.. _sec_phys_pkg_gad:

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/ptracers).
(see also sections [sec:tracer:sub:`e`\ quations] to
[sec:tracer:sub:`a`\ dvection\ :sub:`s`\ chemes]).

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.

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.

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)

Experiments and tutorials that use GAD
++++++++++++++++++++++++++++++++++++++

-  Offline tutorial, in tutorial\_offline verification directory,
   described in section [sec:eg-offline]

-  Baroclinic gyre experiment, in tutorial\_baroclinic\_gyre
   verification directory, described in section [sec:eg-fourlayer]

-  Tracer Sensitivity tutorial, in tutorial\_tracer\_adjsens
   verification directory, described in section
   [sec:eg-simple-tracer-adjoint]