Which combinations of environmental conditions and microphysical parameter values produce a given orographic precipitation distribution?

This study applies an idealized modeling framework, alongside a Bayesian Markov chain Monte Carlo (MCMC) algorithm, to explore which combinations of upstream environmental conditions and cloud microphysical parameter values can produce a particular precipitation distribution over an idealized two-dimensional, bell-shaped mountain. Simulations focus on orographic precipitation produced when an atmospheric river interacts with topography. MCMC-based analysis reveals that different combinations of parameter values produce a similar precipitation distribution, with the most influential parameters being relative humidity (RH), horizontal wind speed (U), surface potential temperature (θsfc), and the snow fallspeed coefficient (As). RH, U, and As exhibit inter-dependence: changes in one or more of these factors can be mitigated by compensating changes in the other(s) to produce similar orographic precipitation rates. The results also indicate that the parameter sensitivities and relationships can vary for spatial sub-regions and given different environmental conditions. In particular, high θsfc values are more likely to produce the target precipitation rate and spatial distribution, and thus the ensemble of simulations shows a preference for liquid precipitation at the surface. The results presented here highlight the complexity of orographic precipitation controls, and have implications for flood and water management, observational efforts, and climate change.

本研究采用理想化建模框架结合贝叶斯马尔可夫链蒙特卡洛（Bayesian Markov chain Monte Carlo, MCMC）算法，探究在理想化二维钟形山地地形上，上游环境条件与云微物理参数的何种组合能够生成特定的降水分布。模拟实验聚焦于大气河流与地形相互作用时产生的地形降水。基于MCMC的分析结果表明，不同的参数组合可生成相似的降水分布，其中影响最为显著的参数为相对湿度（relative humidity, RH）、水平风速（U）、地表位温（θsfc）以及降雪落速系数（As）。RH、U与As三者存在相互依赖关系：调整其中一项或多项参数时，可通过对其余参数进行补偿性调整来抵消其变化，从而获得相近的地形降水速率。研究结果同时表明，参数敏感性及其相互关系会随空间子区域以及不同环境条件发生变化。具体而言，较高的θsfc值更易生成目标降水速率与空间分布，因此模拟集合更倾向于在地表形成液态降水。本研究结果凸显了地形降水控制机制的复杂性，其结论可为洪水与水资源管理、观测工作以及气候变化研究提供参考依据。