Spatial and temporal variability of martian water-ice cloud effective radius in EMIRS thermal infrared observations.

New analyses of thermal infrared spectra obtained from the Emirates Mars Infrared Spectrometer (EMIRS) allow for retrieval of water-ice cloud effective radius, r eff,ice , in the lower atmosphere of Mars across spatial, seasonal, and daytime diurnal scales. Using a one Martian-year observational dataset the mean retrieved r eff,ice is 3.1 μm (standard deviation, σ = 1.5 μm) for an assumed effective variance, ν eff,ice = 0.1, and with most individual values between 1 and 10 μm. Sensitivity analyses show the uncertainty associated with r eff,ice is largely inversely proportional to water-ice cloud optical depth, τ ice. As a result, our retrieval is generally applicable to water-ice clouds with τ ice of at least ∼0.02 to 0.08 at 825 cm−1. Seasonal results show mean r eff,ice values of 4.3 μm (σ = 1.9 μm) during aphelion season (Ls = 40°–140°), associated with larger particle sizes in the aphelion cloud belt (ACB), and 2.5 μm (σ = 0.8 μm) during perihelion season (Ls = 225°–360°). Diurnal analyses between roughly 06:00 and 20:00 local true solar time indicate the largest variability occurs in ACB clouds, with effective radii tending to be smaller during midday (zonal-mean r eff,ice of 4–5 μm) as compared to the mornings and afternoons (zonal-mean r eff,ice of 6–7 μm). Relatively large spatial gradients in r eff,ice within the ACB are observed, including a seasonal mean r eff,ice of ∼8 μm in the Tharsis region and regions in the southern mid-latitudes with seasonal mean r eff,ice of ∼2 μm. Spatial variability in r eff,ice tends to be smaller throughout the rest of the year, though with some similar patterns. This includes several time periods between regional dust storms when a low-latitude band of water-ice clouds formed in the afternoons near Tharsis, which grew in both optical depth and effective radius to maxima in the early evenings. Lastly, changes in r eff,ice are found to be generally positively correlated with changes in τ ice and negatively correlated with estimated cloud height, though r eff,ice distributions also show this to be a more complex relationship than simple averages might suggest. • Mars water-ice cloud particle effective radius is retrieved from EMIRS observations. • Individual retrieved effective radius values range from 1 to 10 μm. • Mean water-ice cloud effective radius is 3.1 μm for all results. • Variability is investigated across spatial, diurnal, and seasonal scales. [ABSTRACT FROM AUTHOR]