PROGRAM SUMMARY
Title of program:
VERDI
Catalogue identifier:
ADPB
Ref. in CPC:
141(2001)149
Distribution format: tar gzip file
Operating system: Linux (Kernel 2.2.12-20, RedHat 6.1); Windows 98
High speed store required:
1MK words
Number of bits in a word:
32
Number of lines in distributed program, including test data, etc:
3541
Keywords:
Geophysics, Diffusion equation, Backward time centred space scheme,
Radon, Radon progeny, Aerosol, Dry deposition, Rainout,
Turbulent transport.
Programming language used: C++
Computer:
Pentium III (Coppermine).
Nature of physical problem:
Radon (222Rn) and thoron (220Rn) are inert radioactive gases produced in
the Earth's crust by decay of 226Ra and 224Ra, respectively.
Transported through soil, these gases enter the atmosphere where they
undergo meteorological processes. Their decay products react with trace
gases and water vapors. These freshly formed clusters attach to
existing aerosol particles. As a consequence of radioactive decay, a
cluster may desorb from its host by recoil. All progeny, whether
attached or not, are removed from the atmosphere by dry deposition,
rainout and washout. Assuming horizontal homogeneity and gradient
transport theory valid, processes taking place in the atmosphere are
modeled into a set of coupled one-dimensional time-dependent diffusion
equations which, in the mean, can provide a working approximation of the
boundary layer concentration activities of radon isotopes and their
decay products as a function of time.
Method of solution:
The set of coupled diffusion equations is differenced by using a
backward time centred space scheme. In order to have sufficiently fine
resolution near the surface, a logarithmic scale is chosen for the
vertical coordinate. The resulted tridiagonal system is solved at each
timestep by the method of backsubstitution.
Restrictions:
When condition of homogeneity does no longer hold (e.g., near large
bodies of water, where advection may have important effects on the
activity concentrations), the one-dimensional model will fail.
Moreover, due to limitations of gradient transport theory, the model
cannot adequately describe non-local events (e.g., free convection).
Typical running time:
Running times for the test runs described in Appendix A are: ~0.05 s per
nuclide for the steady-state cases and ~3.6 s per nuclide and hour of
simulation for the time-dependent case on a Pentium III 750 MHz PC
running Linux.