The measurement of gravity wave momentum flux in the lower atmosphere using VHF radar

Abstract

Radar measurements have often been used to estimate gravity wave momentum flux profiles in the troposphere, lower stratosphere, and mesosphere. The reliability of the measurements in the troposphere and lower stratosphere is investigated, with a comparison of three techniques employing various radar beam directions and zenith angles, including the standard "symmetric-beam" method. The study uses the 46.5 MHz MST radar system at Aberystwyth, Wales (52.4øN, 4.1øW). For short-period waves, errors in momentum flux profiles arising, for example, from differing data quality in the two radial beams sometimes prove difficult to eliminate in the symmetric-beam method. For long-period waves, including inertia-gravity waves, problems arise from the vertical wind measurement which is implicit in any estimate of momentum flux. Furthermore, a difference is sometimes found between the echo powers of the two symmetric radial beams, and this fluctuates with the horizontal wind perturbations from long-period waves. The possible influence in this power difference of tilted anisotropic scattering layers, which might also bias radial velocity measurements, is discussed. A momentum flux method which measures the vertical wind with a vertical beam, and the horizontal wind with a pair of radial beams, avoids some of the problems at both short and long periods. Agreement of this method with the symmetric-beam results would allow improved confidence in momentum flux profiles. 1. Introduction Early estimates of the energy and momentum fluxes associated with gravity waves suggested that their effects at upper heights could be important [e.g., Gossard, 1962; Hines, 1972]. However, the role of such waves in balancing the momentum and energy budgets of the mesosphere awaited studies such as those of Houghton [1978] and Lindzen [1981]. Interest in the momentum budget at lower heights was inspired by the observation of substantial fluxes associated with mountain waves [e.g., Lilly and Kennedy, 1973] and the realization that the flux divergence could affect the large-scale circulation at the lower heights [Lilly, 1972]. Observational techniques to measure the vertical transport of horizontal momentum have included the use of aircraft-mounted systems [e.g., Lilly and Kennedy, 1973; Bougeault et al., 1993; Alexander and Pfister, 1995], radiosonde flights [e.g., Mobbs and Rees, 1989; Shutts et al., 1994], and mesospherestratosphere-troposphere (MST) radar [e.g., Fukao et al., 1988a; Fritts et al., 1990]. The symmetric-beam radar method of Vincent and Reid [1983] has been applied to measure momentum flux in both the lower atmosphere [e.g., Fukao et al., 1988a; McAfee et al., 1989; $ato, 1990, 1993; Fritts et al., 1990; Thomas et al., 1992; Prichard and Thomas, 1993] and the mesosphere [e.g., Tsuda et al., 1990; Reid and Vincent, 1987]. In this method, the momentum flux in a vertical plane is obtained using two radial radar beams in that plane, pointing at symmetric zenith angles +0 and -0: uw --r}ø -r2-ø 2 sin 20 (1) where u, w represent the horizont...

Momentum flux associated with gravity waves in the low-latitude troposphere

“…It is noticed that the magnitude and pattern of the momentum flux value, in general, is comparable to the results of early radar and non-radar studies of vertical flux (Fukao et al, 1988;McAfee et al, 1989;Sato, 1989;Worthington et al, 1996;Alexander and Pfister, 1995;Chang et al, 1997). In general, the momentum flux pattern and its general characteristics observed in the present work are similar to that of the observations mentioned above.…”

“…They found that variations in zonal momentum flux seem to show good correlation with shear and mean zonal wind each day and thereby suggested that day-to-day variations of gravity waves are caused by variations of wind shear. Vertical flux of horizontal momentum in the troposphere and lower stratosphere have been studied by Prichard and Thomas (1993) and Worthington and Thomas (1996) using MST radar at Aberystwyth. They found that momentum fluxes for long period waves show little changes with activity, whereas momentum flux values of shorter periods are enhanced substantially during active periods.…”

An investigation of tilted aspect‐sensitive scatterers in the lower atmosphere using the MU and Aberystwyth VHF radars

“…Kelvin-Helmholtz instability offers an useful conceptual model [Worthington and Thomas, 1996;Muschinski, 1996 The agreement between the azimuth of tilted layers found using Gaussian moments (with tilt angles <1ø), and using power imbalances (apparently much larger tilts) shows they are closely related, both to If the existence of large power imbalances between symmetric off-vertical beams is inconsistent with a simple tilt of <1 ø (section 3), it may be the case that AOA measurements using SI must be performed for each beam pointing direction individually. In other words, it may be impossible to define a single AOA which indicates the effect of tilted layers on both vertical and off-vertical beams, and also not possible for DBS systems to measure vertical-beam AOA.…”

Long-period unstable gravity-waves and associated VHF radar echoes

“…Pro®les based on 1-h sections of the horizontal wind, power di erence, and echo power data, using various beams, are plotted in Figs Worthington and Thomas (1996b) in connection with the possible e ects of tilted layers on velocity and momentum¯ux measurements. Standard errors on the pro®les of velocities, power di erences, and powers are less than 0.5 m s À1 , 1.3 dB and 0.9 dB respectively.…”