z-boundary backward FD stencil uses fields(2) instead of fields(3) #1194
Description
Bug
In src/common/m_finite_differences.fpp, the backward finite-difference stencil for the z-direction upper boundary references fields(2) (the y-direction field) instead of fields(3) (the z-direction field) for the z - 2 term.
File: src/common/m_finite_differences.fpp
Impact: Corrupts z-direction derivative values at the upper z-boundary cells. The stencil silently mixes y-direction data into z-direction derivatives. This affects any 3D simulation using the backward FD stencil at the z-boundary (e.g., viscous stress, gradient computations).
Severity: Critical — produces silently wrong results in 3D simulations near the z upper boundary.