H. S. Porter

Equatorial oscillations in the middle atmosphere generated by small scale gravity waves

A realistic parameterization scheme for the deposition of gravity-wave momentum in the middle atmosphere has been incorporated into the 2D version of a global-scale Numerical Spectral Model of the Earths middle atmosphere. Here we present early results, obtained with only the simplest assumptions for the incident gravity-wave spectrum—that it is azimuthally isotropic (i. e., identical flux in the four cardinal directions), globally uniform, and unchanging with season—and with essentially “untuned” values of tunable parameters. This model reproduces reasonably well the observed anomalous latitudinal temperature distribution and the zonal circulation of the upper mesosphere during solstice, just as other models do. It also produces relatively large oscillations in the mean zonal circulation of the middle atmosphere at low latitudes, descending in altitude with time. In the mesosphere and upper stratosphere, the dominant period is semi-annual and the maximum amplitude is about 20 m/s near a height of 50 km. At lower levels, the dominant period is about 20 months and the maximum amplitude is about 8 m/s near 25 km. These values resemble those associated with the observed Semi-Annual Oscillation and Quasi-Biennial Oscillation, respectively, leading us to conclude that small scale gravity waves may contribute significantly to both.

Upper Limits to the Nightside Ionosphere of Mars

The nightside ionosphere of Mars could be produced by electron precipitation or by plasma transport from the dayside, by analogy to the Venus, but few measurements are available. We report here model calculations of upper limits to the nightside ion densities on Mars that would be produced by both mechanisms. For the auroral model, we have adopted the downward traveling portions of the electron spectra measured by the HARP instrument on the Soviet Phobos spacecraft in the Martian plasma sheet and in the magnetotail lobes. For the plasma transport case, we have imposed on a model of the nightside thermosphere, downward fluxes of O+, C+, N+, NO+ and O+2 that are near the maximum upward fluxes that can be sustained by the dayside ionosphere. The computed electron density peaks are in the range (1.3–1.9) × 104 cm−3 at altitudes of 159 to 179 km. The major ion for all the models is O+2, but significant differences in the composition of the minor ions are found for the ionospheres produced by auroral precipitation and by plasma transport. The calculations reported here provide a guide to the data that should be acquired during a future aeronomy mission to Mars, in order to determine the sources of the nightside ionosphere.

Densities and vibrational distribution of H3 + in the Jovian auroral ionosphere

Observations of the H3+ infrared emission at 2 and 4 μm have suggested that H3+ is in local thermodynamic equilibrium (LTE) in the region of the Jovian ionosphere from which the emissions originate. We have tested this assumption by calculating the vibrational distribution of H3+ over the altitude range of 350 to 1500 km above the methane cloud tops (1 to 4 × 10−3 μbar). We have constructed a model of the Jovian auroral ionosphere in which the neutral temperatures are enhanced over those of the mid-latitude ionosphere, as suggested by observations and models of the auroral region. We have modeled the precipitation of 10-keV electrons with an energy flux of 1 erg cm−2 s−1. Both the energy and energy flux are less than those that are implicated in the production of the UV aurora. We have computed the densities and vibrational distribution of H3+ and find that the distribution of the six lowest states of H3+ can be determined fairly well in spite of uncertainties in the atomic and molecular data. Since the nearly resonant transfer of vibration from H2(υ=1) is an important process in populating the H3+(υ1=0,υ2=2) state, it is necessary to model the vibrational distribution of H2 as well. The computed altitude profiles and vibrational distributions of H3+ and H2 are consistent with the observations of infrared emission in the 2-and 4-μm regions. The H3+ is not in LTE near and above the H3+ peak, since loss of the H3+(υ1=0,υ2=1) and H3+(υ1=0,υ2=2) states by radiation is approximately equal to the collisional loss rate.

