High Resolution Doppler Imager Research Articles

Validation of mesosphere and lower thermosphere winds from the high resolution Doppler imager on UARS

Horizontal wind fields in the mesosphere and lower thermosphere are obtained with the high resolution Doppler imager (HRDI) on the Upper Atmosphere Research Satellite (UARS) by observing the Doppler shifts of emission lines in the O2 atmospheric band. The validity of the derived winds depends on an accurate knowledge of the positions on the detector of the observed lines in the absence of a wind‐induced Doppler shift. Relative changes in these positions are readily identified in the routine measurements of onboard calibration lines. The determination of the absolute values relies on the comparison of HRDI observations with those obtained by MF radars and rockets. In addition, the degrees of horizontal and vertical smoothing of the recovered wind profiles have been optimized by examining the effects of these parameters both on the amplitude of the HRDI‐derived diurnal tidal amplitude and on the variance of the wind differences with correlative measurements. This paper describes these validation procedures and presents comparisons with correlative data. The main discrepancy appears to be in the relative magnitudes measured by HRDI and by the MF radar technique. Specifically, HRDI generally observes larger winds than the MF radars, but the size of the discrepancy varies significantly between different stations. HRDI wind magnitudes are found to be somewhat more consistent with measurements obtained by the rocket launched falling sphere technique and are in very good agreement with the wind imaging interferometer (WINDII), also flown on UARS.

Measurements of stratospheric winds by the high resolution Doppler imager

The high resolution Doppler imager (HRDI) on the Upper Atmosphere Research Satellite (UARS) determines winds in the stratosphere (10–40 km) by measuring the Doppler shift of the rotational lines in the O2 atmospheric B and γ bands. These lines are observed as absorption features in scattered sunlight. The Doppler shifts are found by fitting the observed high‐resolution spectrum to a single‐scattering model. The model includes effects due to scattering from clouds and the ground. Results are compared to radiosonde measurements and the standard deviation between the two measurement systems is found to be between 8 m/s and 12 m/s. This standard deviation includes the errors in both measurement systems as well as geophysical noise due to the different sampling times, sampling locations, and sampling volumes. Monthly averages of HRDI stratospheric winds near the equator are examined from December 1991 to April 1995. These averages reveal the three‐dimensional structure of the quasi‐biennial oscillation (QBO) and its evolution through one full cycle. In particular, we note that the QBO extends over a wide latitude and altitude range and that the easterly and westerly descent rates are similar above 25 km. HRDI measurements show that there is strong meridional shear in the zonal winds above 35 km during the solstices, indicating that inertial instability may play a role in the dynamics of the upper stratosphere. The HRDI wind measurements also show that there is a significant annual cycle in the tropics and that there is substantial interannual variability in the semiannual oscillation in the upper stratosphere.

Combined mesosphere/thermosphere winds using WINDII and HRDI data from the Upper Atmosphere Research Satellite

This paper examines the combined mesospheric and thermospheric (50 to 200 km) longitudinally averaged winds measured by the wind imaging interferometer (WINDII) and the high‐resolution Doppler imager (HRDI) onboard the Upper Atmosphere Research Satellite. The data analyzed cover 2 years from February 1992 to February 1994 and consist of both day and nighttime WINDII winds obtained from the O(1S) green line emission and mesosphere/lower thermosphere daytime HRDI winds from the O2 atmospheric band. The combination of the WINDII and HRDI data sets is first justified by comparing all the data in the lower‐thermosphere overlap region for days and orbits when both instruments were observing the same volume of atmosphere. This comparison shows good agreement between the two instruments. An analysis of the combined WINDII and HRDI winds during equinox and solstice periods is then performed. The amplification with height of the diurnal tide at equinox and its subsequent decay in the lower thermosphere is clearly demonstrated by the observations. The corresponding background (i.e., diurnal mean) zonal wind component exhibits a broad region of easterlies at lower latitudes in the upper mesosphere and lower thermosphere and westerlies at midlatitudes. Above 120 km the mean winds revert to easterlies in the zonal component and a two‐celled equator to pole meridional circulation. The solstice circulation is highly asymmetric about the equator in accordance with the interhemispheric difference in solar heating. The reversal of the mesospheric jets as well as the summer to winter hemisphere meridional flow in the middle thermosphere are clearly shown. At solstice a significantly weaker and more hemispherically asymmetric propagating diurnal tide is also evident.

