Abstract
Abstract. We study the momentum flux of the atmospheric motions in the height ranges between 6 and 22 km observed using the MU radar at Shigaraki in Japan during a 3 day period in January 1988. The data were divided by double Fourier transformation into data set of waves with downward- phase- velocity and data set of waves with upward-phase-velocity for independent momentum flux calculation. The result showed that both the 72 h averaged upward flux and downward flux of zonal momentum were negative at nearly each height, meaning that the upward flux was dominated by westward propagating waves while the downward flux was dominated by eastward propagating waves. The magnitude of the downward flux was approximately a factor of 1.5 larger than the upward flux for waves in the 2~7 h and 7~24 h period bands, and about equal to the upward flux in the 10–30 min and 30 min–2 h period bands. It is also observed that the vertical flux of zonal momentum tended to be small in each frequency band at the altitudes below the jet maximum (10~12 km), and the flux increased toward more negative values to reach a negative maximum at some altitude well above the jet maximum. Daily averaged flux showed tremendous variation: The 1st 24 h (quiet day) was relatively quiet, and the fluxes of the 2nd and 3rd 24 h (active days) increased sharply. Moreover, the upward fluxes of zonal momentum below 17 km in the quiet day for each period band (10~30 min, 30 min~2 h, 2~7 h, and 7~24 h) were dominantly positive, while the corresponding downward fluxes were dominantly negative, meaning that the zonal momentum below 17 km in each period band under study were dominantly eastward (propagating along the mean wind). In the active days, both the upward fluxes and downward fluxes in each frequency band were dominantly negative throughout the whole altitude range 6.1–18.95 km.
Highlights
Gravity wave momentum fluxes observed by radars have been used to estimate the body force due to gravity waves in the lower stratosphere and in the mesosphere and lower thermosphere (Fritts and Alexander, 2003)
Mean values of momentum fluxes of ∼1–10 m2 s−2 have been measured with mean forcing in the range ∼10– 70 ms−1 d−1 that generally oppose the mean winds (Reid and Vincent, 1987; Fritts and Yuan, 1989; Tsuda et al, 1990; Nakamura et al, 1993)
At the tropical Jicamarca radar site (12◦ S) (Hitchman et al, 1992), momentum fluxes of ∼2–8 m2 s−2 were observed with mean flow forcing ∼10– 60 ms−1 d−1
Summary
Gravity wave momentum fluxes observed by radars have been used to estimate the body force due to gravity waves in the lower stratosphere and in the mesosphere and lower thermosphere (Fritts and Alexander, 2003). The technique of velocity tracing was applied to analyze the data observed by SOUSY-Svalbard Radar in October 2000 in the altitude range between 2.4 km and 17.4 km (Kuo et al, 2003), in which hundreds of wave packets with characteristic wave periods in the range 15∼45 min were identified from Range-Time plot of (δV ): About 85% of them were type 1 and about 15% were type 2 wave packets, revealing the typical characteristics of the propagation of atmospheric gravity waves in the linear regime. The upward flux and downward flux will be treated separately, and the vertical group velocities of as many as possible sampled wave packets will be measured
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