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- Author or Editor: Hsiao-ming Hsu x
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Abstract
Mesoscale lake-effect snowstorms in the vicinity of Lake Michigan are studied by a linear steady-state analytic model and a nonlinear time-dependent numerical model with parameterized subgrid-scale physics. The solutions of the linear model show that the orientation of the mean wind field to the surface heating pattern is crucial to the shapes of the disturbances. The results indicate that the relative warmth of the lake surface can induce three updraft centers under a westerly wind, two updraft centers/bands under a northwesterly wind, and a convergence band under a northerly wind. Such convergences are caused by the interaction between the mean wind and the local circulations forced by the curved thermal contrasts. The numerical results from the nonlinear model not only produce convergence patterns very similar to these found in the linear theory and in other numerical studies, but also capture the transient property in some of the lake-induced disturbances. All of these results are qualitatively confirmed by satellite images.
Abstract
Mesoscale lake-effect snowstorms in the vicinity of Lake Michigan are studied by a linear steady-state analytic model and a nonlinear time-dependent numerical model with parameterized subgrid-scale physics. The solutions of the linear model show that the orientation of the mean wind field to the surface heating pattern is crucial to the shapes of the disturbances. The results indicate that the relative warmth of the lake surface can induce three updraft centers under a westerly wind, two updraft centers/bands under a northwesterly wind, and a convergence band under a northerly wind. Such convergences are caused by the interaction between the mean wind and the local circulations forced by the curved thermal contrasts. The numerical results from the nonlinear model not only produce convergence patterns very similar to these found in the linear theory and in other numerical studies, but also capture the transient property in some of the lake-induced disturbances. All of these results are qualitatively confirmed by satellite images.
Abstract
The steady-state atmospheric responses to a finite surface heating through thermal eddy diffusion are studied. The effects of the mean wind, the earth's rotation, and the thermal stratification are considered in a linear system. Scale analysis reveals the limit of the hydrostatic approximation. It shows that the classical aspect ratio is not sufficient in determining the validity of the hydrostatic approximation. It shows that the classical aspect ratio is not sufficient in determining the validity of the hydrostatic approximation, and what the diffusive aspect ratio, the stratification aspect ratio and the Rossby number should be included. Various circulation patterns are presented for different horizontal heating scales and shapes and for different atmospheric mean conditions. In a comparison paper, this study I In a composition paper, this study is extended to investigate the lake-effect nownstorms in the vicinity of Lake Michigan.
Abstract
The steady-state atmospheric responses to a finite surface heating through thermal eddy diffusion are studied. The effects of the mean wind, the earth's rotation, and the thermal stratification are considered in a linear system. Scale analysis reveals the limit of the hydrostatic approximation. It shows that the classical aspect ratio is not sufficient in determining the validity of the hydrostatic approximation. It shows that the classical aspect ratio is not sufficient in determining the validity of the hydrostatic approximation, and what the diffusive aspect ratio, the stratification aspect ratio and the Rossby number should be included. Various circulation patterns are presented for different horizontal heating scales and shapes and for different atmospheric mean conditions. In a comparison paper, this study I In a composition paper, this study is extended to investigate the lake-effect nownstorms in the vicinity of Lake Michigan.
Abstract
Future-generation, operational, weather prediction systems will likely include storm-scale, limited-area models that will explicitly resolve convective precipitation. However, the high-resolution convection-resolving grids will need to be embedded, or nested, within coarser-resolution grids that will provide lateral-boundary conditions. It is the purpose of this study to illustrate how the convective environment on a convection-resolving storm-scale model grid, and therefore the convection itself, can be significantly influenced by the treatment of convection on the coarser grids within which the fine grid is embedded.