The gravity wave Doppler spread theory applied in a numerical spectral model of the middle atmosphere .2. Equatorial oscillations

Mayr et al. [this issue] discussed a two-dimensional version of the numerical spectral model (NSM) of Chan et al. [1994a, b] that incorporates the Doppler spread parameterization (DSP) for momentum deposition by small-scale gravity waves (GW) developed by Hines [1997a, b] and presented numerical results describing the global scale seasonal variations in the temperature and wind fields of the middle atmosphere. Even with the simplest assumptions for the GW flux emanating from the troposphere, to be isotropic and independent of latitude and season, this model also produces significant oscillations in the equatorial zonal circulation which are discussed here. Our model results lead to the following conclusions: (1) At altitudes above 40 km, a periodicity of 6 month dominates, resembling the observed semiannual oscillation (SAO). The peak amplitude of this oscillation is close to 18 m/s near 50 km (20-30 m/s observed). A secondary maximum is excited near 80 km with an amplitude of about 11 m/s (15-25 m/s observed), whose phase is opposite to that at 50 km. In this altitude range, the downward phase progression is about 9 km/month, in agreement with observations. The computed SAO is confined to equatorial latitudes, as observed. (2) At altitudes below 40 km, the period of the computed oscillation is almost 21 months, approaching that of the observed quasi-biennial oscillation (QBO). The maximum wind amplitudes are close to 8 m/s (20 m/s observed), and the downward phase progression is about 1.6 km/month (1.3 km/month observed). The model also produces a QBO in the upper mesosphere, in qualitative agreement with recent UARS measurements [Burrage et al., 1996]. (3) When the eddy diffusivity is reduced by a factor of two, the QBO period increases to 30 months and the maximum wind amplitude approaches 13 m/s. Computer experiments are discussed for constant, equinoctial solar heating to elucidate the GW excitation mechanism for the equatorial oscillations in the zonal circulation.

Chemistry of the Jovian auroral ionosphere

We have investigated the chemistry of the Jovian auroral thermosphere-/ionosphere by modeling the precipitation of high-energy electrons into the auroral zones using a multistream electron-transport code and three model thermospheres: a standard model based on pressure and temperature data from Galileo, and two additional models that are characterized by enhanced eddy diffusion coefficients. We have predicted the effects of precipitation of monoenergetic electrons with energies between 20 and 100 keV with energy fluxes of about 11 ergs cm -2 s -1 . We have derived the column densities of H 2 , H, CH 4 , and C 2 H 2 above the altitudes of peak energy deposition. For methane column densities similar to those determined from IUE and Hubble Space Telescope H 2 spectral data, we find that for our standard model, the most likely electron energies are in the 45-55 keV range. For the enhanced eddy diffusion models the energies are lower. We present ion density profiles, H density profiles, and production profiles for the most important H 2 and H emissions. The predicted H column densities are in the range (1 - 6) x 10 18 cm -2 for the standard model and are smaller for the enhanced eddy diffusion models. We find that the temperatures near the altitude of peak energy deposition vary from 160 to 200 K and are significantly lower than those derived from rotational analyses of auroral H 2 emissions, which average 400-500 K. This indicates that the auroral thermosphere is considerably warmer than those at lower latitudes that were measured by the Voyager spacecraft or the Galileo probe.

Mesosphere dynamics with gravity wave forcing: Part I. Diurnal and semi-diurnal tides

Abstract We present results from a non-linear, 3D, time dependent numerical spectral model, which extends from the ground up to the thermosphere and incorporates Hines’ Doppler spread parameterization for small-scale gravity waves (GWs). Our focal point is the mesosphere, which is dominated by wave interactions. We discuss diurnal and semi-diurnal tides in the present paper (Part I) and planetary waves (PWs) in a companion paper (Part II). To study the seasonal variations of tides, in particular with regard to GW forcing, numerical experiments are performed that lead to the following conclusions: (1) The large semi-annual variations in the diurnal tide (DT), with peak amplitudes observed around equinox, are produced to a significant extent by GW interactions that involve, in part, PWs. (2) The DT, like PWs, is amplified by GW momentum deposition, which reduces also the vertical wavelength. (3) Variations in eddy viscosity associated with GW interactions may also influence the DT. (4) The semidiurnal tide (SDT), and its phase in particular, is strongly influenced by the zonal mean circulation. (5) Without the DT present, the SDT is amplified by GWs; but the DT filters out GWs such that the wave interaction significantly reduces the amplitude of the SDT during equinox, effectively producing a strong non-linear interaction between the DT and the SDT. (6) PWs generated internally by the baroclinic instability and GW forcing produce large amplitude modulations of the DT and SDT.

