Raindrop

Gas-phase mass transfer coefficient K_c to a falling raindrop, from Seinfeld & Pandis (2006), Atmospheric Chemistry and Physics, 2nd ed., Chapter 20 (Wet Deposition), Eq. 20.12. K_c is computed from a Sherwood-number correlation K_c = (D_g/D_p)·[2 + 0.6·Re^(1/2)·Sc^(1/3)], where the drop Reynolds number Re = ρ_air·U_t·D_p/μ_air measures the convective flow past the falling drop and the gas Schmidt number Sc = μ_air/(ρ_air·D_g) is the ratio of the momentum to the mass boundary-layer thickness. The leading 2 is the still-air diffusive limit (Sh→2 for an isolated sphere); the 0.6·Re^(1/2)·Sc^(1/3) term is the ventilation enhancement from the drop's fall. K_c sets the gas-uptake rate at the raindrop surface and feeds the below-cloud gas scavenging coefficient computed by the sibling GasScavengingCoeff / BelowCloudGasScavenging components. Migrated from AtmosphericDeposition.jl@e4468c79 src/seinfeld_pandis_2006_ch20.jl:60 (`MassTransferCoeff` @component). Algebraic (observed-only) system: K_c is the single algebraic output (the upstream component's lone `@variable`); D_g, D_p, U_t are input parameters and ρ_air_sp, μ_air_sp are the migrated upstream module `@constants`.