It was confirmed that, as in the actual atmosphere, mass-field adjustments resulting from convection in one area (the outer grids, in this case) affect the convection in other areas (the inner, convection-resolving grid). That is, the errors in precipitation timing, precipitation intensity, and the vertical distribution of latent heating, associated with the treatment of convection on the outer grids, greatly affect the explicit convection on the inner grid. In this case, the different precipitation parameterizations on the outer two grids produce up to a factor of 3 difference in the 24-h amount of explicit rainfall simulated on the inner grid. Even when the parameterization is limited to only the outer grid, with explicit precipitation on both the middle and inner grids, over a factor of 2 difference in 24-h total explicit rainfall is produced on the inner grid. The different precipitation parameterizations on the outer grids appear to differently modulate the intensity and the timing of the explicit convection on the inner grid through induced subsidence.
Abstract
Future-generation, operational, weather prediction systems will likely include storm-scale, limited-area models that will explicitly resolve convective precipitation. However, the high-resolution convection-resolving grids will need to be embedded, or nested, within coarser-resolution grids that will provide lateral-boundary conditions. It is the purpose of this study to illustrate how the convective environment on a convection-resolving storm-scale model grid, and therefore the convection itself, can be significantly influenced by the treatment of convection on the coarser grids within which the fine grid is embedded.
It was confirmed that, as in the actual atmosphere, mass-field adjustments resulting from convection in one area (the outer grids, in this case) affect the convection in other areas (the inner, convection-resolving grid). That is, the errors in precipitation timing, precipitation intensity, and the vertical distribution of latent heating, associated with the treatment of convection on the outer grids, greatly affect the explicit convection on the inner grid. In this case, the different precipitation parameterizations on the outer two grids produce up to a factor of 3 difference in the 24-h amount of explicit rainfall simulated on the inner grid. Even when the parameterization is limited to only the outer grid, with explicit precipitation on both the middle and inner grids, over a factor of 2 difference in 24-h total explicit rainfall is produced on the inner grid. The different precipitation parameterizations on the outer grids appear to differently modulate the intensity and the timing of the explicit convection on the inner grid through induced subsidence.
Abstract
A stretched coordinate technique for semispectral hydrodynamic models is described that allows for greater flexibility in the placement of model grid points. Stretching is implemented here for the vertical coordinate of an oceanic model that employs Chebyshey polynomials to represent the vertical structure. A three-dimensional test demonstrates how, for a fixed number of vertical levels, the technique may permit greater accuracy in the simulation of linear internal waves, by allowing the placement of grid points closer to the regions of maximum curvature in the represented velocity fields. A one-dimensional test demonstrates enhanced resolution of mixed-layer dynamics by allowing the placement of more of the available grid points, evenly spaced, near the ocean surface, with broader spacing below. These improvements are achieved with negligible computational overhead. While the method cannot yield improved accuracy for all situations, in appropriate cases it permits reduced computation for an accurate result by reducing the number of basis functions necessary for adequate resolution of the modeled fields. Some guidelines are presented for its application, along with cautions as to where the technique is disadvantageous.
Abstract
A stretched coordinate technique for semispectral hydrodynamic models is described that allows for greater flexibility in the placement of model grid points. Stretching is implemented here for the vertical coordinate of an oceanic model that employs Chebyshey polynomials to represent the vertical structure. A three-dimensional test demonstrates how, for a fixed number of vertical levels, the technique may permit greater accuracy in the simulation of linear internal waves, by allowing the placement of grid points closer to the regions of maximum curvature in the represented velocity fields. A one-dimensional test demonstrates enhanced resolution of mixed-layer dynamics by allowing the placement of more of the available grid points, evenly spaced, near the ocean surface, with broader spacing below. These improvements are achieved with negligible computational overhead. While the method cannot yield improved accuracy for all situations, in appropriate cases it permits reduced computation for an accurate result by reducing the number of basis functions necessary for adequate resolution of the modeled fields. Some guidelines are presented for its application, along with cautions as to where the technique is disadvantageous.
