Search Results

You are looking at 1 - 10 of 37 items for

  • Author or Editor: F. Wu x
  • Refine by Access: All Content x
Clear All Modify Search
Xingren Wu, Ian Simmonds, and W. F. Budd

Abstract

A dynamic-thermodynamic sea ice model is developed and coupled with the Melbourne University general circulation model to simulate the seasonal cycle of the Antarctic sea ice distribution. The model is efficient, rapid to compute, and useful for a range of climate studies. The thermodynamic part of the sea ice model is similar to that developed by Parkinson and Washington, the dynamics contain a simplified ice rheology that resists compression. The thermodynamics is based on energy conservation at the top surface of the ice/snow, the ice/water interface, and the open water area to determine the ice formation, accretion, and ablation. A lead parameterization is introduced with an effective partitioning scheme for freezing between and under the ice floes. The dynamic calculation determines the motion of ice, which is forced with the atmospheric wind, taking account of ice resistance and rafting. The simulated sea ice distribution compares reasonably well with observations. The seasonal cycle of ice extent is well simulated in phase as well as in magnitude. Simulated sea ice thickness and concentration are also in good agreement with observations over most regions and serve to indicate the importance of advection and ocean drift in the determination of the sea ice distribution.

Full access
Wan-Shu Wu, R. James Purser, and David F. Parrish

Abstract

In this study, a global three-dimensional variational analysis system is formulated in model grid space. This formulation allows greater flexibility (e.g., inhomogeneity and anisotropy) for background error statistics. A simpler formulation, inhomogeneous only in the latitude direction, was chosen for these initial tests. The background error statistics are defined as functions of the latitudinal grid and are estimated with the National Meteorological Center (NMC) method. The horizontal scales of the variables are obtained through the variances of the variables and of their Laplacian. The vertical scales are estimated through the statistics of the vertical correlation of each variable and are applied locally using recursive filters. For the multivariate correlation between wind and mass fields, a statistical linear relationship between the streamfunction and the balanced part of temperature and surface pressure is assumed. A localized correlation between the velocity potential and the streamfunction is also used to account for the positive correlation between the vorticity and divergence in the planetary boundary layer.

Horizontally, the global domain is divided into three pieces so that efficient spatial recursive filters can be used to spread out the information from the observation locations. This analysis system is tested against the operational Spectral Statistical-Interpolation analysis system used at the National Centers for Environmental Prediction. The results indicate that 3DVAR in physical space is as effective as 3DVAR in spectral space in the extratropics and yields superior results in the Tropics as a result of the latitude dependence of the background error statistics.

Full access
Zhao Jing, Ping Chang, Steven F. DiMarco, and Lixin Wu

Abstract

Moored ADCP data collected in the northern Gulf of Mexico are analyzed to examine near-inertial internal waves and their contribution to subthermocline diapycnal mixing based on a finescale parameterization of deep ocean mixing. The focus of the study is on the impact of near-inertial internal waves generated by an extreme weather event—that is, Hurricane Katrina—and by month-to-month variation in weather patterns on the diapycnal mixing. The inferred subthermocline diapycnal mixing exhibits pronounced elevation in the wake of Katrina. Both the increased near-inertial (0.8–1.8f, where f is the Coriolis frequency) and superinertial (>1.8f) shear variances contribute to the elevated diapycnal mixing, but the former plays a more dominant role. The intense wind work on near-inertial motions by the hurricane is largely responsible for the energetic near-inertial shear variance. Energy transfer from near-inertial to superinertial internal waves, however, appears to play an important role in elevating the superinertial shear variance. The inferred subthermocline diapycnal mixing in the region also exhibits significant month-to-month variation with the estimated diffusivity in January 2006 about 3 times the values in November and December 2005. The subseasonal change in the diapycnal mixing mainly results from the subseasonal variation of the near-inertial wind work that causes intensification of the near-inertial shear in January 2006.

