The Experts below are selected from a list of 288 Experts worldwide ranked by ideXlab platform
Kun Yang - One of the best experts on this subject based on the ideXlab platform.
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dynamical impact of parameterized turbulent orographic Form Drag on the simulation of winter precipitation over the western tibetan plateau
Climate Dynamics, 2019Co-Authors: Xu Zhou, Anton Beljaars, Kun Yang, Huidong Li, Bo Huang, Yan WangAbstract:Sub-grid orographic Drag directly acts on wind and impacts the regional water cycle through control of atmospheric water vapor (AWV) transport. The effect of turbulent orographic Form Drag (TOFD) on wind and precipitation is investigated in this study using the WRF model for a winter month over the western Tibetan Plateau (TP), where solid precipitation supplies large amounts of water resources. The diurnal cycle of wind components and atmospheric circulation simulated with TOFD are consistent with observations and ERA-Interim data, whereas stronger westerlies exist in the simulation without the TOFD scheme. The latter results in more zonal AWV transport from the west and more precipitation over the western TP and surroundings. The implementation of the TOFD scheme leads to reduced biases, when evaluated with two observation-based precipitation products. It is therefore concluded that this scheme has a clear dynamical control on the regional atmospheric water recharge and thus the parameterization of the small-scale orographic Drag in the model helps to improve the prediction of wintertime precipitation in the western TP region.
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implementation of a turbulent orographic Form Drag scheme in wrf and its application to the tibetan plateau
Climate Dynamics, 2018Co-Authors: Xu Zhou, Kun Yang, Yan WangAbstract:Sub-grid-scale orographic variation (smaller than 5 km) exerts turbulent Form Drag on atmospheric flows and significantly retards the wind speed. The Weather Research and Forecasting model (WRF) includes a turbulent orographic Form Drag (TOFD) scheme that adds the Drag to the surface layer. In this study, another TOFD scheme has been incorporated in WRF3.7, which exerts an exponentially decaying Drag from the surface layer to upper layers. To investigate the effect of the new scheme, WRF with the old scheme and with the new one was used to simulate the climate over the complex terrain of the Tibetan Plateau from May to October 2010. The two schemes were evaluated in terms of the direct impact (on wind fields) and the indirect impact (on air temperature and precipitation). The new TOFD scheme alleviates the mean bias in the surface wind components, and clearly reduces the root mean square error (RMSEs) in seasonal mean wind speed (from 1.10 to 0.76 m s−1), when referring to the station observations. Furthermore, the new TOFD scheme also generally improves the simulation of wind profile, as characterized by smaller biases and RMSEs than the old one when referring to radio sounding data. Meanwhile, the simulated precipitation with the new scheme is improved, with reduced mean bias (from 1.34 to 1.12 mm day−1) and RMSEs, which is due to the weakening of water vapor flux at low-level atmosphere with the new scheme when crossing the Himalayan Mountains. However, the simulation of 2-m air temperature is little improved.
Yan Wang - One of the best experts on this subject based on the ideXlab platform.
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dynamical impact of parameterized turbulent orographic Form Drag on the simulation of winter precipitation over the western tibetan plateau
Climate Dynamics, 2019Co-Authors: Xu Zhou, Anton Beljaars, Kun Yang, Huidong Li, Bo Huang, Yan WangAbstract:Sub-grid orographic Drag directly acts on wind and impacts the regional water cycle through control of atmospheric water vapor (AWV) transport. The effect of turbulent orographic Form Drag (TOFD) on wind and precipitation is investigated in this study using the WRF model for a winter month over the western Tibetan Plateau (TP), where solid precipitation supplies large amounts of water resources. The diurnal cycle of wind components and atmospheric circulation simulated with TOFD are consistent with observations and ERA-Interim data, whereas stronger westerlies exist in the simulation without the TOFD scheme. The latter results in more zonal AWV transport from the west and more precipitation over the western TP and surroundings. The implementation of the TOFD scheme leads to reduced biases, when evaluated with two observation-based precipitation products. It is therefore concluded that this scheme has a clear dynamical control on the regional atmospheric water recharge and thus the parameterization of the small-scale orographic Drag in the model helps to improve the prediction of wintertime precipitation in the western TP region.