Abstract
A physically based distributed hydrological model was applied to simulate typhoon floods over a mountainous watershed in Taiwan. The meteorological forcings include the observed gauge rainfall data and the predicted rainfall data from a mesoscale meteorological model, the fifth-generation Pennsylvania State University–National Center for Atmospheric Research (PSU–NCAR) Mesoscale Model (MM5). This study investigates the flood responses of three Typhoons: Zeb (1998), Nari (2001), and Herb (1996), which possessed unique meteorological features and that all produced severe floods. The predicted basin-averaged rainfall hydrographs by the MM5 are compared with that interpreted by rain gauge data to reveal the discrepancies in rainfall peak amounts and time lags, and to explore their subsequent effects on flood generation. The simulated flood hydrographs at the Hsia-Yun station, which is upstream of the Shihmen Reservoir, are compared with observed flood discharges in terms of the amount and time lag of flood peaks. It is shown that the small discrepancy in rainfall peaks and phase lags could be significantly amplified in simulated flood responses of a mountainous watershed. The overall predictive skill of the distributed hydrological model with different rainfall inputs is examined with three parameters, which include the runoff ratio (RR), root-mean-square error (rmse), and goodness of fit (GOF). Although the runoff ratio for the MM5-predicted rainfall is superior to that for the observed gauge rainfall, the simulated hydrographs with observed gauge rainfall have smaller rmse and GOF values for three events. This study shows that the error in flood prediction with the mesoscale-modeled rainfall is mainly caused by the rainfall–peak difference, which arises from the inherent uncertainties in the mesoscale-modeled rainfalls over a mountainous terrain during the typhoon landfall periods.
Abstract
A physically based distributed hydrological model was applied to simulate typhoon floods over a mountainous watershed in Taiwan. The meteorological forcings include the observed gauge rainfall data and the predicted rainfall data from a mesoscale meteorological model, the fifth-generation Pennsylvania State University–National Center for Atmospheric Research (PSU–NCAR) Mesoscale Model (MM5). This study investigates the flood responses of three Typhoons: Zeb (1998), Nari (2001), and Herb (1996), which possessed unique meteorological features and that all produced severe floods. The predicted basin-averaged rainfall hydrographs by the MM5 are compared with that interpreted by rain gauge data to reveal the discrepancies in rainfall peak amounts and time lags, and to explore their subsequent effects on flood generation. The simulated flood hydrographs at the Hsia-Yun station, which is upstream of the Shihmen Reservoir, are compared with observed flood discharges in terms of the amount and time lag of flood peaks. It is shown that the small discrepancy in rainfall peaks and phase lags could be significantly amplified in simulated flood responses of a mountainous watershed. The overall predictive skill of the distributed hydrological model with different rainfall inputs is examined with three parameters, which include the runoff ratio (RR), root-mean-square error (rmse), and goodness of fit (GOF). Although the runoff ratio for the MM5-predicted rainfall is superior to that for the observed gauge rainfall, the simulated hydrographs with observed gauge rainfall have smaller rmse and GOF values for three events. This study shows that the error in flood prediction with the mesoscale-modeled rainfall is mainly caused by the rainfall–peak difference, which arises from the inherent uncertainties in the mesoscale-modeled rainfalls over a mountainous terrain during the typhoon landfall periods.
Abstract
Recent studies have shown the importance of high-resolution wind in coastal ocean modeling. This paper tests the Coupled Ocean–Atmosphere Mesoscale Prediction System (COAMPS) at the 9-, 27-, and 81-km grid resolutions in simulating wind off the central and southern California coasts, including the Santa Barbara Channel (SBC). The test period is March–May (1999) when the wind changes from its characteristics more typical of winter, to spring when strong gradients exist in the SBC. The model results were checked against wind station time series, Special Sensor Microwave Imager wind speeds, and the European Centre for Medium-Range Weather Forecasts (ECMWF) reanalysis. The high-resolution (9-km grid) COAMPS wind shows expansion fans downwind of major capes where speed increases. The large-scale [O(100 km)] wind turns onshore in the Southern California Bight where both wind and wind stress curl weaken southward along the coast. The formation and evolution of the Catalina eddies are also simulated. These general features agree with observations. The turning appears to be the cumulative effect of synoptic cyclones shed downwind of Point Conception during periods of intense northerly wind. The turning and eddies are much weaker in the ECMWF reanalysis or the COAMPS field at the 81-km grid. Near the coast, observed small-scale (tens of kilometers) structures are reasonably reproduced by COAMPS at the 9-km grid. Results from the 9-km grid generally compare better with observations than the 27-km grid, suggesting that a more accurate model wind may be obtained at even higher resolution. However, in the SBC, simulated winds at both the 9- and 27-km grids show along-channel coherency during May, contrary to observations. The observed winds in the channel appear to be of small localized scales (≈<10 km) and would require an improved model grid and perhaps also boundary layer physics to simulate.
