Some aspects of the circulation of Mars
Estimates of vertical temperature structure, heat balance, solstice circulation, and thermally driven tides on Mars in comparison with like conditions on Earth
HEAT BALANCE↗
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Venus' Free Obliquity
The predicted orientation of Venus' rotation axis relative to its orbit ran be uniquely determined given knowledge of its J2 gravity coefficient and polar moment of inertia C if its free obliquity is fully damped. This assumption seems warranted given the dominant damping mechanism: turbulent fluid friction at a core mantle boundary (CMB). This skin friction results from differential obliquity of mantle and core spin axes, and the associated damping rate could be as short as 1/10(exp 6) year. However, the observed pole orientation indicates a free obliquity amplitude epsilon approx. = 2.1 deg compared with a nominal forced amplitude of 0.5 deg. There are two plausible explanations. The most likely is that the observed obliquity is a tidally evolved end state in which core friction, modulated by CMB ellipticity and core obliquity amplitude, counterbalances solid and atmospheric tidal torques. This concept is similar to the explanation for the retrograde spin omega as an end state in which solid and atmospheric thermal tidal torques balance at the present spin rate because of the omega(sup -1) dependence of the axial thermal torque. Weaker effects such as solid and thermal tides can then compete with core friction and for plausible models, their sum tends to increase free obliquity. The obliquity balance is controlled by the nonlinear (and nearly quadratic) dependence of the CMB turbulent "skin friction" torque on obliquity.Core ellipticity also has a profound effect on tidal evolution of Venus obliquity.
Yoder, Charles F.↗
The meteor radar as a tool for upper atmosphere research
Meteor radar provide measurements of the upper mesosphere and lower thermosphere neutral wind field by using the reflection of electromagnetic waves from meteor trails. These radars are relatively inexpensive and provide an excellent means of monitoring the mean winds and tides in the 80 to 100 km region. Recently new techniques were developed to detect meteor echoes from other ground based radar systems operating in the HF/VHF frequency range. The meteor echo information augments the data that are routinely collected by these radars. These new techniques are discussed.
Avery, S. K.↗
Thermospheric dynamics during September 18-19, 1984. I - Model simulations
The September 18-19, 1984 Equinox Transient Study (ETS) interval was simulated using the National Center for Atmospheric Research thermospheric GCM with inputs guided by observations. High-latitude forcings for the model were defined with precision for this period. The importance of upward-propagating semidiurnal tides is emphasized, the first-realistic time-dependent simulation to incorporate such tides. A description of the simulated quiet time thermospheric structure for September 18 is contrasted with the storm time behavior of September 19. Several features of the storm response are described, including the generation of large-scale equatorward propagating disturbances. Longer-lived perturbations of the neutral temperature and composition are also discussed. A new feature discovered in the model predictions for altitudes around 200 km comprises a quasi-fixed, four-cell pattern of high-and low-density regions in the polar cap.
Crowley, G.↗
Mass Transport in Global Geophysical Fluids
Mass transports occurring in the atmosphere-hydrosphere-solid Earth-core system (the "global geophysical fluids") are important geophysical phenomena. They occur on all temporal and spatial scales. Examples include air mass and ocean circulations, tides, hydrological water redistribution, mantle processes such as post-glacial rebound, earthquakes and tectonic motions, and core geodynamo activities. With only a few exceptions on the Earth surface, the temporal history and spatial pattern of such mass transport are often not amenable to direct observations. Space geodesy techniques, however, have the capability of monitoring certain direct consequences of the mass transport, including Earth's rotation variations, gravitational field variations, and the geocenter motion. These techniques include the very-long-baseline interferometry, satellite laser ranging and Doppler tracking, and the Global Positioning System, all entail global observational networks. While considerable advances have been made in observing and understanding of the dynamics of Earth's rotation, only the lowest-degree gravitational variations have been observed and limited knowledge of geocenter motion obtained. New space missions, projects and initiatives promise to further improve the measurements and hence our knowledge about the global mass transports. The latter contributes to our understanding and modeling capability of the geophysical processes that produce and regulate the mass transports, as well as the solid Earth's response to such changes in constraining the modeling of Earth's mechanical properties.
