SHOC User Manual

4.30.15 Alerts

The alert diagnostic attempts to detect early signs of instability by monitoring maximum absolute velocities, divergences and, if invoked, maximum absolute momentum tendencies.

These maximums are written to an alert diagnostic file every time-step, along with the maximums encountered during the whole simulation. A time series file containing the history of the maximum values is also created. To invoke alert tracking in passive mode:

ALERT PASSIVE <alert_file>

ALERT_DT 1 hour

Where

<alert_file>

is an optional filename to write the maximum value information to.

This file name is appended with ‘

.txt

’. If

<alert_file>

is absent then output is written to the file ‘ alert.txt

’ by default. If

ALERT_DT

is present then a time series file is created with the alert filename appended with ‘

.ts

’, and maximum values are output at the specified time interval. The maximum values printed to file are absolute maximum values. Note that maximum tendencies are only printed if the

MOM_TEND

flag is invoked. The mechanical

Excess Mass

=

1

A

A

∫

η

dA

(m)

OBC Energy Flux

=

1

W

∫

W

DU

1

(

g

η

+

0 .

5

U

2

2

)

∆

y

(m

4 s

-3

)

Where A = domain area, W = OBC width (the energy flux is for a u1

boundary in this case, see Section 3.11). Note that

ρ

(OBC Energy Flux) has units Wm

-1

.

Alert tracking may be made active using the following:

ALERT ACTIVE <alert_file>

Using the

ACTIVE

alert mode, when a maximum value exceeds defined thresholds then specific action is taken. This action falls into three categories:

LEVEL1

: 2D or 3D velocities exceed the

VELMAX

parameter. Horizontal diffusion is increased to the value

∆ x

2

/4

∆ t which most effectively damps the shortest waves possible in the grid (Kowalik and Murty, 1993, eqn. 3.141). Velocity thresholds may be defined using the following parameters:

VELMAX 2 # Maximum horizontal velocity (ms

-1

)

WMAX 1e-3 # Maximum vertical velocity (ms

-1

)

LEVEL2

: 2D divergence =

∂

η

/

∂

t

, or 3D divergence =

∂

w

/

∂

z

exceed a (hardwired) threshold. The Shapiro (1970) smoothing filter is applied to the relevant velocity field (Kowalik and Murty, 1993, eqn. 3.136)

LEVEL3

: momentum tendencies exceed (hardwired) thresholds. The relevant process is omitted during the next time-step.

Individual alert actions can be turned on or off by prescribing

0

or

1

for each individual action:: eta vel2d vel3d w tend 2d_div 3d_div cfl ts shear hdiff

ALERT_CODE 1 1 1 0 0 0 0 0 0 1 1

20/08/2015 Page 110 of 185 Version: 1.00