Werner Singer

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.

An explanation for the seasonal dependence of midlatitude sporadic E layers

[1] The midlatitude sporadic E layers form when metallic ions of meteoric origin in the lower thermosphere are converged vertically in a wind shear. The occurrence and strength of sporadic E follow a pronounced seasonal dependence marked by a conspicuous summer maximum. Although this is known since the early years of ionosonde studies, its cause has remained a mystery as it cannot be accounted for by the windshear theory of E s formation. We show here that the marked seasonal dependence of sporadic E correlates well with the annual variation of sporadic meteor deposition in the upper atmosphere. The later has been established recently from long-term measurements using meteor radar interferometers in the Northern and Southern Hemispheres. Knowing that the occurrence and strength of sporadic E layers depends directly on the metal ion content, which apparently is determined primarily by the meteoric deposition, the present study offers a cause-and-effect explanation for the long-going mystery of sporadic E layer seasonal dependence.

MF radar observations of seasonal variability of semidiurnal motions in the mesosphere at high northern and southern latitudes

Abstract The semidiurnal tide (SDT) is investigated through comparative analysis of horizontal winds measured at Poker Flat (65°N, 147°W), Andenes (69°N, 16°E), Davis (69°S, 78°E), and Rothera (68°S, 69°W). At the northern hemisphere sites the SDT maximizes around the autumn equinox. Poker Flat and Andenes results from 1999–2001 are used to demonstrate that there is a clear repeatable enhancement in SDT amplitudes around the autumn equinox, and that the maximum is localized in height around 86 km . In the southern hemisphere seasonal dependence of the SDT during 1997–1998 is more complicated, and the autumn enhancement is less pronounced. Many competing mechanisms might contribute to the observed seasonal dependence of the SDT, but this study focuses on the refractive effects of shears in the mean zonal wind and gradients in temperature. The main evidence for a refractive influence is that the seasonal enhancement in the SDT amplitude is accompanied by a dramatic shortening in the waves vertical scale. This shortening of the vertical scale is consistent with refraction of the SDT energy into the horizontal wind component. Simplified linear tidal theory equations are used to estimate the expected magnitude of the refractive effects using wind and temperature fields observed over Andenes, Norway. The predicted refractive effects are shown to be potentially significant and qualitatively consistent with the observations. In addition to a seasonal dependence, the SDT amplitudes obtained at all the radar sites exhibit a deep amplitude modulation on a time scale characteristic of planetary waves. This sort of modulation has most often been attributed to nonlinear interactions between the tides and planetary waves. We suggest that refraction might instead produce, or at least contribute to, the observed modulation. Although the planetary waves are of weak ( m s −1 ) amplitude, the SDT (particularly the gravest S(2,2) mode) is only marginally propagating at high latitudes. Thus, small perturbations to the background are enough to periodically inhibit propagation of the SDT to higher levels.

First simultaneous and common volume observations of noctilucent clouds and polar mesosphere summer echoes by lidar and radar

We present the results of first simultaneous and common volume observations of noctilucent clouds (NLC) and polar mesosphere summer echoes (PMSE) by two ground-based lidars and one VHF radar. The measurements were performed at the ALOMAR facility (69°N, 16°E) during the time period July 23 through August 18, 1994. Throughout this period, NLC layers were observed by the lidars on four occasions. During each of these four lidar-observed NLC events, PMSE layers were also observed overhead by the radar. These joint NLC/PMSE events divide into two distinct types : (1) tightly coupled and (2) loosely coupled NLC and PMSE layers. In one of the events, the PMSE layer developed into a well-defined double layer with only 0.8-km vertical separation, while the lidar-observed NLC layer matched closely in time and space the lower PMSE sublayer. Within all four NLC/PMSE layers, the wind speed increased sharply with increasing altitude with wind shears amounting to 0.03 s -1 and larger. Current theories can explain some of the observed layer features.

Planetary waves in coupling the stratosphere and mesosphere during the major stratospheric warming in 2003/2004

[1] The vertical coupling of the stratosphere-mesosphere system through quasi-stationary and traveling planetary waves during the major sudden stratospheric warming (SSW) in the Arctic winter of 2003/2004 has been studied using three types of data. The UK Met Office (UKMO) assimilated data set was used to examine the features of the global-scale planetary disturbances present in the winter stratosphere of the Northern Hemisphere. Sounding the Atmosphere using Broadband Emission Radiometry (SABER) satellite measurements were used as well for extracting the stationary planetary waves in the zonal and meridional winds of the stratosphere and mesosphere. Radar measurements at eight stations, four of them situated at high latitudes (63–69N) and the other four at midlatitudes (52–55N) were used to determine planetary waves in the mesosphere-lower thermosphere (MLT). The basic results show that prior to the SSW, the stratospheremesosphere system was dominated by an upward and westward propagating � 16-day wave detected simultaneously in the UKMO and MLT zonal and meridional wind data. After the onset of the SSW, longer-period (� 22–24 days) oscillations were observed in the zonal and meridional MLT winds. These likely include the upward propagation of stationary planetary waves from below and in situ generation of disturbances by the dissipation and breaking of gravity waves filtered by stratospheric winds. Citation: Pancheva, D., et al. (2008), Planetary waves in coupling the stratosphere and mesosphere during the major stratospheric warming in 2003/2004, J. Geophys. Res., 113, D12105, doi:10.1029/2007JD009011.

