H. U. Widdel

Neutral air turbulence in the upper atmosphere observed during the Energy Budget Campaign

A number of different experimental techniques employed in the campaign provided measurements on the fine scale structure of the upper atmosphere, from which information about turbulent intensity, eddy transport and eddy dissipation rates may be extracted. The turbulent state of the mesosphere was shown to be highly variable and significant differences were found between observations obtained during the four salvoes launched during different degrees of geomagnetic disturbance.

Small scale structure and turbulence in the mesosphere and lower thermosphere at high latitudes in winter

Abstract The MAP/WINE campaign has yielded information on small scale structure and turbulence in the winter mesosphere and lower thermosphere by a number of very different remote and in situ techniques. We have assimilated the data from the various sources and thus attempted to present a coherent picture of the small scale dynamics of the atmosphere between 60 and 100 km. We review physical mechanisms which could be responsible for the observed effects, such as ion density fluctuations, radar echoes and wind corners. Evidence has been found for the existence of dynamic structures extending over distances of the order of 100 km; these may be turbulent or non-turbulent. The results indicate that gravity wave saturation is a plausible mechanism for the creation of turbulence and that laminar flows, sharply defined in height and widespread horizontally, may exist.

Mean state densities, temperatures and winds during the MAC/SINE and MAC/EPSILON campaigns

Abstract During 1987 two major field campaigns were conducted, mainly in northern Norway (in summer and late autumn), in which a total of 41 (26+15) in-situ temperature profiles were obtained by different techniques such as passive falling spheres, ionizalion gauges and mass spectrometers. Simultaneously, ground-based measurements of OH-temperatures and sodium lidar temperatures were performed for approximately 85 h and 104 h, respectively. In addition, a total of 67 (37 + 30) wind profiles were measured by in-situ techniques. Several radar systems measured winds almost continuously before, during and after the campaigns. The mean temperature profile for the summer campaign showed major deviations from a recently published reference atmosphere (CIRA 1986), whereas the differences between observations and model are smaller in autumn. In general, both the summer and autumn mean wind profiles agreed with CIRA 1986. Minor differences were attributed to tidal biases of the observations and ageostrophic components.

Vertical movements in the middle atmosphere derived from foil cloud experiments

Abstract Results obtained on vertical velocities of air in the mesosphere are presented which were measured by small foil clouds tracked by radar at Andenes (69°) during January and February 1984. The results (typically ± 4–6 m s −1 , up to 10 m s −1 , and oscillatory in nature) are in good agreement with those obtained by ground-based remote sensing methods. Supplementary observation techniques of the radar return signal show that the interactions between background wind and waves quite often cause small-scale flow separation effects which escape detection when conventional radar tracking is the sole source of information.

Morphology and sources of turbulence in the mesosphere during DYANA

Abstract During the DYANA campaign in early 1990 turbulent parameters were measured at various places by means of in situ and ground supported techniques. Rocket borne instruments detected small-scale fluctuations of neutral (TOTAL instrument) and ion (PIP instrument) number densities in the mesosphere and lower thermosphere. A total of six flights was successfully performed in Andoya (69°N) and two in Biscarosse (44°N). Altitude profiles of turbulent parameters, such as turbulent energy dissipation rates e. and turbulent diffusion coefficients K were derived from the fluctuations. Thanks to improvements in the instrumental capabilities, the reliability of the absolute values is unprecedented. The mean turbulent energy dissipation rates measured both by TOTAL and PIP in Andoya show very low values ( Nearly simultaneously with the sounding rocket flights, temperatures and winds were measured by meteorological rockets and by lidars. This allows study of the relationship between the occurrence of turbulence and the atmospheric stability, parameterized by the Richardson number. In 14 out of 18 cases strong turbulent layers were accompanied by low Richardson numbers. Both wind shear and convectively generated instabilities were observed. Given a particular altitude in the mesosphere, the TOTAL and PIP instrument detected turbulence in approximately 40–60% of all flights. In addition to the in situ measurements, energy dissipation rates were measured around 75 km by the chaff dispersion technique at Heiss Island (81°N) and at Volgograd (48°N). Much higher e values were observed (typically 100 mW/kg) using this procedure than those values obtained by in situ measurements. However, due to non-turbulent dispersion processes, such as small-scale gravity waves and wind shears, the absolute e values from this technique are considered upper limits.

Observational evidence of a saturated gravity wave spectrum in the mesosphere

Abstract Five vertical profiles of scalar horizontal winds have been measured at high resolution (25m) in the range from 80–95 km during the last salvo of the MAC/SINE campaign in the summer 1987 at Andenes, Northern Norway (69.3°N). Our purpose in this study is to examine the consistency of the motion spectrum with the saturated spectrum of gravity waves proposed by Smith S. A. , Fritts D. C. and Van Zandt T.E. , (1987, J. atmos. Sci . 44 , 1404). An analysis of vertical wavenumber spectra of the five horizontal wind profiles is presented and it is found that (a) the average slope of the five vertical wavenumber spectra is −3.0 ± 0.2 for wavelengths in the range from 6.4 km to 100 m. The slope is considerably steeper than the vertical wavenumber spectra of the horizontal velocity discussed in the literature, (b) the average vertical wavenumber spectrum shows that there is excellent agreement between the observed spectrum and the saturated spectrum in both slope and amplitude, suggesting that saturation processes do indeed act to control spectral amplitudes at large wavenumbers, and (c) a dominant vertical wavelength of 6.4 km is found in the mesosphere. Taken together, our observations provide further support for the saturated spectrum theory.