Chao, B. F.↗
Seasonal Variations of the Earth's Gravitational Field: An Analysis of Atmospheric Pressure, Ocean Tidal, and Surface Water Excitation
Seasonal variations in the Earth's gravitational field are investigated through the analysis of LAGEOS I satellite laser ranging measurements spanning 1984 to 1992. Global surface pressure data are analyzed; a self-consistent equilibrium ocean tide model is used to compute the effect of the gravitational field on tides. Laser ranging measurements are used to model gravitational field effects.
Laser↗
The Forced Annual Wobble in Earth's Polar Motion
The annual wobble in Earth's polar motion is a forced motion, as opposed to an excited natural oscillation which is the Chandler wobble in the case of polar motion. It is forced by the combination of many angular momentum variations in the geophysical fluids that exchange these variations with the solid Earth, hence changing its rotation. Among all forcing sources of the annual wobble the geophysical fluid that has the dominant contribution is the atmosphere, while the oceans and the land hydrology make up the remaining budget together with tidal influences. The latter include that from the solid Earth deformation and that from the ocean tides at the annual period. The combined forcing produces both prograde and retrograde wobbles; the prograde wobble gets magnified substantially by the near-by presence of the natural Chandler wobble resonance. On the other hand, the closeness of the prograde annual forcing power to the Chandler period is an indication that some of the power leakage into the Chandler period band becomes the main excitation source for the Chandler wobble. In this paper we will review our knowledge about annual wobble and show the status in the effort of closing the budget with the annual angular momentum variations from the various geophysical fluids.
Chao, Benjamin F.↗
Mean zonal winds on Venus from Doppler tracking of the Vega balloons
Doppler measurements of the two Vega balloons yield the following provisional estimates for the mean zonal wind velocity at the 53-54 km level in the Venus atmosphere: 69 + or - 1 m/sec for Vega 1 and 66 + or - 1 m/sec for Vega 2, with westward flow. The wind data show a perturbation which might be an evidence of solar tides.
Andreev, R. A.↗
Geoscience instrumentation
Geoscience instrumentation systems are considered along with questions of geoscience environment, signal processing, data processing, and design problems. Instrument platforms are examined, taking into account ground platforms, airborne platforms, ocean platforms, and space platforms. In situ and laboratory sensors described include acoustic wave sensors, age sensors, atmospheric constituent sensors, biological sensors, cloud particle sensors, electric field sensors, electromagnetic field sensors, precision geodetic sensors, gravity sensors, ground constituent sensors, horizon sensors, humidity sensors, ion and electron sensors, magnetic field sensors, tide sensors, and wind sensors. Remote sensors are discussed, giving attention to sensing techniques, acoustic echo-sounders, gamma ray sensors, optical sensors, radar sensors, and microwave radiometric sensors.
Wolff, E. A.↗
Understanding Geospace on a Grand Scale: The Global Ionoshphere/Thermosphere Constellation
We present the concept of a constellation of polar orbiting satellites equally spaced in longitude (local time) to systematically sample both the neutral and ionzed gas components of the Earth in circular orbits near 350 km, including their density, temperature, and velocities. The instrumentation would include techniques to measure the height of the ionospheric "F-peak" and its variations along the orbit. The number of satellites (l2? 24? 48?) and their configuration would be determined from modeling analysis and expected geophysical phenomena, including their drivers and characteristic time scales. Together with imaging data from separate satellites, the array of satellites with in situ probes would be expected to provide a new picture of (1) high latitude electrodynamics and atmospheric processes and associated coupling with magnetospheric mass and momentum input, (2) the response of the global ionosphere and thermosphere to magnetic storms, and (3) global neutral wind circulation patterns, neutral density structure, tides, planetary waves, and gravity waves. The comprehensive measurements gathered by the IT-Constellation envisioned here would provide a major leap forward in each of these areas, addressing global physical processes and providing fundamental, new knowledge of Geospace. In particular, by its very nature, the constellation addresses "system science", revealing how the ionosphere-thermosphere connects globally to the magnetosphere above and the troposphere below. We present this concept as the next logical step in observing the "whole" space environment using in situ probes in conjunction with imagers. We invite modelers to not only comment on this concept but also to become actively engaged in helping to define it.
