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Microwave Radar/Radiometer for Arctic Clouds (Mirac): First Insights from the ACLOUD Campaign

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Microwave Radar/Radiometer for Arctic Clouds (Mirac): First Insights from the ACLOUD Campaign Atmos. Meas. Tech., 12, 5019–5037, 2019 https://doi.org/10.5194/amt-12-5019-2019 © Author(s) 2019. This work is distributed under the Creative Commons Attribution 4.0 License. Microwave Radar/radiometer for Arctic Clouds (MiRAC): first insights from the ACLOUD campaign Mario Mech1, Leif-Leonard Kliesch1, Andreas Anhäuser1, Thomas Rose2, Pavlos Kollias1,3, and Susanne Crewell1 1Institute for Geophysics and Meteorology, University of Cologne, Cologne, Germany 2Radiometer-Physics GmbH, Meckenheim, Germany 3School of Marine and Atmospheric Sciences, Stony Brook University, NY, USA Correspondence: Mario Mech ([email protected]) Received: 12 April 2019 – Discussion started: 23 April 2019 Revised: 26 July 2019 – Accepted: 11 August 2019 – Published: 18 September 2019 Abstract. The Microwave Radar/radiometer for Arctic vertical resolution down to about 150 m above the surface Clouds (MiRAC) is a novel instrument package developed is able to show to some extent what is missed by Cloud- to study the vertical structure and characteristics of clouds Sat when observing low-level clouds. This is especially im- and precipitation on board the Polar 5 research aircraft. portant for the Arctic as about 40 % of the clouds during MiRAC combines a frequency-modulated continuous wave ACLOUD showed cloud tops below 1000 m, i.e., the blind (FMCW) radar at 94 GHz including a 89 GHz passive chan- zone of CloudSat. In addition, with MiRAC-A 89 GHz it is nel (MiRAC-A) and an eight-channel radiometer with fre- possible to get an estimate of the sea ice concentration with a quencies between 175 and 340 GHz (MiRAC-P). The radar much higher resolution than the daily AMSR2 sea ice prod- can be flexibly operated using different chirp sequences to uct on a 6.25 km grid. 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 1 Introduction the influence of the strong surface backscatter the radar is mounted with an inclination of about 25◦ backward in a belly In the rapidly changing Arctic climate (e.g., Serreze et al., pod under the Polar 5 aircraft. Procedures for filtering ground 2009; Graversen et al., 2008), the role of clouds and as- return and range side lobes have been developed. MiRAC-P sociated feedback remain unclear (Osborne et al., 2018; frequencies are especially adopted for low-humidity condi- Wendisch et al., 2017). In particular, understanding the ef- tions typical for the Arctic to provide information on wa- fect of mixed-phase clouds whose persistence is controlled ter vapor and hydrometeor content. MiRAC has been oper- by a complex interaction of microphysical, radiative, and dy- ated on 19 research flights during the ACLOUD campaign in namic processes is still challenging (Morrison et al., 2012). the vicinity of Svalbard in May–June 2017 providing in to- Information on their vertical structure and phase partition- tal 48 h of measurements from flight altitudes > 2300 m. The ing, which control their radiative impact (Curry et al., 2002), radar measurements have been carefully quality controlled is currently available from the few ground-based profiling and corrected for surface clutter, mounting of the instrument, sites in the Arctic, e.g., Utqiagvik˙ (formerly known as Bar- and aircraft orientation to provide measurements on a uni- row), Alaska (Shupe et al., 2015); Ny-Ålesund, Svalbard fied, geo-referenced vertical grid allowing the combination (Nomokonova et al., 2019); and Summit, Greenland (Shupe with the other nadir-pointing instruments. An intercompari- et al., 2013). The use of synergistic lidar and cloud radar son with CloudSat shows good agreement in terms of cloud measurements is key for the study of these cloud systems. top height of 1.5 km and radar reflectivity up to −5 dBz and Passive microwave measurements further provide informa- demonstrates that MiRAC with its more than 10 times higher tion on the vertically integrated liquid water path (LWP). The profiling sites provide important long-term statistics; how- Published by Copernicus Publications on behalf of the European Geosciences Union. 