  By the sample generated by the AMPT with string melting model, we compare the
azimuthal distributions of multiplicity, total transverse rapidity, and mean
transverse rapidity. It is demonstrated that the azimuthal distribution of mean
transverse rapidity is a good probe of the radial kinetic expansion. The
anisotropic part of the distribution characterizes the anisotropic nature of
the radial expansion, and isotropic part is combinations of thermal motion and
isotropic radial expansion.