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implementation of a turbulent orographic Form Drag scheme in wrf and its application to the tibetan plateau
Climate Dynamics, 2018Co-Authors: Xu Zhou, Kun Yang, Yan WangAbstract:Sub-grid-scale orographic variation (smaller than 5 km) exerts turbulent Form Drag on atmospheric flows and significantly retards the wind speed. The Weather Research and Forecasting model (WRF) includes a turbulent orographic Form Drag (TOFD) scheme that adds the Drag to the surface layer. In this study, another TOFD scheme has been incorporated in WRF3.7, which exerts an exponentially decaying Drag from the surface layer to upper layers. To investigate the effect of the new scheme, WRF with the old scheme and with the new one was used to simulate the climate over the complex terrain of the Tibetan Plateau from May to October 2010. The two schemes were evaluated in terms of the direct impact (on wind fields) and the indirect impact (on air temperature and precipitation). The new TOFD scheme alleviates the mean bias in the surface wind components, and clearly reduces the root mean square error (RMSEs) in seasonal mean wind speed (from 1.10 to 0.76 m s−1), when referring to the station observations. Furthermore, the new TOFD scheme also generally improves the simulation of wind profile, as characterized by smaller biases and RMSEs than the old one when referring to radio sounding data. Meanwhile, the simulated precipitation with the new scheme is improved, with reduced mean bias (from 1.34 to 1.12 mm day−1) and RMSEs, which is due to the weakening of water vapor flux at low-level atmosphere with the new scheme when crossing the Himalayan Mountains. However, the simulation of 2-m air temperature is little improved.
Xu Zhou - One of the best experts on this subject based on the ideXlab platform.
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dynamical impact of parameterized turbulent orographic Form Drag on the simulation of winter precipitation over the western tibetan plateau
Climate Dynamics, 2019Co-Authors: Xu Zhou, Anton Beljaars, Kun Yang, Huidong Li, Bo Huang, Yan WangAbstract:Sub-grid orographic Drag directly acts on wind and impacts the regional water cycle through control of atmospheric water vapor (AWV) transport. The effect of turbulent orographic Form Drag (TOFD) on wind and precipitation is investigated in this study using the WRF model for a winter month over the western Tibetan Plateau (TP), where solid precipitation supplies large amounts of water resources. The diurnal cycle of wind components and atmospheric circulation simulated with TOFD are consistent with observations and ERA-Interim data, whereas stronger westerlies exist in the simulation without the TOFD scheme. The latter results in more zonal AWV transport from the west and more precipitation over the western TP and surroundings. The implementation of the TOFD scheme leads to reduced biases, when evaluated with two observation-based precipitation products. It is therefore concluded that this scheme has a clear dynamical control on the regional atmospheric water recharge and thus the parameterization of the small-scale orographic Drag in the model helps to improve the prediction of wintertime precipitation in the western TP region.
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implementation of a turbulent orographic Form Drag scheme in wrf and its application to the tibetan plateau
Climate Dynamics, 2018Co-Authors: Xu Zhou, Kun Yang, Yan WangAbstract:Sub-grid-scale orographic variation (smaller than 5 km) exerts turbulent Form Drag on atmospheric flows and significantly retards the wind speed. The Weather Research and Forecasting model (WRF) includes a turbulent orographic Form Drag (TOFD) scheme that adds the Drag to the surface layer. In this study, another TOFD scheme has been incorporated in WRF3.7, which exerts an exponentially decaying Drag from the surface layer to upper layers. To investigate the effect of the new scheme, WRF with the old scheme and with the new one was used to simulate the climate over the complex terrain of the Tibetan Plateau from May to October 2010. The two schemes were evaluated in terms of the direct impact (on wind fields) and the indirect impact (on air temperature and precipitation). The new TOFD scheme alleviates the mean bias in the surface wind components, and clearly reduces the root mean square error (RMSEs) in seasonal mean wind speed (from 1.10 to 0.76 m s−1), when referring to the station observations. Furthermore, the new TOFD scheme also generally improves the simulation of wind profile, as characterized by smaller biases and RMSEs than the old one when referring to radio sounding data. Meanwhile, the simulated precipitation with the new scheme is improved, with reduced mean bias (from 1.34 to 1.12 mm day−1) and RMSEs, which is due to the weakening of water vapor flux at low-level atmosphere with the new scheme when crossing the Himalayan Mountains. However, the simulation of 2-m air temperature is little improved.
