Abstract
Abstract. Cirrus clouds contribute to the general radiation budget of the Earth and play an important role in climate projections. Of special interest are optically thin cirrus clouds close to the tropopause due to the fact that their impact is not yet well understood. Measuring these clouds is challenging as both high spatial resolution as well as a very high detection sensitivity are needed. These criteria are fulfilled by the infrared limb sounder GLORIA (Gimballed Limb Observer for Radiance Imaging of the Atmosphere). This study presents a characterization of observed cirrus clouds using the data obtained by GLORIA aboard the German research aircraft HALO during the WISE (Wave-driven ISentropic Exchange) campaign in September and October 2017. We developed an optimized cloud detection method based on the cloud index and the extinction coefficient retrieved at the microwindow 832.4–834.4 cm−1. We derived macro-physical characteristics of the detected cirrus clouds such as cloud top height, cloud bottom height, vertical extent and cloud top position with respect to the tropopause. The fraction of cirrus clouds detected above the tropopause is on the order of 13 % to 27 %. In general, good agreement with the clouds predicted by the ERA5 reanalysis dataset is obtained. However, cloud occurrence is ≈ 50 % higher in the observations for the region close to and above the tropopause. Cloud bottom heights are also detected above the tropopause. However, considering the uncertainties, we cannot confirm the formation of unattached cirrus layers above the tropopause.
Highlights
High clouds composed of ice crystals are formed in the upper troposphere, where the temperatures are lower than −38 ◦C
HALO can fly to a maximum altitude of 15 km, which means that the vertical coverage of GLORIA observations during this campaign ranged from ∼ 15 down to ∼ 5 km
A fraction of 60 % is considerably larger than the ≈ 17 % reported by Sassen et al (2008) for CloudAerosol Lidar and Infrared Pathfinder Satellite Observations (CALIPSO) measurements and the International Satellite Cloud Climatology Project (ISCCP) for mid-latitudes
Summary
High clouds composed of ice crystals are formed in the upper troposphere, where the temperatures are lower than −38 ◦C. Spang et al (2008) detected optically thin clouds with ice water content (IWC) down to 0.01 ppmv using the airborne limb instrument Cryogenic Infrared Spectrometers and Telescopes for the Atmosphere – New Frontiers (CRISTA–NF) This IWC matches the lower limit of the expected IWC for mid-latitude cirrus clouds, 0.01–200 ppmv (Luebke et al, 2016). Our study uses data from the airborne Gimballed Limb Observer for Radiance Imaging of the Atmosphere (GLORIA) instrument (Riese et al, 2014; Friedl-Vallon et al, 2014) This instrument possesses the technical characteristics necessary for the detection of thin cirrus and SVCs. This instrument possesses the technical characteristics necessary for the detection of thin cirrus and SVCs It has a spatial sampling of 140 m × 140 m (horizontal sampling × vertical sampling) at a tangent point altitude (i.e., closest point of the LOS to the Earth’s surface) of 10 km for a flight altitude of 15 km. The tropopause was computed following the definition of the first thermal tropopause from WMO (1957)
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