Full access
Zhao Jing, Ping Chang, S. F. DiMarco, and Lixin Wu

Abstract

A long-term mooring array deployed in the northern Gulf of Mexico is used to analyze energy exchange between internal waves and low-frequency flows. In the subthermocline (245–450 m), there is a noticeable net energy transfer from low-frequency flows, defined as having a period longer than six inertial periods, to internal waves. The magnitude of energy transfer rate depends on the Okubo–Weiss parameter of low-frequency flows. A permanent energy exchange occurs only when the Okubo–Weiss parameter is positive. The near-inertial internal waves (NIWs) make major contribution to the energy exchange owing to their energetic wave stress and relatively stronger interaction with low-frequency flows compared to the high-frequency internal waves. There is some evidence that the permanent energy exchange between low-frequency flows and NIWs is attributed to the partial realization of the wave capture mechanism. In the periods favoring the occurrence of the wave capture mechanism, the horizontal propagation direction of NIWs becomes anisotropic and exhibits evident tendency toward that predicted from the wave capture mechanism, leading to pronounced energy transfer from low-frequency flows to NIWs.

Open access
Gerald F. Herman, Man-Li C. Wu, and Winthrop T. Johnson

Abstract

The effect of global cloudiness on the solar and infrared components of the earth's radiation balance is studied in general circulation model experiments. A wintertime simulation is conducted in which the cloud radiative transfer calculations use realistic cloud optical properties and are fully interactive with model-generated cloudiness. This simulation is compared to others in which the clouds are alternatively non-interactive with respect to the solar or thermal radiation calculations. Other cloud processes (formation, latent heat release, precipitation, vertical mixing) were accurately simulated in these experiments.

We conclude that on a global basis clouds increase the global radiation balance by 40 W m−2 by absorbing longwave radiation, but decrease it by 56 W m−2 by reflecting solar radiation to space. The net cloud effect is therefore a reduction of the radiation balance by 16 W m−2, and is dominated by the cloud albedo effect.

Changes in cloud frequency and distribution and in atmospheric and land temperatures are also reported for the control and for the non-interactive simulations. In general, removal of the clouds’ infrared absorption cools the atmosphere and causes additional cloudiness to occur, while removal of the clouds’ solar radiative properties warms the atmosphere and causes fewer clouds to form. It is suggested that layered clouds and convective clouds over water enter the climate system as positive feedback components, while convective clouds over land enter as negative components.

Full access
Wan-Shu Wu, David F. Parrish, Eric Rogers, and Ying Lin

Abstract

At the National Centers for Environmental Prediction, the global ensemble forecasts from the ensemble Kalman filter scheme in the Global Forecast System are applied in a regional three-dimensional (3D) and a four dimensional (4D) ensemble–variational (EnVar) data assimilation system. The application is a one-way variational method using hybrid static and ensemble error covariances. To enhance impact, three new features have been added to the existing EnVar system in the Gridpoint Statistical Interpolation (GSI). First, the constant coefficients that assign relative weight between the ensemble and static background error are now allowed to vary in the vertical. Second, a new formulation is introduced for the ensemble contribution to the analysis surface pressure. Finally, in order to make use of the information in the ensemble mean that is disregarded in the existing EnVar in GSI, the trajectory correction, a novel approach, is introduced. Relative to the application of a 3D variational data assimilation algorithm, a clear positive impact on 1–3-day forecasts is realized when applying 3DEnVar analyses in the North American Mesoscale Forecast System (NAM). The 3DEnVar DA system was operationally implemented in the NAM Data Assimilation System in August 2014. Application of a 4DEnVar algorithm is shown to further improve forecast accuracy relative to the 3DEnVar. The approach described in this paper effectively combines contributions from both the regional and the global forecast systems to produce the initial conditions for the regional NAM system.

Full access
Yao-Chu Wu, Ming-Jen Yang, and Robert F. Rogers

Abstract

Typhoon Fanapi (2010) made landfall in Hualien in Taiwan on 0100 UTC 19 September 2010 and left Taiwan on 1200 UTC 19 September 2010, producing heavy rainfall and floods. Fanapi’s eyewall was disrupted by the Central Mountain Range (CMR) and reorganized after leaving the CMR. High-resolution simulations (nested down to 1-km horizontal grid size) using the Advanced Research Weather Research and Forecast (WRF) model, one simulation using the full terrain (CTL) and another set of simulations where the terrain on Taiwan was removed, were analyzed. Precipitation areas were classified into different sub-regions by a convective-stratiform separation algorithm to assess the impact of precipitation structure on Fanapi’s eyewall evolution. The percentage of deep convection increased from 9% to 20% when Fanapi underwent an eyewall reorganization process while departing the CMR. In the absence of terrain, moderate convection occupied most of the convective regions during the period when Fanapi moved across Taiwan Island. The low-level total vorticity stretching within the convective, stratiform and weak echo regions in the no-terrain experiment were of similar magnitudes, but the total vorticity stretching within the convective region at low levels was dominant in the CTL experiment. Total vorticity stretching in the region of deep convection increased after eyewall reorganization, and later became stronger than that in the moderate convection region. In the absence of the CMR, total vorticity stretching in moderate convection dominated. The total vorticity stretching within the deep convective region in the CTL experiment played an essential role in the reorganization of Fanapi’s eyewall through a bottom-up process.

