  We study the dynamics of shock waves observed in the umbra of a sunspot using
the spectroscopic observations from the Interface Region Imaging Spectrometer
(IRIS). The presence of the shock significantly deforms the shape of the
spectral lines of Mg II , C II , and Si IV . We found that C II 1335.66 {\AA}
and Si IV 1393.75 {\AA} show double-peaked profiles that change to a single
peak later on. However, the Mg II h 2803.53 {\AA} line first shows flat-top
profiles that change into double-peaked followed by the single peak. To study
the shock dynamics, we isolate the shock component from the spectra by fitting
two Gaussians. We find that the lifetime of the shock is largest in Mg II h
2803.53 {\AA} line. Moreover, the plasma motion shows both acceleration and
deceleration phase of the shock. Yet, in C II 1335.66 {\AA} and Si IV 1393.75
{\AA}, only deceleration phase is observed. We observe a strong correlation
between the largest blueshift of the shock and deceleration for all three
spectral lines. We find a positive (negative) correlation between intensities
contributed due to the shocks in Mg II and C II (Si IV ). This is suggestive
that the shocks are first amplified in C II , followed by a decline in the
height range corresponding to Si IV . These results may indicate the
dissipation of shocks above the formation height of C II , and the shocks may
have important roles in the dynamics of the upper chromosphere and transition
region above sunspots.
