Roles of Surface Latent Heat Flux and Gravity Waves in Offshore MCS Development in the Coastal Eastern Tropical Pacific

The Eastern Tropical Pacific (ETP) coastal region is the rainiest place on Earth, characterized by pronounced diurnal offshore rainfall propagation primarily contributed by mesoscale convective systems (MCSs). Previous studies showed that MCS initiation in the offshore region peaks in the early morning, with diurnally generated gravity waves from the Andes proposed as a key triggering mechanism. Additionally, enhanced nocturnal low-level moisture from wind-induced surface evaporation had been hypothesized as a contributing factor for convective development. However, the relative roles of these processes in triggering MCSs have not been well quantified.
In this study, we investigated these mechanisms using an ensemble-based satellite data assimilation experiment focused on a representative nocturnal MCS event. Results reveal a notable pre-MCS cooling trend in the lower troposphere linked to gravity waves generated by afternoon inland convection. Concurrently, substantial low-level moistening occurs, driven by enhanced surface latent heat flux and horizontal moisture advection associated with the Panama low-level jet. Together, these processes destabilize the lower troposphere, creating favorable thermodynamic conditions for convection. Moreover, an anabatic surface frontal structure is identified offshore, enhancing low-level convergence and promoting vertical lifting of unstable air, thereby providing dynamic support for MCS initiation. Sensitivity experiments further show that MCS development is highly sensitive to low-level moisture availability, underscoring the critical role of surface wind-induced evaporation.

东热带太平洋（Eastern Tropical Pacific, ETP）沿海区域是地球上降雨最多的区域，其典型特征为显著的昼夜离岸降雨传播，这一现象主要由中尺度对流系统（mesoscale convective systems, MCSs）驱动。此前研究表明，离岸区域的MCS触发峰值出现在清晨，而来自安第斯山脉的昼夜生成重力波被认为是关键触发机制。此外，由风力驱动的地表蒸发所增强的夜间低层水汽，曾被假设为对流发展的促进因素。但目前尚未对这些过程在触发MCSs中的相对作用进行充分量化。
在本研究中，我们针对一次典型夜间MCS事件，开展基于集合的卫星数据同化试验，对上述机制展开探究。试验结果显示，MCS发生前的低层对流层存在显著降温趋势，该现象与午后内陆对流生成的重力波密切相关。与此同时，受巴拿马低空急流相关的增强地表潜热通量与水平水汽平流驱动，低层出现显著增湿现象。上述过程共同作用使低层对流层变得不稳定，为对流活动创造了有利的热力学条件。此外，研究还在离岸区域识别出上坡型地表锋面结构，该结构增强了低层辐合，促进不稳定空气的垂直抬升，为MCS触发提供了动力支撑。敏感性试验进一步表明，MCS发展对低层水汽供给高度敏感，凸显了风力驱动的地表蒸发的关键作用。