Edgar L Andreas - One of the best experts on this subject based on the ideXlab platform.
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a parametrization based on sea ice morphology of the neutral atmospheric Drag coefficients for weather prediction and climate models
Journal of Geophysical Research, 2012Co-Authors: Christof Lüpkes, Vladimir M. Gryanik, Jorg Hartmann, Edgar L AndreasAbstract:[1] A hierarchy of parametrizations of the neutral 10 m Drag coefficients over polar sea ice with different morphology regimes is derived on the basis of a partitioning concept that splits the total surface Drag into contributions of skin Drag and Form Drag. The new derivation, which provides Drag coefficients as a function of sea ice concentration and characteristic length scales of roughness elements, needs fewer assumptions than previous similar approaches. It is shown that Form Drag variability can explain the variability of surface Drag in the marginal sea ice zone (MIZ) and in the summertime inner Arctic regions. In the MIZ, Form Drag is generated by floe edges; in the inner Arctic, it is generated by edges at melt ponds and leads due to the elevation of the ice surface relative to the open water surface. It is shown that an earlier fit of observed neutral Drag coefficients is obtained as a special case within the new concept when specific simplifications are made which concern the floe and melt pond geometry. Due to the different surface morphologies in the MIZ and summertime Arctic, different functional dependencies of the Drag coefficients on the sea ice concentration result. These differences cause only minor differences between the MIZ and summertime Drag coefficients in average conditions, but they might be locally important for atmospheric momentum transport to sea ice. The new parametrization Formulae can be used for present conditions but also for future climate scenarios with changing sea ice conditions.
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a physically based model of the Form Drag associated with sastrugi
This Digital Resource was created in Microsoft Word and Adobe Acrobat, 1995Co-Authors: Edgar L AndreasAbstract:Abstract : On Ice Station Weddell, some characteristics of the neutral-stability-air-iceDrag coefficient at a reference height of 10 m (C sub DNl0) were observed that had not been documented before. The main finding was that wind driven snow continually alters the sea ice surface; the resulting snowdrifts determine how large C sub DNl0 is. In particular, this report describes three observations and attempts to explain them: (1) C sub DNl0 is near 0.0015 when the wind is well aligned with the drifted snow; (2) C sub DN10 is near 0.0025 when the wind makes a large angle with the dominant orientation of the snowdrifts; (3) C sub DNl0 can increase by 20% if, after being well aligned with the drift patterns, the mean wind direction shifts by as little as 20 deg. To investigate this behavior of C sub DN10, this report adapts a model developed by Raupach that partitions the total surface stress into contributions from Form Drag and skin friction. With reasonable choices for free model parameters and with little fine-tuning, this physically based model can reproduce the three main observations. In other words, the model seems to include the basic physics of air-ice momentum exchange. This modeling implies that 10- cm-high sastrugi-like roughness elements, rather than pressure ridges, sustain most of the Form Drag over compact sea ice in the western Weddell Sea. Lastly, the report speculates on what the observations and this model say about how to parameterize C sub DN10 over snow-covered sea ice. (MM)
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air ice Drag coefficients in the western weddell sea 2 a model based on Form Drag and drifting snow
Journal of Geophysical Research, 1995Co-Authors: Edgar L AndreasAbstract:In part 1 (Andreas and Claffey, this issue) we observed some characteristics of the neutral stability air-ice Drag coefficient at a reference height of 10 m (CDN10) that had not been documented before. Our main conclusion was that wind-driven snow continually alters the sea ice surface; the resulting snowdrifts determine how large CDN10 is. In particular, part 1 reported three observations that I would like to explain. (1) CDN10 is near 1.5×10−3 when the wind is well aligned with the drifted snow. (2) CDN10 is near 2.5×10−3 when the wind makes a large angle with the dominant orientation of the snowdrifts. (3) CDN10 can increase by 20% if, after being well aligned with the drift patterns, the mean wind direction shifts by as little as 20°. To investigate this behavior of CDN10 here I adapt a model developed by Raupach (1992) that partitions the total surface stress into contributions from Form Drag and skin friction. An essential part of this development was extending Raupach's model to the more complex geometry of sastrugi-like roughness elements. Assuming that 10-cm high sastrugi cover 15% of the surface, this physically based model reproduces the three main observations listed above. Thus the model seems to include the basic physics of air-ice momentum exchange. The main conclusion from this modeling is that 10-cm, sastrugilike snowdrifts, rather than pressure ridges, sustain most of the Form Drag over compact sea ice in the western Weddell Sea. Secondly, the modeling suggests that skin friction accounts for about 60% of the surface stress when the wind is well aligned with the sastrugi; but when the wind is not well aligned, Form Drag accounts for about 80% of the stress. The sastrugi are thus quite effective in streamlining the surface.
