Task: Class Definition Understanding (ontology: Earth_Environment/SWEET)

Which of the following definitions best describes 'foehn wall'?

A. A low-level, horizontal, wedge-shaped arcus cloud associated with a convective storm's gust front.
B. 1. Cloud forms that arise from Kelvin?Helmholtz waves. 2. Vortical structures that result from the growth and nonlinear development of unstable waves in a shear flow. The billows get their name from the instability responsible for the growth of the unstable waves, Kelvin?Helmholtz instability.
C. A local, often abrupt lowering from a cumulonimbus cloud base into a low-hanging accessory cloud, normally a kilometer or more in diameter.
D. The leeward edge of the orographic stratiform cap cloud as seen from the lee side of a mountain barrier, preceding or during a foehn or chinook event. The edge is generally abrupt and resembles a wall of cloud (?foehnwand? in German). This cloud often signifies the occurrence of orographic precipitation, especially snowfall in the cold season, over the peaks. See foehn cloud.

Answer: The leeward edge of the orographic stratiform cap cloud as seen from the lee side of a mountain barrier, preceding or during a foehn or chinook event. The edge is generally abrupt and resembles a wall of cloud (?foehnwand? in German). This cloud often signifies the occurrence of orographic precipitation, especially snowfall in the cold season, over the peaks. See foehn cloud.