Temperature and wind tides around the summer mesopause at middle and arctic latitudes

Abstract Temperatures and winds have been determined with a meteor radar at middle and arctic latitudes. Temperatures are estimated as daily averages around 90 km and winds as hourly means between about 82 km and 98 km. Tides are extracted by a superposed epoch analysis using data from periods of typically 10 or 20 days. The variability of meteor radar temperatures and winds obtained at mid-latitudes during summer in 2000 and 2001 as well as at arctic latitudes in summer 2002 is discussed. The observed low temperatures in early summer (150–170K at 54°N and 120–130K at 69°N) are correlated with the appearance of strong mesospheric radar echoes in the VHF range and of noctilucent clouds at arctic latitudes. At mid-latitudes the amplitudes of the diurnal and semidiurnal temperature tide are in the order of 5 K during summer. The tidal amplitudes at arctic latitudes are smaller with about 4 K for the diurnal tide and 2 K for the semidiurnal tide. The steep temperature decrease from spring to summer at mid-latitudes is accompanied by an enhanced semi-diurnal temperature tide (7–10 K) between middle of May and middle of June.

DROPPS: A study of the polar summer mesosphere with rocket, radar and lidar

DROPPS (The Distribution and Role of Particles in the Polar Summer Mesosphere) was a highly coordinated international study conducted in July, 1999 from the Norwegian rocket range (Andoya, Norway). Two sequences of rockets were launched. Each included one NASA DROPPS payload, containing instruments to measure the electrodynamic and optical properties of dust/aerosol layers, accompanied by European payloads (MIDAS, Mini-MIDAS, and/or Mini-DUSTY) to study the same structures in a complementary manner. Meteorological rockets provided winds and temperature. ALOMAR lidars and radars (located adjacent to the launch site) monitored the mesosphere for noctilucent clouds (NLCs) and polar mesosphere summer echoes (PMSEs), respectively. EISCAT radars provided PMSE and related information at a remote site (Tromso, Norway). Sequence 1 (5–6 July) was launched into a strong PMSE with a weak NLC present; sequence 2 (14 July) occurred during a strong NLC with no PMSE evident. Here we describe program details along with preliminary results.

General circulation model results on migrating and nonmigrating tides in the mesosphere and lower thermosphere. Part I: comparison with observations

Abstract The general circulation model of the Department of Numerical Mathematics of the Russian Academy of Science ( Volodin and Schmitz, 2001 , Tellus 53A (2001) 300) from the surface to mesospheric and lower thermospheric heights has been used to analyse the diurnal and semi-diurnal tides. The GCM includes tropospheric and stratospheric tidal forcings due to absorption of the radiation and latent heat release and uses the gravity wave breaking parameterization of Hines (J. Atmos. Sol. Terr. Phys. 59 (1997a) 371; J. Atmos. Sol. Terr. Phys. 59 (1997b) 387). The model tides describe the observed tidal amplitudes and phases of eastward wind components at different northern hemispheric medium frequency radar sites (Andenes, Juliusruh, Saskatoon, Yamagawa and Hawaii) for January and July conditions. The separation of model tides into migrating and nonmigrating components shows that the nonmigrating part forms the total tide to a large extent, especially for the diurnal tide at low latitudes. The variability of diurnal and semi-diurnal tides is mostly determined by the variability of the nonmigrating part; the variability due to migrating tidal oscillations contributes only a small amount to the total variability. The nonmigrating diurnal model tide is strongly dependent on the longitude, with maxima in the western hemisphere at middle southern latitudes in January. In July, these tidal amplitudes are much weaker with maxima in the subtropics of the eastern hemisphere.

Large electric potential perturbations in PMSE during DROPPS

Comprehensive vector electric field detectors were flown during the DROPPS rocket experiment to study electrodynamic processes near the mesopause. This paper will discuss the first DROPPS rocket flight, which penetrated a strong polar mesosphere summer echo (PMSE) event that also included a weak noctilucent cloud (NLC) layer. During this flight, strong potential perturbations were observed which at first appeared to be caused by large geophysical electric fields. However, as shown here, the potential perturbations resulted from the rocket wake, and were not caused by an environmental electric field. This result strongly differs from other previous in-situ experiments, which have reported large electric fields in PMSE regions.

In‐situ measurement of the Schmidt number within a PMSE layer

During the SCALE campaign in July and August 1993, rocket borne in situ measurements of neutral and electron small scale density fluctuations were performed in the mesosphere over Andoya (69° N16°E) at the same time as the EISCAT 224 MHz radar in Tromso observed strong polar mesosphere summer echoes (PMSE). Strong neutral air turbulence was observed in the height of the PMSE with a turbulent energy dissipation rate of ϵ = 630mW/kg. In the same altitude range, electron density fluctuations were observed extending to scales smaller than found in the neutrals. From a comparison of the neutral and electron density fluctuation spectrum we obtained a Schmidt number Sc of 6.8 (Sc = ν/D ; ν = kinematic viscosity ; D = molecular diffusion coefficient for the electrons). From the above numbers we deduced loD&K = 5.2m (loD&K is the ‘break-off’ scale between the viscous-convective and the viscous-diffusive subrange of the turbulence spectrum). The half wavelength of the EISCAT 224 MHz radar (λ/2 = 0.67m) is therefore well located in the viscous-diffusive subrange of the turbulence spectrum, despite the fact that Sc > 1, which shifts loD&K towards smaller scales. Our results indicate that the theoretical model of Driscoll and Kennedy [1985], frequently used in this context, is not appropriate under PMSE conditions at scales significantly smaller than loD&K.