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- Author or Editor: B. B. Balsley x
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Abstract
Monthly-averaged horizontal and vertical mesospheric wind fields have been measured using the Poker Flat Radar at 65°N latitude during 15 months in 1980–81. The horizontal wind fields are reasonably consistent with previous observations and with some of the current theoretical models that take into account enhanced turbulence and eddy transport observed at mesospheric heights. However, the observed vertical wind field has a quasi-sinusoidal seasonal variation with peak values of 25 cm s−1 both downward near summer solstice and upward near winter solstice and is inconsistent with current models of mean circulation in the meridional plane. Because there are no other comparable vertical wind observations, the possibility of error in the vertical wind measurements was carefully considered and rejected. We conclude that the actual mean circulation is more complex than that implied by current models. Possible complicating factors include nonuniform zonal flow, multicellular meridional structure and an enhanced vertical Stokes drift that would arise from strong eddy activity.
Abstract
Monthly-averaged horizontal and vertical mesospheric wind fields have been measured using the Poker Flat Radar at 65°N latitude during 15 months in 1980–81. The horizontal wind fields are reasonably consistent with previous observations and with some of the current theoretical models that take into account enhanced turbulence and eddy transport observed at mesospheric heights. However, the observed vertical wind field has a quasi-sinusoidal seasonal variation with peak values of 25 cm s−1 both downward near summer solstice and upward near winter solstice and is inconsistent with current models of mean circulation in the meridional plane. Because there are no other comparable vertical wind observations, the possibility of error in the vertical wind measurements was carefully considered and rejected. We conclude that the actual mean circulation is more complex than that implied by current models. Possible complicating factors include nonuniform zonal flow, multicellular meridional structure and an enhanced vertical Stokes drift that would arise from strong eddy activity.
Abstract
Results of an analysis of the summertime wind field between 80–93 km over Poker Flat, Alaska are presented. The data were obtained using the large, but as yet incomplete, MST radar at Poker Flat, and cover the periods 17 June–13 July 1979, and 22 June–22 July 1980. Zonal and meridional mean wind profiles and “tidal” components during these periods are examined and are compared with other observations at comparable latitudes as well as with current theoretical profiles. While our results agree reasonably closely with other observations and theory, some discrepancies exist and are discussed. In addition to the expected 8, 12 and 24 h components of atmospheric motions, we find strong evidence for a 16 h component, particularly during the 1979 data period. In addition, we show examples of the average power spectrum of wind fluctuations from 3 min to 8 days. These spectra not only show the expected “tidal” peaks, but show also that the spectral energy density is continuous between the shortest (8 h) tidal period and the shortest observed period (3 min), falling off with an f −5/3 power law. Some aspects of these spectral results are discussed, including the possible processes that may operate to produce such a spectral shape.
Abstract
Results of an analysis of the summertime wind field between 80–93 km over Poker Flat, Alaska are presented. The data were obtained using the large, but as yet incomplete, MST radar at Poker Flat, and cover the periods 17 June–13 July 1979, and 22 June–22 July 1980. Zonal and meridional mean wind profiles and “tidal” components during these periods are examined and are compared with other observations at comparable latitudes as well as with current theoretical profiles. While our results agree reasonably closely with other observations and theory, some discrepancies exist and are discussed. In addition to the expected 8, 12 and 24 h components of atmospheric motions, we find strong evidence for a 16 h component, particularly during the 1979 data period. In addition, we show examples of the average power spectrum of wind fluctuations from 3 min to 8 days. These spectra not only show the expected “tidal” peaks, but show also that the spectral energy density is continuous between the shortest (8 h) tidal period and the shortest observed period (3 min), falling off with an f −5/3 power law. Some aspects of these spectral results are discussed, including the possible processes that may operate to produce such a spectral shape.
