  We constrain the velocity spectral distribution of global-scale solar
convective cells at depth using techniques of local helioseismology. We
calibrate the sensitivity of helioseismic waves to large-scale convective cells
in the interior by analyzing simulations of waves propagating through a
velocity snapshot of global solar convection via methods of time-distance
helioseismology. Applying identical analysis techniques to observations of the
Sun, we are able to bound from above the magnitudes of solar convective cells
as a function of spatial convective scale. We find that convection at a depth
of $r/R_\odot = 0.95$ with spatial extent $\ell <20$, where $\ell$ is the
spherical harmonic degree, comprise weak flow systems, on the order of 15 m/s
or less. Convective features deeper than $r/R_\odot = 0.95$ are more difficult
to image due to the rapidly decreasing sensitivity of helioseismic waves.
