TAU0
TAU0
is an input in the fort.15 file that influences the degree of numerical diffusion in ADCIRC's governing equations. Specifically, it influences the weighting factor that determines the relative contribution of the primitive and wave portions of the the Generalized WaveContinuity Equation (GWCE). The weighting factor, Failed to parse (MathML with SVG or PNG fallback (recommended for modern browsers and accessibility tools): Invalid response ("Math extension cannot connect to Restbase.") from server "https://wikimedia.org/api/rest_v1/":): {\displaystyle \tau_0}
, is affected by values in both the fort.15 file and the fort.13 file, if the primitive weighting in continuity equation or min and max primitive weighting in continuity equation nodal attributes are specified. This page addresses both the TAU0
value in the fort.15 file and the Failed to parse (MathML with SVG or PNG fallback (recommended for modern browsers and accessibility tools): Invalid response ("Math extension cannot connect to Restbase.") from server "https://wikimedia.org/api/rest_v1/":): {\displaystyle \tau_0}
parameter more generally.
Contents
 1 TAU0 Values
 2 Outputting Failed to parse (MathML with SVG or PNG fallback (recommended for modern browsers and accessibility tools): Invalid response ("Math extension cannot connect to Restbase.") from server "https://wikimedia.org/api/rest_v1/":): {\displaystyle \tau_0}
 3 Selecting Failed to parse (MathML with SVG or PNG fallback (recommended for modern browsers and accessibility tools): Invalid response ("Math extension cannot connect to Restbase.") from server "https://wikimedia.org/api/rest_v1/":): {\displaystyle \tau_0}
TAU0
Values
Because the TAU0
value specified in the fort.15 file can be either a flag (indicating how ADCIRC should operate) or the actual Failed to parse (MathML with SVG or PNG fallback (recommended for modern browsers and accessibility tools): Invalid response ("Math extension cannot connect to Restbase.") from server "https://wikimedia.org/api/rest_v1/":): {\displaystyle \tau_0}
value used in solving the GWCE, it is important to distinguish between the two. All negative TAU0
are flags, all positive TAU0
are Failed to parse (MathML with SVG or PNG fallback (recommended for modern browsers and accessibility tools): Invalid response ("Math extension cannot connect to Restbase.") from server "https://wikimedia.org/api/rest_v1/":): {\displaystyle \tau_0}
. The following table is a summary of possible TAU0
values and their meaning. Note that for TAU0 = x.1
where x
is an integer, behavior is the same as x
, but the Failed to parse (MathML with SVG or PNG fallback (recommended for modern browsers and accessibility tools): Invalid response ("Math extension cannot connect to Restbase.") from server "https://wikimedia.org/api/rest_v1/":): {\displaystyle \tau_0}
values are written to the fort.90 file. More on this below in Outputting.
fort.15 TAU0

>= 0  1  2  3  5  6  7 

Varies in space  no  yes  yes  yes  yes  yes  yes 
Varies in time  no  no  no  yes  yes  yes  yes 
Spaceaveraged  no  no  no  yes  yes  yes  yes 
Timeaveraged  no  no  no  no  no  yes  yes 
Requires nodal attribute  no  no  no  yes  no  yes  no 
Minimum  N/A  0.002  0.002  N/A  Tau0FullDomainMin

N/A  N/A 
Maximum  N/A  0.005  1  0.2  Tau0FullDomainMax

0.2  0.2 
Code Flags  
HighResTimeVaryingTau0

.FALSE.  .FALSE.  .FALSE.  .TRUE.  .FALSE.  .TRUE.  .TRUE. 
FullDomainTimeVaryingTau0

.FALSE.  .FALSE.  .FALSE.  .FALSE.  .TRUE.  .FALSE.  .FALSE. 
TimeAveragedTau0

.FALSE.  .FALSE.  .FALSE.  .FALSE.  .FALSE.  .TRUE.  .FALSE. 
BackLoadedTimeAveragedTau0

