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  • Author or Editor: Conway B. Leovy x
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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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Geoffrey A. Briggs
and
Conway B. Leovy

Mariner 9 photographs showing variations in the behavior of the Mars atmosphere over 21 consecutive days during northern winter are displayed. The photographs show the north polar cloud hood to be highly variable from day to day, with variations suggestive of development, motion, and decay of baroclinic waves. Lee wave clouds apparently composed primarily of water ice, convective cloud lines inferred to consist of CO2 ice, and dust clouds all occur frequently in the active region between latitudes 40 and 60N. The wave clouds show that persistent surface westerlies and strong westerly shear occur in this region.

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Joel R. Norris
and
Conway B. Leovy

Abstract

No abstract available

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Joel R. Norris
and
Conway B. Leovy

Abstract

Marine stratiform cloudiness (MSC) (stratus, stratocumulus, and fog) is widespread over subtropical oceans west of the continents and over midlatitude oceans during summer, the season when MSC has maximum influence on surface downward radiation and is most influenced by boundary-layer processes. Long-term datasets of cloudiness and sea surface temperature (SST) from surface observations from 1952 to 1981 are used to examine interannual variations in MSC and SST. Linear correlations of anomalies in seasonal MSC amount with seasonal SST anomalies are negative and significant in midlatitude and eastern subtropical oceans, especially during summer. Significant negative correlations between SST and nimbostratus and nonprecipitating midlevel cloudiness are also observed at midlatitudes during summer, suggesting that summer storm tracks shift from year to year following year-to-year meridional shifts in the SST gradient. Over the 30-yr period, there are significant upward trends in MSC amount over the northern midlatitude oceans and a significant downward trend off the coast of California. The highest correlations and trends occur where gradients in MSC and SST are strongest.

During summer, correlations between SST and MSC anomalies peak at zero lag in midlatitudes where warm advection prevails, but SST lags MSC in subtropical regions where cold advection predominates. This difference is attributed to a tendency for anomalies in latent heat flux to compensate anomalies in surface downward radiation in warm advection regions but not in cold advection regions.

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Thomas P. Ackerman
,
Kuo-Nan Liou
, and
Conway B. Leovy

Abstract

A four-stream, multi-layered radiative transfer model has been developed to treat the problem of the transfer of infrared radiation in an atmosphere containing both scatterers and absorbers. Each atmospheric layer is isothermal and contains a uniform concentration of scatterers and absorbers. To facilitate the computations, the infrared spectrum was divided into four bands. In each band empirical transmission functions were fitted by a series of exponential functions. Test calculations of the infrared cooling rate were made using the empirical transmission functions and the fitted transmission functions. The resulting cooling rate profiles exhibit good agreement with each other. A model of a typical urban aerosol was developed using recent experimental results on the spectral dependence of the complex refractive index and the size distributions of aerosols. Atmospheric cooling rates as a function of height were computed on a band by band basis, with and without aerosols, in order to compare the effect of aerosols to the effect of H2O and CO2 in cooling the boundary layer. The presence of large, but realistic, concentrations of aerosol can substantially increase the infrared cooling of the aerosol-containing layer.

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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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Varavut Limpasuvan
,
Conway B. Leovy
, and
Yvan J. Orsolini

Abstract

The two-day wave is observed in the Upper Atmosphere Research Satellite Microwave Limb Sounder temperature data around 40–58 km. Between December 1991 and September 1994, the two-day wave temperature signature is most significant after each solstice when the derived easterly winds near the stratopause extend across the equator to at least 20° latitude in the winter hemisphere, and the zonal mean winds near the equator are inertially unstable with observed inertial instability disturbances. The observed two-day wave consists of a 2.0-day period zonal wavenumber-3 and a 1.8-day period zonal wavenumber-4 component, named (3, 2.0) and (4, 1.8), respectively. The (3, 2.0) component is dominant during two of the three available austral summers, but its amplitude is much weaker than the (4, 1.8) component during the two observed boreal summers.

