First results of tropospheric gravity wave momentum flux measurements over Christmas Island

Abstract

Abstract. The importance of gravity wave effe. cts on various dynamical processes has been well documented in the upper and middle atmosphere. However, less is known about these waves in the lower atmosphere. Whereas many studies of longer-period waves have previously been conducted using wind profiler data, this paper is an effort to quantify the short-term motions of the troposphere. We will present some of the initial tropospheric gravity wave horizontal variance and momentum flux estimates calculated from Christmas Island wind profiler data collected during 1994. A coplanar antenna beam configuration was used for this analysis. In addition to the high-frequency results, we will discuss some of the difficulties we encountered while processing the data. In order to determine the robustness of applying a coplanar beam analysis method to tropospheric data, we have compared the results of two data sets that were created using two independently developed cleaning algorithms on the same original raw data. These results show that the high-frequency calculations are sensitive to the quality of the cleaned data used for this analysis. As long as the uncertainties associated with cleaning the data are considered, it is possible to establish the magnitude of the vertical momentum fluxes and the resulting accelerations in the troposphere.

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.…”

“…Alexander and Pfister (1995) have shown that gravity waves propagate away from deep convection. The effect of the transfer of momentum from one region of the atmosphere to another can be seen from the changes in the mean flow acceleration (Chang et al, 1997). Fritts et al (1990) calculated vertical fluxes of zonal momentum per unit mass as −0.1 m 2 s −2 to 0.1 m 2 s −2 and found that near the tropopause jet, it is associated with a net acceleration of approximately 1-2 ms −1 d −1 .…”

“…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. Chang et al (1997) have estimated the vertical fluxes of zonal and meridional momentum and found them to be in the range ± 0.02 kg m −1 s −2 and ± 0.01 kg m −1 s −2 , respectively. Gravity wave generation and propagation in the tropical easterly jet region was studied by calculating momentum fluxes and KH instability at different tropospheric and lower stratospheric heights using Indian MST radar by Sasi et al (1998).…”

Studies on atmospheric gravity wave activity in the troposphere and lower stratosphere over a tropical station at Gadanki

“…Alexander and Pfister (1995) studied the vertical flux of horizontal momentum carried by short horizontal scale gravity waves (λ x =10-100 km) using aircraft measured winds in the lower stratosphere. Chang et al (1997) used ST radar data from Christmas Island to study tropospheric gravity waves, and they found a broad range of frequency associated with gravity wave activity in the troposphere. The derived flux values agree well with the values obtained from MU radar and by Rao et al (1997), Dhaka et al (2001).…”

“…Owing to smaller temporal and spatial scale sizes of cumulus convection, its observations at meso-scale using existing techniques like rockets and satellites have been extremely difficult. Significant improvements in the MST radar technique have led to study these wave phenomena with large spatial and temporal resolutions (Balsley and Gage, 1980;Worthington and Thomas, 1997;Chang et al, 1997). VHF radar is a promising tool to study such events and their dynamical effects, because MST radars offer the unique capability of directly measuring the vertical velocity and also horizontal velocities with high spatial and temporal resolutions.…”