  Retrieving the optical depth of the Martian clouds ($\tau_\mathrm{cld}$) is a
powerful way to monitor their spatial and temporal evolution. However, such
retrievals from nadir imagery rely on several assumptions, including the
vertical structure of the clouds in the atmosphere. Here we compare the results
of cloud optical depth retrievals from the Emirates eXploration Imager (EXI)
onboard the Emirates Mars Mission (EMM) "Hope" orbiter performed using an
empirical cloud profile (typically used in similar studies) and using derived
cloud profiles obtained from near-simultaneous Solar Occultation observations
from the Middle-Infrared channel of the Atmospheric Chemistry Suite (ACS)
instrument onboard the ESA Trace Gas Orbiter (TGO). We show that the
latitudinal dependence of the cloud vertical profiles can have a strong impact
on the nadir retrievals; neglecting it can lead to a significant
underestimation of $\tau_\mathrm{cld}$ in the polar regions (up to 25 %). We
also discuss the impact of a vertically-dependent particle size profile, as
previous studies have shown the presence of very small water ice particles at
the top of the clouds. From this analysis, we provide recommendations for the
improvement of water ice cloud parameterization in radiative transfer
algorithms in nadir atmospheric retrievals.
