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Conway B. Leovy

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Conway B. Leovy

Abstract

The first four diurnal surface pressure harmonics have been analysed over major portions of two Martian annual cycles (Mars years) at the two Viking lander sites. The diurnal harmonies of surface wind have also been analysed at one of the sites. Since the atmospheric tides have previously been shown to provide a good indication of Martian global dust storms, these results provide a basis for comparing dust storm activity in the two years. Two global dust storms occurred during the first year. During the second year, there was only one major storm, and it closely followed the pattern of the first storm of the first year. A significant feature of the results is the brief, but nearly complete. vanishing of the diurnal pressure tide at the onset of one of the global dust storms. It is proposed that this may have been due to interference between the normal westward-propagating diurnal tide and a topographically forced eastward propagating tide, and that the latter may have helped initiate the global storm.

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Mathew H. Hitchman
and
Conway B. Leovy

Abstract

The behavior of Kelvin waves in the equatorial middle atmosphere is investigated with the use of daily mapped temperature derived from the Limb Infrared Monitor of the Stratosphere (LIMS) experiment. Diagrams of wave activity per unit mass and wave activity flux density concisely illustrate bulk properties of Kelvin waves and facilitate tracing of packets to source times near the tropopause. Kelvin wave packets of different zonal wavenumbers propagate separately and appear to be forced separately. During the LIMS data period (25 October 1978–28 May 1979) two Kelvin wave regimes are found. Packets of wave one, wave two, or wave three Kelvin waves occur at irregular intervals prior to April. During April and May a nearly continuous upward flux of wave one activity dominates.

For very tall Kelvin waves the observed dependence of vertical wavelength on zonal wind is weaker than predicted by the slowly-varying theory for internal gravity waves. However, most properties of the observed waves are consistent with slowly-varying theory, and the zonal mean body force per unit mass due to Kelvin waves is estimated from observed temperatures and application of the WKBJ approximation. Both a flux convergence and radiative damping formulation yield westerly wave driving which is smaller than that required to satisfy the zonal momentum budget. A comparison of the residual of terms in the zonal momentum equation, estimated from LMS data, with gravity wave driving, estimated by Lindzen's breaking parameterization, suggests that gravity waves may contribute significantly to the equatorial stratopause semiannual oscillation.

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Matthew H. Hitchman
and
Conway B. Leovy

Abstract

The evolution of the zonal mean state in the equatorial middle atmosphere is investigated with the use of daily mapped temperatures derived from the Limb Infrared Monitor of the Stratosphere (LIMS) experiment. These quasi-global, high vertical resolution data cover the pressure range 100-05 mb and the period 25 October 1978-28 May 1979. The equatorial semiannual oscillation (SAO) in zonal mean temperature, derived zonal wind and meridional shear of the zonal wind is described in detail. Rocket profiles are used to validate features seen in LIMS data. These include ranges in temperature and zonal wind of 20 K and 100 m s−1, and cross-equatorial shears of at 3 day−1

Consistent with the theory that the wave-mean flow interaction is essential to the SAO, flow acclerations over the equator exhibit strong week-to-week variations. While easterly accelerations are moderate and occur in deep cool layers, westerly accelerations are generally stronger and occur in shallow warm layers which descend with time at a mean rate of about 0.3 cm s−1.

A detailed heating algorithm is used to estimate residual circulations. Wave-driven residual mean circulation cells associated with the SAO are found to extend well into midlatitudes their latitudinal scale expanding from December through February as newly formed SAO westerlies descend in the lower mesosphere. In the descending branch of the SAO circulation over the equator, estimated downward advection is very similar to observed westerly acceleration in pattern and magnitude. Cross-equatorial shear and mean meridional wind both maximize beneath the descending zero wind line, which is also the level of maximum penetration of easterlies into the winter hemisphere. Inertial instability may enhance meridional circulation especially during November through mid-January in the lower mesosphere. The vertical distribution of wave driving over the equator, inferred as a residual in the zonal momentum equation, is compatible with expectations from gravity wave theory.

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Matthew H. Hitchman
and
Conway B. Leovy

Abstract

The distribution of day-night temperature differences in the middle atmosphere determined by the Nimbus 7 LIMS experiment is described. Day-night differences maximize at and are approximately symmetric about the equator. Successive centers of opposite sign increase in amplitude with altitude, the pattern having a vertical wavelength of ∼25 km. Profiles of rocket meridional wind at Kwajalein (8.7°N) and Ascension Island (8.0°S) taken near local noon and averaged over the LIMS data period, exhibit maxima which support the tidal interpretation of the equatorial temperature pattern. These characteristics are in general agreement with previous observational and theoretical results for the solar driven diurnal tide. Substantial time variations in amplitude and in location of the temperature maxima are observed. The diurnal tide near the equatorial stratopause appears to be influenced by the phase of the semiannual oscillation.

