Observations of vertical velocities in the tropical upper troposphere and lower stratosphere using the Arecibo 430‐MHZ radar

Abstract

We report the first clear-air observations of vertical velocities in the tropical upper troposphere and lower stratosphere (8-22 km) using the Arecibo 430-MHz radar. Oscillations in the vertical velocity near the Brunt-V•iis•il•i period are observed in the lower stratosphere during the 12-hour observation period. Frequency power spectra from the vertical velocity time series show a slope between -0.5 and -1.0. Vertical wave number spectra computed from the height profiles of vertical velocities have slopes between -1.0 and -1.5. These observed slopes do not agree well with the slopes of +1/3 and -2.5 for frequency and vertical wave number spectra, respectively, predicted by a universal gravity-wave spectrum model. The spectral power of wave number spectra of a radial beam directed 15 ø off-zenith is enhanced by an order of magnitude over the spectral power levels of the vertical beam. This enhancement suggests that other geophysical processes besides gravity waves are present in the horizontal flow. The steepening of the wave number spectrum of the off-vertical beam in the lower stratosphere to near -2.0 is attributed to a quasi-inertial period wave, which was present in the horizontal flow during the observation period. 1. •ODUCTION The development of the stratospheric tropospheric (ST) radar technique in the past decade has afforded the capability of direct measurement of vertical velocities in the lower middle atmosphere. Such measurements are useful in studies of gravity waves and mesoscale flows, both of which can have a significant true or apparent vertical component. The first radar observations of vertical velocities in the stratosphere were made using the Jicamarca 50-MHz radar in Peru by Woodman and Guillen [1974], who pointed out the potential for studies of gravity waves near the Brunt-V•iis•il•i frequency by mesospheric-stratospheric-tropospheric (MST) radar. Further observations at Jicamarca by Fukao et al. [1981] showed that oscillations near the Brunt-V•iis•il•i period were a dominant and frequent feature in the tropical lower stratosphere. Observations of vertical velocities by VHF radars elsewhere have shown pronounced variability in the activity of vertical winds. Observations over a 30-day period at Poker Flat by Ecklund et al. [1981] revealed that the magnitude of the vertical velocity fluctuations in the troposphere and lower stratosphere is controlled by propagating planetary waves, which modulate the large-scale wind field. In addition, Ecklund et al. [1981] observed active periods, during which the power in vertical fluctuations increased to about 25 times over that observed during quiet conditions, when horizontal winds were weak. They attributed the increase during active periods to enhanced gravity wave activity in the vertical winds. Ecklund et al. [ 1982] compared vertical velocities measured simultaneously at the Sunset and Platteville radars, located east of the continental divide in Colorado. Their observations indicated that lee wave effects were present. In addition, they noted ...

Simultaneous lidar observations of vertical wind, temperature, and density profiles in the upper mesophere: Evidence for nonseparability of atmospheric perturbation spectra

“…The o-spectrum of vertical winds is very shallow with a spectral index near 0. This result is consistent with the predictions of gravity wave theory and previous observations of vertical winds in the lower atmosphere [e.g., Kuo et al, 1985;Larsen et al, 1986 andCornish, 1988].…”

“…The m-spectrum of vertical winds was also very shallow with a spectral index of-l.4. This result is also consistent with previous observations in the lower atmosphere [Kuo et al, 1985;Larsen et al, 1986 andCornish, 1988]. However, it is inconsistent with the predictions of linear instability theory and the concept of separable (m, to) spectra for the horizontal wind, relative density, and relative temperature perturbations.…”

Interpretation of gravity waves observed in the mesopause region at Starfire Optical Range, New Mexico: Strong evidence for nonseparable intrinsic (m, ω) spectra

“…For our data the co o spectra of vertical winds are very shallow with a mean spectral index of-0.76. This result is also consistent with gravity wave theory and previous observations of the lower atmosphere [e.g., Kuo et al, 1985;Larson et al, 1986Larson et al, , 1987Cornish, 1988].…”

“…Horizontal velocities can approach and even exceed 100 m/s, so pointing accuracies of a few tenths of a degree or a few milliradians are required to be certain that horizontal contamination does not exceed a few tenths of a meter per second. Among the few reports of vertical wind measurements are the papers of Kuo et al [1985], Larsen et al [1986Larsen et al [ , 1987, and Cornish [1988]. These authors used stratosphere/troposphere (ST) radars to measure vertical winds and their spectra in the lower atmosphere.…”

Mean winds in the tropical stratosphere and mesosphere during January 1993, March 1994, and August 1994

“…al. 1986;Cornish 1988;Hoppe and Fritts, 1995], it is important to understand what they mean. This problem is compounded by the fact that nonzero time mean vertical and horizontal motions imply a nonzero momentum flux, requiring a careful interpretation of body force estimates.…”