Properties of QBO and SAO generated by gravity waves

Abstract In this paper we present an extension for the 2D (zonal mean) version of our numerical spectral mode (NSM) that incorporates Hines’ Doppler spread parameterization (DSP) for small-scale gravity waves (GW). This model is applied to describe the seasonal variations and the semi-annual and quasi-biennial oscillations (SAO and QBO). Our earlier model reproduced the salient features of the mean zonal circulation in the middle atmosphere, including the QBO extension into the upper mesosphere inferred from UARS measurements. The model is extended to reproduce the upwelling at equatorial latitudes that is associated with the Brewer–Dobson circulation — which affects significantly the dynamics of the stratosphere as Dunkerton had pointed out. In the presence of GW, this upwelling is produced in our model with tropospheric heating, which generates also zonal jets outside the tropics similar to those observed. The resulting upward vertical winds increase the period of the QBO. To compensate for that, one needs to increase the eddy diffusivity and the GW momentum flux, bringing the latter closer to values recommended in the DSP. The QBO period in the model is 30 months (mo), which is conducive to synchronize this oscillation with the seasonal cycle of solar forcing. Associated with this QBO are interannual and interseasonal variations that become increasingly more important at higher altitudes — and this variability is interpreted in terms of GW filtering that effectively couples the dynamical components of the mesosphere. The computed temperature amplitudes for the SAO and QBO are in substantial agreement with observations at equatorial and extra-tropical latitudes. At high latitudes, however, the observed QBO amplitudes are significantly larger, which may be a signature of propagating planetary waves not included in the present model. The assumption of hydrostatic equilibrium not being imposed, we find that the effects from the vertical Coriolis force associated with the equatorial oscillations are large for the vertical winds and significant for the temperature variations even outside the tropics, but the effects are small for the zonal winds.

The gravity wave Doppler spread theory applied in a numerical spectral model of the middle atmosphere: 1. Model and global scale seasonal variations

Hines [1997a, b] has developed a Doppler spread parameterization (DSP) for the deposition of small-scale gravity wave (GW) momentum and energy in the middle atmosphere. We have incorporated this DSP into the two-dimensional (2-D) version of the numerical spectral model (NSM) of Chan et al [1994a, b] which is applied to the Earths middle atmosphere. With a globally uniform flux of (quasi) isotropically propagating GW emanating from the troposphere, the NSM has been integrated for several model years to describe seasonal variations and equatorial oscillations. Here, after a review of the NSM and DSP, we discuss numerical results that describe the temperature and wind fields during solstice and equinox conditions, emphasizing the role played by the GW spectrum. That spectrum is filtered as it ascends through the stratosphere and provides, at the solstices, a highly anisotropic wave and momentum flux at mesospheric heights. Upon further filtering there, with attendant momentum deposition, the waves decelerate and then reverse the zonal circulation. In quasi-geostrophic balance latitudinally, this reversal is accompanied by a reversal of the latitudinal temperature gradient, one that leads to a temperature minimum in the summer polar mesopause region, as is observed. Corresponding results for equinox are obtained, and all are discussed. Our results differ only in detail from those of similar analyses that employ other GW parameterizations. They are presented here in part to exhibit the success of the DSP at this elementary level and in part to provide a point of departure against which future refinements may be judged. Our first extension of the modeling concerns the semiannual and quasi-biennial oscillations that are produced by the DSP in the NSM at equatorial latitudes with the same, constant and uniform, incident GW flux. Initial results are presented in the companion paper and are compared there with observations.