.FALSE.  .FALSE.  .FALSE.  .FALSE.  .FALSE.  .FALSE.  .TRUE. 
Positive values
For all positive values, the value specified in the fort.15 file is spatially and temporally constant and applied directly, i.e. TAU0 = Failed to parse (MathML with SVG or PNG fallback (recommended for modern browsers and accessibility tools): Invalid response ("Math extension cannot connect to Restbase.") from server "https://wikimedia.org/api/rest_v1/":): {\displaystyle \tau_0}
, so:
TAU0 = 0
, the GWCE is a pure wave equationTAU0 = 1
, the GWCE behaves like a pure primitive continuity equation0 < TAU0 < 1
, the GWCE is a weighted sum of the primitive and wave portions of the equation
Negative Values
Spatially varying, temporally constant
TAU0 = 1
, Failed to parse (MathML with SVG or PNG fallback (recommended for modern browsers and accessibility tools): Invalid response ("Math extension cannot connect to Restbase.") from server "https://wikimedia.org/api/rest_v1/":): {\displaystyle \tau_0} is computed as follows:If depth >= 10; then Failed to parse (MathML with SVG or PNG fallback (recommended for modern browsers and accessibility tools): Invalid response ("Math extension cannot connect to Restbase.") from server "https://wikimedia.org/api/rest_v1/":): {\displaystyle \tau_0} = 0.005
If depth < 10; then Failed to parse (MathML with SVG or PNG fallback (recommended for modern browsers and accessibility tools): Invalid response ("Math extension cannot connect to Restbase.") from server "https://wikimedia.org/api/rest_v1/":): {\displaystyle \tau_0} = 0.020
TAU0 = 2
, Failed to parse (MathML with SVG or PNG fallback (recommended for modern browsers and accessibility tools): Invalid response ("Math extension cannot connect to Restbase.") from server "https://wikimedia.org/api/rest_v1/":): {\displaystyle \tau_0} is is computed according to depth as follows:If depth >= 200; then Failed to parse (MathML with SVG or PNG fallback (recommended for modern browsers and accessibility tools): Invalid response ("Math extension cannot connect to Restbase.") from server "https://wikimedia.org/api/rest_v1/":): {\displaystyle \tau_0} = 0.005
If 1 < depth < 200; then Failed to parse (MathML with SVG or PNG fallback (recommended for modern browsers and accessibility tools): Invalid response ("Math extension cannot connect to Restbase.") from server "https://wikimedia.org/api/rest_v1/":): {\displaystyle \tau_0} = 1/depth
If depth < 1; then Failed to parse (MathML with SVG or PNG fallback (recommended for modern browsers and accessibility tools): Invalid response ("Math extension cannot connect to Restbase.") from server "https://wikimedia.org/api/rest_v1/":): {\displaystyle \tau_0 = 1}
Spatially and temporally varying
TAU0 = 3
, Failed to parse (MathML with SVG or PNG fallback (recommended for modern browsers and accessibility tools): Invalid response ("Math extension cannot connect to Restbase.") from server "https://wikimedia.org/api/rest_v1/":): {\displaystyle \tau_0} is computed fromTAU0Base
(read in from the nodal attribute file) as follows:If TAU0Base < 0.025; then Failed to parse (MathML with SVG or PNG fallback (recommended for modern browsers and accessibility tools): Invalid response ("Math extension cannot connect to Restbase.") from server "https://wikimedia.org/api/rest_v1/":): {\displaystyle \tau_0} = TAU0Base
(constant in time)If TAU0Base >= 0.025; then Failed to parse (MathML with SVG or PNG fallback (recommended for modern browsers and accessibility tools): Invalid response ("Math extension cannot connect to Restbase.") from server "https://wikimedia.org/api/rest_v1/":): {\displaystyle \tau_0} = TAU0Base+1.5*TK
whereTK=speed*Cd/depth
TAU0 = 5
, Failed to parse (MathML with SVG or PNG fallback (recommended for modern browsers and accessibility tools): Invalid response ("Math extension cannot connect to Restbase.") from server "https://wikimedia.org/api/rest_v1/":): {\displaystyle \tau_0} is computed similar toTAU0 = 3
