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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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Conway B. Leovy

Abstract

No abstract available.

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Sungsu Park and Conway B. Leovy

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As a contribution to understanding the possible impact of altered climate regimes on marine clouds, and hence on cloud radiative forcing, ship-observed marine low clouds and precipitation frequency for individual seasons are regressed at zero lag on an index of El Niño–Southern Oscillation (ENSO) for the period December 1955– January 1996 for ocean areas between 40°S and 70°N. Seasonal anomalies of atmospheric circulation parameters, static stability, and SST are also examined in order to illuminate physical mechanisms responsible for observed ENSO cloud variations.

The following extratropical regions exhibit significant ENSO cloud anomalies and are discussed in detail: winter–spring North Pacific, summer North Pacific, winter western North Atlantic, autumn northeastern Atlantic, and western Mediterranean Sea. In all of these regions except the summer North Pacific, cloud anomalies are related to jet stream and storm track anomalies associated with atmospheric teleconnection patterns. The summer North Pacific anomalies are also connected with jet stream and storm track anomalies, but these are associated with a persistent SST anomaly rather than an atmospheric teleconnection. ENSO anomalies in the western and eastern equatorial Pacific are analyzed in greater detail than in previous work, as well as those in the Arabian Sea during winter and summer monsoons. With the exception of the Arabian Sea region in winter, cloud anomalies are consistently related to 1) changes in storm tracks and/or 2) changes in low-level static stability and temperature advection.

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Conway B. Leovy and Peter J. Webster

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No abstract available.

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Louis J. Bajuk and Conway B. Leovy

Abstract

The dominant interdecadal signal in normalized frequency of occurrence of cumulonimbus reported by volunteer observing ships is globally uniform over the period 1952–92 over all ocean areas between 40°S and 50°N. Globally uniform signals in both normalized frequency of occurrence and amount-when-present also dominate interdecadal variations for other low cloud types. This pattern is inconsistent with plausible physical mechanisms and is apparently due to slow changes in observational practice. Eight ocean weather ships with approximately fixed positions also reported gradual changes in low cloud occurrence frequencies between 1952 and 1969 that were similar in pattern for all eight ships, but for most cloud types these variation patterns differed markedly from those at nearly collocated volunteer observing ships. These apparently spurious variations make it difficult to identify real interdecadal variations in marine clouds from ship observations. However, over the tropical Indian Ocean and central and eastern Pacific Ocean, small but widespread decreases in stratocumulus frequency and increases in deep convective cloud frequency between 1955 and 1978, and 1979 and 1991 tend to be consistently related to changes in sea surface temperature and are likely to be real. Over the western Pacific Ocean, ship reports indicate an increase in the frequency of deep convective clouds between these two periods that is not consistently related to SST changes and is less likely to be real.

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Matthew H. Hitchman and Conway B. Leovy

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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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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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Louis J. Bajuk and Conway B. Leovy

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Anomalies in frequency of occurrence of stratiform and convective cloud types identified by volunteer observing ships are related to anomalies in SST and surface wind divergence for the tropical Pacific and Indian Oceans for the period December 1952–December 1992. Cloud type frequency anomalies have also been related to outgoing longwave radiation (OLR) anomalies for the period 1979–91. The strongest cloud frequency signals are associated with the seasonal shift in the ITCZ and with the first empirical orthogonal function of SST after removal of the annual cycle. The latter corresponds to the annually averaged SST signature of ENSO. Data are separated into two seasons to display these signatures: January–May corresponding to warm eastern Pacific equatorial SST and July–November corresponding to cool eastern Pacific equatorial SST. Relationships between cloud type frequencies, SST, and divergence are generally similar for spatial variations within each season, and for seasonal and ENSO-related differences. The major cloud frequency shifts are between stratiform clouds and large cumulus east of 130°W and between small cumulus and deep convective clouds west of 130°W. East of 160°E, frequency of deep convective cloud increases rapidly above a knee in the curve of frequency versus SST located near 25.5°C in July–November and near 27°C January–May. Since this temperature difference is similar to the difference in midtropospheric mean temperature between the same seasons in the same region, this relationship suggests strong control of deep convection by mean static stability in this region. As expected, a strong and linear relationship exists between anomalies in OLR and in the frequency of deep convective clouds observed at the surface.

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Ruth S. Lieberman and Conway B. Leovy

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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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Mathew H. Hitchman and Conway B. Leovy

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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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