The role of gravity waves in maintaining the QBO and SAO at equatorial latitudes

Abstract We present a numerical spectral model (NSM) that describes the equatorial oscillations in the zonal circulation, the Quasi-biennial Oscillations (QBO) and Semi-annual Oscillations (SAO). The oscillations are generated by the momentum deposition of small scale gravity waves (GW) described with the Doppler spread parameterization (DSP) of Hines. We discuss here the GW mechanism and some of the conditions that are favorable for generating the QBO and SAO. Our model reproduces the salient features observed and leads to the following conclusions: (1) In 2D, a QBO can be generated with a period of about 30 months, peak amplitude close to 20 m/s near 30 km, and downward propagation velocity of 1.2 km/months, close to the observed values. The QBO phase reverses near 70 km and extends with significant amplitude into the upper mesosphere. (2) SAO amplitudes between 5 and 30 m/s are generated that peak at about 55 km with eastward phase during equinox. A second peak with opposite phase is produced above 70 km. (3) The computed oscillations depend on the rate of radiative cooling and on the chosen eddy viscosity that is tied in the DSP to the GW source but is uncertain to some degree. (4) GW filtering by the QBO at lower altitudes can account for the large variability in the SAO that is observed. The SAO in turn also affects the QBO, and under certain conditions the seasonal (semiannual) variations in the solar forcing can act as a pacemaker to seed and synchronize the QBO. (5) In a 3D version of the model that computes also tides and planetary waves, the amplitudes of the SAO, and even the QBO, are significantly reduced compared to 2D. The tides and planetary waves apparently absorb some of the GW momentum that otherwise in 2D goes into the equatorial oscillations of the zonal circulation. (7) The QBO is also generated in 3D with Kelvin and Rossby gravity waves that deposit momentum through nonlinear advection and radiative cooling. However, the wave amplitudes need to be carefully tuned in order to generate such an oscillation, in contrast to the GW mechanism which is robust.

Mesosphere dynamics with gravity wave forcing: Part II. Planetary waves

Abstract We present results from a non-linear, 3D, time dependent numerical spectral model (NSM), which extends from the ground up to the thermosphere and incorporates Hines’ Doppler spread parameterization for small-scale gravity waves (GWs). Our focus is the mesosphere where wave interactions play a prominent role. We discuss planetary waves (PWs) in the present paper (Part II) and diurnal and semi-diurnal tides in the companion paper (Part I). Without external, time-dependent energy or momentum sources, PWs are generated in the model for zonal wavenumbers 1–4, which have amplitudes in the mesosphere above 50 km as large as 30 m / s and periods between 2 and 15 days. The waves are generated primarily during solstice, which indicates that the baroclinic instability (associated with the GW driven reversal in the latitudinal temperature gradient) plays an important role. Results from a numerical experiment show that GWs are also significantly involved directly in generating the PWs. For the zonal wavenumber m =1, the predominant wave periods at mid latitudes in summer cover a wide range around 7 days and peak in winter around 4 days. For m =2, the periods in summer and winter are around 4 and 3 days, respectively. For m =3, 4 the predominant wave periods are in both seasons close to 2 days. At low latitudes, these waves have the characteristics of Rossby gravity waves, with large meridional winds at the equator and propagating westward. A common feature of the PWs ( m =1 to 4) generated in the summer and winter hemispheres is that their vertical wavelengths throughout the mesosphere are large, which indicates that the waves are not propagating freely but are generated (and absorbed) throughout the region. Another common feature is that the PWs propagate preferentially westward in summer and eastward in winter, being launched from the westward and eastward zonal winds that prevail, respectively, in the summer and winter hemispheres at altitudes below 80 km . As shown in Part I, the PWs generated in the model produce large amplitude modulations of the diurnal and semi-diurnal tides above 80 km and contribute to their seasonal variations.