Assessing Arctic low-level clouds and precipitation from above - a radar perspective

According to satellite-based estimations, a lot of clouds over the Arctic Ocean occur below2 km. Most information on Arctic low-level clouds come from CloudSat radar measurements.However, CloudSat lacks a complete representation of low-level clouds because the blindzone masks the lowest kilometer and the coarse spatial sampling conceals cloud patterns.Thus, higher resolved observations of cloud characteristics are needed to determine howthe cloud fraction varies close to the ground and how it depends on surface characteristicsand meteorological situation.Our study investigates the low-level hydrometeor fraction of Arctic clouds over the oceanusing airborne remote sensing measurements by the Microwave Radar/radiometer for ArcticClouds (MiRAC) flown on the Polar 5 aircraft. Four campaigns have been conducted in thevicinity of Svalbard during different seasons: ACLOUD, AFLUX, MOSAiC-ACA, and HALO-AC3. We convolute the MiRAC radar reflectivity measurements to adapt the fine MiRAC andcoarse CloudSat resolution. The convoluted measurements are compared with the originalairborne observations over all campaigns to investigate the effects of CloudSat’s spatial res-olution, clutter mask, and sensitivity on the low-level hydrometeor fraction. Measurementsreveal high hydrometeor fractions of up to 60% in the lowest 1.5 km, which CloudSat wouldmiss due to the blind zone. CloudSat would especially underestimate half of the total pre-cipitation. During cold air outbreaks, when rolling cloud structures evolve, CloudSat over-estimates the hydrometeor fraction most. Moreover, CloudSat does not resolve the separatelayers of multilayer clouds but rather merges them because of its coarse vertical resolution.