Abstract
Abstract. As knowledge about the cirrus clouds in the lower stratosphere is limited, reliable long-term measurements are needed to assess their characteristics, radiative impact and important role in upper troposphere and lower stratosphere (UTLS) chemistry. We used 6 years (2006–2012) of Michelson Interferometer for Passive Atmospheric Sounding (MIPAS) measurements to investigate the global and seasonal distribution of stratospheric cirrus clouds and compared the MIPAS results with results derived from the latest version (V4.x) of the Cloud-Aerosol Lidar and Infrared Pathfinder Satellite Observations (CALIPSO) data. For the identification of stratospheric cirrus clouds, precise information on both the cloud top height (CTH) and the tropopause height is crucial. Here, we used lapse rate tropopause heights estimated from the ERA-Interim global reanalysis. Considering the uncertainties of the tropopause heights and the vertical sampling grid, we define CTHs more than 0.5 km above the tropopause as stratospheric for CALIPSO data. For MIPAS data, we took into account the coarser vertical sampling grid and the broad field of view so that we considered cirrus CTHs detected more than 0.75 km above the tropopause as stratospheric. Further sensitivity tests were conducted to rule out sampling artefacts in MIPAS data. The global distribution of stratospheric cirrus clouds was derived from night-time measurements because of the higher detection sensitivity of CALIPSO. In both data sets, MIPAS and CALIPSO, the stratospheric cirrus cloud occurrence frequencies are significantly higher in the tropics than in the extra-tropics. Tropical hotspots of stratospheric cirrus clouds associated with deep convection are located over equatorial Africa, South and Southeast Asia, the western Pacific, and South America. Stratospheric cirrus clouds were more often detected in December–February (15 %) than June–August (8 %) in the tropics (±20∘). At northern and southern middle latitudes (40–60∘), MIPAS observed about twice as many stratospheric cirrus clouds (occurrence frequencies of 4 %–5 % for MIPAS rather than about 2 % for CALIPSO). We attribute more frequent observations of stratospheric cirrus clouds with MIPAS to the higher detection sensitivity of the instrument to optically thin clouds. In contrast to the difference between daytime and night-time occurrence frequencies of stratospheric cirrus clouds by a factor of about 2 in zonal means in the tropics (4 % and 10 %, respectively) and at middle latitudes for CALIPSO data, there is little diurnal cycle in MIPAS data, in which the difference of occurrence frequencies in the tropics is about 1 percentage point in zonal mean and about 0.5 percentage point at middle latitudes. The difference between CALIPSO day and night measurements can also be attributed to their differences in detection sensitivity. Future work should focus on better understanding the origin of the stratospheric cirrus clouds and their impact on radiative forcing and climate.
Highlights
Cirrus clouds are ice clouds that form at cold temperatures in the middle and upper troposphere
Considering a typical ±0.3 km bias of the ERA-Interim lapse rate tropopause (LRT) data with respect to Global Positioning System (GPS) measurements and the 0.2 km vertical grid sampling of the Cloud-Aerosol Lidar and Infrared Pathfinder Satellite Observations (CALIPSO) data, an uncertainty of 0.5 km was used for stratospheric cirrus cloud detections, which is comparable to the approach of Homeyer et al (2010) and Pan and Munchak (2011)
The means of the distances of the cloud top height (CTH) to the tropopause in Fig. 9 are larger in the tropics (1.1 to 1.3 km at the tropical hotspots) than at middle latitudes (0.75 to 1.0 km). These differences might relate to the 300 m low bias of the ERA-Interim tropopause heights in the tropics compared to GPS measurements and the different underlying causes for stratospheric cirrus clouds in the tropics, such as overshooting convection (De Reus et al, 2009; Iwasaki et al, 2015) and wave activity (Alexander et al, 2000) and, in the extra-tropics, such as double tropopause events (Noël and Haeffelin, 2007), we introduced an additional criterion for the MD-TP so that it is more homogeneous at all latitudes to rule out sampling artefacts
Summary
Cirrus clouds are ice clouds that form at cold temperatures in the middle and upper troposphere. About twice as many stratospheric cirrus clouds (∼ 5 %) were detected during 2 weeks of infrared limb emission measurements in boreal summer 1997 with the CRISTA instrument over 40–60◦ N (Spang et al, 2015). Potential reasons for these differences could be the higher detection sensitivity of infrared (IR) limb emission measurements compared to the standard CALIPSO data product and a sampling bias due to the comparison of a 2 week period in 1996 versus a 4-year 3-monthly mean between 2006 and 2010. High detection accuracy and sensitivity measurements are of significant importance for investigating the global occurrence and distribution of stratospheric cirrus clouds.
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