Climatological mean wind observations from the UARS high‐resolution Doppler imager and wind imaging interferometer: Comparison with current reference models

Direct wind measurements from the high‐resolution Doppler imager (HRDI) and wind imaging interferometer (WINDII) instruments onboard the Upper Atmosphere Research Satellite (UARS) represent a substantial increase in the number of available wind observations in the mesosphere‐to lower‐thermosphere region. These unique data sets provide an opportunity to evaluate current climatological reference models in the 80‐ to 120‐km region which are based on limited empirical data coverage available during the pre‐UARS time period. Below 80 km the general climatological features have been fairly well established by previous observational databases. The reference models therefore provide a means for validating the climatological mean UARS observations in the lower mesosphere, especially at midlatitudes. We have used 2–3 years of UARS HRDI and WINDII observations to construct a monthly and diurnal mean zonally averaged climatology of zonal and meridional wind for the 50‐ to 120‐km region. We use this to compare with the current CIRA‐86 and HWM‐93 reference models. Comparisons are limited to regions where the tidal oscillations can be adequately removed from the UARS observations using simple binning and averaging methods. At 50–80 km the reference models, which are based on gradient winds derived from temperatures, compare well with the HRDI data in reproducing the overall seasonal variations, including the magnitudes and positions of the winter westerly and summer easterly jets at midlatitudes. At 95 km, HRDI and WINDII show year‐round easterlies at low latitudes and indicate a strong westerly jet during summer and weaker, more variable flow during winter at midlatitudes. The CIRA‐86 gradient winds at 95 km show some qualitative agreement with the UARS data at midlatitudes, especially in simulating the summertime jet. However, the comparison is poor at low latitudes. We attribute this to the lack of available temperature measurements in this region from which the gradient winds are derived. The HWM‐93 model, which is based mainly on medium frequency and meteor radar winds at 95 km, shows qualitatively similar seasonal variations in both zonal and meridional wind compared with the UARS data in this region. However, the HWM model is consistently smaller by a factor of 2–3 in wind magnitude relative to the UARS measurements at 95 km.

Validation of O(1S) wind measurements by WINDII: the WIND Imaging Interferometer on UARS

This paper describes the current state of the validation of wind measurements by the wind imaging interferometer (WINDII) in the O(1S) emission. Most data refer to the 90‐to‐110‐km region. Measurements from orbit are compared with winds derived from ground‐based observations using optical interferometers, MF radars and the European Incoherent‐Scatter radar (EISCAT) during overpasses of the WINDII fields of view. Although the data from individual passes do not always agree well, the averages indicate good agreement for the zero reference between the winds measured on the ground and those obtained from orbit. A comparison with winds measured by the high resolution Doppler imager (HRDI) instrument on UARS has also been made, with excellent results. With one exception the WINDII zero wind reference agrees with all external measurement methods to within 10 m s−1 at the present time. The exception is the MF radar winds, which show large station‐to‐station differences. The subject of WINDII comparisons with MF radar winds requires further study. The thermospheric O(1S) emission region is less amenable to validation, but comparisons with EISCAT radar data give excellent agreement at 170 km. A zero wind calibration has been obtained for the O(1D) emission by comparing its averaged phase with that for O(1S) on several days when alternating 1D/1S measurements were made. Several other aspects of the WINDII performance have been studied using data from on‐orbit measurements. These concern the instrument's phase stability, its pointing, its responsivity, the phase distribution in the fields of view, and the behavior of two of the interference filters. In some cases, small adjustments have been made to the characterization database used to analyze the atmospheric data. In general, the WINDII characteristics have remained very stable during the mission to date. A discussion of measurement errors is included in the paper. Further study of the instrument performance may bring improvement, but the utimate limitation for wind validation appears to be atmospheric variability and this needs to be better understood.

