Global cloud top heights and thermodynamic phase classification as derived from SCIAMACHY visible and near infrared limb spectra: Verification and examples
Kai-Uwe Eichmann(1), Christian von Savigny(1), Alexander Kokhanovsky(1) and Heinrich Bovensmann(1)
(1) University of Bremen, Postfach 33 04 40, 28334 Bremen, Germany
Global cloud top heights (CTH) and cloud thermodynamic phase (CTP)
discrimination using SCIAMACHY limb measurements in the visible and near
infrared is presented in this paper. The effective CTH is an important
parameter for extending the limb trace gas retrievals towards the
troposphere. Use of CTH information reduces the error above clouds
significantly as was shown in theoretical studies (Sonkaev, 2009). The cloud thermodynamic phase is important for both cloud modelling and cloud properties remote sensing.
A simple scheme has been designed based on work from von Savigny et al.(2005) and Acarreta et al.(2004) to determine the quantities CTH and CTP for the limb viewing geometry for tropospheric clouds, polar stratospheric clouds and noctilucent clouds. The detection of cloud top heights is performed by their signature found using the colour index ratio (CIR) in the vertical radiance profiles at wavelengths around 750 and 1090 nm. The cloud phase is derived from spectra around 1.6 micron, where the absorption of water and ice have different spectral signatures. The height resolution is due to the limb scanning concept of SCIAMACHY about 3.3 km. Flagging of cloudy pixels was done using a threshold method.
This study shows global maps of the parameters CTH and CTP. Verification with the SACURA model (Kokhanovsky,2005) using nadir measurements of SCIAMACHY gives good global agreement of CTH. The sensitivity of the method and problems are shown with an outbreak of a volcano, that leads to semi-global, longer lasting detection of false PSCs in the northern hemisphere in autumn 2008.