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X. H. Wang

Abstract

Sediment transport and bottom boundary layer (BBL) in an idealized estuary with a muddy bed were studied by numerical simulations. The focus was placed on description and prediction of the dynamics of nepheloid layer (a fluid–mud layer) developed in the estuary because of the coupling effect of the seawater and resuspended sediment concentration. The Princeton Ocean Model was coupled to a sediment transport model to conduct the numerical experiments. A semidiurnal tide with a spring–neap cycle was used to force the model at the estuary entrance. A stability function was introduced to the bottom drag coefficient C d for a slip bottom boundary condition in order to consider the effects of sediment-induced stratification. When the seawater density is not affected by the resuspended sediments, spring tides resuspend sediments to the sea surface near the estuary entrance where the bottom stress is larger than the critical stress value. The sediment distribution in the BBL near the entrance is dominantly affected by the vertical eddy diffusion, and the time series of the sediment concentration presents two high value peaks within a tidal cycle. Above the BBL the sediment concentration is primarily controlled by the horizontal tidal advection; thus a semidiurnal oscillation in sediment concentration is predicted. When the seawater density and the sediment concentration are coupled, the sediments resuspended by the spring tides are only distributed in the bottom layer with a thickness of a few meters. A lutocline is developed above a nepheloid layer where the vertical sediment concentration gradient is of maximum. The settlement of the nepheloid layer gives rise to the resuspension events that are characterized with an abnormally high value in sediment concentration within a thin wall layer that is overlaid by a thicker layer with much smaller concentration. This two-layer sediment distribution structure was observed on the continental shelf off the mouth of the Amazon River. These resuspension events may be referred to as “resuspension hysteresis” with respect to the tidal forcing frequency. The frequency of the resuspension hysteresis is controlled by both the sediment settling velocity and the turbulence intensity, and is lower than that of the tidal forcing. A hyperpycnal plume is also established near the entrance, generating a cross-estuary tidal mean flow on the order of 1 cm s−1 there. Variability in C d between the spring and neap tides is predicted because of the sediment-induced stratification, and the prediction agrees, in general term, with observations in south San Francisco Bay.

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Qing Wang and D. H. Lenschow

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Isolated cumuli penetrating through marine stratocumulus clouds were documented during the Atlantic Stratocumulus Transition Experiment. This paper aims at understanding the role of the penetrating cumulus in regulating stratocumulus and boundary-layer structure through analysis of data from the NCAR Electra aircraft. When penetrating cumulus clouds are present, the boundary layer is generally decoupled from the near-surface air except in the cumulus region. Therefore, air in the cumulus region includes air entrained at the cloud top, as well as air modified by surface processes. In the stratocumulus region, however, entrained inversion air and moist surface air are confined to separate layers. As a result, large horizontal variations are found in scalars, such as ozone and water vapor. Turbulence statistics and conditional sampling of entrainment events in the cumulus and stratocumulus regions indicate that stronger entrainment may occur at the cumulus top compared to the surrounding stratocumulus. This analysis is, however, complicated by insufficient sampling of cloud-top jump conditions in both regions.

Convergent flow in the lower boundary layer and compensating diverging flow in the upper boundary layer were identified along the flight trark. This flow field, together with the vertical coupling of surface air with the cloud layer in the cumulus region, helps to transport moisture upwards from the sea surface and disperse it to the surrounding stratocumulus sheet, thus helping to maintain the stratocumulus cover.

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Chunzai Wang and Robert H. Weisberg

Abstract

The stability, periodicity, and horizontal structure of equatorial modes in a coupled ocean-atmosphere model, simplified by the assumption that zonal wind stress anomalies are proportional to sea surface temperature anomalies lagged by a zonal phase difference, are examined analytically in an unbounded basin. The gravest coupled Rossby and Kelvin modes coexist with additional westward and eastward slow modes whose phase speeds are smaller than the former. Two of these four modes, one propagating westward and the other eastward, are destabilized in each case depending upon the model parameters. For some particular parameter choices. coupled Rossby and Kelvin modes merge with westward and eastward slow modes, respectively. For other parameters, however. they separate and remain distinct from the slow modes. For all of these modes the primary modifications by coupling relative to uncoupled oceanic equatorial waves are a decrease in phase speed and an increase in meridional scale.

