Abstract
Abstract. Here we present observations of the Cryogenic Infrared Spectrometers and Telescopes for the Atmosphere (CRISTA) of cirrus cloud and water vapour in August 1997 in the upper troposphere and lower stratosphere (UTLS). The observations indicate a considerable flux of moisture from the upper tropical troposphere into the extratropical lowermost stratosphere (LMS), resulting in the occurrence of high-altitude optically thin cirrus clouds in the LMS. The locations of the LMS cloud events observed by CRISTA are consistent with the tropopause height determined from coinciding radiosonde data. For a hemispheric analysis in tropopause relative coordinates an improved tropopause determination has been applied to the European Centre for Medium-Range Weather Forecasts (ECMWF) temperature profiles. We found that a significant fraction of the cloud occurrences in the tropopause region are located in the LMS, even if a conservative overestimate of the cloud top height (CTH) determination by CRISTA of 500 m is assumed. The results show rather high occurrence frequencies (~ 5%) up to high northern latitudes (70° N) and altitudes well above the tropopause (> 500 m at ~350 K and above) in large areas at mid- and high latitudes. Comparisons with model runs of the Chemical Lagrangian Model of the Stratosphere (CLaMS) over the CRISTA period show a reasonable consistency in the retrieved cloud pattern. For this purpose a limb ray tracing approach was applied through the 3-D model fields to obtain integrated measurement information through the atmosphere along the limb path of the instrument. The simplified cirrus scheme implemented in CLaMS seems to cause a systematic underestimation in the CTH occurrence frequencies in the LMS with respect to the observations. The observations together with the model results demonstrate the importance of isentropic, quasi-horizontal transport of water vapour from the subtropics and the potential for the occurrence of cirrus clouds in the lowermost stratosphere and tropopause region.
Highlights
A large proportion of the uncertainties of climate change projections by general circulation models (GCMs) arises from poorly understood and represented interactions and feedbacks between dynamic, microphysical, and radiative processes affecting cirrus clouds (IPCC, 2014)
There are a couple of ground-based lidar observations suggesting the presence of cirrus clouds in the lowermost stratosphere (LMS), a region strongly influenced by isentropic transport of air masses from the tropics (Gettelman et al, 2011), there are open questions: which microphysical process and specific meteorological conditions foster the formation of ice particles in this specific region, how frequently do these cirrus clouds occur on global scales, and are clouds tops or even complete clouds significantly above the tropopause?
This is in coincidence with the slightly higher cloud layer detected by CRISTA, where cloud top height (CTH) and tropopause height (TPH) suggest a cloud above the local tropopause
Summary
A large proportion of the uncertainties of climate change projections by general circulation models (GCMs) arises from poorly understood and represented interactions and feedbacks between dynamic, microphysical, and radiative processes affecting cirrus clouds (IPCC, 2014). There are a couple of ground-based lidar observations suggesting the presence of cirrus clouds in the lowermost stratosphere (LMS), a region strongly influenced by isentropic (quasi-horizontal) transport of air masses from the tropics (Gettelman et al, 2011), there are open questions: which microphysical process and specific meteorological conditions foster the formation of ice particles in this specific region, how frequently do these cirrus clouds occur on global scales, and are clouds tops or even complete clouds significantly above the tropopause?. A 100 km or even 1 km horizontally extended cirrus cloud is detectable by an IR limb sounder with an ice water content (IWC) of 3 × 10−6 and 3 × 10−4 g m−3 respectively (Spang et al, 2012), assuming that the cloud fills the vertical field of view of the instrument completely These values represent even better detection sensitivity than the current CALIOP cloud products.
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