Abstract
AbstractCombining vertically resolved lidar retrievals of water vapor and cloud detection, we document a 2‐day subtropical cirrus case study over La Réunion (20.9°S–55.5°E) in March 2005, focusing on the conditions of ice supersaturation inside and near the observed cloud. Using satellite observations, we describe the synoptic conditions leading to cloud formation. Supersaturation occurs 25% of the time within the cirrus, up to 35% in its middle segment, where relative humidity goes beyond 150%. In clear‐sky areas, relative humidity stays consistently low, especially in profiles without clouds. High‐troposphere atmospheric waves could initiate the formation of supersaturation conditions, especially on 16 March.
Highlights
Cirrus clouds form mostly at temperatures colder than 240 K and altitudes higher than 7 km (Heintzenberg and Charlson, 2009)
We evaluated water vapor saturation over ice within and around subtropical cirrus clouds, by combining profiles of temperature, pressure, cloud detection and water vapor mixing ratio observed from Raman lidar over La Réunion
We find that supersaturation occurs frequently within the observed cirrus clouds (25% of the time), and most frequently in the middle segment of clouds (34% of the time), in agreement with similar results in the North Hemisphere (C04)
Summary
Cirrus clouds form mostly at temperatures colder than 240 K and altitudes higher than 7 km (Heintzenberg and Charlson, 2009). Microphysical and radiative processes drive their formation and evolution (Dinh and Fueglistaler, 2014) Those are controlled, on first approximation, by the temperature and amount of humidity available in the environment: the relative humidity over ice crossing 100%, or supersaturation. Gayet et al, 2004; Spichtinger et al, 2004), but because their autonomy is limited aircraft cannot document all synoptic conditions everywhere on the globe They can hardly document the lifetime of a cloud system over several days. The difficulty to document the processes responsible for the persistence of supersaturation at a large-scale contributes to a poor representation of cirrus clouds in climate models (Zhang et al, 2005; Ringer et al, 2006). Satellite observations, we examine ice supersaturation conditions within and around the cirrus clouds (Section 3).
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