as follows:Failed to parse (MathML with SVG or PNG fallback (recommended for modern browsers and accessibility tools): Invalid response ("Math extension cannot connect to Restbase.") from server "https://wikimedia.org/api/rest_v1/":): {\displaystyle \tau_0} =Tau0Min+1.5*TK
 It is limited by
Tau0FullDomainMin <= Failed to parse (MathML with SVG or PNG fallback (recommended for modern browsers and accessibility tools): Invalid response ("Math extension cannot connect to Restbase.") from server "https://wikimedia.org/api/rest_v1/":): {\displaystyle \tau_0} <= Tau0FullDomainMax
, which are specified on the following line of the fort.15 file (only whenTAU0 = 5
)  If the min and max primitive weighting in continuity equation nodal attribute is used, then the nodal minimum and maximum values replace the fulldomain values in the above calculation.
Spatial and Temporal Updating
For TAU0=3, 5, 6, or 7
, Failed to parse (MathML with SVG or PNG fallback (recommended for modern browsers and accessibility tools): Invalid response ("Math extension cannot connect to Restbase.") from server "https://wikimedia.org/api/rest_v1/":): {\displaystyle \tau_0}
is updated in space and in time.
Outputting Failed to parse (MathML with SVG or PNG fallback (recommended for modern browsers and accessibility tools): Invalid response ("Math extension cannot connect to Restbase.") from server "https://wikimedia.org/api/rest_v1/":): {\displaystyle \tau_0}
For TAU0 formulations that vary spatially or temporally, ADCIRC can output the internallycalculated nodal Failed to parse (MathML with SVG or PNG fallback (recommended for modern browsers and accessibility tools): Invalid response ("Math extension cannot connect to Restbase.") from server "https://wikimedia.org/api/rest_v1/":): {\displaystyle \tau_0} values. They are written to the fort.90 file, which has the same format and output frequency as the water surface elevation output file (fort.63). fort.90 output is activated by placing a 1 in the tenths place of TAU0 in the fort.15 file. For example, if TAU0=3.1, the calculation of Failed to parse (MathML with SVG or PNG fallback (recommended for modern browsers and accessibility tools): Invalid response ("Math extension cannot connect to Restbase.") from server "https://wikimedia.org/api/rest_v1/":): {\displaystyle \tau_0} is still carried out according to the description of TAU0=3 above, and the fort.90 output file will also be produced.
Selecting Failed to parse (MathML with SVG or PNG fallback (recommended for modern browsers and accessibility tools): Invalid response ("Math extension cannot connect to Restbase.") from server "https://wikimedia.org/api/rest_v1/":): {\displaystyle \tau_0}
For positive values (TAU0=Failed to parse (MathML with SVG or PNG fallback (recommended for modern browsers and accessibility tools): Invalid response ("Math extension cannot connect to Restbase.") from server "https://wikimedia.org/api/rest_v1/":): {\displaystyle \tau_0}
), a good rule of thumb for setting TAU0
is to set it equal to the largest value of an equivalent linear friction factor (e.g, for linear friction TAU0 = TAU
; for quadratic friction TAU0
= maximum (speed*Cd/depth). Typical values for TAU0
are in the range of 0.005 – 0.1.
TAU0 = 3
, paired with the primitive weighting in continuity equation nodal attribute is generally the most popular formulation. In this case, TAU0Base
nodal attribute values can be generated with the ADCIRC utility program tau0_gen.f. The program bases generation on the following logic:
 If the avg. distance between a node and its neighbors < 1750 m TAU0Base = 0.03
 If the avg. distance between a node and its neighbors > 1750 m AND depth < 10m; TAU0Base = 0.02 (TAU0 is constant in time)
 If the avg. distance between a node and its neighbors > 1750 m AND depth > 10m; TAU0Base = 0.005 (TAU0 is constant in time)