Stellar alignment of the High Resolution Doppler Imager

A method is described for determining the angular misalignment between the High Resolution Doppler Imager instrument and its spacecraft, the Upper Atmospheric Research Satellite, using observations of stars by the instrument. Roll, pitch, and yaw misalignments have been measured to within 0.0025 deg (l<r), which is an order of magnitude better than the largest tolerable alignment error. The misalignment results are validated using observations of the nighttime 5577-A O^S) green line emission. In addition, the long-term behavior of the derived alignment offsets, which shows a clear dependence of the alignment on the solar (3 angle, is presented. Nomenclature px = vector in the instrument's viewing direction expressed in the x coordinate frame Tx/y = transformation matrix from the y to the x coordinate frame t = measured time of the star crossing, s to = calculated star crossing time for the known star position and spacecraft position and attitude, s rjo = known angular misalignment vector, deg ?7i = angular misalignment between the instrument and spacecraft frame about the roll axis of the spacecraft, deg 772 = angular misalignment between the instrument and spacecraft frame about the pitch axis of the spacecraft, deg 773 = angular misalignment between the instrument and spacecraft frame about the yaw axis of the spacecraft, deg 3rj/31 = derivative of the misalignment with respect to crossing time expressed as a three-element column vector (in roll, pitch, and yaw), deg/s

Long‐term variability in the solar diurnal tide observed by HRDI and simulated by the GSWM

Observations of the mesosphere and lower thermosphere winds obtained by the High Resolution Doppler Imager (HRDI) on the Upper Atmosphere Research Satellite (UARS) during 1991 to 1995 reveal a semiannual variation in the amplitude of the (1, 1) diurnal tide. The global‐scale wave model (GSWM) represents the first numerical modeling attempt at simulating this seasonal variability, and a preliminary comparison of the GSWM tidal results with HRDI measurements is presented. The results of the comparison and of numerical tests point to some vital and unresolved questions regarding tidal dissipation and tropospheric forcing. In addition to the seasonal variability, HRDI has revealed a strong interannual modulation of the diurnal tide with amplitudes observed to change by nearly a factor of 2 from 1992 to 1994.

A Least Squares Method for Spectral Analysis of Space-Time Series

Common methods in spectral analyses of satellite data are the discrete Fourier transform (DFT) type of approaches, which generally require regular sampling and uniform spacing. These conditions sometimes cannot be met in the satellite applications, for example, such as one made by the High Resolution Doppler Imager (HRDI) on board the Upper Atmosphere Research Satellite (UARS). To be able to handle irregular sampling cases, a least squares fitting method is established here for a space-time Fourier analysis and has been applied to the HRDI sampling as well as other regular sampling cases. This method can resolve space-time spectra as robustly and accurately as DFT-type methods for the regular cases. In the same fashion, given an appropriate sampling pattern, it can also handle the irregular cases in which there exist large data gaps, frequent mode changes, and varying weight samples. Various sampling schemes and the associated aliasing spectra are examined. A better sampling plan than those currently used by the UARS instruments to reduce spectral aliasing is proposed, which leads to the question of how to optimize satellite sampling in the future.

Latitude and seasonal dependence of the semidiurnal tide observed by the high‐resolution Doppler imager

The high resolution Doppler imager (HRDI) on the upper atmosphere research satellite (UARS) has provided measurements of the horizontal wind field in the mesosphere and lower thermosphere (MLT) since November 1991. The observed dynamical patterns are dominated by the (1,1) diurnal tide, but semidiurnal perturbations are also very prominent in the HRDI data, particularly at latitudes greater than 40°. This paper presents a preliminary study of the semidiurnal tide and discusses latitudinal and seasonal variations in the phenomenon. Regular seasonal changes are clearly detected, and significant interhemispheric asymmetries have been identified. In general, the observed features are consistent with previous measurements and model results, but some discrepancies have been found. This new technique, which brings a global perspective to the observation of MLT dynamics, is complementary to the very detailed but localized radar data sets currently available and should provide important constraints for numerical models.

