We present a time-dependent semiempirical model of the chromospheric umbral oscillation in sunspots. This model has been obtained by applying recently developed non-LTE inversion techniques to a time series of spectropolarimetric observations. The model consists of two optically thick unresolved atmospheric components: a ``quiet'' component with downward velocities that covers most of the resolution element and an ``active'' component with upward velocities as high as 10 km s-1 that covers a smaller filling factor and has a higher temperature at the same chromospheric optical depth. This semiempirical model accounts for all the observational signatures of the chromospheric oscillation when the filling factor of the active component oscillates between a few percent and 20% of the resolution element. We discuss a plausible physical scenario in which upward-propagating waves in a downflowing magnetized environment lead to periodic mass ejections from the atmospheric layers where the waves become nonlinear. Based on observations obtained with the Gregory Coudé Telescope, operated on the island of Tenerife by the Observatory of Göttingen University in the Spanish Observatorio del Teide of the Instituto de Astrofísica de Canarias.