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- Author or Editor: C-Y. J. Kao x
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Abstract
A high-resolution one-dimensional version of a second-order turbulence closure radiative-convective model, developed at Los Alamos National Laboratory, is used to simulate the interactions among turbulence, radiation, and bulk cloud parameters in stratiform clouds observed during the Arctic Stratus Experiment conducted during June 1980 over the Beaufort Sea. The fidelity of the model to the underlying physics is assessed by comparing the modeled evolution of the cloud-capped boundary layer against data reported for two particular days of observations. Over the period encompassed by these observations, the boundary layer evolved from a well-mixed cloud-capped boundary layer overlying a stable cloudy surface layer to a shallower well-mixed boundary layer with a single upper cloud deck and a clear, diminished, stable surface layer. The model was able to reproduce the observed profiles of the liquid water content, cloud-base height, radiative heating rates, and the mean and turbulence variables over the period of observation fairly well. The formation and eventual dissipation of the surface cloud feature over the period of the simulation was found to be caused by the formation of a stable surface layer as the modeled air mass moved over the relatively cold Beaufort Sea region. Condensation occurred as heat in the surface layer was transported downward toward the sea surface. Eventual dissipation of the surface cloud layer resulted from the transport of moisture in the surface layer downward toward the sea surface. The results show that the subsidence was the major influence on the evolution of the cloud-top height but was not a major factor for dissipation of either cloud layer during the simulation.
Abstract
A high-resolution one-dimensional version of a second-order turbulence closure radiative-convective model, developed at Los Alamos National Laboratory, is used to simulate the interactions among turbulence, radiation, and bulk cloud parameters in stratiform clouds observed during the Arctic Stratus Experiment conducted during June 1980 over the Beaufort Sea. The fidelity of the model to the underlying physics is assessed by comparing the modeled evolution of the cloud-capped boundary layer against data reported for two particular days of observations. Over the period encompassed by these observations, the boundary layer evolved from a well-mixed cloud-capped boundary layer overlying a stable cloudy surface layer to a shallower well-mixed boundary layer with a single upper cloud deck and a clear, diminished, stable surface layer. The model was able to reproduce the observed profiles of the liquid water content, cloud-base height, radiative heating rates, and the mean and turbulence variables over the period of observation fairly well. The formation and eventual dissipation of the surface cloud feature over the period of the simulation was found to be caused by the formation of a stable surface layer as the modeled air mass moved over the relatively cold Beaufort Sea region. Condensation occurred as heat in the surface layer was transported downward toward the sea surface. Eventual dissipation of the surface cloud layer resulted from the transport of moisture in the surface layer downward toward the sea surface. The results show that the subsidence was the major influence on the evolution of the cloud-top height but was not a major factor for dissipation of either cloud layer during the simulation.
Abstract
A high-resolution one-dimensional version of a second-order turbulence radiative–convective model, developed at Los Alamos National Laboratory, is used to simulate the diurnal cycle of the marine stratocumulus cloud-capped boundary layer. The fidelity of the model to the underlying physics is assessed by comparing the model simulation to data taken at San Nicolas Island during the intensive field observation (IFO) of the First International Satellite Cloud Climatology Project (ISCCP) Regional Experiment (FIRE), conducted during June and July 1987. The model is able to reproduce the observed diurnal cycle of the liquid water content, cloud-base height, radiative heating or cooling rates, and the mean and turbulence variables fairly well. The mechanisms that cause the diurnal variation and the decoupling of the boundary layer are examined.
The possible role of an imposed diurnal cycle for the subsidence in inducing the cloud-top diurnal cycle observed during the FIRE IFO is also addressed. Three regimes of subsidence influence are identified for the stratocumulus-capped boundary layer. Regimes I and III are characterized by vertical propagation of the inversion height and erratic fluctuation of turbulence in the region of the inversion. Regime II is characterized by a continuum of quasi-equilibrium states that can exist for a range of subsidence values. In this regime, the boundary layer height is fairly insensitive to changes in the subsidence. The boundary layer behavior implied for these regimes is used to explore the effect of a diurnally varying subsidence rate on the diurnal cycle for the cloud-top height.
Abstract
A high-resolution one-dimensional version of a second-order turbulence radiative–convective model, developed at Los Alamos National Laboratory, is used to simulate the diurnal cycle of the marine stratocumulus cloud-capped boundary layer. The fidelity of the model to the underlying physics is assessed by comparing the model simulation to data taken at San Nicolas Island during the intensive field observation (IFO) of the First International Satellite Cloud Climatology Project (ISCCP) Regional Experiment (FIRE), conducted during June and July 1987. The model is able to reproduce the observed diurnal cycle of the liquid water content, cloud-base height, radiative heating or cooling rates, and the mean and turbulence variables fairly well. The mechanisms that cause the diurnal variation and the decoupling of the boundary layer are examined.
