Synergy of stereo cloud top height and ORAC optimal estimation cloud retrieval: evaluation and application to AATSR

Abstract

Abstract. In this paper we evaluate the impact on the cloud parameter retrievals of the ORAC (Optimal Retrieval of Aerosol and Cloud) algorithm following the inclusion of stereo-derived cloud top heights as a priori information. This is performed in a mathematically rigorous way using the ORAC optimal estimation retrieval framework, which includes the facility to use such independent a priori information. Key to the use of a priori information is a characterisation of their associated uncertainty. This paper demonstrates the improvements that are possible using this approach and also considers their impact on the microphysical cloud parameters retrieved. The Along-Track Scanning Radiometer (AATSR) instrument has two views and three thermal channels, so it is well placed to demonstrate the synergy of the two techniques. The stereo retrieval is able to improve the accuracy of the retrieved cloud top height when compared to collocated Cloud-Aerosol Lidar and Infrared Pathfinder Satellite Observations (CALIPSO), particularly in the presence of boundary layer inversions and high clouds. The impact of the stereo a priori information on the microphysical cloud properties of cloud optical thickness (COT) and effective radius (RE) was evaluated and generally found to be very small for single-layer clouds conditions over open water (mean RE differences of 2.2 (±5.9) microns and mean COD differences of 0.5 (±1.8) for single-layer ice clouds over open water at elevations of above 9 km, which are most strongly affected by the inclusion of the a priori).

Methods to Retrieve the Cloud-Top Height in the Frame of the JEM-EUSO Mission

“…During the last twenty years, space-based stereoscopic methods have been raising interest among researchers and are improving thanks to new satellite-based instruments [9], [10], [11], [12], [13], [14], [15]. Stereo reconstruction is an alternative way to estimate the CTH compared with the more common radiative methods.…”

Wind Profile Satellite Observation Requirements and Capabilities

“…Complementary to cloud radar, in an ESA satellite convoy study, Stoffelen et al (2014) proposed a tandem thermal multispectral imager, that would be able to sense clouds simultaneously from different directions, enabling geometric cloud wind height and motion estimation from parallax observations. Fisher et al (2016) demonstrated that such stereo retrieval is indeed able to improve the accuracy of retrieved cloud-top height when compared to collocated Cloud-Aerosol Lidar and Infrared Pathfinder Satellite Observations (CALIPSO) data (Winker et al 2010). Recently, the Flow by IR tandem (FLIRt) mission proposal was submitted to ESA (Muller et al 2017), with the objective to retrieve updraft speeds, wind fields, thermal buoyancy, and rates of mixing with ambient air as well as the impact of aerosols on cloud convective processes.…”

Algorithm for Improved Stereoscopic Cloud-Top Height Retrieval Based on Visible and Infrared Bands for Himawari-8 and FY-4A

“…For example, stereoscopic methods have been applied to combinations of two geostationary orbit (GEO) satellites, which are located at two different locations [11][12][13][14][15][16][17], and combinations of GEO and low Earth orbit (LEO) satellites [10,18,19]. Stereoscopic methods have also been used by multi-view instruments onboard a single LEO satellite [9,[20][21][22][23][24][25].…”

“…This algorithm uses a visible (VIS) band to retrieve stereoscopic CTHs (VIS-band-based approach), and the retrieval accuracy generally tends to lie within the theoretical accuracy range. However, stereoscopic CTHs are associated with the strongest image contrast [24], which, in a VIS image, pertains to optically thick dense clouds [9]. Consequently, in a case in which optically thin upper clouds overlie lower clouds, the VIS-band-based stereoscopic CTHs represent the heights of not the upper clouds but the lower clouds [9,13,23,24].…”