The model FIR filter in the PW model, H_PW,mod (black solid lines) and in the current model, H_YSK (red dashed lines) for time constants of (a) 60 days and (b) 10 days. The black and red dotted lines follow the exponential envelope of the respective model FIR filters. Right panel: we limit the plot range to −10 ≤ t ≤ 10 days to highlight the difference in the two model FIR filters from the different envelope functions assumed. See Sect. 3 for details.