Among the model parameter effects, those of the zonal phase lag between the wind stress and SST anomalies and the coefficients of thermal and mechanical damping are the most interesting. Positive and negative phase lags represent the wind stress anomalies located to the west and east of the SST anomalies, respectively. The frequency of all modes is symmetric about zero phase lag, whereas the growth rate is antisymmetric about zero phase lag relative to the uncoupled damping rate. Wind stress anomalies to the west of SST anomalies favor slow mode growth and coupled Rossby and Kelvin mode decay. Dissipation for the slow modes and the coupled Rossby and Kelvin modes is controlled differently. For the slow modes the dissipation is mainly thermal, whereas for coupled Rossby and Kelvin modes the dissipation is mainly mechanical.

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Chunzai Wang and Robert H. Weisberg

Abstract

A linear perturbation, coupled ocean–atmosphere model is revisited for further insights into the El Niño–Southern Oscillation phenomenon. The model oscillates as a slow, eastward propagating mode interpreted as a divergence mode, whose energetics are controlled by the ocean. Growth requires that the work performed by the wind stress minus the work required to effect the ocean divergence exceeds the loss terms. The intrinsic scale of the atmosphere relative to the basin width is important. For sustainable oscillations, the ocean basin must be large enough so that oppositely directed divergence can develop on opposite sides of the basin. The global aspect of the atmospheric pressure field suggests that continental heating may provide either a direct source affecting adjacent oceans, or a connection between oceans. The important model parameters are the coupling and warming coefficients and the ocean Kelvin wave speed. The importance of the Kelvin wave speed derives from its specification of the background buoyancy state for the ocean. Upon further simplification, an analytical solution gives similar parameter dependence as found numerically and shows that growth requires both large zonal wavelength and a zonal phase lag between the anomalies of wind stress and SST.

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Victor Wang and John H. Shaw

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J. H. LaCasce and J. Wang

Abstract

A previously published method by Wang et al. for predicting subsurface velocities and density from sea surface buoyancy and surface height is extended by incorporating analytical solutions to make the vertical projection. One solution employs exponential stratification and the second has a weakly stratified surface layer, approximating a mixed layer. The results are evaluated using fields from a numerical simulation of the North Atlantic. The simple exponential solution yields realistic subsurface density and vorticity fields to nearly 1000 m in depth. Including a mixed layer improves the response in the mixed layer itself and at high latitudes where the mixed layer is deeper. It is in the mixed layer that the surface quasigeostrophic approximation is most applicable. Below that the first baroclinic mode dominates, and that mode is well approximated by the analytical solution with exponential stratification.

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Shuguang Wang and Adam H. Sobel

Abstract

The Madden Julian Oscillation (MJO) and the Boreal Summer Intraseasonal Oscillation (BSISO) are fundamental modes of variability in the tropical atmosphere on the intraseasonal time scale. A linear model, using a moist shallow water equation set on an equatorial beta plane, is developed to provide a unified treatment of the two modes and to understand their growth and propagation over the Indian Ocean. Moisture is assumed to increase linearly with longitude and to decrease quadratically with latitude. Solutions are obtained through linear stability analysis, considering the gravest (n = 1) meridional mode with nonzero meridional velocity.

Anomalies in zonal moisture advection and surface fluxes are both proportional to those in zonal wind, but of opposite sign. With observation-based estimates for both effects, the zonal advection dominates, and drives the planetary-scale instability. With a sufficiently small meridional moisture gradient, the horizontal structure exhibits oscillations with latitude and a northwest-southeast horizontal tilt in the northern hemisphere, qualitatively resembling the observed BSISO. As the meridional moisture gradient increases, the horizontal tilt decreases and the spatial pattern transforms toward the “swallowtail” structure associated with the MJO, with cyclonic gyres in both hemispheres straddling the equatorial precipitation maximum. These results suggest that the magnitude of the meridional moisture gradient shapes the horizontal structures, leading to the transformation from the BSISO-like tilted horizontal structure to the MJO-like neutral wave structure as the meridional moisture gradient changes with the seasons. The existence and behavior of these intraseasonal modes can be understood as a consequence of phase speed matching between the equatorial mode with zero meridional velocity (analogous to the dry Kelvin wave) and a local off-equatorial component that is characterized by considering an otherwise similar system on an f-plane.

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Shuguang Wang and Adam H. Sobel

Abstract

A set of idealized cloud-permitting simulations is performed to explore the influence of small islands on precipitating convection as a function of large-scale wind speed. The islands are situated in a long narrow ocean domain that is in radiative–convective equilibrium (RCE) as a whole, constraining the domain-average precipitation. The island occupies a small part of the domain, so that significant precipitation variations over the island can occur, compensated by smaller variations over the larger surrounding oceanic area.