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Conway B. Leovy
and
Peter J. Webster

Abstract

With the aim of comparing planetary-wave behavior in the Northern and Southern Hemispheres, we have examined stratospheric temperature/thickness data for 1971–72 using Nimbus 4 Selective Chopper Radiometer (SCR) data, National Meteorological Center (NMC) gridded data, and Eole constant-level balloon data. Properties of planetary waves 1 and 2 derived from the SCR and NMC data are shown to agree well. Large-amplitude oscillations with energy in the 20–30 day period range and eastward propagating wave 2 in the Southern Hemisphere have been noted previously and are prominent in these data. Strong vertical coherence in wave phase at all levels between 200 mb and the upper stratosphere is observed. Additional features of interest in this data set include the following: 1) the upper stratosphere in mid-winter appears to satisfy the necessary condition for instability derived by Charney and Stern (1962), especially in the Southern Hemisphere; 2) wave amplification is closely associated with eastward phase propagation; 3) wavenumber 2 amplitude is highly asymmetric with respect to the solstice in both hemispheres and at least in the north the asymmetry is prominent in the upper troposphere as well as the stratosphere; and 4) the meridional extent of the wave amplitude is directly related to zonal wind speed. Some tentative interpretations of these observations are offered.

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Conway B. Leovy
and
Peter J. Webster

Abstract

No abstract available.

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Ruth S. Lieberman
and
Conway B. Leovy

Abstract

Observations of surface pressure and middle atmosphere temperatures and winds indicate that a substantial nonmigrating component is present in the diurnal tide. The nonmigrating tides, which propagate with a zonal phase speed that is different from the earth's rotation, are attributed to the diurnal heating of geographically fixed sources. In this study we utilize a classical tidal model to examine the propagation characteristics of diurnal tides. The global fields of tropospheric sensible, radiative, and latent heating used to drive the model are supplied from summer and winter diurnal climatologies of the NCAR Community Climate Model (CCM2). A novel aspect of this study is the focus on the relative importance of the nonmigrating components.

The classical model successfully reproduces many observed features of the low-latitude diurnal surface pressure tides. In the middle atmosphere, the simulated migrating (or sun-synchronous) tide shows qualitative agreement with November–March LIMS observations. Tropospheric solar heating is clearly the dominant driving force for the migrating tide, with secondary contributions from boundary-layer sensible heating and tropospheric latent heat release. The leading modes of the zonal mean tide are also driven chiefly by tropospheric solar heating. The higher-order modes of the zonal mean and eastward propagating tides may be attributed to the joint effects of tropospheric solar heating, sensible heating, and latent heat release. The LIMS and other data reveal features that cannot be explained or examined within the context of the classical model used in the present study. These include upward phase propagation, vertical attenuation, and temporal variations in the migrating diurnal tide.

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Sungsu Park
,
Conway B. Leovy
, and
Margaret A. Rozendaal

Abstract

A new heuristic model of stratocumulus cloudiness in the inversion-capped marine boundary layer is developed and tested. The essential ingredient is a new method for predicting the statistical distribution of temperature and specific humidity at the inversion base under partially decoupled conditions along steady-state marine boundary layer (MBL) trajectories. MBL decoupling is parameterized as an increasing function of the height difference between the inversion base and lifting condensation level (LCL) of the mixed-layer air. Required inputs are sea surface temperature (SST), free air (above inversion) temperature and humidity, subsidence velocity, and mean boundary layer wind speed. Upstream boundary conditions must also be specified but have little influence at sufficient downstream distances (>2000 km).

The model is applied to the cold advection regime of the northeastern subtropical Pacific and to both warm and cold advection regimes of the eastern equatorial Pacific Ocean. The model is conceptually simple and avoids explicit calculation of several important physical processes. Nevertheless, it is at least qualitatively successful in predicting both the climatological mean properties and climate anomaly variations of MBL stratocumulus in both regions. These results suggest that, regardless of other properties, successful MBL stratocumulus models will need to accurately predict inversion base height and the LCL and they will have to account for downstream memory effects.

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Jordan L. Sutton
,
Conway B. Leovy
, and
James E. Tillman

Abstract

Wind speed, ambient and surface temperatures from both Viking Landers have been used to compute bulk Richardson numbers and Monin-Obukhov lengths during the earliest phase of the Mars missions. These parameters are used to estimate drag and heat transfer coefficients, friction velocities and surface heat fluxes at the two sites. The principal uncertainty is in the specification of the roughness length. Maximum heat flux occur near local noon at both sites, and are estimated to be in the range 15–20 W m−2 at the Viking 1 site and 10–15 W m−2 at the Viking 2 site. Maximum values of friction velocity occur in late morning at Viking 1 and are estimated to be 0.4–0.6 m s−1. They occur shortly after dawn at the Viking 2 site where peak values are estimated to be in the range 0.25–0.35 m s−1. Extension of these calculations to later times during the mission will require allowance for dust opacity effects in the estimation of surface temperature and in the correction of radiation errors of the Viking 2 temperature sensor.

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