Synop&c variability in specific humidity and isotope composi&on over the subtropical North Atlan&c: a numerical modelling study
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Synop&c variability in specific humidity and isotope composi&on
over the subtropical North Atlan&c: a numerical modelling study
COSMO / ICON User Workshop
13 January 2022
Fabienne Dahinden1, Heini Wernli1, Franziska Aemisegger1, Lukas Papritz1,
Peter Knippertz2, Christopher J. Diekmann2, MaLhias Schneider2, Stephan Pfahl3
1 Ins&tute for Atmospheric and Climate Science, ETH Zurich
2 Ins&tute of Meteorology and Climate Research, Karlsruhe Ins&tute of Technology
3 Ins&tute for Meteorology, Freie Universität Berlin
COSMO User Workshop 2022
Subtropical atmospheric water cycle
worldview.earthdata.nasa.gov
Key component of the climate system
à greenhouse effect: free-tropospheric humidity
(e.g., Held and Soden, 2000; Schmidt et al., 2010)
à albedo: low-level cloud cover
(e.g., Bony and Dufresne, 2005; Stephens, 2005)
Considerable uncertainty on how low-level clouds are linked to small-scale mixing processes,
exchange with the free troposphere, and large-scale circula:ons
Complex interac=ons of dynamical and physical processes that control
the free-tropospheric moisture variability in this sensi=ve region not fully understood
The representa:on and coupling of these processes in model simula=ons
cri=cally influences climate projec:ons
à be
Stable water isotopes
Isotopic composiRon of a water sample
light isotope
!!"#$%&
!D = − 1 (‰) (99.731%)
!!'"()"*)
16O
Molecular raRo of isotope concentraRon 1H 1H
!
"#$%& '()*)+# ./!"0
( = = !
,'-"* '()*)+# .#!"0
heavy isotopes
(0.031%) (0.200%)
à Natural tracers of water phase changes
à Record informaRon about evapora*on and 16O 18O
condensa*on processes during the transport 1H D=2H 1H 1H
of air parcels
Gat (2010)
ETH Zurich COSMO User Workshop 2022 2
Methods for inves9ga9ng the subtropical water cycle
Eulerian framework
COSMOisotag
equipped with two diagnos=c tools: Lagrangian framework
Stable water isotopes Moisture tagging Backward trajectories
(COSMOiso: Pfahl et al., 2012) (COSMOtag: Winschall et al., 2014)
à calculated from COSMOisotag
à insight into moist diaba*c à passive tracers represen5ng
output fields
history experienced by air moisture origina5ng from pre- (LAGRANTO: Sprenger and Wernli, 2015)
parcels specified source regions à explicit information about
à limited informa5on about à quan5fica5on of rela*ve long-range moisture transport
origin of moisture contribu*on of different moisture pathways
sources to a given air mass à cannot resolve sub-grid-scale
processes
à direct link between evapora5ve origin of moisture and its isotope signature
à represents all relevant processes of water cycle (incl. mixing)
à does not retain explicit transport pathway of atmospheric moisture
ETH Zurich COSMO User Workshop 2022 3
Setup for COSMOisotag simula9ons
• June–Sept. 2016–2018
• 12 km horizontal resolution
• 60 vertical levels
• Explicit convection
à COSMO performs better for many aspects of the
hydrological cycle
(Vergara-Temprado et al. 2020)
à more realistic representation of the West African
monsoon already with a horizontal resolution of ~10 km
(Marsham et al. 2013, Pante and Knippertz 2019)
• Boundary data: ECHAM5-wiso
ETH Zurich COSMO User Workshop 2022 4
Setup for COSMOisotag simula9ons
surface tracers
boundary tracers
Canary Islands
ETH Zurich COSMO User Workshop 2022 5
Water tracer analysis for the Canary Islands region (CAN)
surface tracers
boundary tracers
max. abs. error of
tagged +! : 0.28 g kg-1
ETH Zurich COSMO User Workshop 2022 6
Backward trajectories for the period of 22–26 July 2018
7-day backward trajectories started at 600 hPa above the Canary Islands region (CAN)
!D "!
• 4 d back in Ime
ETH Zurich COSMO User Workshop 2022 7
Uptakes of tagged moisture in the period of 22–26 July 2018
AFR EUR MED
black contours: location of
the non-stationary Saharan
heat low (SHL) in the
considered time period
TRP ATL ETRP NATL
Lagrangian moisture source
diagnosScs using the method
of Sodemann et al. (2008)
and Pfahl and Wernli (2008)
ETH Zurich COSMO User Workshop 2022 8
Correla9on of !D with frac9on of "! from passive tracers
July–August 2016–2018
Isotope signal good indicator for 600 hPa above CAN
origin of moisture?
à yes!
The higher the frac=on of moisture
origina=ng from evapora=ve source
regions (light coloured tracers), the
higher !D
The higher the frac=on of moisture The higher the fraction of moisture
evaporated from the originating from the extratropical North
Mediterranean, the higher !D Atlantic model boundary, the lower !D
ETH Zurich COSMO User Workshop 2022 9Correla9on of !D with frac9on of "! from passive tracers at 600 hPa
Where is the isotope signal an indicator for the origin of moisture?
max. correla=on July–August 2016–2018 tracer of max. correla=on (|r|>0.6)
hatching: r < 0
ETH Zurich COSMO User Workshop 2022 10Conclusions
• Free-tropospheric moisture budget over the Canary Islands region influenced by
1) dry, low-!D air from the upper-level extratropical North Atlan=c
2) humid, high-!D air from North Africa
à in agreement with González et al. (2016), Schneider et al. (2016), Lacour et al. (2017), Dahinden et al. (2021)
• These two contras:ng circula:on regimes alternate on synop:c :mescales and
control the short-term variability in specific humidity and isotopic composi:on
• Key role of SHL in moistening the free troposphere over the subtropical North Atlan:c in summer
à moisture from mixed source regions advected at low levels into the SHL
à dry convec:vely liFed to the mid troposphere
à transported over the North Atlan=c
• High correla:ons of "D and rela=ve contribu=ons from certain passive water tracers over the
subtropical and tropical North Atlan=c as well as western Sahel
à Water vapour isotopes are indicators for the origin of moisture
ETH Zurich COSMO User Workshop 2022 11Cloud and aerosol distribu/on over West Africa and the eastern
North Atlan/c showing a typical example of the mid-summer
state of the West African Monsoon
Thank you
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