While the prevailing wind speeds vary over flat islands, three distinct flow regimes occur. Rainfall is greatly enhanced, and a local symmetric circulation is formed in the time mean around the island, when the prevailing large-scale wind speed is small. The rainfall enhancement over the island is much reduced when the wind speed is increased to a moderate value. This difference is characterized by a change in the mechanisms by which convection is forced. A thermally forced sea breeze due to surface heating dominates when the large-scale wind is weak. Mechanically forced convection, on the other hand, is favored when the large-scale wind is moderately strong, and horizontal advection of temperature reduces the land–sea thermal contrast that drives the sea breeze. Further increases of the prevailing wind speed lead to strong asymmetry between the windward and leeward sides of the island, owing to gravity waves that result from the land–sea contrast in surface roughness as well as upward deflection of the horizontal flow by elevated diurnal heating. Small-amplitude topography (up to 800-m elevation is considered) has a quantitative impact but does not qualitatively alter the flow regimes or their dependence on wind speed.

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Chunzai Wang and Robert H. Weisberg

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The evolution of the 1997–98 El Niño is described using NCEP SST and OLR data, NCEP–NCAR reanalysis sea level pressure (SLP) fields, and The Florida State University surface wind data. From November 1996 to January 1997, the eastern Pacific is characterized by equatorial cold SST and high SLP anomalies, while the western Pacific is marked by off-equatorial warm SST anomalies and off-equatorial anomalous cyclones. Corresponding to this distribution are high OLR anomalies in the equatorial central Pacific and low OLR anomalies in the off-equatorial far western Pacific. The off-equatorial anomalous cyclones in the western Pacific are associated with a switch in the equatorial wind anomalies over the western Pacific from easterly to westerly. These equatorial westerly anomalies then appear to initiate early SST warmings around the date line in January/February 1997 and around the far eastern Pacific in March 1997. Subsequently, both the westerly wind and warm SST anomalies, along with the low OLR anomalies, grow and progress eastward. The eastward propagating warm SST anomalies merge with the slower westward spreading warm SST anomalies from the far eastern Pacific to form large-scale warming in the equatorial eastern and central Pacific. The anomaly patterns in the eastern and central Pacific continue to develop, reaching their peak values around December 1997. In the western Pacific, the off-equatorial SST anomalies reverse sign from warm to cold. Correspondingly, the off-equatorial SLP anomalies in the western Pacific also switch sign from low to high. These off-equatorial high SLP anomalies initiate equatorial easterly wind anomalies over the far western Pacific. Like the equatorial westerly wind anomalies that initiate the early warming, the equatorial easterly wind anomalies over the far western Pacific appear to have a cooling effect in the east and hence help facilitate the 1997–98 El Niño decay. This paper also compares the 1997–98 El Niño with previous warm events and discusses different ENSO mechanisms relevant to the 1997–98 El Niño.

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Xuguang Wang and Craig H. Bishop

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The ensemble transform Kalman filter (ETKF) ensemble forecast scheme is introduced and compared with both a simple and a masked breeding scheme. Instead of directly multiplying each forecast perturbation with a constant or regional rescaling factor as in the simple form of breeding and the masked breeding schemes, the ETKF transforms forecast perturbations into analysis perturbations by multiplying by a transformation matrix. This matrix is chosen to ensure that the ensemble-based analysis error covariance matrix would be equal to the true analysis error covariance if the covariance matrix of the raw forecast perturbations were equal to the true forecast error covariance matrix and the data assimilation scheme were optimal. For small ensembles (∼100), the computational expense of the ETKF ensemble generation is only slightly greater than that of the masked breeding scheme.

Version 3 of the Community Climate Model (CCM3) developed at National Center for Atmospheric Research (NCAR) is used to test and compare these ensemble generation schemes. The NCEP–NCAR reanalysis data for the boreal summer in 2000 are used for the initialization of the control forecast and the verifications of the ensemble forecasts. The ETKF and masked breeding ensemble variances at the analysis time show reasonable correspondences between variance and observational density. Examination of eigenvalue spectra of ensemble covariance matrices demonstrates that while the ETKF maintains comparable amounts of variance in all orthogonal and uncorrelated directions spanning its ensemble perturbation subspace, both breeding techniques maintain variance in few directions. The growth of the linear combination of ensemble perturbations that maximizes energy growth is computed for each of the ensemble subspaces. The ETKF maximal amplification is found to significantly exceed that of the breeding techniques. The ETKF ensemble mean has lower root-mean-square errors than the mean of the breeding ensemble. New methods to measure the precision of the ensemble-estimated forecast error variance are presented. All of the methods indicate that the ETKF estimates of forecast error variance are considerably more accurate than those of the breeding techniques.

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