MIPAS detection of cloud and aerosol particle occurrence in the UTLS with comparison to HIRDLS and CALIOP

Abstract

Abstract. Satellite infrared emission instruments require efficient systems that can separate and flag observations which are affected by clouds and aerosols. This paper investigates the identification of cloud and aerosols from infrared, limb sounding spectra that were recorded by the Michelson Interferometer for Passive Atmospheric Sounding (MIPAS), a high spectral resolution Fourier transform spectrometer on the European Space Agency's (ESA) ENVISAT (Now inoperative since April 2012 due to loss of contact). Specifically, the performance of an existing cloud and aerosol particle detection method is simulated with a radiative transfer model in order to establish, for the first time, confident detection limits for particle presence in the atmosphere from MIPAS data. The newly established thresholds improve confidence in the ability to detect particle injection events, plume transport in the upper troposphere and lower stratosphere (UTLS) and better characterise cloud distributions utilising MIPAS spectra. The method also provides a fast front-end detection system for the MIPClouds processor; a processor designed for the retrieval of macro-and microphysical cloud properties from the MIPAS data.It is shown that across much of the stratosphere, the threshold for the standard cloud index in band A is 5.0 although threshold values of over 6.0 occur in restricted regimes. Polar regions show a surprising degree of uncertainty at altitudes above 20 km, potentially due to changing stratospheric trace gas concentrations in polar vortex conditions and poor signal-to-noise due to cold atmospheric temperatures. The optimised thresholds of this study can be used for much of the time, but time/composition-dependent thresholds are recommended for MIPAS data for the strongly perturbed polar stratosphere. In the UT, a threshold of 5.0 applies at 12 km and above but decreases rapidly at lower altitudes. The new thresholds are shown to allow much more sensitive detection of particle distributions in the UTLS, with extinction detection limits above 13 km often better than 10 −4 km −1 , with values approaching 10 −5 km −1 in some cases.Comparisons of the new MIPAS results with cloud data from HIRDLS and CALIOP, outside of the poles, establish a good agreement in distributions (cloud and aerosol top heights and occurrence frequencies) with an offset between MIPAS and the other instruments of 0.5 km to 1 km between 12 km and 20 km, consistent with vertical oversampling of extended cloud layers within the MIPAS field of view. We conclude that infrared limb sounders provide a very consistent picture of particles in the UTLS, allowing detection limits which are consistent with the lidar observations. Investigations of MIPAS data for the Mount Kasatochi volcanic eruption on the Aleutian Islands and the Black Saturday fires in Australia are used to exemplify how useful MIPAS limb sounding data were for monitoring aerosol injections into the UTLS. It is shown that the new thresholds allowed such events to be much more effectively derived fro...

Infrared limb emission measurements of aerosol in the troposphere and stratosphere

“…A fixed CI threshold of 6 (yellow, orange, red, dark red) captures the UTLS aerosol layer but mistakes cloud-free regions (profiles 0-3, 56-60, and 92-95; see comparison with IR nadir data below) as cloudy. This feature of the CI of becoming smaller at lower altitudes in cloud-free conditions is addressed by Sembhi et al (2012), providing a variable CI threshold definition at altitudes above 10 km. Here we used a simplified variable CI threshold based on Sembhi et al (2012).…”

“…It is defined as the ratio between the mean radiances around the 792 cm −1 band with strong CO 2 emissions and the atmospheric window region around 833 cm −1 : (Spang et al, 2004Sembhi et al, 2012). The CI detection threshold depends on altitude, latitude, and season, mainly because of the water vapour continuum contributing to the 833 cm −1 window radiance (Spang et al, 2004;Sembhi et al, 2012).…”

Volcanic ash detection with infrared limb sounding: MIPAS observations and radiative transfer simulations

“…CI values in between relate to optically very thin clouds or clouds that expand over only a small part of the MIPAS field of view. For more specialised purposes Spang and Remedios (2003, for CRISTA) and Höpfner et al (2009) use a cloud detection threshold of 4.5 for polar stratospheric clouds and, depending on altitude, season and latitude, Sembhi et al (2012) suggest a CI up to 6 for cloud and aerosol detection.…”

“…Spang et al (2008) showed that the CI is also suited for the cloud evaluation of the air-borne CRISTA -New Frontiers (CRISTA-NF) measurements. Sembhi et al (2012) optimised the CI thresholds to also detect aerosol with MIPAS. Regarding the discrimination between different cloud and aerosol types, , Spang et al (2004Spang et al ( , 2005, Höpfner et al (2009), and have presented methods to distinguish between different types of polar stratospheric clouds (PSCs).…”

Cloud discrimination in probability density functions of limb-scattered sunlight measurements

“…Limb-emission measurements in the infrared have also been employed for studying clouds. Greenhough et al (2005) used MIPAS (Michelson Interferometer for Passive Atmospheric Sounding) measurements to retrieve a cloud detection index, and Sembhi et al (2012) studied MIPAS detection limits for cloud and aerosol particles. We use measurements from the Optical Spectrograph and Infrared Imaging System (OSIRIS), a Canadian satellite instrument that measures atmospheric limb profiles of scattered solar radiation, along with a statistical approach to characterize and discriminate cloud scattering.…”