During the austral summers, correspondence between amplification of the two-day wave temperature signatures, regions of reversed potential vorticity gradient due to meridional curvature of the zonal mean flow, and the critical lines for the (3, 2.0) and (4, 1.8) modes suggest barotropic instability as a source of both wave components. Momentum redistribution by observed inertial instability appears to barotropically destabilize the equatorward flank of the easterly jet where the wave components subsequently grow. During the boreal summers, the (4, 1.8) component appears to be excited by instability that is associated with vertical shear and curvature of the flow seated above the observational domain. The boreal (3, 2.0) mode appears unrelated to the zonal flow instability within the observational domain and may reflect a normal-mode-like response during these periods.

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Margaret A. Rozendaal
,
Conway B. Leovy
, and
Stephen A. Klein

Abstract

Marine stratiform clouds are important and highly variable contributors to earth's radiation budget over the eastern subtropical oceans and over middle- to high-latitude oceans in summer. Because these clouds influence the radiation budget primarily through their albedo, their diurnal cycle has an important influence on their radiative effectiveness.

The authors have analyzed the diurnal cycle in marine low-cloud fraction inferred from the International Satellite Cloud Climatology Project (ISCCP) dataset, after correcting for overlying clouds by using the assumption of random cloud overlap. The results have been compared with the diurnal cycle of low clouds at fixed ships and ships of opportunity. The diurnal cycles of ISCCP low clouds are in good agreement with surface observations of the diurnal cycle of low stratiform clouds almost everywhere. Analysis of the ISCCP data on a 2.5° × 2.5° grid shows that the largest diurnal range in low-cloud fraction occurs downwind in the mean flow, or westward and equatorward, of the subtropical maxima in low-cloud fraction. This is qualitatively consistent with control of the cloud amount by two competing processes in a partially decoupled cloud-topped planetary boundary layer: heating by solar radiation absorption and advection of moist boundary layer air. A radiative transfer code has been used to show that in eastern subtropical ocean regions, where the diurnal cycle of low clouds is large and the cloud has a small optical thickness, calculations with diurnally averaged cloud fraction overestimate total cloud radiative forcing by up to 3 W m−2 (16%) at the surface and 3 W m−2 (7%) at the top of the atmosphere compared with calculations that account for the diurnal cycle.

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James B. Pollack
,
Conway B. Leovy
,
Paul W. Greiman
, and
Yale Mintz

Abstract

A three-layer general circulation model, used to simulate the Martian atmosphere, is described and results are presented. The model assumes a dust-free pure C02 atmosphere and allows for a diurnally- varying convective boundary layer. Smoothed Martian topography and albedo variations are incorporated. The simulation described is for the period near southern winter solstice, season of the Viking landings. The zonally-averaged circulation, mass, heat and momentum balances, and properties of stationary and transient waves are described in some detail, and are compared with results of previous simulations of the Martian general circulation, with related features of the Earth's general circulation, and with observed characteristics of the Martian atmosphere.

The principal conclusions are the following: 1) The simulated zonally-averaged circulation is not very sensitive to differences between this model and the earlier general circulation model of Leovy and Mintz (1969), and compares reasonably well with observations, except for differences attributable to dust and season. 2) The meridional mass flow produced by the seasonal condensation of CO2, in the winter polar region has a major influence on the circulation, but, because of the weak influence of atmospheric heat transport, it is controlled almost entirely by radiation. 3) Quasi-barotropic stationary waves forced kinematically by the topography and resembling topographically-forced terrestrial planetary waves, are generated by the model in the winter hemisphere region of strong eastward flow, while baroclinic stationary waves are thermally forced by topography in the tropics and summer subtropics. 4) Transient baroclinically unstable waves, of somewhat lower dominant wavenumber than those found on the Earth, are generated in winter midlatitudes and their amplitudes, wavenumbers and phase speeds closely agree with what has been deduced from the Viking lander observations.

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