In the past few years, sensitive Doppler radars have been used to probe the clear atmosphere from the boundary layer up to stratospheric altitudes. Routine Doppler radar observations are now attainable over a wide range of frequencies at virtually all altitudes in this height range. Although a number of these systems operate at UHF (300–3000 MHz), sensitive Doppler radars have recently been developed that operate at VHF (30–300 MHz). These new systems employ large, inexpensive antennas constructed of phased arrays of dipole elements. This paper surveys recent developments in the use of both VHF and UHF Doppler radars for probing the clear atmosphere and presents design considerations for new radar systems.
In the past few years, sensitive Doppler radars have been used to probe the clear atmosphere from the boundary layer up to stratospheric altitudes. Routine Doppler radar observations are now attainable over a wide range of frequencies at virtually all altitudes in this height range. Although a number of these systems operate at UHF (300–3000 MHz), sensitive Doppler radars have recently been developed that operate at VHF (30–300 MHz). These new systems employ large, inexpensive antennas constructed of phased arrays of dipole elements. This paper surveys recent developments in the use of both VHF and UHF Doppler radars for probing the clear atmosphere and presents design considerations for new radar systems.
The application of clear-air radars to operational wind profiling is considered. Several alternative techniques for determining atmospheric winds from radars are surveyed and, in light of the current interest in operational applications, the current status of the techniques is summarized and the need for more research and development knowledge before defining any final system configuration stressed. Finally, some possibilities for portable automated systems of the future are suggested.
The application of clear-air radars to operational wind profiling is considered. Several alternative techniques for determining atmospheric winds from radars are surveyed and, in light of the current interest in operational applications, the current status of the techniques is summarized and the need for more research and development knowledge before defining any final system configuration stressed. Finally, some possibilities for portable automated systems of the future are suggested.
Abstract
We compare vertical wind fluctuations observed by VHF radar wind profilers in the tropical troposphere over a large, mountainous island (Pohnpei, at 7°N, 158°E) and a large, low-profile atoll (Christmas Island, at 2°N, 157°W). The major difference in the two datasets appears to be due to the frequent occurrence of mountain waves over Pohnpei and to their absence over Christmas Island. These waves are generated continuously at low levels over Pohnpei by the steady ENE trade winds acting on the 800 m high terrain, and can extend at least into the lower stratosphere. We find the occurrence of mountain waves at middle- and upper-tropospheric levels over Pohnpei to be governed primarily by a “critical-layer” relationship between the winds aloft and the near-surface winds: a reversal in the direction of the upper level winds relative to the lower-level trade winds precludes the upward propagation of mountain waves, whereas a nonreversed wind profile allows the waves to propagate freely into the lower stratosphere, and possibly to much higher heights, depending on the phase of the QBO.
Abstract
We compare vertical wind fluctuations observed by VHF radar wind profilers in the tropical troposphere over a large, mountainous island (Pohnpei, at 7°N, 158°E) and a large, low-profile atoll (Christmas Island, at 2°N, 157°W). The major difference in the two datasets appears to be due to the frequent occurrence of mountain waves over Pohnpei and to their absence over Christmas Island. These waves are generated continuously at low levels over Pohnpei by the steady ENE trade winds acting on the 800 m high terrain, and can extend at least into the lower stratosphere. We find the occurrence of mountain waves at middle- and upper-tropospheric levels over Pohnpei to be governed primarily by a “critical-layer” relationship between the winds aloft and the near-surface winds: a reversal in the direction of the upper level winds relative to the lower-level trade winds precludes the upward propagation of mountain waves, whereas a nonreversed wind profile allows the waves to propagate freely into the lower stratosphere, and possibly to much higher heights, depending on the phase of the QBO.
