  This paper presents a characteristic-based flux partitioning for the
semi-implicit time integration of atmospheric flows. Nonhydrostatic models
require the solution of the compressible Euler equations. The acoustic
time-scale is significantly faster than the advective scale, yet it is
typically not relevant to atmospheric and weather phenomena. The acoustic and
advective components of the hyperbolic flux are separated in the characteristic
space. High-order, conservative additive Runge-Kutta methods are applied to the
partitioned equations so that the acoustic component is integrated in time
implicitly with an unconditionally stable method, while the advective component
is integrated explicitly. The time step of the overall algorithm is thus
determined by the advective scale. Benchmark flow problems are used to
demonstrate the accuracy, stability, and convergence of the proposed algorithm.
The computational cost of the partitioned semi-implicit approach is compared
with that of explicit time integration.
