Synthetic observables of the chromospheric and transition region spectral lines obtained from the numerical simulation. Top row (left to right): a cutout of the 2D temperature map, the signed vertical velocity around spicule 1, and the Ca II K, Mg II k, and Si IV 1393 Å synthetic spectra corresponding to the upflowing phase of the spicule at t = 140 s. Bottom row: corresponding temperature, vertical velocity, and synthetic spectra for the downflowing phase of the spicule at t = 370 s in the same format as the top row. Temperatures beyond 30 kK have been masked in the 2D temperature maps and are indicated with a black contour in the vertical velocity maps. The dashed vertical blue line corresponds to the LOS of the observer along which optically thick (thin) radiative transfer is performed.