  Modelling the integrated H I spectra of galaxies has been a difficult task
due to their diverse shapes, but more dynamical information is waiting to be
explored in Hi line profiles. Based on simple assumptions, we construct a
physically motivated model for the integrated Hi spectra: Parametrized
Asymmetric Neutral hydrogen Disk Integrated Spectrum Characterization
(PANDISC). The model shows great flexibility in reproducing the diverse Hi
profiles. We use Monte-Carlo Markov Chain (MCMC) for fitting the model to
global H I profiles, producing statistically robust quantitative results.
Comparing with several samples of H I data available in the literature , we
find the model-fitted results agree with catalogued velocity widths (e.g., W50)
down to the lowest S/N. The model is also shown to be useful for applications
like the baryonic Tully-Fisher relation (BTFR) and profile-based sample
control. By comparing v_r to v_flat , we uncover how the H I width is affected
by the structure of the rotation curve, following a trend consistent with the
difference in the BTFR slope. We also select a sample of spectra with broad
wing-like features suggestive of a population of galaxies with unusual gas
dynamics. The PANDISC model bears both promise and limitations for potential
use beyond H I lines. Further application on the whole ALFALFA sample will
enable us to perform large scale ensemble studies of the H I properties and
dynamics in nearby galaxies.
