The purpose of the paper is to investigate charge transfer modes in upward lightning flashes by means of numerical simulation constrained by concurrent observations of electromagnetic fields and currents. In particular, we focus on different types of pulses occurring in upward negative flashes. The MTLE return stroke model is used to compute the electric fields associated with return strokes and mixed-mode pulses, while the M-component model of Rakov et al. (1995) is used to compute electric fields associated with M-components and M-component-type ICC pulses. The simulation results are constrained by experimental data consisting of simultaneous records of lightning currents and electric fields associated with upward flashes at the Santis tower. The inferred velocities for M-component and M-component-type ICC pulses range from 2.0x10(7) m/s to 9.0x10(7) m/s, and the corresponding junction point heights range from 1.0 km to 2.0 km. The inferred pulse velocities for return strokes and mixed-mode pulses range from 1.3x10(8) m/s to 1.65x10(8) m/s. The inferred current attenuation constants of the MTLE model obtained in this study range from 0.3 km to 0.8 km, lower than the value of 2.0 km suggested in previous studies. The obtained results confirm the similarity of mixed-mode charge transfer to ground with return strokes on the one hand, and of the M-component-type ICC with classical M-components mode of charge transfer on the other hand.