Microwave Radar/radiometer for Arctic Clouds (MiRAC): first insights from the ACLOUD campaign
The Microwave Radar/radiometer for Arctic Clouds (MiRAC) is a novel instrument package developed to study the vertical structure and characteristics of clouds and precipitation on board the Polar 5 research aircraft. MiRAC combines a frequency-modulated continuous wave (FMCW) radar at 94 GHz including a 89 GHz passive channel (MiRAC-A) and an eight-channel radiometer with frequencies between 175 and 340 GHz (MiRAC-P). The radar can be flexibly operated using different chirp sequences to provide measurements of the equivalent radar reflectivity with different vertical resolution down to 5 m. MiRAC is mounted for down-looking geometry on Polar 5 to enable the synergy with lidar and radiation measurements. To mitigate the influence of the strong surface backscatter the radar is mounted with an inclination of about 25∘ backward in a belly pod under the Polar 5 aircraft. Procedures for filtering ground return and range side lobes have been developed. MiRAC-P frequencies are especially adopted for low-humidity conditions typical for the Arctic to provide information on water vapor and hydrometeor content. MiRAC has been operated on 19 research flights during the ACLOUD campaign in the vicinity of Svalbard in May–June 2017 providing in total 48 h of measurements from flight altitudes >2300 m. The radar measurements have been carefully quality controlled and corrected for surface clutter, mounting of the instrument, and aircraft orientation to provide measurements on a unified, geo-referenced vertical grid allowing the combination with the other nadir-pointing instruments. An intercomparison with CloudSat shows good agreement in terms of cloud top height of 1.5 km and radar reflectivity up to −5 dBz and demonstrates that MiRAC with its more than 10 times higher vertical resolution down to about 150 m above the surface is able to show to some extent what is missed by CloudSat when observing low-level clouds. This is especially important for the Arctic as about 40 % of the clouds during ACLOUD showed cloud tops below 1000 m, i.e., the blind zone of CloudSat. In addition, with MiRAC-A 89 GHz it is possible to get an estimate of the sea ice concentration with a much higher resolution than the daily AMSR2 sea ice product on a 6.25 km grid.