate falling hail in the midlatitudes and tropics. We also explore how sensitive the falling hail is to temperature, moisture, and in-cloud vertical velocity changes. For hail smaller than 40 mm, simulations are frequently at least as sensitive, and often more sensitive, to which parameterization is used than to changes in the environmental conditions. In tropical oceanic environments, results suggest that hail initially smaller than 10 mm requires unrealistically cool or dry conditions to reach the surface. However, initially larger hailstones in tropical environments are shown to be capable of reaching the surface. These results indicate that there are likely larger uncertainties than previously understood in our hail simulations of the current and future climate. Additionally, they suggest all the hail identified by satellite-based hail climatologies in the tropics entirely melts before reaching the surface in some cases. Significance Statement This study is the first systematic comparison of different hail melt and hail fall speed relationships in the midlatitudes and tropics. It explores the environmental sensitivity of these relationships for hail smaller than 40 mm. The amount that hailstones melt is often more sensitive to which relationship is being used than to changes in environmental conditions. This sensitivity likely adds additional uncertainty to simulations that predict the size of hail. We also show that the initial in-cloud hailstone sizes have a larger impact on determining if hail will reach the surface in the tropics than the environmental conditions.