Abstract
The automatic contrail tracking algorithm (ACTA) – developed to automatically follow contrails as they age, drift and spread – enables the study of a large number of contrails and the evolution of contrail properties with time. In this paper we present a year's worth of tracked contrails, from August 2008 to July 2009 in order to derive statistically significant mean values. The tracking is performed using the 5 min rapid-scan mode of the Spinning Enhanced Visible and Infrared Imager (SEVIRI) on board the Meteosat Second Generation (MSG) satellites. The detection is based on the high spatial resolution of the images provided by the Moderate Resolution Imaging Spectroradiometer on board the Terra satellite (Terra/MODIS), where a contrail detection algorithm (CDA) is applied. The results show the satellite-derived average lifetimes of contrails and contrail-cirrus along with the probability density function (PDF) of other geometric characteristics such as mean coverage, distribution and width. In combination with specifically developed algorithms (RRUMS; Rapid Retrieval of Upwelling irradiance from MSG/SEVIRI and COCS (Cirrus Optical properties derived from CALIOP and SEVIRI), explained below) it is possible to derive the radiative forcing (RF), energy forcing (EF), optical thickness (τ) and altitude of the tracked contrails. Mean values here retrieved are duration, 1 h; length, 130 km; width, 8 km; altitude, 11.7 km; optical thickness, 0.34. Radiative forcing and energy forcing are shown for land/water backgrounds in day/night situations.
Highlights
One of the least understood influences that the air traffic has in the atmosphere results from contrail formation
Two recently developed algorithms to derive cloud properties are used in this paper to retrieve contrail radiative forcing, energy forcing, optical thickness and cloud top height: RRUMS (Rapid Retrieval of Upwelling irradiance from Meteosat Second Generation (MSG)/Spinning Enhanced Visible and Infrared Imager (SEVIRI), Vázquez-Navarro et al, 2013) and COCS (Cirrus Optical properties derived from CALIOP and SEVIRI, Kox et al, 2014)
The data set consists of 25 179 contrail events issued from the 2375 contrails detected by the contrail detection algorithm (CDA) on Terra/MODIS and tracked by automatic contrail tracking algorithm (ACTA) on MSG/SEVIRI (Rapid Scan Service)
Summary
One of the least understood influences that the air traffic has in the atmosphere results from contrail formation. A modified version of the algorithm has recently been used by Duda et al (2013); Bedka et al (2013) and Spangenberg et al (2013) to estimate several linear contrail properties in the Northern Hemisphere When they are past the linear stage, contrails spread and lose their distinct shape and the CDA can no longer detect them. At first sight, these aged contrails are indistinguishable from natural cirrus clouds, but it is possible to continue tracking them by combining the information on location and displacement when such data are available. The COCS algorithm retrieves optical thickness and top altitude of cirrus clouds through a combination of SEVIRI infrared channels. Note that throughout this paper, the word contrail refers both to linear contrail and contrail cirrus
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