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Air‐ice Drag coefficients in the western Weddell Sea: 2. A model based on Form Drag and drifting snow
Journal of Geophysical Research, 1995Co-Authors: Edgar L AndreasAbstract:In part 1 (Andreas and Claffey, this issue) we observed some characteristics of the neutral stability air-ice Drag coefficient at a reference height of 10 m (CDN10) that had not been documented before. Our main conclusion was that wind-driven snow continually alters the sea ice surface; the resulting snowdrifts determine how large CDN10 is. In particular, part 1 reported three observations that I would like to explain. (1) CDN10 is near 1.5×10−3 when the wind is well aligned with the drifted snow. (2) CDN10 is near 2.5×10−3 when the wind makes a large angle with the dominant orientation of the snowdrifts. (3) CDN10 can increase by 20% if, after being well aligned with the drift patterns, the mean wind direction shifts by as little as 20°. To investigate this behavior of CDN10 here I adapt a model developed by Raupach (1992) that partitions the total surface stress into contributions from Form Drag and skin friction. An essential part of this development was extending Raupach's model to the more complex geometry of sastrugi-like roughness elements. Assuming that 10-cm high sastrugi cover 15% of the surface, this physically based model reproduces the three main observations listed above. Thus the model seems to include the basic physics of air-ice momentum exchange. The main conclusion from this modeling is that 10-cm, sastrugilike snowdrifts, rather than pressure ridges, sustain most of the Form Drag over compact sea ice in the western Weddell Sea. Secondly, the modeling suggests that skin friction accounts for about 60% of the surface stress when the wind is well aligned with the sastrugi; but when the wind is not well aligned, Form Drag accounts for about 80% of the stress. The sastrugi are thus quite effective in streamlining the surface.
Parker Maccready - One of the best experts on this subject based on the ideXlab platform.
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the dynamics of pressure and Form Drag on a sloping headland internal waves versus eddies
Journal of Geophysical Research, 2014Co-Authors: Sally J Warner, Parker MaccreadyAbstract:Topographically generated eddies and internal waves have traditionally been studied separately even though bathymetry that creates both phenomena is abundant in coastal regions. Here a numerical model is used to understand the dynamics of eddy and wave generation as tidal currents flow past Three Tree Point, a 1 km long, 200 m deep, sloping headland in Puget Sound, WA. Bottom pressure anomalies due to vertical perturbations of the sea surface and isopycnals are used to calculate Form Drag in different regions of the topography to assess the relative importance of eddies versus internal waves. In regions where internal waves dominate, sea surface and isopycnal perturbations tend to work together to create Drag, whereas in regions dominated by eddies, sea surface, and isopycnal perturbations tend to counteract each other. Both phenomena are found to produce similar amounts of Form Drag even though the bottom pressure anomalies from the eddy have much larger magnitudes than those created by the internal waves. Topography like Three Tree Point is common in high latitude, coastal regions, and therefore the findings here have implications for understanding how coastal topography removes energy from tidal currents.
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measurement of tidal Form Drag using seafloor pressure sensors
Journal of Physical Oceanography, 2013Co-Authors: Sally J Warner, Parker Maccready, James N Moum, Jonathan D NashAbstract:AbstractAs currents flow over rough topography, the pressure difference between the up- and downstream sides results in Form Drag—a force that opposes the flow. Measuring Form Drag is valuable because it can be used to estimate the loss of energy from currents as they interact with topography. An array of bottom pressure sensors was used to measure the tidal Form Drag on a sloping ridge in 200 m of water that Forms a 1-km headland at the surface in Puget Sound, Washington. The Form Drag per unit length of the ridge reached 1 × 104 N m−1 during peak flood tides. The tidally averaged power removed from the tidal currents by Form Drag was 0.2 W m−2, which is 30 times larger than power losses to friction. Form Drag is best parameterized by a linear wave Drag law as opposed to a bluff body Drag law because the flow is stratified and both internal waves and eddies are generated on the sloping topography. Maximum turbulent kinetic energy dissipation rates of 5 × 10−5 W kg−1 were measured with a microstructure pr...
