Mult-Scale Jet/Front Interactions that Organize Typical Cloud Seeding Environments in the Sierra Nevada
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Jet/Front systems that propagate every winter season across the Pacific Ocean basin create a favorable environment for supercooled cloud water above the complex terrain over the Central Sierra Nevada Mountains. This environment favors cloud seeding which can enhance the local orographic precipitation. Most of the time, however, water vapor is not that significant and therefore forecasting cloud seeding conditions is quite challenging. Two mutually interacting frontal structures that contribute to the formation of the supercooled cloud water over the mountains during the typical cloud seeding case study are: (1) a large-scale (primary) front that forms with the confluence of polar and subtropical jet streams at the synoptic to meso-α scale and (2) a small-scale (secondary) front at the meso-β/γ scale caused by orographic form and wave drag embedded within the first front. There were four typical quasi-marginal cloud seeding case studies that were investigated in this research study and they all occurred in the 2014 water year. Observations, including GFS and NARR analyses, satellite data, and SNOTEL precipitation as well as WRF multi-scale numerical simulations were employed to understand the dynamical and thermodynamical processes that caused frontogenetical circulations and the subsequent lift resulting in supercooled cloud water above the Sierra Nevada. Also, a sensitivity study was performed on all four case studies to diagnose the background frontogenetical signal independent of complex terrain forcing. This study has diagnosed the characteristics and relative roles of both scales of frontogenesis, i.e., due to jet interactions and complex terrain and how they mutually interact to focus the ascent for favorable cloud seeding conditions in typical cloud seeding case studies with relatively low amounts of available water vapor. This represents a paradigm for cloud seeding forecasting when upstream water vapor fluxes are not excessive.