Deviations of geopotential height from a 3-day time average are depicted in a vertical cross section along 42 degrees north. Fields are shown at 12-hour intervals: (a) 0000 UTC 19 March 1994; (b) 1200 UTC 19 March 1994; (c) 0000 UTC 20 March 1994, and (d) 1200 UTC 20 March 1994. The heavy solid line shows the topography which exceeds 1 km, as resolved at 60 km horizontal resolution. The contour interval is 20 m. Initially, the wave shows little vertical tilt, but as it interacts with the mountain, potential temperature anomalies are induced at the lower boundary. These are situated nearly out of phase with the primary wave, causing the vertical tilt in the lower troposphere to increase, slightly weakening the wave.