Visible wavelength Doppler lidar for measurement of wind and aerosol profiles during day and night

An incoherent (direct detection) Doppler lidar is developed that operates in the middle of the visible spectrum and measures wind and aerosol profiles during the day and night from the planetary boundary layer to the lower stratosphere. The primary challenge of making a lidar measurement in the visible spectrum during daylight hours is the strong presence of background light from the sun. To make a measurement of this type, the laser line must be isolated spectrally to the greatest extent possible. This has been accomplished through the use of a multiple etalon Fabry-Perot interferometer in combination with a narrow-band filter. The incoherent technique and system are a modified version of the Fabry-Perot interlerometer and image-plane detector technology developed for an earlier Doppler lidar developed at the University of Michigan and for the High-Resolution Doppler Imager (HRDI) now flying on the Upper Atmosphere Research Satellite. The incoherent Doppler analysis is discussed and sample measurements are shown. Winds are measured in the boundary layer with 100-m vertical resolution and 5-mm temporal resolution with 1 to 3 m s-1 accuracy.

Atmospheric wind measurements with the high-resolution Doppler imager

Covers advancements in spacecraft and tactical and strategic missile systems, including subsystem design and application, mission design and analysis, materials and structures, developments in space sciences, space processing and manufacturing, space operations, and applications of space technologies to other fields.

Observations of the 5‐day wave in the mesosphere and lower thermosphere

The 5‐day planetary wave has been detected in the winds measured by the High Resolution Doppler Imager (HRDI) on the Upper Atmosphere Research Satellite (UARS) in the mesosphere and lower thermosphere (50–110 km). The appearances of the 5‐day wave are transient, with a lifetime of 10–20 days in the two‐year data set. The structures of selected 5‐day wave events are in generally good agreement with the (1,1) Rossby normal mode for both zonal and meridional components. A climatology of the 5‐day wave is presented for an altitude of 95 km and latitudes mainly between 40°S and 40°N.

An Estimate of the Momentum Deposition in the Lower Thermosphere by the Observed Diurnal Tide

This paper reports a calculation of the acceleration of the zonal mean flow induced by dissipating tides in the equatorial lower thermosphere. Estimates of the gravest symmetric gravitiational Hough mode (1,1) of the migrating diurnal tide are obtained from monthly composites of global winds observed by the Upper Atmosphere Research Satellite (UARS) High Resolution Doppler Imager (HRDI). Using the principles of classical tidal theory, the tidal momentum flux divergence is computed for a series of monthly mean (1,1) fields from January 1992 to May 1993. The contribution to the mean flow by the leading mode of the migrating tide ranges between -5 and -20 (easterly) m/s/day in the equatorial lower thermosphere. A semiannual variation is noted in the tidal amplitudes and the inferred tidal accelerations. These variations are consistent with observed trends in the zonal mean flow of the lower thermosphere.

Observations of the Diurnal Tide from Space

This study presents a climatology of mesospheric and lower-thermospheric diurnal tidal winds obtained with the High Resolution Doppler Imager (HRDI) on the Upper Atmosphere Research Satellite (UARS). The observations reveal that although tidal structures are present at all times, like the prevailing zonal winds, they exhibit significant semiannual as well as other shorter-term variations in amplitude. Results are presented for a period extending over more than one year from November 1991 to July 1993. The 1,1 diurnal tidal amplitude of the meridional component, characterized by the value at an altitude of 90 km and a latitude of 22 deg, ranges from a minimum at solstice of less than 20 m per sec to an equinox maximum of over 70 m per sec. The vertical wavelength and phase of the tide show only slight variations throughout the year, with a suggestion of semiannual variations in both.