5020 M. Mech et al.: Microwave Radar/radiometer for Arctic Clouds: MiRAC ever, they might be limited in their representativity for the ronmental Research (HIAPER) Cloud Radar, HCR; Rauber wider Arctic. et al., 2017; Wyoming Cloud Radar, WCR; Khanal and Polar-orbiting passive satellite imagery provides coverage Wang, 2015; High Altitude and LOng range research aircraft of the Arctic region; however, the retrieval of cloud proper- Microwave Package, HAMP; Mech et al., 2014), which use ties is challenged by the surface properties and suffers from short microwave pulses for ranging, the radar of the MiRAC limited vertical information. Active spaceborne measure- package employs a frequency-modulated continuous wave ments by lidar and radar, i.e., by the combination of Cloud- (FMCW) radar. Thus, a lower peak power transmitter is used; Aerosol Lidar and Infrared Pathfinder Satellite Observation however, careful consideration on handling the surface return (Winker et al., 2003, CALIPSO) and CloudSat (Stephens is required. Therefore, in the past, airborne FMCW radar has et al., 2009), have been fundamental in better understand- been mounted in uplooking geometry (Fang et al., 2017). ing the vertical structure of clouds around the globe. How- The purpose of this study is twofold. First, the MiRAC ever, the CloudSat Cloud Profiling Radar (CPR) provides package, which consists of a unique 94 GHz FMCW radar limited information in the lowest 0.75 to 1.25 km due to the (MiRAC-A) and an eight-channel passive microwave ra- presence of strong surface echo (Maahn et al., 2014; Burns diometer with channels between 170 and 340 GHz (MiRAC- et al., 2016), while the CALIPSO lidar observations are often P), is introduced. The instrument specifications and integra- fully attenuated by the presence of supercooled liquid layers. tion into the Polar 5 aircraft in downward-looking geometry Using CALIPSO and CloudSat measurements Mioche et al. are provided in Sect.2 followed by the methodology used to (2015) identified the region around Svalbard to be particu- quality control and map the observations to a geo-referenced larly interesting to study mixed-phase clouds as these show a coordinate system in Sect.3. The performance of MiRAC higher frequency of occurrence (55 %) here compared to the during its first deployment within ACLOUD will be demon- Arctic average (30 % in winter and early spring, 50 % May strated via a comparison with CloudSat within a case study to October). in Sect.4, and a short statistical analysis of the ACLOUD Airborne platforms have the advantage of high spatial flex- measurements is shown in Sect.5. Conclusions and outlook ibility and accessibility of remote places comparable to satel- to further analysis and deployments of MiRAC are given in lite observations. While a number of airborne campaigns Sect.6. have been performed in the Arctic since the 1980s (An- dronache, 2017; Wendisch et al., 2018), the use of a radar– lidar system in these aircraft campaigns is rather limited. 2 Instruments and aircraft installation One notable exception was during the Polar Study using Air- craft, Remote Sensing, Surface Measurements and Models, MiRAC is composed of an active (MiRAC-A) and passive of Climate, Chemistry, Aerosols, and Transport (POLAR- (MiRAC-P) part. MiRAC-A is mounted between the wings CAT) campaign in spring 2008, in which Delanoë et al. of the research aircraft Polar 5 and MiRAC-P is mounted (2013) studied an Arctic nimbostratus ice cloud using the inside of the aircraft measuring through a sufficiently large French airborne radar–lidar instrument in detail. aperture (see Fig.3). Since the FMCW radar needs a differ- During May–June 2017 the Arctic CLoud Observations ent measuring angle, MiRAC-A is tilted by 25◦ backwards Using airborne measurements during polar Day (ACLOUD; with respect to nadir, whereas MiRAC-P is nadir-looking. Wendisch et al., 2018; Knudsen et al., 2018) aircraft cam- The following three sections will describe the instruments paign was performed as part of the ArctiC Amplifica- and aircraft installation in detail. tion: Climate Relevant Atmospheric and SurfaCe Processes, and Feedback Mechanisms project ((AC)3; Wendisch et al., 2.1 FMCW W-band radar 2017). The research aircraft Polar 5 and 6 of the Al- fred Wegener Institute (AWI) operating from Longyear- MiRAC-A is based on the novel single vertically polar- byen, Svalbard, deployed a remote sensing and in situ mi- ized cloud radar RPG-FMCW-94-SP manufactured by RPG- crophysics instrument package, respectively. Polar 5 was Radiometer Physics GmbH, which is described in detail by equipped with the Airborne Mobile Aerosol Lidar for Arc- Küchler et al.(2017). It basically consists of a transmitter tic research (AMALi; Stachlewska et al., 2010) and spec- with adjustable power to protect the receiver from satura- tral solar radiation measurement already operated during tion, a Cassegrain two-antenna system for continuous signal the VERtical Distribution of Ice in Arctic clouds (VERDI; transmission and reception, and a receiver containing both Schäfer et al., 2015) campaign. During ACLOUD, the re- the radar receiver channel at 94 GHz and the passive broad- mote sensing package was complemented by the novel Mi- band channel at 89 GHz. To guarantee accurate measure- crowave Radar/radiometer for Arctic Clouds (MiRAC). In ments both channels are thermally stabilized within a few contrast to most other millimeter radars employed on re- millikelvin. The FMCW principle allows us to achieve high search aircraft (e.g., Radar Aéroporté et Sol de Télédétec- sensitivity for short-range resolutions down to 5 m with low tion des propriétés nuAgeuses, RASTA; Delanoë et al., 2012; transmitter power of about 1.5 W from solid state amplifiers.
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