Synchrotron radiation is one of few experimental signals from runaway electrons which is sensitive to the details of the runaway electron distribution function. Recent advances in the simulation of synchrotron radiation from runaway electrons have enabled us to benchmark aspects of kinetic theory against experiments, but in order to aid our understanding of the runaway dynamics in experiments, it would be desirable to also develop simple models for the distribution function, which can be used to interpret synchrotron observations in different scenarios. In this contribution, we use combined fluid-kinetic simulations to aid the interpretation of synchrotron radiation measurements in an ASDEX Upgrade disruption. Based on the simulations, we develop a simple, two-parameter model for the momentum distribution function, which allows us to fit the synchrotron radiation measurements and draw conclusions about the runaway current density profile evolution.