Abstract
Recent studies have shown the importance of high-resolution wind in coastal ocean modeling. This paper tests the Coupled Ocean–Atmosphere Mesoscale Prediction System (COAMPS) at the 9-, 27-, and 81-km grid resolutions in simulating wind off the central and southern California coasts, including the Santa Barbara Channel (SBC). The test period is March–May (1999) when the wind changes from its characteristics more typical of winter, to spring when strong gradients exist in the SBC. The model results were checked against wind station time series, Special Sensor Microwave Imager wind speeds, and the European Centre for Medium-Range Weather Forecasts (ECMWF) reanalysis. The high-resolution (9-km grid) COAMPS wind shows expansion fans downwind of major capes where speed increases. The large-scale [O(100 km)] wind turns onshore in the Southern California Bight where both wind and wind stress curl weaken southward along the coast. The formation and evolution of the Catalina eddies are also simulated. These general features agree with observations. The turning appears to be the cumulative effect of synoptic cyclones shed downwind of Point Conception during periods of intense northerly wind. The turning and eddies are much weaker in the ECMWF reanalysis or the COAMPS field at the 81-km grid. Near the coast, observed small-scale (tens of kilometers) structures are reasonably reproduced by COAMPS at the 9-km grid. Results from the 9-km grid generally compare better with observations than the 27-km grid, suggesting that a more accurate model wind may be obtained at even higher resolution. However, in the SBC, simulated winds at both the 9- and 27-km grids show along-channel coherency during May, contrary to observations. The observed winds in the channel appear to be of small localized scales (≈<10 km) and would require an improved model grid and perhaps also boundary layer physics to simulate.
Abstract
An investigation of the generation of turbulence above deep convection is presented. This investigation is motivated by an encounter between a commercial passenger aircraft and severe turbulence above a developing thunderstorm near Dickinson, North Dakota, on 10 July 1997. Very high-resolution two- and three-dimensional numerical simulations are used to investigate the possible causes of the turbulence encounter. These simulations explicitly resolve the convection and the turbulence-causing instabilities. The configurations of the models are consistent with the meteorological conditions surrounding the event.
The turbulence generated in the numerical simulations can be placed into two general categories. The first category includes turbulence that remains local to the cloud top, and the second category includes turbulence that propagates away from the convection and owes its existence to the breakdown of convectively generated gravity waves. In both the two- and three-dimensional calculations, the local turbulence develops rapidly and occupies a layer about 1 km deep above the top of convective updrafts after their initial overshoot into the stratosphere. This local turbulence is generated by the highly nonlinear interactions between the overshooting convective updrafts and the tropopause. Gravity wave breakdown is only present in the two-dimensional calculation and occurs in a layer about 3 km deep and 30 km long. This gravity wave breakdown is attributed to an interaction between the gravity waves and a critical level induced by the background wind shear and cloud-induced wind perturbations above cloud top.
Abstract
An investigation of the generation of turbulence above deep convection is presented. This investigation is motivated by an encounter between a commercial passenger aircraft and severe turbulence above a developing thunderstorm near Dickinson, North Dakota, on 10 July 1997. Very high-resolution two- and three-dimensional numerical simulations are used to investigate the possible causes of the turbulence encounter. These simulations explicitly resolve the convection and the turbulence-causing instabilities. The configurations of the models are consistent with the meteorological conditions surrounding the event.
