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 below
2 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 blind
zone masks the lowest kilometer and the coarse spatial sampling conceals cloud patterns.
Thus, higher resolved observations of cloud characteristics are needed to determine how
the cloud fraction varies close to the ground and how it depends on surface characteristics
and meteorological situation.

Our study investigates the low-level hydrometeor fraction of Arctic clouds over the ocean
using airborne remote sensing measurements by the Microwave Radar/radiometer for Arctic
Clouds (MiRAC) flown on the Polar 5 aircraft. Four campaigns have been conducted in the
vicinity of Svalbard during different seasons: ACLOUD, AFLUX, MOSAiC-ACA, and HALO-
AC3. We convolute the MiRAC radar reflectivity measurements to adapt the fine MiRAC and
coarse CloudSat resolution. The convoluted measurements are compared with the original
airborne observations over all campaigns to investigate the effects of CloudSat’s spatial res-
olution, clutter mask, and sensitivity on the low-level hydrometeor fraction. Measurements
reveal high hydrometeor fractions of up to 60% in the lowest 1.5 km, which CloudSat would
miss 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 separate
layers of multilayer clouds but rather merges them because of its coarse vertical resolution.