Observations of the O2 atmospheric band nightglow by the high resolution Doppler imager

During nighttime operation the High Resolution Doppler Imager (HRDI) on the Upper Atmosphere Research Satellite (UARS) measures both the horizontal wind field at about 94 km altitude and the limb brightness of the O2(b¹Σg+ − X³Σg+) (0,0) atmospheric band airglow. The dominant feature of the observed emission is a latitudinal and local time dependence which is consistent with the (1,1) diurnal tidal mode. A survey of the available data set from November 1991 to July 1993 reveals a semiannual variation in the peak brightness observed at the equator, with maxima observed at the equinoxes and minima at the solstices. These results are consistent with the long‐term variations in the diurnal tidal amplitudes detected in HRDI wind measurements.

Zonal mean winds in the equatorial mesosphere and lower thermosphere observed by the High Resolution Doppler Imager

This paper presents analyses of mesospheric and lower thermospheric zonal mean winds observed by the High Resolution Doppler Imager (HRDI) on the Upper Atmosphere Research Satellite (UARS). Monthly averages of the equatorial zonal mean zonal winds are presented for January 1992 through June 1993. Equatorial zonal winds in the 70–90 km region are dominated by a semiannual oscillation (SAO), ranging from 30 m s−1 (westerly) to −100 m s−1 (easterly). At high latitudes the zonal wind variations are predominantly annual. Above 90 km, the low‐latitude flow is easterly at all times, punctuated by a small semiannual variation. This behavior may be related to the deposition of momentum by the diurnal tides.

Observations of the quasi 2‐day wave from the High Resolution Doppler Imager on Uars

A strong westward traveling oscillation, with a period of 2 days and zonal wave number 3, is observed in the mesospheric and lower thermospheric winds from the High Resolution Doppler Imager on the Upper Atmosphere Research Satellite. The important events happen in January, July, and September/October, of which the occurrence in January is the strongest with an amplitude over 60ms−1. Detailed analyses for the periods of January 1992 and January 1993 reveal a cause‐and‐effect relationship in the wave developing process at 95km. The global structures of the wave amplitude and phase are also presented.

The high‐resolution doppler imager on the Upper Atmosphere Research Satellite

The high‐resolution Doppler imager (HRDI) on the Upper Atmosphere Research Satellite (UARS) is a triple‐etalon Fabry‐Perot interferometer designed to measure winds in the stratosphere, mesosphere, and lower thermosphere. Winds are determined by measuring the Doppler shifts of rotational lines of the O2 atmospheric band, which are observed in emission in the mesosphere and lower thermosphere and in absorption in the stratosphere. The interferometer has high resolution (0.05 cm−1), good offband rejection and excellent stability. This paper provides details of the design and capabilities of the HRDI instrument.

Comparison of HRDI wind measurements with radar and rocket observations

Wind fields in the mesosphere and lower thermosphere are obtained with the High Resolution Doppler Imager (HRDI) on the Upper Atmosphere Research Satellite (UARS) by observing the Doppler shifts of emission lines in the O2 Atmospheric band. The validity of the measured winds depends on an accurate knowledge of the positions on the detector of the observed lines in the absence of a wind induced Doppler shift. These positions have been determined to an accuracy of approximately 5 ms−1 from the comparison of winds measured by HRDI with those obtained by MF radars. Excellent agreement is found between HRDI measured winds and winds observed with radars and rockets. In addition, the sensitivity of HRDI to migrating tides and other large scale waves is demonstrated.

Global mesospheric tidal winds observed by the high resolution Doppler imager on board the Upper Atmosphere Research Satellite

This paper presents preliminary results of a study of mesospheric and lower thermospheric diurnal tidal winds obtained with the High Resolution Doppler Imager (HRDI) on the Upper Atmosphere Research Satellite (UARS). Zonal averages of the meridional and zonal wind fields are performed at fixed local times. These analyses reveal tidal structures in the mean meridional wind field during the months of February and March 1992. Results are presented for a typical day in February 1992. The observed maximum amplitude of meridional wind is approximately 75 m s−1. The observed vertical wavelength is about 20 km, with amplitudes increasing linearly with height. In addition to the tides, the zonal wind field reveals many features of the mesospheric climatological mean zonal wind.