Search Results
You are looking at 1 - 4 of 4 items for
- Author or Editor: Chiu-Wai Yuen x
- Refine by Access: All Content x
Abstract
A primitive equation planetary boundary layer (PBL) model is constructed and applied to simulate the downwind evolution of coupled dynamic, thermodynamic and cloud properties in the PBL over warmer mean. A multilayered approach is adopted to model the inversion-capped convective PBL filled with shallow cumuli, stratocumulus clouds, or cloud-free air. Vertically varying turbulent and convective cloud fluxes are constructed from recently published parameterization schemes. This model is designed to simulate some influences of cloud, vertical mixing, and horizontal advection on surface winds over much of the world's oceans.
An initial application is the local simulation of trade wind flow using a one-dimensional version of the model. Modeled vertical momentum flux and wind in the cumulus-filled baroclinic PBL agree well with observations, confirming the validity of the momentum parameterization scheme. Applications to two-dimensional simulation of a moderate cold surge over a warm ocean show the enhancement of mixing processes by shallow moist convection. The resulting changes in the PBL mass field show a significant effect on the PBL horizontal pressure gradient forces and wind fields during air mass transformation.
Abstract
A primitive equation planetary boundary layer (PBL) model is constructed and applied to simulate the downwind evolution of coupled dynamic, thermodynamic and cloud properties in the PBL over warmer mean. A multilayered approach is adopted to model the inversion-capped convective PBL filled with shallow cumuli, stratocumulus clouds, or cloud-free air. Vertically varying turbulent and convective cloud fluxes are constructed from recently published parameterization schemes. This model is designed to simulate some influences of cloud, vertical mixing, and horizontal advection on surface winds over much of the world's oceans.
An initial application is the local simulation of trade wind flow using a one-dimensional version of the model. Modeled vertical momentum flux and wind in the cumulus-filled baroclinic PBL agree well with observations, confirming the validity of the momentum parameterization scheme. Applications to two-dimensional simulation of a moderate cold surge over a warm ocean show the enhancement of mixing processes by shallow moist convection. The resulting changes in the PBL mass field show a significant effect on the PBL horizontal pressure gradient forces and wind fields during air mass transformation.
Abstract
Comparative numerical experiments and observational verifications for cold surges over the ocean are conducted using a primitive equation planetary boundary layer (PBL) model. The comparative experiments illustrate the importance of large-scale environmental conditions on air mass transformation. Such factors as baroclinity, static stability, moisture content, upwind inversion strength and height exert strong controls on the downwind evolution of the PBL and clouds. Most results are obtained for clouds in the cumulus mode, but two comparisons are made with simpler stratocumulus simulations. The PBL flow is influenced by the synoptic geostrophic wind distribution and the PBL depth is also sensitive to large-scale vertical velocity. The responses of this advective PBL to stronger wind are found to be different from that of a horizontally homogeneous PBL.
A simulation of an intense cold air outbreak observed during the Air Mass Transformation Experiment, 1975 (AMTEX'75) is performed. Downwind variations of dynamic and thermodynamic PBL properties and cloud distribution and type are well reproduced. The steep sea-surface temperature gradient produces strong PBL baroclinity and a strongly divergent PBL flow. The simulated large cross-isobar angle associated with intense cold air advection and vigorous momentum mixing is in favorable agreement with both observation and theory.
Abstract
Comparative numerical experiments and observational verifications for cold surges over the ocean are conducted using a primitive equation planetary boundary layer (PBL) model. The comparative experiments illustrate the importance of large-scale environmental conditions on air mass transformation. Such factors as baroclinity, static stability, moisture content, upwind inversion strength and height exert strong controls on the downwind evolution of the PBL and clouds. Most results are obtained for clouds in the cumulus mode, but two comparisons are made with simpler stratocumulus simulations. The PBL flow is influenced by the synoptic geostrophic wind distribution and the PBL depth is also sensitive to large-scale vertical velocity. The responses of this advective PBL to stronger wind are found to be different from that of a horizontally homogeneous PBL.
A simulation of an intense cold air outbreak observed during the Air Mass Transformation Experiment, 1975 (AMTEX'75) is performed. Downwind variations of dynamic and thermodynamic PBL properties and cloud distribution and type are well reproduced. The steep sea-surface temperature gradient produces strong PBL baroclinity and a strongly divergent PBL flow. The simulated large cross-isobar angle associated with intense cold air advection and vigorous momentum mixing is in favorable agreement with both observation and theory.
Abstract
Theoretical responses of boundary layer flow for winter cold surges crossing a coastline are determined. Analytical solutions for linearized equations describing the mixed layer adjustments to impulsive heating and surface drag reduction are obtains Dynamical interpretation contrasts with nonturbulent adjustment processes and nonlinear simulations are made.
The response depends upon various physical process length males, two reflecting dynamical adjustments and four reflecting turbulence influences. Upstream wind and inversion strength determine a Froude number Fr which strongly influences the flow, regimes resembling quasi-geostrophic and inertia-gravity behavior (modified by turbulence) are identified. These possess strong two-way coupling between wind and inversion, but primarily a one-way influence between temperature and wind. Surface pressure drops may commence over land (small Fr); their maxima (a few millibars) over the sea are smaller than simple boundary layer warming would imply. Examples show a subsynoptic scale region of coastal subsidence which may extend upwind over land (small Fr) or may be replaced by oscillating reversals over the sea (large Fr). Wind speed adjustments are stronger in large Fr cases. Characteristic wind perturbation scales for land-ocean temperature and drag differences are obtained. Together with the dynamic solutions, they suggest that the change in surface drag may dominate differential heating in altering the flow.
