Turbulence and gravity waves in the vicinity of a midtropospheric warm front: A case study using VHF echo‐intensity measurements and radiosonde data

It is well known that VHF echo‐intensity data obtained at nonvertical beam directions can be used to retrieve the refractive‐index structure parameter C 2 n and that vertical profiles of C 2 n can be used to identify and monitor frontal structures in the free troposphere. In a case study, VHF radar observations of C 2 n are compared with vertical profiles of M 2 g (the square of the vertical gradient of the generalized potential refractive index) obtained from the data of two radiosondes launched from 4 km east of the radar site. The experiment was carried out in the Harz Mountains, north Germany, on October 18, 1993, during a high‐pressure period under clear‐sky conditions. Well‐defined maxima of C 2 n and M 2 g are found at the surface of a warm front at altitudes slightly below 5 km. At altitudes in the immediate vicinity of the front, echo‐intensity oscillations are observed, which have an amplitude of up to 9 dB and a period of about 40 min and which are attributed to gravity wave‐induced vertical displacements of the frontal surface. Vertical profiles of the outer scale of turbulence, L 0 , are obtained from the profiles of C 2 n and M 2 g . In the stably stratified region below the front (at altitudes between 3 and 5 km), 5–20 m is typical, while in the less stably stratified altitude region between 5 and 7 km, 20–50 m is more representative. It is found that large values of L 0 are correlated with low static stability; a relationship between L 0 and the Richardson number (dynamic stability), however, was not observed. A continuous temporal increase of C 2 n at altitudes between 5 and 7 km was observed. It is attributed to a temporal increase of the vertical variability of the humidity above the front.