| 610 | | Advection scheme to be used for the momentum equations. |
| 611 | | |
| 612 | | The user can choose between the following schemes: |
| 613 | | |
| 614 | | |
| 615 | | 'pw-scheme' |
| 616 | | The scheme of Piascek and Williams (1970, J. Comp. Phys., 6, 392-405) with central differences in the form C3 is used. |
| 617 | | If intermediate Euler-timesteps are carried out in case of timestep_scheme = 'leapfrog+euler' the advection scheme is - for the Euler-timestep - automatically switched to an upstream-scheme. |
| 618 | | |
| 619 | | 'ups-scheme' |
| 620 | | The upstream-spline scheme is used (see Mahrer and Pielke, 1978: Mon. Wea. Rev., 106, 818-830). In opposite to the Piascek-Williams scheme, this is characterized by much better numerical features (less numerical diffusion, better preservation of flow structures, e.g. vortices), but computationally it is much more expensive. In addition, the use of the Euler-timestep scheme is mandatory (timestep_scheme = 'euler'), i.e. the timestep accuracy is only of first order. For this reason the advection of scalar variables (see scalar_advec) should then also be carried out with the upstream-spline scheme, because otherwise the scalar variables would be subject to large numerical diffusion due to the upstream scheme. |
| 621 | | |
| 622 | | Since the cubic splines used tend to overshoot under certain circumstances, this effect must be adjusted by suitable filtering and smoothing (see cut_spline_overshoot, long_filter_factor, ups_limit_pt, ups_limit_u, ups_limit_v, ups_limit_w). This is always neccessary for runs with stable stratification, even if this stratification appears only in parts of the model domain. |
| 623 | | With stable stratification the upstream-spline scheme also produces gravity waves with large amplitude, which must be suitably damped (see rayleigh_damping_factor). |
| 624 | | |
| 625 | | Important: The upstream-spline scheme is not implemented for humidity and passive scalars (see humidity and passive_scalar) and requires the use of a 2d-domain-decomposition. The last conditions severely restricts code optimization on several machines leading to very long execution times! The scheme is also not allowed for non-cyclic lateral boundary conditions (see bc_lr and bc_ns). |
| | 610 | Advection scheme to be used for the momentum equations.\\\\ |
| | 611 | The user can choose between the following schemes:\\\\ |
| | 612 | '' 'pw-scheme' ''\\\\ |
| | 613 | The scheme of Piascek and Williams (1970, J. Comp. Phys., 6, 392-405) with central differences in the form C3 is used. |
| | 614 | If intermediate Euler-timesteps are carried out in case of [#timestep_scheme timestep_scheme] = '' 'leapfrog+euler' '' the advection scheme is - for the Euler-timestep - automatically switched to an upstream-scheme.\\\\ |
| | 615 | '' 'ups-scheme' ''\\\\ |
| | 616 | The upstream-spline scheme is used (see Mahrer and Pielke, 1978: Mon. Wea. Rev., 106, 818-830). In opposite to the Piascek-Williams scheme, this is characterized by much better numerical features (less numerical diffusion, better preservation of flow structures, e.g. vortices), but computationally it is much more expensive. In addition, the use of the Euler-timestep scheme is mandatory ([#timestep_scheme timestep_scheme] = '' 'euler' ''), i.e. the timestep accuracy is only of first order. For this reason the advection of scalar variables (see [#scalar_advec scalar_advec]) should then also be carried out with the upstream-spline scheme, because otherwise the scalar variables would be subject to large numerical diffusion due to the upstream scheme.\\\\ |
| | 617 | Since the cubic splines used tend to overshoot under certain circumstances, this effect must be adjusted by suitable filtering and smoothing (see [#cut_spline_overshoot cut_spline_overshoot], [#long_filter_factor long_filter_factor], [#ups_limit_pt ups_limit_pt], [#ups_limit_u ups_limit_u], [#ups_limit_v ups_limit_v], [#ups_limit_w ups_limit_w]). This is always neccessary for runs with stable stratification, even if this stratification appears only in parts of the model domain.\\\\ |
| | 618 | With stable stratification the upstream-spline scheme also produces gravity waves with large amplitude, which must be suitably damped (see [../d3par#rayleigh_damping_factor rayleigh_damping_factor]).\\\\ |
| | 619 | '''Important:'''\\ |
| | 620 | The upstream-spline scheme is not implemented for humidity and passive scalars (see [#humidity humidity] and [#passive_scalar passive_scalar]) and requires the use of a 2d-domain-decomposition. The last conditions severely restricts code optimization on several machines leading to very long execution times! The scheme is also not allowed for non-cyclic lateral boundary conditions (see [#bc_lr bc_lr] and [#bc_ns bc_ns]). |
| 730 | | Usually the mixing length in LES models l,,LES,, depends (as in PALM) on the grid size and is possibly restricted further in case of stable stratification and near the lower wall (see parameter [#wall_adjustment |
| | 725 | Usually the mixing length in LES models l,,LES,, depends (as in PALM) on the grid size and is possibly restricted further in case of stable stratification and near the lower wall (see parameter [#wall_adjustment wall_adjustment]). With '''adjust_mixing_length''' = ''.T.'' the Prandtl' mixing length l,,PR,, = kappa * z/phi is calculated and the mixing length actually used in the model is set l = MIN (l,,LES,, , l,,PR,,). This usually gives a decrease of the mixing length at the bottom boundary and considers the fact that eddy sizes decrease in the vicinity of the wall.\\\\ |
