Orographic-Convective Flows and Gravity Waves

Mountains and convective clouds both induce internal gravity waves. When they act together, the flow and the vertical flux of horizontal momentum are not the sum of the two. I study these orographic–convective flows with analytic theory and cloud-resolving simulations, with an eye to gravity-wave drag parameterizations. Near the ground, mountain flows can turn into severe downslope windstorms and gap winds, such as the springtime windstorms east of the Taebaek Mountains that drive rapid wildfire spread.

Figure. Theoretically obtained orographic–convective flows in single- and two-layer atmospheres, without and with basic-state wind shear.

Theory

Linear solutions in single- and two-layer, hydrostatic and nonhydrostatic atmospheres show how wave reflection at a stability interface and nonhydrostatic effects select propagating or evanescent modes, and how the momentum flux splits into a mountain part, a convective part and a nonlinear interaction part of the same order as the forced parts.

Figure. The governing equations, forcings and basic state used in the analytic theory of orographic–convective flows (Seo et al. 2018, Tellus A; Phys. Fluids; Seo & Baik 2026, Geosci. Lett.).

Multiple convective cells

Convective gravity-wave drag schemes usually treat subgrid convection as one source. An analytic framework shows that the per-cell momentum flux from n cells equals the single-cell flux times [1 + Jₙ(β, δ)], where β is the ratio of compensating-cooling width to heating-core width and δ the spacing between cells. Closely packed cells with broad cooling enhance the flux; widely spaced cells reduce it. The factor can be added to existing parameterizations.

Figure. Nonlinear modification factor Jₙ(β, δ) of the per-cell momentum flux for n = 2, 5, 101 and ∞ convective cells (Seo & Baik 2026, Geosci. Lett.).

Aerosol effects in simulations

WRF simulations with aerosol-aware microphysics show that the nonlinear modification is stronger when the convectively forced wave is stronger, which grows with aerosol number concentration. The nonlinear interaction weakens the total momentum flux. Without basic-state wind, an additional convective cell forms near the mountain top and the weakening is stronger than with wind.

Figure. Simulated orographic–convective flows over a bell-shaped mountain, single layer without shear, for different aerosol concentrations.

Mountain flows and downslope windstorms

Mountains also drive strong flows near the ground. The Yeongdong region east of the Taebaek Mountains often sees severe springtime windstorms (Yang-Gan winds) that can spread forest fires rapidly. A high-resolution WRF simulation of the 8 April 2012 event showed that the generation mechanism depends on local terrain: a hydraulic jump, helped by an elevated inversion near mountain-top height, produces the downslope windstorm at Yangyang; the sea-level pressure difference across a gap drives plume-shaped gap winds at Jangjeon; and downslope winds passing over a deep boundary layer create banded wind patterns at Uljin.

Figure. Vertical cross-sections over the Yangyang area of potential temperature (blue lines), wind speed and wind vectors (a, c) and the vertical temperature gradient (b, d) at 0900 and 1500 LT, showing the steep descent of isentropes on the lee slope and their recovery farther leeward (Lee et al. 2020).