8.2.1. Generic Advection/Diffusion

The generic_advdiff package contains high-level subroutines to solve the advection-diffusion equation of any tracer, either active (potential temperature, salinity or water vapor) or passive (see pkg/ptracer). (see also Section 2.16 and Section 2.17).

8.2.1.1. Introduction

Package “generic_advdiff” provides a common set of routines for calculating advective/diffusive fluxes for tracers (cell centered quantities on a C-grid).

Many different advection schemes are available: the standard centered second order, centered fourth order and upwind biased third order schemes are known as linear methods and require some stable time-stepping method such as Adams-Bashforth. Alternatives such as flux-limited schemes are stable in the forward sense and are best combined with the multi-dimensional method provided in gad_advection.

8.2.1.2. Key subroutines, parameters and files

There are two high-level routines:

  • GAD_CALC_RHS calculates all fluxes at time level “n” and is used for the standard linear schemes. This must be used in conjuction with Adams–Bashforth time stepping. Diffusive and parameterized fluxes are always calculated here.
  • GAD_ADVECTION calculates just the advective fluxes using the non-linear schemes and can not be used in conjuction with Adams–Bashforth time stepping.
CPP Flag Name Default Description
COSINEMETH_III #define sets the implementation form of \(\cos{\varphi}\) scaling of bi-harmonic terms for tracer diffusivity (note, in pkg/generic_advdiff routines the definition set here overrides whether this is defined in model/inc/CPP_OPTIONS.h, where the setting affects viscous term calculations)
ISOTROPIC_COS_SCALING #undef selects isotropic scaling of harmonic and bi-harmonic terms when using the \(\cos{\varphi}\) scaling (note, in pkg/generic_advdiff routines the definition set here overrides whether this is defined in model/inc/CPP_OPTIONS.h, where the setting affects viscous term calculations)
DISABLE_MULTIDIM_ADVECTION #undef disables compilation of multi-dim. advection code
GAD_MULTIDIM_COMPRESSIBLE #undef use compressible flow method for multi-dim advection instead of older, less accurate method; note option has no effect on SOM advection which always uses compressible flow method
GAD_ALLOW_TS_SOM_ADV #undef enable the use of 2nd-order moment advection scheme (Prather 1986 [Pra86]) for temp. and salinity
GAD_SMOLARKIEWICZ_HACK #undef enables hack to get rid of negatives caused by Redi, see Smolarkiewicz (1989) [Smo89] (for ptracers, except temp and salinity)

8.2.1.3. GAD Diagnostics

------------------------------------------------------------------------
<-Name->|Levs|<-parsing code->|<--  Units   -->|<- Tile (max=80c)
------------------------------------------------------------------------
ADVr_TH | 15 |WM      LR      |degC.m^3/s      |Vertical   Advective Flux of Pot.Temperature
ADVx_TH | 15 |UU   087MR      |degC.m^3/s      |Zonal      Advective Flux of Pot.Temperature
ADVy_TH | 15 |VV   086MR      |degC.m^3/s      |Meridional Advective Flux of Pot.Temperature
DFrE_TH | 15 |WM      LR      |degC.m^3/s      |Vertical Diffusive Flux of Pot.Temperature (Explicit part)
DIFx_TH | 15 |UU   090MR      |degC.m^3/s      |Zonal      Diffusive Flux of Pot.Temperature
DIFy_TH | 15 |VV   089MR      |degC.m^3/s      |Meridional Diffusive Flux of Pot.Temperature
DFrI_TH | 15 |WM      LR      |degC.m^3/s      |Vertical Diffusive Flux of Pot.Temperature (Implicit part)
ADVr_SLT| 15 |WM      LR      |psu.m^3/s       |Vertical   Advective Flux of Salinity
ADVx_SLT| 15 |UU   094MR      |psu.m^3/s       |Zonal      Advective Flux of Salinity
ADVy_SLT| 15 |VV   093MR      |psu.m^3/s       |Meridional Advective Flux of Salinity
DFrE_SLT| 15 |WM      LR      |psu.m^3/s       |Vertical Diffusive Flux of Salinity    (Explicit part)
DIFx_SLT| 15 |UU   097MR      |psu.m^3/s       |Zonal      Diffusive Flux of Salinity
DIFy_SLT| 15 |VV   096MR      |psu.m^3/s       |Meridional Diffusive Flux of Salinity
DFrI_SLT| 15 |WM      LR      |psu.m^3/s       |Vertical Diffusive Flux of Salinity    (Implicit part)

8.2.1.4. Experiments and tutorials that use GAD