Abstract
This paper shows that a very slight tilt of the vertically directed antenna beam of a VHF wind profiler can produce a measurable change in the observed long-term averaged “vertical” velocity profiles. The results are based primarily on data obtained using the NOAA/CU profiler at Piurn, Peru, where phase measurements of individual antenna elements made in 1992 showed that the calculated angle of the 3°-wide vertical beam was skewed only by about 0.06° from true vertical. This small error was corrected in February 1993 by carefully rephasing some of the array feedpoints. Mean vertical velocity profiles obtained prior to the correction were adjusted to account for the slight contamination by the horizontal wind. These corrected vertical profiles compare favorably with vertical profiles obtained after rephasing the antenna, as well as with mean vertical wind profiles from other profiler sites in our tropical Pacific profiler network.
The results show that in order to be confident in long-term averaged vertical wind profiles using VHF profilers in the Tropics, the vertically directed antenna needs to be very carefully phased. The results also suggest that long-term averaging tends to nullify any possible effects of apparent variations of the vertical beam that might arise from short-term echo specularity. In addition, asymmetric biases in the turbulent-scattering process thought to contaminate mean vertical velocity measurements at midlalitudes are not at all apparent in our tropical profiles. This final factor may be due to the much smaller average vertical velocity variances observed at low latitudes.
Abstract
This paper shows that a very slight tilt of the vertically directed antenna beam of a VHF wind profiler can produce a measurable change in the observed long-term averaged “vertical” velocity profiles. The results are based primarily on data obtained using the NOAA/CU profiler at Piurn, Peru, where phase measurements of individual antenna elements made in 1992 showed that the calculated angle of the 3°-wide vertical beam was skewed only by about 0.06° from true vertical. This small error was corrected in February 1993 by carefully rephasing some of the array feedpoints. Mean vertical velocity profiles obtained prior to the correction were adjusted to account for the slight contamination by the horizontal wind. These corrected vertical profiles compare favorably with vertical profiles obtained after rephasing the antenna, as well as with mean vertical wind profiles from other profiler sites in our tropical Pacific profiler network.
The results show that in order to be confident in long-term averaged vertical wind profiles using VHF profilers in the Tropics, the vertically directed antenna needs to be very carefully phased. The results also suggest that long-term averaging tends to nullify any possible effects of apparent variations of the vertical beam that might arise from short-term echo specularity. In addition, asymmetric biases in the turbulent-scattering process thought to contaminate mean vertical velocity measurements at midlalitudes are not at all apparent in our tropical profiles. This final factor may be due to the much smaller average vertical velocity variances observed at low latitudes.
Abstract
The DataHawk small airborne measurement system provides in situ atmospheric measurement capabilities for documenting scales as small as 1 m and can access reasonably large volumes in and above the atmospheric boundary layer at low cost. The design of the DataHawk system is described, beginning with the atmospheric measurement requirements, and articulating five key challenges that any practical measurement system must overcome. The resulting characteristics of the airborne and ground support components of the DataHawk system are outlined, along with its deployment, operating, and recovery modes. Typical results are presented to illustrate the types and quality of data provided by the current system, as well as the need for more of these finescale measurements. Particular focus is given to the DataHawk's ability to make very-high-resolution measurements of a variety of atmospheric variables simultaneously, with emphasis given to the measurement of two important finescale turbulence parameters, 
Abstract
The DataHawk small airborne measurement system provides in situ atmospheric measurement capabilities for documenting scales as small as 1 m and can access reasonably large volumes in and above the atmospheric boundary layer at low cost. The design of the DataHawk system is described, beginning with the atmospheric measurement requirements, and articulating five key challenges that any practical measurement system must overcome. The resulting characteristics of the airborne and ground support components of the DataHawk system are outlined, along with its deployment, operating, and recovery modes. Typical results are presented to illustrate the types and quality of data provided by the current system, as well as the need for more of these finescale measurements. Particular focus is given to the DataHawk's ability to make very-high-resolution measurements of a variety of atmospheric variables simultaneously, with emphasis given to the measurement of two important finescale turbulence parameters,
A recent campaign on Christmas Island in the equatorial Pacific has demonstrated that, at least under some conditions, simple and relatively inexpensive kites can serve as effective platforms for continuous monitoring of the free atmosphere. Individual flights can be anticipated to last for periods of weeks and possibly months. Our results can be considered as a renewal and extension of kite-borne atmospheric research—a technique that was used extensively around the turn of the century, but which has languished in relative obscurity for the past 50 years.
