  Among the most advanced and sophisticated methods for state analysis of an
atmospheric system is the four dimensional variational data assimilation. The
numerically challenging task of this approach is the development and
application of the adjoint model components. For tracer transport in fluid
dynamics accuracy of numerical advection schemes is vital. It is even more
important for applications in space-time variational data assimilation with
adjoint model version. We propose novel straightforward and efficient approach
- artificial source term method - for adjoint advection solver development. It
has several benefits compared to traditional adjoint model building technique.
One of the attractive features of the new approach is that it reuses existing
advection solver code, thus resulting into significant reduction of time needed
for adjoint solver development. The stability, accuracy and convergence of the
adjoint schemes are investigated. The method is implemented and evaluated for
the linear advection equation on the sphere in the Icosahedral Nonhydrostatic
Model (ICON). Adjoint solvers developed by the conventional and the methods are
compared against each other on a collection of standard advection test cases
and variational data assimilation test cases developed here. The advantages of
the artificial source term method is especially obvious in case of monotonic
advection equation solvers, granting, e.g. absence of oscillations and
nonphysical negative concentrations.
