Observations of lee waves by high‐power radar

Abstract

A very high-sensitivity 107 mm radar has been brought into operation for meteorological studies of airflow within the optically clear (or cloudy) atmosphere. One application of this radar has been to observe the detailed three-dimensional structure of lee waves, using backscatter from both refractive index inhomogeneities and cirrus 'clouds' as means of tracing the pattern of airflow. This paper presents case studies showing how the radar has revealed the structure of lee waves downwind from the Welsh mountains. One of the case studies illustrates a steady-state wave pattern, another illustrates an unsteady pattern with waves varying in orientation and wavelength, another illustrates a strongly damped wave pattern, and a final one illustrates two families of waves co-existing at different altitudes. The radar method is shown to provide a direct and elegant technique for measuring lee wave properties. Although it depends on the presence of naturally occurring targets to trace the airflow pattern, such targets are found to be abundant in situations of strong lee wave activity. ' Two-dimensional mountain wavesa method for the rapid computation of lee wavelengths and vertical velocities,' Ibid., 88, pp. 271-285. ' Multiwavelength backscatter from the dear atmosphere,' J Geophys. Res.i7l, pp. 1,5374,552. ' Theory of waves in the lee of mountains,' Quart. J.R. Met. ' High-power radar for meteorological studiea in clear air,' ' The airflow over mountains,' W.M.O. Tech. Note No. 34. Edited by Akka, M.A. 135 pp. ' Detailed observations of mountain lee waves and a comparison with theory,' Colorado State University. Atmot. Sci. Paper No. 138. SOC., 75, pp. 41-56. PCOG. ZEE, 118, pp. 519-528.

Radar and sodar probing of waves and turbulence in statically stable clear-air layers

“…Additional evidence for the existence of clear-air radar layers associated with large variations of refractive index has been given by Saxton et al (1964), Hardy et al (1966), and Kropfli et al (1968). Multiple stratifications have been reported recently by Ottersten (1969a), Katz (1969), Crane (1970), Kropfti (1971) and Starr and Browning (1972). The scattering layers may be quite shallow, perhaps only a few meters thick as reported by , or they may be of the order of 1 km in depth.…”

“…Both the short and long waves maintained fixed positions with respect to the ground in the lee of the Welsh mountains. Reed and Hardy (1972) describe some long waves which, in contrast to the stationary lee waves observed by Starr and Browning (1972), were embedded within the mean flow. A portion of one of the waves is shown in Figure 14.…”

“…Radar detects the lee waves by observing the distortion of clear-air layers by lee-wave flow, offering a direct and elegant method of observing three-dimensional lee-wave patterns. Starr and Browning (1972) have exploited this for some unique lee-wave studies. The Defford radar is situated 60 km east of the lee slope of the South Wales mountains, a suitable location for observation of organized lee waves.…”

Nonstationary Trapped Lee Waves Generated by the Passage of an Isolated Jet

“…Nevertheless, observational evidence readily demonstrates that trapped waves display notable nonstationary behavior (Vergeiner and Lilly 1970;Ralph et al 1992;Worthington and Thomas 1996), with significant changes sometimes occurring on time scales of less than 1 h (Ralph et al 1997). Temporal changes in amplitude (Starr and Browning 1972;Brown 1983), downstream position and extent (Lindsay 1962), and horizontal wavelength (Holmboe and Klieforth 1957;Queney et al 1960;Smith 1976;Mitchell et al 1990;Ralph et al 1997) have all been observed. In particular, in an analysis of 24 observed events, Ralph et al (1997) found that the resonant wavelength of trapped waves gradually increased at an average rate of 9% h 21 .…”

Diurnal variation of mountain waves

“…Mountain waves can modify downwind convection (Hosler et al, 1963;Booker, 1963;Starr and Browning, 1972;Winstead et al, 2002), however mountain-wave clouds can also occur above convection (Pigot and Hill, 1939;Sinha, 1966;Müller, 1983) covering the mountains, as if the wave source region could be higher than the mountain surface. Sea-breeze convection could also form an "additional effective mountain" (Kozhevnikov et al, 1986).…”