  We present a new approach to derive the observed properties of synchrotron
outbursts in relativistic jets. The idea is to use the very well sampled
submillimetre-to-radio long-term light curves of 3C 273 to extract the spectral
and temporal evolution of a typical outburst. The method consists in a
decomposition of these light curves into a series of twelve self-similar
flares. With a model-independent parameterization, we find that the obtained
outburst's evolution is in good qualitative agreement with the expectations of
shock models in relativistic jets. We then derive, by a second approach, the
relevant parameters of three-stage shock models.
  We observe for the first time that the optically thin spectral index is
steeper during the initial rising phase of the evolution than during the final
declining phase as expected by the shock model of Marscher & Gear (1985). We
obtain that this index flattens from alpha=-1.1 to alpha=-0.5, in good
agreement with what is expected from a power law electron energy distribution
with an index of -2.0. The observed flattening gives support to the idea that
radiative (synchrotron and/or Compton) losses are the dominant cooling process
of the electrons during the initial phase of the shock evolution.
  Two other results give us confidence in our decomposition: 1) the outbursts
that we identify do well correspond to the VLBI components observed in the jet
and 2) there is strong evidence that high-frequency peaking outbursts evolve
faster than low-frequency peaking outbursts. We propose that this last
correlation is related to the distance from the core of the jet at which the
shock forms.