Pfaff, Robert↗
Thermal tides and Martian dust storms - Direct evidence for coupling
Evidence for the coupling of diurnal and semidiurnal thermal tides, observed as surface pressure oscillations at the Viking Lander 1 and 2 landing sites, to Martian global dust storms is presented and the implications for models of dust storm generation are discussed. Local atmospheric opacity increases and jumps in atmospheric pressure ranges were found to be essentially simultaneous, and diurnal and semidiurnal components of the pressure and wind oscillations were observed to increase markedly after the onset of a dust storm. The semidiurnal wind component can be related to the observed surface pressure variations by means of a model of a semidiurnally forced Ekman boundary layer. However, a classical atmospheric tidal model shows that the preferential enhancement of the semidiurnal surface pressure oscillation observed at the Lander 1 site can be produced by a tidal heating distribution in which most of the heating is done at an altitude above 10 km. Observations suggest that various mechanisms are important for the generation and decay of global Martian dust storms.
Leovy, C. B.↗
Meteorological and dynamical requirements for MST radar networks: Waves
Studies of wave motions using the MST radar have concentrated on single station time series analyses of gravity waves and tides. Since these radars collect high time resolution data they have the potential to become a significant tool for mesoscale research. In addition, radars are operated almost continuously unattended and, consequently, data sets are available for analyzing longer period wave motions such as tides and planetary scale waves. Although there is much to learn from single station data, the possibilities of new knowledge from a network of radars is exciting. The scales of wave motions in the atmosphere cover a broad range. Consequently the choice of a radar network depends to a large extent on the types of wave motions that are studied. There are many outstanding research problems that would benefit from observations from a MST radar network. In particular, there is a strong need for measurements of gravity wave parameters and equatorial wave motions. Some of the current problems in wave dynamics are discussed.
Avery, S. K.↗
Space Geodesy Monitoring Mass Transport in Global Geophysical Fluids
Mass transports occurring in the atmosphere-hydrosphere-cryosphere-solid Earth-core system (the 'global geophysical fluids') are important geophysical phenomena. They occur on all temporal and spatial scales. Examples include air mass and ocean circulations, oceanic and solid tides, hydrological water and idsnow redistribution, mantle processes such as post-glacial rebound, earthquakes and tectonic motions, and core geodynamo activities. The temporal history and spatial pattern of such mass transport are often not amenable to direct observations. Space geodesy techniques, however, have proven to be an effective tool in monitorihg certain direct consequences of the mass transport, including Earth's rotation variations, gravitational field variations, and the geocenter motion. Considerable advances have been made in recent years in observing and understanding of these geodynamic effects. This paper will use several prominent examples to illustrate the triumphs in research over the past years under a 'Moore's law' in space geodesy. New space missions and projects promise to further advance our knowledge about the global mass transports. The latter contributes to our understanding of the geophysical processes that produce and regulate the mass transports, as well as of the solid Earth's response to such changes in terms of Earth's mechanical properties.
Chao, Benjamin F.↗
Wind Forcing of the North Sea Pole Tide
The Chandler wobble of the earth's rotation has a period near 14 months and sets up the 0.5 cm amplitude pole tide in the deep oceans, However, the pole tide is anomalously large in the North Sea, where the amplitude increases sharply up to 3 cm along the continental coast. It is shown here that the sea levels are well correlated with winds at the pole tide frequency. The Princeton Ocean Model is used to investigate the response of the North Sea to wind forcing. The barotropic numerical ocean model depicts realistic coastlines and bathymetry at 5 ft x 5 ft resolution, with 97 x 73 grid points. The monthly mean wind fields for the 40-year period (1958-1997) from the National Centers for Atmospheric Prediction (NCEP) reanalysis were used to force the model. The winds were converted to stress with a neutral drag coefficient that varied linearly with windspeed (instead of using the NCEP windstress). A 5-day simulation was made for each month until the resulting flow regime came into equilibrium, and model water levels at various station locations were saved for comparison with tidal-gauge observed sea levels from the Permanent Service for Mean Sea Level (PSMSL). The comparison is made for 10 North Sea stations with high quality tide gauge data. We find: (1) good agreement in annual and semi-annual phases and in the trend of amplitude w.r.t. latitude; (2) more importantly for this study, the model-predicted and observed non-seasonal sea level variations show a very significant temporal correlation as well as spectral coherence. However, a large amplitude difference exists between the two sets -- the overall amplitude variability of the observed is generally a factor of 2-3 larger than the model prediction (this same phenomenon has been reported in ocean circulation studies, although the cause is not yet clear.) Our results indicate that the wind forcing is the main cause of the observed large pole tide in the North Sea.
