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Abstract Semi‐idealized simulations are conducted to investigate orographic impacts on deep‐convection initiation (CI) over Hainan Island, prior to island‐scale sea‐breeze convergence. A control simulation and sensitivity tests are conducted to reproduce Hainan's characteristic diurnal cycle and quantify CI‐related processes. In these tests, diurnal heating is found to be necessary for CI, highlighting the importance of thermal (rather than mechanical) forcing. Analysis focuses on a set of Gaussian mountain (GM) simulations that simplify the island terrain greatly but reproduce the CI from the full‐terrain simulations reasonably. The heated GM case develops a much stronger updraft and more favorable thermodynamic conditions for CI in the mountain lee than corresponding unheated cases. This difference stems primarily from up‐mountain directed buoyancy forcing, which opposes the decelerative pressure gradient force (PGF) and friction over the windward slope, allowing more low‐level flow to ascend the mountain. Due to this increased windward adiabatic ascent, the PGF strengthens over the crest to drive stronger cross‐barrier flow. In the lee, the buoyancy and PGF act in concert to force a strong flow reversal and a deep layer of moist, humid air ascending the slope. Along the collision zone between the cross‐barrier flow over the crest and the leeside reversed flow, an intense subcloud updraft forces CI. In contrast, simulations without diurnal heating produce stronger leeside drying and much weaker leeside updrafts in a shallower boundary layer, which fails to cause CI. The insights derived from the GM simulations carry over to simulations with the full Hainan terrain.
Lu et al. (Sun,) studied this question.