In the experiment described herein, we have flown kites to altitudes up to 3.5 km, and have maintained them aloft with ease for the four-day duration of the experiment. In addition, we outline the general capabilities and limitations of state-of-the-art kite platforms, and show that, under some conditions, such systems can be expected to attain much higher altitudes and remain aloft for much longer periods. We also outline the potential for using kites as long-term monitoring platforms for temperature, radiation, ozone, water vapor, ice crystal structure, atmospheric electric fields, and myriad other variables, and show that such measurements are both theoretically possible and practically feasible, at least under some restrictive conditions.
A recent campaign on Christmas Island in the equatorial Pacific has demonstrated that, at least under some conditions, simple and relatively inexpensive kites can serve as effective platforms for continuous monitoring of the free atmosphere. Individual flights can be anticipated to last for periods of weeks and possibly months. Our results can be considered as a renewal and extension of kite-borne atmospheric research—a technique that was used extensively around the turn of the century, but which has languished in relative obscurity for the past 50 years.
In the experiment described herein, we have flown kites to altitudes up to 3.5 km, and have maintained them aloft with ease for the four-day duration of the experiment. In addition, we outline the general capabilities and limitations of state-of-the-art kite platforms, and show that, under some conditions, such systems can be expected to attain much higher altitudes and remain aloft for much longer periods. We also outline the potential for using kites as long-term monitoring platforms for temperature, radiation, ozone, water vapor, ice crystal structure, atmospheric electric fields, and myriad other variables, and show that such measurements are both theoretically possible and practically feasible, at least under some restrictive conditions.
Abstract
Measurements by VHF wind-profiling radars worldwide have, for many years, shown long-term downward atmospheric motion in most of the troposphere. Various explanations have been proposed, some atmospheric and some caused by the radar measurement method. At many sites, except those in the Tropics, the authors suggest a new model, in which the long-term downward vertical wind (
Abstract
Measurements by VHF wind-profiling radars worldwide have, for many years, shown long-term downward atmospheric motion in most of the troposphere. Various explanations have been proposed, some atmospheric and some caused by the radar measurement method. At many sites, except those in the Tropics, the authors suggest a new model, in which the long-term downward vertical wind (
Abstract
Recently published data (Ecklund and Balsley) describing VHF radar echo characteristics from the Arctic mesosphere and lower thermosphere show a remarkable seasonal dependence of both the echo height and echo intensity: during the three-month period around the summer solstice, intense and nearly continuous echoes are returned from a narrow (±2 km half-power) region centered at 86 km; during the remainder of the year, however, the echoes are much weaker, more sporadic and occur at a much lower altitude (70 km ± 9 km). In this paper, we present additional data that suggest that the summer echoes are primarily the result of shear instability of low-frequency (tidal) motions in the region of high stratification above the Arctic summer mesopause, while the winter echoes arise from the nonlinear breakup of upward-propagating gravity waves.
Abstract
Recently published data (Ecklund and Balsley) describing VHF radar echo characteristics from the Arctic mesosphere and lower thermosphere show a remarkable seasonal dependence of both the echo height and echo intensity: during the three-month period around the summer solstice, intense and nearly continuous echoes are returned from a narrow (±2 km half-power) region centered at 86 km; during the remainder of the year, however, the echoes are much weaker, more sporadic and occur at a much lower altitude (70 km ± 9 km). In this paper, we present additional data that suggest that the summer echoes are primarily the result of shear instability of low-frequency (tidal) motions in the region of high stratification above the Arctic summer mesopause, while the winter echoes arise from the nonlinear breakup of upward-propagating gravity waves.
