Previously Neglected Effects of Strong Horizontal Winds on Raindrop Collisions in Tropical Cyclones

Microphysical processes play a critical role in determining rainfall intensity, so accurately simulating these processes is essential for improving the accuracy of rainfall forecasts. For instances, the self-collection process of raindrops, including collisional coalescence and breakup, is widely recognized for playing a crucial role in influencing the evolution of RSDs and the eventual rainfall intensity. However, in current state-of-art microphysics schemes of numerical models, the inaccurate parameterization of this self-collection process is thought to be a major source of uncertainty in precipitation forecasts, such as excluding the effect of horizontal winds on RSDs. Based on multi-layer observations of the evolution of RSDs as raindrops descend within the near-surface layer during the landfall of Typhoons, this study elucidated the additional impact of horizontal winds (including horizontal wind speeds, VWS and small-scale turbulence) on the self-collection process and RSDs. It further incorporates this effect into a bin microphysics scheme, thereby improving the accuracy in simulating the observed changes in RSDs.It was observed that the number concentration of small and mid-sized raindrops increases significantly at the expense of large-sized ones as the height decreases, suggesting the occurrence of raindrop breakup process. Furthermore, this study found that raindrops have smaller fall speeds than their terminal velocity (sub-terminal velocity), as influenced by strong horizontal winds in TCs. However, the rainshaft model using the original bin microphysics scheme (RAINSHAFT_ORI) did not successfully reproduce these RSD characteristics. In response, after considering the influence of horizontal winds on the vertical and horizontal component of raindrop motion in the stochastic collection equation (SCE-SBE), this study ultimately established the relationship between the wind effects and the collisional outcomes of raindrops.Results from sensitive experiments showed that if only considering the wind effects on the fall velocity of raindrops, such as using fall velocity difference  to replace terminal velocity difference   in RAINSHAFT_ORI, the RAINSHAFT_V still failed to reproduce the observed evolution of RSDs with decreasing height. However, the RAINSHAFT_NEW, which accounted for the wind impact on the   of raindrops (including  and ) and used  instead of (if the direction of the vector is consistent, =), interestingly captured the intrinsic changes in RSDs with decreasing height under strong-wind conditions.In summary, current state-of-the-art microphysics schemes face challenges in precisely simulating the formation of clouds and precipitation across different types of synoptic systems, due to limited understanding of the intrinsic processes governing the evolution of hydrometeors. Although efforts have been made to only incorporate wind effects on raindrop sedimentation and collisional coalescence/breakup to improve bin microphysics schemes, this study represents a preliminary but significant step in advancing our understanding of rain microphysics under strong-wind conditions.