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

Abstract The eastern tropical Pacific (ETP) coastal region, one of the rainiest places on Earth, is characterized by distinct diurnal offshore rainfall propagation primarily driven by mesoscale convective systems (MCSs). Previous studies have shown that MCS initiation in this region peaks in the early morning, with diurnally generated gravity waves from the Andes proposed as a key triggering mechanism. Additionally, enhanced low‐level moisture has been hypothesized to be a crucial contributing factor in MCS development. However, the relative roles of these processes in triggering MCSs over the region remain insufficiently quantified. This study explores these mechanisms using an ensemble‐based satellite data assimilation experiment focused on a representative nocturnal MCS event. Our results reveal a clear 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 increased surface latent heat flux and horizontal moisture advection from the Panama low‐level jet. These processes together destabilize the lower troposphere, creating favorable thermodynamic conditions for convection. Additionally, an offshore anabatic surface front enhances low‐level convergence, promoting vertical lifting of unstable air and providing dynamic support for MCS initiation. Sensitivity experiments further demonstrate that MCS development is highly sensitive to low‐level moisture availability, which is strongly influenced by surface wind‐induced evaporation.