CRISTA observations of cirrus clouds around the tropopause

Abstract

[1] The Cryogenic Infrared Spectrometers and Telescopes for the Atmosphere (CRISTA) instrument observed thin cirrus clouds at and above the tropopause during its two missions in November 1994 and August 1997. A simple cloud detection scheme was developed for extinctions greater than 2 Â 10 À3 km À1 through analysis of the measured infrared spectra in the 12-mm range. Horizontal and vertical distributions of cloud occurrence frequencies are in good agreement with the Stratospheric Aerosol and Gas Experiment (SAGE) II subvisual cirrus cloud (SVC) climatology as well as SAGE measurements for the 1997 period. Seasonal variations, strong longitudinal variability, and indications of enhanced cloud occurrence frequencies in separated regions caused by El Niño events were detected in the CRISTA data set. A substantial day-to-day variability could be found throughout the tropics, and several regions with enhanced variability have been identified. In addition, a significant amount of cloud was found above the midlatitude tropopause. Backward trajectories in relation to outgoing longwave radiation (OLR) measurements and cloud observation in the troposphere by meteorological satellites suggest that about three fourths of the high clouds (>15 km) observed by CRISTA in the tropics stem from deep convection systems and the outflow of these systems. This would imply that on the order of at least one fourth of the observed cloud events are originated by other mechanisms, such as in situ formation due to cooling events on synoptic and/or gravity wave scales. For the convective generated cirrus clouds, a maximum lifetime of around 3-4 days was estimated over a wide range of latitudes. Such a long lifetime could be important for modeling the impact of cirrus clouds on radiation budget (climate) and heterogeneous chemical processes around the tropopause.

Freezing thresholds and cirrus cloud formation mechanisms inferred from in situ measurements of relative humidity

“…The occurrence of thin cirrus clouds extending well into the lower stratosphere in the wintertime polar regions is a well known phenomenon (Kärcher and Solomon, 1999). Recent satellite data provide further compelling evidence for the widespread existence of subvisible cirrus at the tropopause (Spang et al, 2002).…”

Sea surface temperature as a proxy for convective gravity wave excitation: a study based on global gravity wave observations in the middle atmosphere

“…In studies which focus on monsoon regions, convective clouds dominate. In addition, in their outflow high-reaching convective clouds generate drifting high-level clouds (e.g., Spang et al, 2002). These clouds are opaque at high altitudes, i.e., cause low OLR values, but do not indicate convection and the corresponding latent heat release which could excite GWs.…”

“…High cloud top altitudes may indicate deep convection. Accordingly, high cloud top altitude has been used as a proxy for convective wave generation in previous studies (Ricciardulli and Garcia, 2000;Preusse, 2001;Spang et al, 2002;Wright and Gille, 2011). Strong convection generates high-reaching clouds, but high-level clouds may be also generated by different processes, as the extreme example of PSCs elucidates.…”

“…While OLR (or cloud top height in general, cf. also Preusse, 2001;Spang et al, 2002) is a well-suited proxy for convective GW excitation in regions dominated by deep convection, drifting high-level clouds make it very problematic for global application. We discuss this in Appendix A.…”

MIPAS-STR measurements in the Arctic UTLS in winter/spring 2010: instrument characterization, retrieval and validation

“…Also cirrus cloud particles affect mid-infrared limb-emission spectra (e.g. Spang et al, 2002) and the corresponding effects can be taken into account by dedicated radiative transfer models (Ewen et al, 2005). Further types of stratospheric aerosolparticles were measured by in-situ instruments, whereas the sources for some particle types remain unclear (e.g.…”