The turbulence generated in the numerical simulations can be placed into two general categories. The first category includes turbulence that remains local to the cloud top, and the second category includes turbulence that propagates away from the convection and owes its existence to the breakdown of convectively generated gravity waves. In both the two- and three-dimensional calculations, the local turbulence develops rapidly and occupies a layer about 1 km deep above the top of convective updrafts after their initial overshoot into the stratosphere. This local turbulence is generated by the highly nonlinear interactions between the overshooting convective updrafts and the tropopause. Gravity wave breakdown is only present in the two-dimensional calculation and occurs in a layer about 3 km deep and 30 km long. This gravity wave breakdown is attributed to an interaction between the gravity waves and a critical level induced by the background wind shear and cloud-induced wind perturbations above cloud top.
Abstract
Directionally averaged time series of precipitation rates for eight warm seasons (1996–2003) over the continental United States derived from Next Generation Weather Radar (NEXRAD) measurements are analyzed using spectral decomposition methods. For the latitudinally averaged data, in addition to previously identified diurnal and semidiurnal cycles, the temporal spectra show cross-scale self-similarity and periodicity. This property is revealed by a power-law scaling with an exponent of −4/3 for the frequency band higher than semidiurnal and −3/4 for the 1–3-day band. For the longitudinally averaged series the scaling exponent for the frequency band higher than semidiurnal changes from −4/3 to −5/3 revealing anisotropic properties.
The dominant periods and propagation speeds display temporal variability on about 1/2, 1, 4, 11, and 25 days. Composite patterns describing periods of <5 days display the eastward propagation characteristic of classical mesoscale convective organization. The lower-frequency (>5 days) patterns propagate westward suggesting the influence of large-scale waves, and both dominant periods and propagation speeds show marked interannual variability. The implied dependence between propagation and mean-flow for <5 days is consistent with the macrophysics of warm-season convective organization, and extends known dynamical mechanisms to a statistical framework.
Abstract
Directionally averaged time series of precipitation rates for eight warm seasons (1996–2003) over the continental United States derived from Next Generation Weather Radar (NEXRAD) measurements are analyzed using spectral decomposition methods. For the latitudinally averaged data, in addition to previously identified diurnal and semidiurnal cycles, the temporal spectra show cross-scale self-similarity and periodicity. This property is revealed by a power-law scaling with an exponent of −4/3 for the frequency band higher than semidiurnal and −3/4 for the 1–3-day band. For the longitudinally averaged series the scaling exponent for the frequency band higher than semidiurnal changes from −4/3 to −5/3 revealing anisotropic properties.
The dominant periods and propagation speeds display temporal variability on about 1/2, 1, 4, 11, and 25 days. Composite patterns describing periods of <5 days display the eastward propagation characteristic of classical mesoscale convective organization. The lower-frequency (>5 days) patterns propagate westward suggesting the influence of large-scale waves, and both dominant periods and propagation speeds show marked interannual variability. The implied dependence between propagation and mean-flow for <5 days is consistent with the macrophysics of warm-season convective organization, and extends known dynamical mechanisms to a statistical framework.
Abstract
Numerical simulations of terrain-induced turbulence associated with airflow over Lantau Island of Hong Kong are presented. Lantau is a relatively small island with three narrow peaks rising to between 700 and 950 m above mean sea level. This research was undertaken as part of a project to better understand and predict the nature of turbulence and shear at the new airport site on the island of Chek Lap Kok, which is located to the lee of Lantau. Intensive ground and aerial observations were taken from May through June 1994, during the Lantau Experiment (LANTEX). This paper focuses on flow associated with the passage of Tropical Storm Russ on 7 June 1994, during which severe turbulence was observed.
The nature of the environmental and topographic forcing on 7 June 1994 resulted in the turbulence and shear being dominated by the combination of topographic effects and surface friction. High-resolution numerical simulations, initialized using local sounding data, were performed using the Clark model. The simulation results indicate that gravity-wave dynamics played a very minor role in the flow distortion and generation of turbulence. As a result of this flow regime, relatively high vertical and horizontal resolution was required to simulate the mechanically generated turbulence associated with Tropical Storm Russ.
