David A. Ortland

High-Resolution Doppler Imager on the Upper Atmosphere Research Satellite

The high resolution Doppler imager (HRDI) on the Upper Atmosphere Research Satellite has been providing measurements of the wind field in the stratosphere, mesosphere, and lower thermosphere since November 1991. Examination of various calibration data indicates the instrument has remained remarkably stable since launch. The instrument has a thermal drift of about 30 m/s/ degree(s)C (slightly dependent on wavelength) and a long-term temporal drift that has amounted to about 80 m/s since launch. These effects are removed in the data processing leaving an uncertainty in the instrument stability of approximately 2 m/s. The temperature control of the instrument has improved significantly since launch as a new method was implemented. The initial temperature control held the instrument temperature at about +/- 1 degree(s)C. The improved method, which holds constant the temperature of the optical bench instead of the radiator, keeps the instrument temperature at about 0.2 degree(s)C. The calibrations indicate very little change in the sensitivity of the instrument. The detector response has shown no degradation and the optics have not changed their transmittance.

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.

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.

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.

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.

Remote sensing of mesospheric temperature and O2(1Σ) band volume emission rates with the high-resolution Doppler imager

An algorithm for obtaining temperatures and volume emission rate profiles in the mesosphere and lower thermosphere from HRDI measurements is described. Temperature and volume emission rates are derived via perturbation theory by comparing measured brightness to brightness computed from the forward model described here. Error analysis shows that the method recovers the temperatures with an error of 7° K and volume emission rates with an error of 3% from 80 to 100 km, providing global measurements of these fields in this altitude range for the first time. Results show the structure of the cold summer mesopause near the poles, the semiannual oscillation, and the diurnal tidal oscillations at the equator.

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 thermosphedc diurnal tidal winds obtained with the High Resolution Doppler Imager (HI) on the Upper Atmosphere Research Satellite (UARS). Zonal averages of the meddional 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

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.

An estimation of the dynamical isolation of the tropical lower stratosphere using UARS wind and trace gas observations of the quasi-biennial oscillation

Upper Atmosphere Research Satellite tropical wind and constituent measurements are used to estimate an upper limit of mid-latitude to tropical trace gas exchange by examining the phase of the N 2 O/CH 4 ratio with respect to the Quasi-Biennial Oscillation winds. Assuming a simple diffusive model for tropical - mid-latitude exchange in the 20-28 km region, the mixing times are estimated to be at least 18 months, and, we estimate an eddy mixing coefficient of 7×10 8 cm 2 /sec. This low mixing rate suggests that if significant chemical mixing into the lower tropical stratosphere occurs, as has been suggested by observational data, then this mixing must take place below 20 km.

Absorption and emission line shapes in the O 2 atmospheric bands: Theoretical model and limb viewing simulations

A multiple scattering radiative transfer model has been developed to carry out a line by line calculation of the absorption and emission limb measurements that will be made by the High Resolution Doppler Imager to be flown on the Upper Atmosphere Research Satellite. The multiple scattering model uses the doubling and adding methods to solve the radiative transfer equation, modified to take into account a spherical inhomogeneous atmosphere. Representative absorption and emission line shapes in the O(2)((1)Sigma(+)(g)-(3)Sigma(-)(g)) atmospheric bands (A, B, and gamma) and their variation with altitude are presented. The effects of solar zenith angle, aerosol loading, surface albedo, and cloud height on the line shapes are also discussed.