Spatial and temporal variations of the atmospheric sodium layer observed with a steerable laser radar
LASER radar observations of resonance scattering from atmospheric sodium during the night have, on occasions, shown evidence of short term changes in the height distribution1–3. It has been suggested that these changes arise from the movement of a horizontally structured sodium layer, but the measurements have all been limited to the vertical direction, and have provided no direct evidence of a horizontal variation. Evidence of horizontal structure at mesospheric heights has been provided by observations of hydroxyl emissions in the airglow, both from the ground4 and from an aircraft5,6. In addition, the observed fluctuations of intensities and infrared rotational temperatures of the hydroxyl bands have been associated with the passage of atmospheric gravity waves7,8. We report here observations intended to examine the short term and small scale horizontal variations of the sodium layer which might also be associated with such waves.
High-resolution lidar system for measuring the spatial and temporal structure of the mesospheric sodium layer
One of the first remote sensing applications of tunable dye lasers was in the study of mesospheric sodium. The layer, which is believed to be of meteoric origin, is confined to altitudes between approximately 80 and 100 km. The layer has been investigated using a variety of techniques since its discovery in the late 1930s. A review of early measurements and the theory of sodium layer chemistry is contained in Brown (1973). Before dye laser based lidar systems were developed, sodium measurements were largely restricted to studying resonantly scattered sunlight. Ground-based measurements of this type were able to define seasonal variations in column abundance, but the sharp layer boundaries were not revealed until rocket-borne dayglow measurements were made (Hunten and Wallace, 196 7). Lidar observations of the vertical structure of the sodium layer were first made in England (Bowman et al., 1969). Since then, similar measurements have been reported from a variety of locations including France (Blamont et al., 1972), Brazil (Kirchoff and Clemesha, 1973), California (Hake et al., 1972), Illinois (Rowlett et al., 1978) and at high latitudes (Franz Joseph Land, USSR), (Juramy et al., 1981). Because the daytime sky produces very high background photocount rates in most sodium lidar receivers, the majority of the lidar observations of the layer have been restricted to nighttime.
Turbulent boundary layer wall‐pressure fluctuations downstream from an abrupt change in surface roughness
Spectra of wall‐pressure fluctuations beneath a turbulent boundary layer have been investigated immediately downstream from an abrupt change in surface roughness, from slightly rough to very rough. Over the measurable frequency band (400 Hz to 7 kHz), the spectra in the first 150 mm scale on wall variables. This indicates that the motions responsible for the pressure fluctuations measured in this region are very close to the wall. Farther downstream this scaling procedure breaks down. It is suggested that the initially thin internal layer behaves independently of the flow external to it until it has grown appreciably and only then interacts with the outer boundary layer. It was found that, for all frequencies measured, the dimensional spectral level decreases initially then rises to a maximum at 250 mm from the roughness change before decreasing once more. The position of the maximum is close to where the internal layer ceases to have a distinct edge.