Abstract
Theoretical responses of boundary layer flow for winter cold surges crossing a coastline are determined. Analytical solutions for linearized equations describing the mixed layer adjustments to impulsive heating and surface drag reduction are obtains Dynamical interpretation contrasts with nonturbulent adjustment processes and nonlinear simulations are made.
The response depends upon various physical process length males, two reflecting dynamical adjustments and four reflecting turbulence influences. Upstream wind and inversion strength determine a Froude number Fr which strongly influences the flow, regimes resembling quasi-geostrophic and inertia-gravity behavior (modified by turbulence) are identified. These possess strong two-way coupling between wind and inversion, but primarily a one-way influence between temperature and wind. Surface pressure drops may commence over land (small Fr); their maxima (a few millibars) over the sea are smaller than simple boundary layer warming would imply. Examples show a subsynoptic scale region of coastal subsidence which may extend upwind over land (small Fr) or may be replaced by oscillating reversals over the sea (large Fr). Wind speed adjustments are stronger in large Fr cases. Characteristic wind perturbation scales for land-ocean temperature and drag differences are obtained. Together with the dynamic solutions, they suggest that the change in surface drag may dominate differential heating in altering the flow.
Abstract
Land surface models (LSMs) need to be coupled with atmospheric general circulation models (GCMs) to adequately simulate the exchanges of energy, water, and carbon between the atmosphere and terrestrial surfaces. The heterogeneity of the land surface and its interaction with temporally and spatially varying meteorological conditions result in nonlinear effects on fluxes of energy, water, and carbon, making it challenging to scale these fluxes accurately. The issue of up-scaling remains one of the critical unsolved problems in the parameterization of subgrid-scale fluxes in coupled LSM and GCM models.
A new distributed LSM, the Ecosystem–Atmosphere Simulation Scheme (EASS) was developed and coupled with the atmospheric Global Environmental Multiscale model (GEM) to simulate energy, water, and carbon fluxes over Canada’s landmass through the use of remote sensing and ancillary data. Two approaches (lumped case and distributed case) for handling subgrid heterogeneity were used to evaluate the effect of land-cover heterogeneity on regional flux simulations based on remote sensing. Online runs for a week in August 2003 provided an opportunity to investigate model performance and spatial scaling issues.
Comparisons of simulated results with available tower observations (five sites) across an east–west transect over Canada’s southern forest regions indicate that the model is reasonably successful in capturing both the spatial and temporal variations in carbon and energy fluxes, although there were still some biases in estimates of latent and sensible heat fluxes between the simulations and the tower observations. Moreover, the latent and sensible heat fluxes were found to be better modeled in the coupled EASS–GEM system than in the uncoupled GEM. There are marked spatial variations in simulated fluxes over Canada’s landmass. These patterns of spatial variation closely follow vegetation-cover types as well as leaf area index, both of which are highly correlated with the underlying soil types, soil moisture conditions, and soil carbon pools. The surface fluxes modeled by the two up-scaling approaches (lumped and distributed cases) differ by 5%–15% on average and by up to 15%–25% in highly heterogeneous regions. This suggests that different ways of treating subgrid land surface heterogeneities could lead to noticeable biases in model output.
Abstract
Land surface models (LSMs) need to be coupled with atmospheric general circulation models (GCMs) to adequately simulate the exchanges of energy, water, and carbon between the atmosphere and terrestrial surfaces. The heterogeneity of the land surface and its interaction with temporally and spatially varying meteorological conditions result in nonlinear effects on fluxes of energy, water, and carbon, making it challenging to scale these fluxes accurately. The issue of up-scaling remains one of the critical unsolved problems in the parameterization of subgrid-scale fluxes in coupled LSM and GCM models.
A new distributed LSM, the Ecosystem–Atmosphere Simulation Scheme (EASS) was developed and coupled with the atmospheric Global Environmental Multiscale model (GEM) to simulate energy, water, and carbon fluxes over Canada’s landmass through the use of remote sensing and ancillary data. Two approaches (lumped case and distributed case) for handling subgrid heterogeneity were used to evaluate the effect of land-cover heterogeneity on regional flux simulations based on remote sensing. Online runs for a week in August 2003 provided an opportunity to investigate model performance and spatial scaling issues.
Comparisons of simulated results with available tower observations (five sites) across an east–west transect over Canada’s southern forest regions indicate that the model is reasonably successful in capturing both the spatial and temporal variations in carbon and energy fluxes, although there were still some biases in estimates of latent and sensible heat fluxes between the simulations and the tower observations. Moreover, the latent and sensible heat fluxes were found to be better modeled in the coupled EASS–GEM system than in the uncoupled GEM. There are marked spatial variations in simulated fluxes over Canada’s landmass. These patterns of spatial variation closely follow vegetation-cover types as well as leaf area index, both of which are highly correlated with the underlying soil types, soil moisture conditions, and soil carbon pools. The surface fluxes modeled by the two up-scaling approaches (lumped and distributed cases) differ by 5%–15% on average and by up to 15%–25% in highly heterogeneous regions. This suggests that different ways of treating subgrid land surface heterogeneities could lead to noticeable biases in model output.
