Abstract
Abstract. Ice supersaturation in the upper troposphere is a complex and important issue for the understanding of cirrus cloud formation. On one hand, infrared sounders have the ability to provide cloud properties and atmospheric profiles of temperature and humidity. On the other hand, they suffer from coarse vertical resolution, especially in the upper troposphere and therefore are unable to detect shallow ice supersaturated layers. We have used data from the Measurements of OZone and water vapour by AIrbus in-service airCraft experiment (MOZAIC) in combination with Atmospheric InfraRed Sounder (AIRS) relative humidity measurements and cloud properties to develop a calibration method for an estimation of occurrence frequencies of ice supersaturation. This method first determines the occurrence probability of ice supersaturation, detected by MOZAIC, as a function of the relative humidity determined by AIRS. The occurrence probability function is then applied to AIRS data, independently of the MOZAIC data, to provide a global climatology of upper-tropospheric ice supersaturation occurrence. Our climatology is then compared to ice supersaturation occurrence statistics from MOZAIC alone and related to high cloud occurrence from the Cloud-Aerosol Lidar with Orthogonal Polarization (CALIOP). As an example of application it is compared to model climatologies of ice supersaturation from the Integrated Forecast System (IFS) of the European Centre for Medium-Range Weather Forecasts (ECMWF) and from the European Centre HAmburg Model (ECHAM4). This study highlights the benefits of multi-instrumental synergies for the investigation of upper tropospheric ice supersaturation.
Highlights
Ice supersaturation is a prerequisite for the nucleation of cirrus clouds (e.g. Haag et al, 2003) and for the persistence of condensation trails (Schumann, 1996)
We have presented a climatology of ice supersaturation occurrence from Atmospheric InfraRed Sounder (AIRS) observations covering the period 2003– 2009
The relatively coarse vertical resolution of the AIRS retrieved water vapour profiles prohibits the direct detection of shallow supersaturated layers
Summary
Ice supersaturation (or “ISS”, relative humidity with respect to ice larger than 100 %) is a prerequisite for the nucleation of cirrus clouds (e.g. Haag et al, 2003) and for the persistence of condensation trails (Schumann, 1996). Nine AIRS measurements (3 × 3) correspond to one footprint of the Advanced Microwave Sounder Unit (AMSU) within which atmospheric profiles of temperature T and specific humidity q are retrieved from cloud-cleared AIRS radiances at a spatial resolution of about 40 km at nadir (Chahine et al, 2006). In Montoux et al (2009) data higher than 200 hPa in altitude have shown consistency with in-situ measurements in the tropics and subtropics These authors insist on the strong influence of the a priori profiles used in the retrieval at such altitudes. No sensitivity threshold is used to remove low values of water vapor as it would certainly lead to even more biased results by considering all data
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