The possible role of an imposed diurnal cycle for the subsidence in inducing the cloud-top diurnal cycle observed during the FIRE IFO is also addressed. Three regimes of subsidence influence are identified for the stratocumulus-capped boundary layer. Regimes I and III are characterized by vertical propagation of the inversion height and erratic fluctuation of turbulence in the region of the inversion. Regime II is characterized by a continuum of quasi-equilibrium states that can exist for a range of subsidence values. In this regime, the boundary layer height is fairly insensitive to changes in the subsidence. The boundary layer behavior implied for these regimes is used to explore the effect of a diurnally varying subsidence rate on the diurnal cycle for the cloud-top height.
Abstract
A three-dimensional ensemble-mean atmospheric model, with simplified second-moment turbulence closure equations and a statistical treatment for the condensation process, is used to simulate a fair weather marine boundary layer observed during the GATE [GARP (Global Research Program) Atlantic Tropical Experiment]. The data deduced from airborne and surface-based instrumentation provided not only comprehensive initial and boundary conditions for the model but also permitted detailed comparisons between modeled and observed turbulence quantities.
The modeled mean variables and turbulence quantities are found to agree well with observations and the results obtained by a large-eddy simulation. The cloud properties are also well reproduced in terms of the amount of cloud liquid water and fraction of cloud coverage. Other significant modeled features include high correlations between cloud liquid water and turbulence quantities; the entrainment process associated with negative virtual temperature flux near the top of the mixed-layer, and the well-balanced turbulence kinetic energy budget.
Abstract
A three-dimensional ensemble-mean atmospheric model, with simplified second-moment turbulence closure equations and a statistical treatment for the condensation process, is used to simulate a fair weather marine boundary layer observed during the GATE [GARP (Global Research Program) Atlantic Tropical Experiment]. The data deduced from airborne and surface-based instrumentation provided not only comprehensive initial and boundary conditions for the model but also permitted detailed comparisons between modeled and observed turbulence quantities.
The modeled mean variables and turbulence quantities are found to agree well with observations and the results obtained by a large-eddy simulation. The cloud properties are also well reproduced in terms of the amount of cloud liquid water and fraction of cloud coverage. Other significant modeled features include high correlations between cloud liquid water and turbulence quantities; the entrainment process associated with negative virtual temperature flux near the top of the mixed-layer, and the well-balanced turbulence kinetic energy budget.
Abstract
The impact of using grid-averaged thermodynamic properties (i.e., neglecting their subgrid variability due to partial cloudiness) to represent forcings for condensation or evaporation has long been recognized. In particular, numerical difficulties in terms of spurious oscillations and/or diffusion in vicinity of a cloud–environment interface have been encountered in most of the conventional finite-difference Eulerian advection schemes. This problem is equivalent to the inability of models to accurately track the cloud boundary within a grid cell, which eventually leads to spurious production or destruction of cloud water at leading or trailing edges of clouds. This paper employs a specialized technique called the “volume-of-fluid” (VOF) method to better parameterize the subgrid-scale advection process that accounts for the transport of material interfaces. VOF also determines the actual location of the partial cloudiness within a grid box. Consequently, relevant microphysical parameterizations in mixed cells can be consistently applied in “cloudy” and “clear” regions. The VOF technique is incorporated in a two-dimensional hydrodynamic model to simulate the diurnal cycle of the marine stratocumulus-capped boundary layer. The fidelity of VOF to advection–condensation processes under a diurnal radiative forcing is assessed by comparing the model simulation with data taken during the ISCCP FIRE observational period as well as with results from simulations without VOF. This study shows that the VOF method indeed suppresses the spurious cloud boundary instability and supports a multiday cloud evolution as observed. For the case without VOF, the spurious instability near the cloud top causes the dissipation of the entire cloud layer within a half of a diurnal cycle.
Abstract
The impact of using grid-averaged thermodynamic properties (i.e., neglecting their subgrid variability due to partial cloudiness) to represent forcings for condensation or evaporation has long been recognized. In particular, numerical difficulties in terms of spurious oscillations and/or diffusion in vicinity of a cloud–environment interface have been encountered in most of the conventional finite-difference Eulerian advection schemes. This problem is equivalent to the inability of models to accurately track the cloud boundary within a grid cell, which eventually leads to spurious production or destruction of cloud water at leading or trailing edges of clouds. This paper employs a specialized technique called the “volume-of-fluid” (VOF) method to better parameterize the subgrid-scale advection process that accounts for the transport of material interfaces. VOF also determines the actual location of the partial cloudiness within a grid box. Consequently, relevant microphysical parameterizations in mixed cells can be consistently applied in “cloudy” and “clear” regions. The VOF technique is incorporated in a two-dimensional hydrodynamic model to simulate the diurnal cycle of the marine stratocumulus-capped boundary layer. The fidelity of VOF to advection–condensation processes under a diurnal radiative forcing is assessed by comparing the model simulation with data taken during the ISCCP FIRE observational period as well as with results from simulations without VOF. This study shows that the VOF method indeed suppresses the spurious cloud boundary instability and supports a multiday cloud evolution as observed. For the case without VOF, the spurious instability near the cloud top causes the dissipation of the entire cloud layer within a half of a diurnal cycle.
