  We consider the effect of energy dependent propagation of relativistic
electrons in a spatially inhomogeneous medium in order to interpret the
broad-band nonthermal radiation of the young supernova remnant (SNR) Cassiopeia
A. A two-zone model is proposed that distinguishes between compact, bright
steep-spectrum radio knots and the bright fragmented radio ring on the one
hand, and the remainder of the shell - the diffuse `plateau' - on the other
hand. In the framework of this model it is possible to explain the basic
features of the spectral and temporal evolution of the synchrotron radiation of
Cas A if one assumes that these compact structures correspond to sites of
efficient electron acceleration producing hard spectra of accelerated particles
with power-law indices ~2.2. The resulting energy distribution of radio
electrons in these compact structures becomes significantly steeper than the
electron production spectrum on timescales of the energy dependent escape of
these electrons into the surrounding diffuse plateau region. We argue that the
steepness, rather than the hardness, of the radio spectra of compact bright
structures in clumpy sources can in general be considered as a typical
signature of sites where strong electron acceleration has built up high
gradients in the spatial distribution of radio electrons. Subsequent diffusive
escape then modifies their energy distribution, leading to spatial variations
of spectral indices within the radio source. Qualitative and quantitative
interpretations of a number of observational data of Cas A are given.
Predictions following from the model are discussed.