OConnor, W.↗
Correlated environmental corrections in TOPEX/POSEIDON, with a note on ionospheric accuracy
Estimates of the effectiveness of an altimetric correction, and interpretation of sea level variability as a response to atmospheric forcing, both depend upon assuming that residual errors in altimetric corrections are uncorrelated among themselves and with residual sea level, or knowing the correlations. Not surprisingly, many corrections are highly correlated since they involve atmospheric properties and the ocean surface's response to them. The full corrections (including their geographically varying time mean values), show correlations between electromagnetic bias (mostly the height of wind waves) and either atmospheric pressure or water vapor of -40%, and between atmospheric pressure and water vapor of 28%. In the more commonly used collinear differences (after removal of the geographically varying time mean), atmospheric pressure and wave height show a -30% correlation, atmospheric pressure and water vapor a -10% correlation, both pressure and water vapor a 7% correlation with residual sea level, and a bit surprisingly, ionospheric electron content and wave height a 15% correlation. Only the ocean tide is totally uncorrelated with other corrections or residual sea level. The effectiveness of three ionospheric corrections (TOPEX dual-frequency, a smoothed version of the TOPEX dual-frequency, and Doppler orbitography and radiopositioning integrated by satellite (DORIS) is also evaluated in terms of their reduction in variance of residual sea level. Smooth (90-200 km along-track) versions of the dual-frequency altimeter ionosphere perform best both globally and within 20 deg in latitude from the equator. The noise variance in the 1/s TOPEX inospheric samples is approximately (11 mm) squared, about the same as noise in the DORIS-based correction; however, the latter has its error over scales of order 10(exp 3) km. Within 20 deg of the equator, the DORIS-based correction adds (14 mm) squared to the residual sea level variance.
Zlotnicki, V.↗
Improving NOAA's NWLON Through Enhanced Data Inputs from NASA's Ocean Surface Topography
This report assesses the benefit of incorporating NASA's OSTM (Ocean Surface Topography Mission) altimeter data (C- and Ku-band) into NOAA's (National Oceanic and Atmospheric Administration) NWLON (National Water Level Observation Network) DSS (Decision Support System). This data will enhance the NWLON DSS by providing additional inforrnation because not all stations collect all meteorological parameters (sea-surface height, ocean tides, wave height, and wind speed over waves). OSTM will also provide data where NWLON stations are not present. OSTM will provide data on seasurface heights for determining sea-level rise and ocean circulation. Researchers and operational users currently use satellite altimeter data products with the GSFCOO NASA data model to obtain sea-surface height and ocean circulation inforrnation. Accurate and tirnely inforrnation concerning sea-level height, tide, and ocean currents is needed to irnprove coastal tidal predictions, tsunarni and storm surge warnings, and wetland restoration.
Guest, DeNeice C.↗
Variability of the observed temperature, 20-60 km at 80 deg N to 40 deg S
Results are presented for a periodic analysis of atmospheric temperature variations at heights of from 20 to 60 km between 80 deg N and 40 deg S. The analysis is based on Meteorological Rocket Network temperatures not corrected for solar radiation or aliasing by the diurnal tide, and the frequencies examined include the long-term mean, the quasi-biennial oscillation (QBO), and the first six harmonics of the annual wave. Amplitudes are plotted for the long-term mean and QBO as well as for the annual, semiannual, and terannual components. The results show two distinct annual oscillations (the high-latitude one and another above the tropical stratopause) and a polar semiannual wave with two centers of large amplitude that are 90 deg out of phase and separated by a zone of minimum amplitude near 45 km.
Nastrom, G. D.↗
NASA ground-based and space-based laser ranging systems
The development of NASA laser ranging systems is discussed with reference to applications such as orbit determination, gravity-field studies, and analyses of polar motion, earth tides, and tectonic plate motion. Several laser ranging designs are described noting the first satellite laser ranging experiment, the Moblas II and III systems, and the laser ranging satellites presently in orbit. Primary error sources are identified as atmospheric delay, optical signal-to-noise ratios, the electron multiplier, time-interval measurements, and target noise. Plans for Shuttle-based laser ranging systems are reviewed noting simulation studies of 12-day Shuttle missions.
Fitzmaurice, M. W.↗