  We present a simple, new criterion to extract line plumes from the velocity
fields, without using the temperature field, in a horizontal plane close to the
plate in turbulent convection. The existing coherent structure detection
criteria from velocity fields, proposed for shear driven wall turbulence, are
first shown to be inadequate for turbulent convection. Based on physical
arguments, we then propose that the negative values of
$\overline{\nabla_H}\cdot\overline{u}$, the horizontal divergence of the
horizontal velocity field extracts the plume regions from the velocity field.
The $\overline{\nabla_H}\cdot\overline{u}$ criterion is then shown to predict
the total length of plumes correctly over a Rayleigh number range of $7.7\times
10^5\le Ra_w\le 1.43\times 10^9$ and a Prandtl number range of $3.96\le Pr\le
5.3$. The thickness of the plume region predicted by the present criterion is
approximately same as that obtained by a threshold of RMS temperature
fluctuations, $\sqrt{T'^2}=0.2 \Delta T$, where $\Delta T$ is the temperature
drop across the fluid layer.