Restricted access
Chun-Chieh Wu, Shun-Nan Wu, Ho-Hsuan Wei, and Sergio F. Abarca

Abstract

The purpose of this study is to analyze the role of diabatic heating in tropical cyclone ring structure evolution. A full-physics three-dimensional modeling framework is used to compare the results with two-dimensional modeling approaches and to point to limitations of the barotropic instability theory in predicting the storm vorticity structure configuration. A potential vorticity budget analysis reveals that diabatic heating is a leading-order term and that it is largely offset by potential vorticity advection. Sawyer–Eliassen integrations are used to diagnose the secondary circulation (and corresponding vorticity tendency) forced by prescribed heating. These integrations suggest that diabatic heating forces a secondary circulation (and associated vorticity tendency) that helps maintain the original ring structure in a feedback process. Sensitivity experiments of the Sawyer–Eliassen model reveal that the magnitude of the vorticity tendency is proportional to that of the prescribed heating, indicating that diabatic heating plays a critical role in adjusting and maintaining the eyewall ring.

Full access
Chuanhao Wu, Pat J.-F. Yeh, Haichun Wu, Bill X. Hu, and Guoru Huang

Abstract

Recent studies have extended the applicability of the Budyko framework from the long-term mean to annual or shorter time scales. However, the effects of water storage change ΔS on the overall water balance estimated from the Budyko models (BM) at annual-to-monthly time scales were less investigated, particularly at the continental or global scales, due to the lack of large-scale ΔS data. Here, based on a 25-yr (1984–2008) global gridded terrestrial water budget dataset and by using an analytical error-decomposition framework, we analyzed the effects of ΔS in evapotranspiration (ET) predicted from BM at both grid and basin scales under diverse climates for the annual, wet-seasonal, dry-seasonal, and monthly time scales. Results indicated that the BM underperforms in the short dry (wet) seasons of predominantly humid (dry) basins, with lower accuracy under more humid climates (at annual, dry-seasonal, and monthly scales) and under more arid climates (at wet-seasonal scale). When the effects of ΔS are incorporated into BM, improvements can be found mostly at annual and dry-seasonal scales, but not notable at wet-seasonal and monthly scales. The magnitudes of ΔS are positively correlated with the errors in BM-predicted ET for most global regions at annual and monthly scales, especially under arid climates. Under arid climates, the variability of ET prediction errors is controlled mainly by the ΔS variability at annual and monthly time scales. In contrast, under humid climates the effect of ΔS on ET prediction errors is generally limited, particularly at the wet-seasonal scale due to the more dominant influences of other climatic factors (precipitation and potential ET) and catchment responses (runoff).

Full access
A. Gettelman, W. J. Randel, S. Massie, F. Wu, W. G. Read, and J. M. Russell III

Abstract

The interannual variability of the tropical tropopause region between 14 and 18 km is examined using observations of convection, winds, and tropopause temperatures from reanalyses and water vapor from satellites. This variability is compared to a simulation using the Community Climate Model version 3 (CCM3) general circulation model forced by observed sea surface temperatures. A coherent picture of the effect of the El Niño–Southern Oscillation (ENSO) on the tropopause region is presented in the NCEP–NCAR reanalyses and CCM3. ENSO modifies convection in the Tropics, and the temperature and circulation of the tropical tropopause region, in agreement with idealized models of tropical heating. CCM3 reproduces most details of these changes, but not the zonal mean temperature variations present in the analysis fields, which are not related to ENSO. ENSO also forces significant changes in observed and simulated water vapor fields. In the upper troposphere water vapor is at maximum near convection, while in the tropopause region water vapor is at minimum in the regions of convection and surrounding it. Convection, cirrus clouds, temperatures, and transport are all linked to describe the water vapor distribution and highlight the role of transport in the tropopause region.

Full access