Abstract
A 40-yr integration is conducted using the National Center for Atmospheric Research (NCAR) Community Climate Model Version 2 (CCM2). The simulation was forced by observed monthly global sea surface temperature (SST) changes during 1950–89. The January climates of the model results are presented in the paper. The modeled means and interannual variability are analyzed and compared with observations based on different accounts. Firm, the authors concentrate on the period of 1951–79. The monthly varying SSTs of this period were used to construct the SST climatology for an earlier 20-yr simulation conducted by NCAR researchers. The difference of the model climatology between the two simulations, respectively, forced by monthly varying SST and annually repeating SST, is examined. The modeled mean fields do not significantly differ between the two simulations especially for the Northern Hemisphere. The magnitude of interannual variability is enhanced in the current simulation especially for the northern Pacific due to the tropical SST forcing. The authors then concentrate on the remaining part of the simulation-the period from 1979 to 1989. The global climate during this period analyzed by the European Centre for Medium-Range Weather Forecasts (ECMWF) has been widely used for validation purposes by various general circulation model (GCM) studies including the CCM2 simulation mentioned above. The model performance in terms of basic circulation features for the period 1979–89 is actually quite impressive. Some earlier recognized model deficiencies in the above 20-yr simulation are improved simply because they were identified based upon mismatched time periods between the ECMWF analysis and the model simulation.
The model results of the entire simulation are finally compared with the multidecadal data of sea level pressure and 700-mb geopotential height analyzed by the National Meteorological Center. The decadal analysis of the model results reveals that the model has different performance for different decades. It is found that the simulated circulations are in better agreement with the observations during warmer decades in terms of the evolution of the El Niño-Southern Oscillation. The analysis of tropical/extratropical teleconnection patterns based on the SST index over the central equatorial Pacific and the Northern Hemisphere 700-mb height shows that the negative correlation between these two fields over the northern Pacific takes place somewhat too far west compared with observations. The net result is that CCM2 tends to produce a ridge of the height field also too far west from the west coast of North America. This deficiency may well be due to an unrealistic beating anomaly associated with condensation processes over the western tropical Pacific as indicated by earlier CCCM2 studies and linear steady-state model results.
Abstract
A 40-yr integration is conducted using the National Center for Atmospheric Research (NCAR) Community Climate Model Version 2 (CCM2). The simulation was forced by observed monthly global sea surface temperature (SST) changes during 1950–89. The January climates of the model results are presented in the paper. The modeled means and interannual variability are analyzed and compared with observations based on different accounts. Firm, the authors concentrate on the period of 1951–79. The monthly varying SSTs of this period were used to construct the SST climatology for an earlier 20-yr simulation conducted by NCAR researchers. The difference of the model climatology between the two simulations, respectively, forced by monthly varying SST and annually repeating SST, is examined. The modeled mean fields do not significantly differ between the two simulations especially for the Northern Hemisphere. The magnitude of interannual variability is enhanced in the current simulation especially for the northern Pacific due to the tropical SST forcing. The authors then concentrate on the remaining part of the simulation-the period from 1979 to 1989. The global climate during this period analyzed by the European Centre for Medium-Range Weather Forecasts (ECMWF) has been widely used for validation purposes by various general circulation model (GCM) studies including the CCM2 simulation mentioned above. The model performance in terms of basic circulation features for the period 1979–89 is actually quite impressive. Some earlier recognized model deficiencies in the above 20-yr simulation are improved simply because they were identified based upon mismatched time periods between the ECMWF analysis and the model simulation.
The model results of the entire simulation are finally compared with the multidecadal data of sea level pressure and 700-mb geopotential height analyzed by the National Meteorological Center. The decadal analysis of the model results reveals that the model has different performance for different decades. It is found that the simulated circulations are in better agreement with the observations during warmer decades in terms of the evolution of the El Niño-Southern Oscillation. The analysis of tropical/extratropical teleconnection patterns based on the SST index over the central equatorial Pacific and the Northern Hemisphere 700-mb height shows that the negative correlation between these two fields over the northern Pacific takes place somewhat too far west compared with observations. The net result is that CCM2 tends to produce a ridge of the height field also too far west from the west coast of North America. This deficiency may well be due to an unrealistic beating anomaly associated with condensation processes over the western tropical Pacific as indicated by earlier CCCM2 studies and linear steady-state model results.
