  Context: There is a wide discrepancy in current estimates of the strength of
convection flows in the solar interior obtained using different helioseismic
methods applied to observations from SDO/HMI. The cause for these disparities
is not known. Aims: As one step in the effort to resolve this discrepancy, we
aim to characterize the multi-ridge fitting code for ring-diagram helioseismic
analysis that is used to obtain flow estimates from local power spectra of
solar oscillations. Methods: We updated the multi-ridge fitting code developed
by Greer et al.(2014) to solve several problems we identified through our
inspection of the code. In particular, we changed the merit function to account
for the smoothing of the power spectra, model for the power spectrum, and noise
estimates. We used Monte Carlo simulations to generate synthetic data and to
characterize the noise and bias of the updated code by fitting these synthetic
data. Results: The bias in the output fit parameters, apart from the parameter
describing the amplitude of the p-mode resonances in the power spectrum, is
below what can be measured from the Monte-Carlo simulations. The amplitude
parameters are underestimated; this is a consequence of choosing to fit the
logarithm of the averaged power. We defer fixing this problem as it is well
understood and not significant for measuring flows in the solar interior. The
scatter in the fit parameters from the Monte-Carlo simulations is well-modeled
by the formal error estimates from the code. Conclusions: We document and
demonstrate a reliable multi-ridge fitting method for ring-diagram analysis.
The differences between the updated fitting results and the original results
are less than one order of magnitude and therefore we suspect that the changes
will not eliminate the aforementioned orders-of-magnitude discrepancy in the
amplitude of convective flows in the solar interior.