Relationships between the local dynamical structure of the atmosphere and ionospheric absorption during the Western European Winter Anomaly Campaign 1975/76

During the period December 1975–mid-February 1976 of the Western European Winter Anomaly Campaign, the local dynamical and thermal structure of the stratosphere and mesosphere was determined by some 31 rocket launches at El Arenosillo, Spain, while the mesospheric thermal structure in the winter (northern) hemisphere was derived from Nimbus 6 satellite observations. An analysis of these data is presented here. As determined by the A3 technique and the parameter Ld[= L0(n +1)], ionospheric absorption is significantly correlated with many features of the dynamical and thermal structure of the atmosphere. The strongest correlations are, on this occasion, with local winds, at a level between 85 and 92 km altitude, the sense being that high absorption corresponds to northward and eastward wind tendencies, while low absorption corresponds to southward and westward wind tendencies. Additionally, there is a significant correlation with local temperature above 60 km, high absorption corresponding to high mesospheric temperatures. These local variations are corroborated by data obtained from the PMR 3000 channel of the Nimbus 6 satellite (centred about 80km). A trivariate analysis of absorption (Ld), temperature (40° latitude) and meridional temperature gradient along the 0° longitude meridian, which corresponds to the pressure gradient driving a ‘geostrophic’ zonal wind, shows extremely high significance. High ionospheric absorption in Spain is associated with disruption of the normally strong winter vortex around a warm winter mesospheric pole. As the polar temperature decreases, temperatures at low- and mid-latitudes increase and, responding to a decrease of meridional temperature gradient, the mid-latitude zonal and meridonal wind regime changes. Disturbances of the winter mesospheric polar vortex are triggered by the upward propagation of earlier perturbations within the troposphere and/or stratosphere. Delays of 2–4 days are indicated between the onset of stratospheric circulation disturbances, perturbation of the mesospheric polar vortex, and the consequent dynamical and thermal effects in the mid-latitude mesosphere which are associated with anomalously high ionospheric absorption.

Results of concentration and mobility measurements of positively and negatively charged particles taken by a rocket-borne parachutet aspiration (Gerdien) probe in the height region from 72 to 39 km

SummaryWith a self-neutralizing, rocket-borne parachute aspiration probe concentration and mobility measurements of positively and negatively charged carriers were made between 72 and 40 km over Sardinia. Between 72 km and 60 km a ‘heavy’ and a ‘light’ group of positively charged particles were found. The ‘heavy’ group was more abundant than the ‘hight’ group in the greater heights. For the negatively charged particles only one mobility group was found which roughly corresponded to the mobility of the ‘light’ group of the positively charged carriers. The probe was designed so that no electrons could enter the analyzer section of the aspiration system. By this the electron concentration was given by the difference in concentration between the positively and negatively charged particles.

Turbulent energy dissipation rates and eddy diffusion coefficients derived from foil cloud measurements

Abstract Turbulent energy dissipation rates and eddy diffusion coefficients in the 66–90 km altitude range using the foil cloud technique are presented. Turbulent energy dissipation rates vary between 3.2E-5 and 8.6E-2 W kg −1 , and between 0.1 and 170 m 2 s −1 for eddy diffusion coefficients. These derived quantities are in good agreement with the values discussed in the literature. The average value of the calculated spectral index for the vertical velocity is −1.689 ±0.569, which is very close to −5 3 . Richardson number profiles obtained show that the presence of turbulence is associated with a range of dynamic instability.

Mean winds in the winter middle atmosphere above northern Scandinavia

Abstract Wind measurements which were carried out during the MAP/WINE Campaign in northern Scandinavia between 2 December 1983 and 24 February 1984 are used to derive background winds and monthly as well as winter mean values from the ground up to 90 km altitude. These mean winds compare favourably to the wind field proposed for the revised CIRA 86, which is deduced from satellite measurements. The vertical structure of the zonal monthly means is similar in both data sets during January and February. The winter mean zonal winds are observed to be slightly stronger in the stratosphere and lower mesosphere during the MAP/WINE winter than the satellite winds proposed for CIRA 86. The long term mean meridional winds are in good agreement up to 60 km. They indicate a dominant influence of quasistationary planetary waves up to 90 km and an ageostrophic poleward flow between 60 km and 85 km over northern Scandinavia, which maximizes at 76 km at about 8 m s −1 . The observed short term variability of the wind is discussed with respect to a possible impact of saturating gravity waves on the momentum budget of the middle atmosphere.