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- Author or Editor: DAVID WILLIAMSON x
- Journal of Applied Meteorology and Climatology x
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Abstract
The accuracy of finite-diffrerence approximations to the shallow water equations on a sphere is examined for flow cases having an analytic solution. Approximations over grids with the longitudinal grid increment (Δλ) increasing near the poles such that the distance between grid points is nearly constant have large errors near the poles. These large polar errors are caused by the large longitudinal grid increment used in the approximations and are reduced by using a grid with Δλ constant. The normally severe limit on the time step caused by the small distance between grid points near the pole can be relaxed by removing the short-wave-length, fast-moving waves by Fourier analysis. With our test case, which contains only large scales, this filtering method produced a solution which is almost identical to that obtained over the uniform grid using a small time step. In comparing second- and fourth-order schemes applied to the above test case, we find that the fourth-order schemes offer more improvement per computer time than second-order Themes with mesh reduction.
Abstract
The accuracy of finite-diffrerence approximations to the shallow water equations on a sphere is examined for flow cases having an analytic solution. Approximations over grids with the longitudinal grid increment (Δλ) increasing near the poles such that the distance between grid points is nearly constant have large errors near the poles. These large polar errors are caused by the large longitudinal grid increment used in the approximations and are reduced by using a grid with Δλ constant. The normally severe limit on the time step caused by the small distance between grid points near the pole can be relaxed by removing the short-wave-length, fast-moving waves by Fourier analysis. With our test case, which contains only large scales, this filtering method produced a solution which is almost identical to that obtained over the uniform grid using a small time step. In comparing second- and fourth-order schemes applied to the above test case, we find that the fourth-order schemes offer more improvement per computer time than second-order Themes with mesh reduction.
Abstract
The Limited-Area Model (LAM) developed at the National Center for Atmospheric Research for use in conjunction with the NCAR Global Circulation Model is described including details of the lateral boundary conditions. One set of experiments is described for which a 2½° glow simulation provides the correct or control data against which 2½° LAM forecasts are compared. Three cases are considered in which the LAM inflow boundary values are provided by the 2½° global forecast, a 5° global forecast, or are held fixed equal to the initial values. Forecasts produced by the LAM with finer gods (up to ⅝°) are also shown. Although based on simulated data, the results indicate that the Limited-Area Model shows good potential for short-range forecasting.
Abstract
The Limited-Area Model (LAM) developed at the National Center for Atmospheric Research for use in conjunction with the NCAR Global Circulation Model is described including details of the lateral boundary conditions. One set of experiments is described for which a 2½° glow simulation provides the correct or control data against which 2½° LAM forecasts are compared. Three cases are considered in which the LAM inflow boundary values are provided by the 2½° global forecast, a 5° global forecast, or are held fixed equal to the initial values. Forecasts produced by the LAM with finer gods (up to ⅝°) are also shown. Although based on simulated data, the results indicate that the Limited-Area Model shows good potential for short-range forecasting.
Abstract
This paper presents results of experiments designed to test the importance of arithmetic precision in short-term forecasts and long-term climate simulations with the NCAR global circulation model. It is expected that the next-generation computers will have a sizable speed gain when using lower-precision arithmetic. To determine how precision affects the model results, we compare several short- and long-term experiments using 48-bit mantissa arithmetic (normal for the CDC 6600 and 7600 computers) with corresponding experiments using 24- and 21-bit mantissa arithmetic. The errors due to the lower precision are much smaller than typical observational errors. In addition, it appears that in the short-term experiments the rapid error growth of the model dominates the round-off error accumulation resulting from the lower-precision arithmetic. Therefore, the lower precision used by the next-generation computers should not have a detrimental effect on short-term forecast accuracy. The long-term climate simulation experiments indicated a very similar conclusion. Even though there were some differences in the results of the experiments, climate indicators such as zonal wind, zonal temperature or eddy transport are quite similar.
Abstract
This paper presents results of experiments designed to test the importance of arithmetic precision in short-term forecasts and long-term climate simulations with the NCAR global circulation model. It is expected that the next-generation computers will have a sizable speed gain when using lower-precision arithmetic. To determine how precision affects the model results, we compare several short- and long-term experiments using 48-bit mantissa arithmetic (normal for the CDC 6600 and 7600 computers) with corresponding experiments using 24- and 21-bit mantissa arithmetic. The errors due to the lower precision are much smaller than typical observational errors. In addition, it appears that in the short-term experiments the rapid error growth of the model dominates the round-off error accumulation resulting from the lower-precision arithmetic. Therefore, the lower precision used by the next-generation computers should not have a detrimental effect on short-term forecast accuracy. The long-term climate simulation experiments indicated a very similar conclusion. Even though there were some differences in the results of the experiments, climate indicators such as zonal wind, zonal temperature or eddy transport are quite similar.
Abstract
Boundary layer wind data observed by a Doppler lidar and sonic anemometers during the mornings of three intensive observational periods (IOP2, IOP3, and IOP7) of the Joint Urban 2003 (JU2003) field experiment are analyzed to extract the mean and turbulent characteristics of airflow over Oklahoma City, Oklahoma. A strong nocturnal low-level jet (LLJ) dominated the flow in the boundary layer over the measurement domain from midnight to the morning hours. Lidar scans through the LLJ taken after sunrise indicate that the LLJ elevation shows a gradual increase of 25–100 m over the urban area relative to that over the upstream suburban area. The mean wind speed beneath the jet over the urban area is about 10%–15% slower than that over the suburban area. Sonic anemometer observations combined with Doppler lidar observations in the urban and suburban areas are also analyzed to investigate the boundary layer turbulence production in the LLJ-dominated atmospheric boundary layer. The turbulence kinetic energy was higher over the urban domain mainly because of the shear production of building surfaces and building wakes. Direct transport of turbulent momentum flux from the LLJ to the urban street level was very small because of the relatively high elevation of the jet. However, since the LLJ dominated the mean wind in the boundary layer, the turbulence kinetic energy in the urban domain is correlated directly with the LLJ maximum speed and inversely with its height. The results indicate that the jet Richardson number is a reasonably good indicator for turbulent kinetic energy over the urban domain in the LLJ-dominated atmospheric boundary layer.
Abstract
Boundary layer wind data observed by a Doppler lidar and sonic anemometers during the mornings of three intensive observational periods (IOP2, IOP3, and IOP7) of the Joint Urban 2003 (JU2003) field experiment are analyzed to extract the mean and turbulent characteristics of airflow over Oklahoma City, Oklahoma. A strong nocturnal low-level jet (LLJ) dominated the flow in the boundary layer over the measurement domain from midnight to the morning hours. Lidar scans through the LLJ taken after sunrise indicate that the LLJ elevation shows a gradual increase of 25–100 m over the urban area relative to that over the upstream suburban area. The mean wind speed beneath the jet over the urban area is about 10%–15% slower than that over the suburban area. Sonic anemometer observations combined with Doppler lidar observations in the urban and suburban areas are also analyzed to investigate the boundary layer turbulence production in the LLJ-dominated atmospheric boundary layer. The turbulence kinetic energy was higher over the urban domain mainly because of the shear production of building surfaces and building wakes. Direct transport of turbulent momentum flux from the LLJ to the urban street level was very small because of the relatively high elevation of the jet. However, since the LLJ dominated the mean wind in the boundary layer, the turbulence kinetic energy in the urban domain is correlated directly with the LLJ maximum speed and inversely with its height. The results indicate that the jet Richardson number is a reasonably good indicator for turbulent kinetic energy over the urban domain in the LLJ-dominated atmospheric boundary layer.