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dissecting the pressure field in tidal flow past a headland when is Form Drag real
Journal of Physical Oceanography, 2009Co-Authors: Sally J Warner, Parker MaccreadyAbstract:In the few previous measurements of topographic Form Drag in the ocean, Drag that is much larger than a typical bluff body Drag estimate has been consistently found. In this work, theory combined with a numerical model of tidal flow around a headland in a channel gives insight into the mechanisms that create Form Drag in oscillating flow situations. The total Form Drag is divided into two parts: the inertial Drag, which is derived from a local potential flow solution, and the separation Drag, which accounts for flow features such as eddies. The inertial Drag can have a large magnitude, yet it cannot do work on the flow because its phase is in quadrature with the velocity. The separation Drag has a magnitude that is nearly equal to the bluff body Drag and accounts for all of the energy removed from the flow by the topography. In addition, the dependence of the Form Drag on the tidal excursion distance and the aspect ratio of the headlands were determined with a series of numerical experiments. This theory explains why Form Drag can be so large in the ocean, and it provides a method for separating the pressure field into the parts that can and cannot extract energy from the flow.
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Form Drag due to Flow Separation at a Headland
Journal of Physical Oceanography, 2006Co-Authors: Ryan M. Mccabe, Parker Maccready, Geno PawlakAbstract:Observational and model estimates of the Form Drag on Three Tree Point, a headland located in a tidal channel of Puget Sound, Washington, are presented. Subsurface, Three Tree Point is a sloping ridge. Tidal flow over this ridge gives rise to internal lee waves that lead to wave Drag and enhanced mixing. At the same time, horizontal flow separation produces a headland eddy that distorts the surface height field in the lee of the point. Two observational methods for estimating the portion of the Form Drag associated with deFormation of the surface height field, referred to here as the “external” Form Drag, are also introduced. Drogued drifters and ship-mounted acoustic current profiles from different days are used to indirectly map the flood-tide surface height field. Data are derived from a depth shallow enough that baroclinic pressure gradient forcing may be neglected, and yet deep enough that wind stress may also be ignored. This leaves an approximate balance between the acceleration and surface height pressure gradient, permitting, in this case, two independent estimates of the surface height (to within a constant). These fields are used to calculate the external Form Drag at the headland. Drag estimates from both observational datasets agree well. External Form Drag decreases offshore of the headland as expected, and is highly dependent on tidal phase, with maximum Drag leading peak flood currents by 1–2 h at this location. Form Drag is much larger than model estimates of the frictional Drag, implying that it is the dominant mechanism extracting energy from the barotropic tide. A kinematic argument is also presented to show why the external Form Drag should increase in importance relative to the frictional Drag as the topographic slope and tidal excursion increase.
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Form Drag due to flow separation at a headland
Journal of Physical Oceanography, 2006Co-Authors: Ryan M. Mccabe, Parker Maccready, Geno PawlakAbstract:Abstract Observational and model estimates of the Form Drag on Three Tree Point, a headland located in a tidal channel of Puget Sound, Washington, are presented. Subsurface, Three Tree Point is a sloping ridge. Tidal flow over this ridge gives rise to internal lee waves that lead to wave Drag and enhanced mixing. At the same time, horizontal flow separation produces a headland eddy that distorts the surface height field in the lee of the point. Two observational methods for estimating the portion of the Form Drag associated with deFormation of the surface height field, referred to here as the “external” Form Drag, are also introduced. Drogued drifters and ship-mounted acoustic current profiles from different days are used to indirectly map the flood-tide surface height field. Data are derived from a depth shallow enough that baroclinic pressure gradient forcing may be neglected, and yet deep enough that wind stress may also be ignored. This leaves an approximate balance between the acceleration and surfac...