[1] Long-term thermospheric neutral density trends near 400 km altitude are analyzed using high accuracy satellite drag measurements over the common time period 1970 -2000. Data coverage is over all latitudes and local times and an extensive range of solar and geomagnetic conditions. Densities are compared to empirical models that remove known variations related to solar activity, latitude, local time, day of year and altitude. An average unmodeled secular neutral density decrease of 1.7% per decade is detected. This result is qualitatively consistent with predictions of thermospheric cooling related to anthropogenic causes deduced by theoretical models, and in general agreement with global cooling estimates determined from previous analyses of satellite orbital decay.
Vertical profiles of scalar horizontal winds have been measured at high resolution (10m) in the 13 to 37 km region of the stratosphere. This resolution (at that range of altitude) represents the state‐of‐the‐art, and is unique. Our goal was to ascertain whether or not the internal waves of the stratosphere behave consistently with the Garrett‐Munk model which was originally created for oceanic internal waves. The power spectral densities (PSD's) of five profiles are presented and it is found that (a) they closely fit a straight line on a log‐log graph even to wavelengths as small as 40m, and (b) the average slope is −2.7 ± .2 (standard error = 0.1). We conclude that (a) stratospheric internal waves obey the Garrett‐Munk model for vertical wave numbers, and (b) there is no statistically significant evidence for a break in the curve at high wave numbers when due allowance is made for aliasing effects.
Abstract. There are several theories of atmospheric gravity wave power spectral densities (PSDs) which have been published. These, in turn, have inspired numerous experimental tests. The spectra involved are in the class denoted "stochastic, red noise spectra." This means that most of the power is at the low-frequency end and obeys a power law falloff in going to higher frequency. The present paper describes how some published experimental spectra are flawed by an artifact of spectral analysis which has not heretofore been recognized in the literature. It involves both an amplitude fluctuation enhancement and a coupling between spectral amplitude and slope, and it can be avoided only by stringent control of spectral leakage. Because of "trade-off' considerations every data set, depending on its length and signal characteristics, requires a different method of analysis. It is therefore required that PSD analysis programs must be adjusted and tested to fit each situation. For this purpose a simple method is described to simulate data of known general characteristics for test purposes (to avoid the pitfalls). Since the papers by Nastrom et al. [1997] and de la Torre et al. [1997] have the unfortunate artifact in their analyses, their conclusions regarding saturated gravity wave theories should be reexamined. IntroductionThe discovery [Dewan et al., 1984] that the horizontal velocity vertical wavenumber spectrum appears to be "universal" has inspired a number of theories to be formulated in order to explain this finding. These include those of Dewan and Good [1986] waves. In particular, they showed "scatter diagrams" that display correlations between spectral slope and spectral amplitude. As we shall show, their results are contaminated by an artifact not previously discussed in the literature; and this was the original motivation of the present investigation.In recent years, many "canned programs" and "recipes" have been published which perform spectral analysis of experimental data. Unfortunately, such publications give little hint of the vast number of pitfalls that are involved, and they do not give much information about the literature available which might be helpful in this regard. On the basis of our own experience and that of our colleagues, we are very much aware that there is no single procedure (due to trade-offs) which is appropriate to every data set, and that therefore in our joint
The crucial parameter used to calculate turbulence effects upon light waves propagating through the atmosphere is known as the structure constant, c 2 n . As Tatarski has shown, this parameter depends upon the "outer scale" of the inertial sub-range of the turbulence. Recently there have been successful predictions of astronomical "seeing" conditions at Mauna Kea Astronomical Observatory which have increased interest in this subject and in the use of the so-called "Dewan Optical Turbulence Model". In the case of the Air Force, there has been a longstanding need for such optical turbulence prediction, especially in the stratosphere. In the past researchers have used a relation due to Tatarski, (which plays a prominent role in this model) in order to deduce values of the "outer scale" from c 2 n measurements. When doing this, they have been surprised to find values very much smaller than expected. The goal of the paper is to explain this unexpected result. As we will show, this result can be explained by two factors: (a) the average turbulent layer thicknesses are smaller than originally believed, and, more importantly, (b) only a minor fraction of the stratosphere is turbulent. In order to arrive at this conclusion, we used the high-resolution (10 m) wind profiles that were originally used to formulate the previously mentioned optical turbulence model.
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