Abstract
Abstract. An improved cloud-index-based method for the detection of clouds in limb sounder data is presented that exploits the spatial overlap of measurements to more precisely detect the location of (optically thin) clouds. A second method based on a tomographic extinction retrieval is also presented. Using CALIPSO data and a generic advanced infrared limb imaging instrument as examples for a synthetic study, the new cloud index method has a better horizontal resolution in comparison to the traditional cloud index and has a reduction of false positive cloud detection events by about 30 %. The results for the extinction retrieval even show an improvement of 60 %. In a second step, the extinction retrieval is applied to real 3-D measurements of the airborne Gimballed Limb Observer for Radiance Imaging in the Atmosphere (GLORIA) taken during the Wave-driven ISentropic Exchange (WISE) campaign to retrieve small-scale cirrus clouds with high spatial accuracy.
Highlights
Clouds, and in particular cirrus clouds, play an important part in the radiative balance of the atmosphere
In addition to an enhancement of the cloud index method, we investigate the capabilities of a tomographic extinction retrieval that employs the same techniques used for the tomographic retrieval of temperature and trace gases
We assume a pixel pitch of 0.014◦, which corresponds roughly to a vertical sampling of ≈ 700 m in the troposphere. These capabilities are in line with those of the GLORIA instrument and certainly achievable. They correspond to the “dynamics mode”, which was envisioned to be used for about half of the instrument measurement time; its primary purpose was a high spatial resolution to reveal processes associated with mixing and convective outflow in the upper troposphere–lower stratosphere (UTLS), as well as threedimensionally resolving gravity waves to determine momentum fluxes driving global circulation systems (ESA, 2012)
Summary
In particular cirrus clouds, play an important part in the radiative balance of the atmosphere. To complement the work with synthetic measurements, the final part of this paper applies the extinction tomography to evaluate a tomographic measurement of cirrus clouds made by the Gimballed Limb Observer for Radiance Imaging in the Atmosphere (GLORIA) (Riese et al, 2014; Friedl-Vallon et al, 2014). This airborne instrument points at a 90◦ angle in relation to the heading of its carrier and has a very different measurement geometry compared to along-track-pointing satellite instruments.
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