Skip to main content

Cirrus and water vapour transport in the tropical tropopause layer – Part 2: Roles of ice nucleation and sedimentation, cloud dynamics, and moisture conditions

Author(s): Dinh, Tra; Fueglistaler, Stephan; Durran, Dale; Ackerman, Thomas

Download
To refer to this page use: http://arks.princeton.edu/ark:/88435/pr1dk3j
Abstract: A high-resolution, two-dimensional numerical model is used to study the moisture redistribution following1 homogeneous ice nucleation induced by Kelvin waves in the tropical tropopause layer (TTL). We compare results for dry/moist initial conditions and three levels of complexity for the representation of cloud processes: complete microphysics and cloud radiative effects, likewise but without radiative effects, and instantaneous removal of moisture in excess of saturation upon nucleation. Cloud evolution and moisture redistribution are found to be sensitive1 to initial conditions and cloud processes. Ice sedimentation leads to a downward flux of water, whereas the cloud radiative heating induces upward advection of the cloudy air. The latter results in an upward (downward) flux of water vapour if the cloudy air is moister (drier) than the environment, which is typically when the environment is subsaturated (supersaturated). Only a fraction (∼25% or less) of the cloud experiences nucleation. Post-nucleation processes (ice depositional growth, sedimentation, and sublimation) are important to cloud morphology, and both dehydrated hydrated layers may be indicators of TTL cirrus occurrence. The calculation with instantaneous removal of moisture not only misses the hydration but also underestimates dehydration due to (i) nucleation before reaching the minimum saturation mixing ratio, and (ii) lack of moisture removal from sedimenting ice particles below the nucleation level. The sensitivity to initial conditions and cloud processes suggests that it is difficult to reach generic, quantitative estimates of cloud-induced moisture redistribution on the basis of case-by-case calculations. Cloud evolution and moisture redistribution are found to be sensitive to initial conditions and cloud processes. Ice sedimentation leads to a downward flux of water, whereas the cloud radiative heating induces upward advection of the cloudy air. The latter results in an upward (downward) flux of water vapour if the cloudy air is moister (drier) than the environment, which is typically when the environment is subsaturated (supersaturated). Only a fraction (similar to 25% or less) of the cloud experiences nucleation. Post-nucleation processes (ice depositional growth, sedimentation, and sublimation) are important to cloud morphology, and both dehydrated and hydrated layers may be indicators of TTL cirrus occurrence. The calculation with instantaneous removal of moisture not only misses the hydration but also underestimates dehydration due to (i) nucleation before reaching the minimum saturation mixing ratio, and (ii) lack of moisture removal from sedimenting ice particles below the nucleation level. The sensitivity to initial conditions and cloud processes suggests that it is difficult to reach generic, quantitative estimates of cloud-induced moisture redistribution on the basis of case-by-case calculations.
Publication Date: 20-Nov-2014
Citation: Dinh, Tra, Stephan Fueglistaler, Dale Durran, and Thomas Ackerman. "Cirrus and water vapour transport in the tropical tropopause layer-Part 2: Roles of ice nucleation and sedimentation, cloud dynamics, and moisture conditions." Atmospheric Chemistry and Physics 14, no. 22 (2014): 12225-12236. doi:10.5194/acp-14-12225-2014.
DOI: doi:10.5194/acp-14-12225-2014
ISSN: 1680-7316
EISSN: 1680-7324
Pages: 12225 - 12236
Type of Material: Journal Article
Journal/Proceeding Title: Atmospheric Chemistry and Physics
Version: Final published version. This is an open access article.



Items in OAR@Princeton are protected by copyright, with all rights reserved, unless otherwise indicated.