Frank T. Huang

Ozone diurnal variations observed by UARS and their model simulation

Several years of ozone measurements from the Microwave Limb Sounder onboard the Upper Atmosphere Research Satellite are analyzed using a two-dimensional Fourier series in day of year and time of day. Because of limited temporal coverage near local noon, only the diurnal and semidiurnal components are included. Data are investigated in detail at 28°N in the middle stratosphere to lower mesosphere, where the data are considered most reliable. The observations show that ozone is a maximum in the afternoon at 3 mbar and a minimum in the afternoon at 1 mbar and above with a narrow transition zone of reduced diurnal variation in between. This strong dependency on altitude in the transition from a maximum in the afternoon to a minimum in the afternoon, coupled with the small percentage changes in ozone, imposes strict requirements on the data and on the analysis of the data. Comparisons are made with results from a photochemical box model run at 11 levels between 0.46 mbar and 21.5 mbar for 28°N at spring equinox and near the solstices. This is the first time that a data analysis and model comparison of this kind has been made, leading to the identification of relatively small diurnal variations, especially in the transition zone. In the middle stratosphere the model results are in poor agreement with the observations because of the influence of stratospheric dynamics which are neglected in the model runs. In the upper stratosphere the model shows the expected underprediction of absolute ozone amounts, although the percentage change from the midnight value is in excellent agreement with the observations and in particular correctly simulates the diurnal variation in the transition zone between 3 and 1 mbar. Model sensitivity studies are performed to determine the effects of major reaction rate changes and simplified tidal effects.

Large-Scale Waves in the Thermosphere Observed by the AE-C Satellite

Atmosphere Explorer C (AE-C) data are analyzed to study wavelike perturbations in the thermosphere at an altitude of about 260 km. The data were measured during one orbit on January 20, 1975. The examples shown are typical of many other orbits of both satellites AE-C and AE-E. Four geophysical parameters are analyzed: nitrogen and oxygen densities, electron density, and ion temperature, as measured by three different instruments. The data are processed by normalizing them to their average values and extracting their trends. Their fluctuations are obtained by passing the normalized detrended data through a high-pass filter. Strong periodicities are apparent that persist throughout the records, regardless of filter cutoff frequency. Fluctuations are compared by computing cross correlation functions. Spectra are obtained using the MEM and FFT procedures. It is demonstrated, for the first time, that relationships between ion variations and neutral variations are coherent over a wide range of scale sizes over global distances. It is also inferred from plane wave modeling studies and from the persistent periodicities over global distances that a quasi-stationary wave structure is present in the thermosphere that may slowly dissipate and be regenerated by auroral region sources. Large-scale structures are found in all four parameters with horizontal scale sizes ranging from about 400 to about 4000 km. The spectra for all parameters contain peaks at wavelength that are confirmed by the periods of the fluctuations, and decrease with decreasing wavelength with a power law type of variation.

“Synoptic” estimates of chemically active species and other diurnally varying parameters in the stratosphere, derived from measurements from the Upper Atmosphere Research Satellite (UARS)

Over the past two decades there have been numerous articles in the literature discussing methods of obtaining global-scale “synoptic” mathematical representations from asynoptic satellite observations. These studies cover a wide range of viewpoints, ranging from claims of presenting synoptic maps to opinions stating that synoptic representations cannot be obtained from asynoptic data alone, and that physical models are needed to augment the measurements. We review and comment on many of these previous studies. We do not believe that previous published representations generated from Upper Atmosphere Research Satellite data alone for variables that exhibit significant diurnal variations, are indeed synoptic. However, we show that UARS data can be used to estimate useful synoptic approximations without the need for physical models. Using a two-dimensional Fourier least squares algorithm, we calculate mathematical approximations to “synoptic maps.” We provide examples for ClO, ClONO2, NO2, O3, CH4, temperature, and winds and discuss how they can provide new information on both the chemistry and dynamics of the atmosphere.

Medium Scale Gravity Waves in the Thermosphere Observed by the AE-C Satellite

Atmospheric Explorer C (AE-C) satellite data were used to investigate the spectral characteristics of medium-scale wavelike structures (horizontal-scale size from tens to hundreds of kilometers) observed in the neutral and ionized components of the thermosphere at about 250-km altitude. Data for the neutral densities of oxygen, nitrogen, and helium from the Neutral Atmosphere Temperature Experiment, the electron density and temperature from the Cylindrical Electrostatic Probe were analyzed for a part of an orbit in the northern hemisphere between 54° and 10° geographic latitudes. Coherent wave systems are shown by employing direct comparison of the fluctuations of the parameters, comnputations of their cross-correlation functions and spectral analysis. Waves from many tens to hundreds of kilometers are found in the fluctuations of all parameters. A power law decrease of the spectrum is also found with different rates for the various densities and electron temperature, -3.2 for oxygen, -4.6 for nitrogen, 0.23 for helium, -1.96 for the electron density, and -1.66 for the electron temperature. These results complement similar studies of large-scale structures reported in another paper [8]. They are typical of those obtained for other parts of this orbit and for other orbits. Unlike largescale waves, medium-scale waves appear to attenuate after about 4-5 cycles, consistent with lower altitude sources and known loss mechanisms. An exception is shown with wave structure throughout the entire time record, however, implying a source in the F region.