Results are presented using a vertical resolution of 10 m near the surface and with horizontal resolutions of both 125 and 62.5 m over local, nested domains of about 13–24 km on a side. The 125-m model resolution simulated highly distorted flow in the lee of Lantau, with streaks emanating downstream from regions of sharp orographic gradients. At this resolution the streaks were nearly steady in time. At the higher horizontal resolution of 62.5 m the streaks became unstable, resulting in eddies advecting downstream within a distorted streaky mean flow similar to the 125-m resolution simulation. The temporally averaged fields changed little with the increase in resolution; however, there was a three- to fourfold increase in the temporal variability of the flow, as indicated by the standard deviation of the wind from a 10-min temporal average. Overall, the higher resolution simulations compared quite well with the observations, whereas the lower resolution cases did not. The high-resolution experiments also showed a much broader horizontal and vertical extent for the transient eddies. The depth of orographic influence increased from about 200 m to over 600 m with the increase in resolution. A physical explanation, using simple linear arguments based on the blocking effects of the eddies, is presented. The nature of the flow separation is analyzed using Bernoulli’s energy form to display the geometry of the separation bubbles. The height of the 80 m2 s−2 energy surface shows eddies forming in regions of large orographic gradients and advecting downstream.
Tests using both buoyancy excitation and stochastic backscatter to parameterize the underresolved dynamics at the 125-m resolution are presented, as well as one experiment testing the influence of static stability suppressing turbulence development. All these tests showed no significant effect. Implications of these results to the parameterization of mechanically induced turbulence in complex terrain are discussed.
Abstract
Numerical simulations of terrain-induced turbulence associated with airflow over Lantau Island of Hong Kong are presented. Lantau is a relatively small island with three narrow peaks rising to between 700 and 950 m above mean sea level. This research was undertaken as part of a project to better understand and predict the nature of turbulence and shear at the new airport site on the island of Chek Lap Kok, which is located to the lee of Lantau. Intensive ground and aerial observations were taken from May through June 1994, during the Lantau Experiment (LANTEX). This paper focuses on flow associated with the passage of Tropical Storm Russ on 7 June 1994, during which severe turbulence was observed.
The nature of the environmental and topographic forcing on 7 June 1994 resulted in the turbulence and shear being dominated by the combination of topographic effects and surface friction. High-resolution numerical simulations, initialized using local sounding data, were performed using the Clark model. The simulation results indicate that gravity-wave dynamics played a very minor role in the flow distortion and generation of turbulence. As a result of this flow regime, relatively high vertical and horizontal resolution was required to simulate the mechanically generated turbulence associated with Tropical Storm Russ.
Results are presented using a vertical resolution of 10 m near the surface and with horizontal resolutions of both 125 and 62.5 m over local, nested domains of about 13–24 km on a side. The 125-m model resolution simulated highly distorted flow in the lee of Lantau, with streaks emanating downstream from regions of sharp orographic gradients. At this resolution the streaks were nearly steady in time. At the higher horizontal resolution of 62.5 m the streaks became unstable, resulting in eddies advecting downstream within a distorted streaky mean flow similar to the 125-m resolution simulation. The temporally averaged fields changed little with the increase in resolution; however, there was a three- to fourfold increase in the temporal variability of the flow, as indicated by the standard deviation of the wind from a 10-min temporal average. Overall, the higher resolution simulations compared quite well with the observations, whereas the lower resolution cases did not. The high-resolution experiments also showed a much broader horizontal and vertical extent for the transient eddies. The depth of orographic influence increased from about 200 m to over 600 m with the increase in resolution. A physical explanation, using simple linear arguments based on the blocking effects of the eddies, is presented. The nature of the flow separation is analyzed using Bernoulli’s energy form to display the geometry of the separation bubbles. The height of the 80 m2 s−2 energy surface shows eddies forming in regions of large orographic gradients and advecting downstream.
Tests using both buoyancy excitation and stochastic backscatter to parameterize the underresolved dynamics at the 125-m resolution are presented, as well as one experiment testing the influence of static stability suppressing turbulence development. All these tests showed no significant effect. Implications of these results to the parameterization of mechanically induced turbulence in complex terrain are discussed.
