Mesoscale Atmospheric Systems
Atmospheric moisture transport and
stable water isotopes
Stephan Pfahl
23 May 2017
MAS Topics
STE
Fronts
Convection
Evaporation
(Extreme)
precipitation
Fronts
Radar
Moisture transport
2
Seasonal mean distribution of water vapour
Vertically
integrated water
vapour (IWV)
Units: kg/m2
DJF
JJA
ERA-40 Atlas (2005)
Vertical distribution of humidity
Mean profiles in NH on 1 June 2001 12 UTC (ERA-Interim data)
Precipitation and evaporation over the ocean
Precipitation DJF
Precipitation JJA
Evaporation DJF
Evaporation JJA
Evaporation minus precipitation
DJF
JJA
Evaporation minus precipitation
DJF
Moisture flux
divergence
=
E-P (freshwater
flux)
JJA
(with w = IWV)
Water vapour fluxes
Columnintegrated vector
fluxes of water
vapour and their
convergence (in
kg m-2s-1)
DJF
JJA
ERA-40 Atlas (2005)
Animation of IWV (72h after 05 Apr 2014)
Vertically integrated water vapour (IWV) from SSM/I and AMSR-E
Source: http://tropic.ssec.wisc.edu/real-time/mimic-tpw/global2/main.html
„Atmospheric
rivers"
-  Poleward water vapour flux in narrow filaments
-  Total flux has similar magnitude as the major rivers
IWV
Ralph et al. (2011)
Atmospheric river concept
Atmospheric rivers are characterised by high values of both IWV
and integrated vapour transport (IVT,
IWV and sea level pressure
)
IVT and wind speed @250 hPa
Cordeira et al. (2013)
Atmospheric rivers and cyclones
Atmospheric rivers typically occur in the warm sector of cyclones
fronts
Cyclone-centred IWV (left) and IVT (right) for North Atlantic cyclone
Dacre et al. (2015)
Atmospheric rivers and cyclones
Schematic view of an atmospheric river in the Northeast Pacific
along-front wind
along-front
moisture flux
specific humidity
Ralph et al. (2004)
Water vapour sources in an atmospheric river
Numerical water vapour tracers released from ocean surface in
regional model simulation
Sodemann and Stohl (2013)
Water vapour sources in an atmospheric river
Strong AR event in southern Norway associated with a cyclone‘s
frontal system on 14 Dec 2006
IWV, SLP, winds @ 700hPa
water vapour tracers, 14mm IWV
Sodemann and Stohl (2013)
Moisture transport with and without ARs
=> more remote moisture
and more intense
precipitation with ARs
Sodemann and Stohl (2013)
Summary
•  Atmospheric water vapour transport provides an important link
between evaporation and precipitation on global scales.
•  Mesoscale processes associated with cyclones and fronts
control poleward moisture transport in atmospheric rivers.
•  It is difficult to assess the details of moisture transport based
on observations.
Stable water isotopes
SWI in atmospheric waters can be
used as diagnostic tools in order to
•  improve our understanding of the
present day water cycle (e.g.
moisture transport, atmospherevegetation feedbacks,
microphysical processes in
clouds, ...)
•  obtain information on past
climates (e.g., via their
concentration in ice cores)
Stable water isotopes: species and molecules
Stable heavy isotopes of O and H:
18O, 17O and 2H (or D)
→ stable water isotopes: H216O, H218O, HD16O, ...
Natural abundances of oxygen and hydrogen isotopes:
Mook (2001)
Isotope ratios and the δ notation
for V-SMOV, i.e. heavy water isotopes are relatively rare
small abundance → δ-notation, e.g.
[‰]
V-SMOV: Vienna standard mean ocean water (defined by IAEA)
Isotope fractionation
Mass difference causes slight changes in physical properties
18R
vap
18R
< 18Rliq
liq
Equilibrium fractionation:
different binding energies
→ heavy isotopes are more abundant in the condensed phase, have
smaller water vapour pressures
(a quantum mechanical effect, basically controlled by temperature)
Equilibrium fractionation during cloud
formation
18R
0
18R
1
=
<
18R
0
Isotope measurements in precipitation
Continental isotope map interpolated from station
measurements (Bowen and Wilkinson, 2002)
Typical isotope ratios in natural reservoirs
by definition: V-SMOV = 0‰
Mook, 2001
Non-equilibrium („kinetic“) fractionation
Lower diffusion velocities of heavy isotopes lead to
additional, diffusion-controlled fractionation during
transport under non-equilibrium conditions.
Non-equilibrium fractionation
atmospheric example:
evaporation from the sea
zl
other examples:
–  formation of ice clouds, when supersaturation occurs
–  re-evaporation of rain drops under the cloud base in
unsaturated air
Deuterium excess
•  for equilibrium fractionation:
δD
18
δ O
≈8
(only slight temperature dependence)
•  relative importance of non-equilibrium fractionation much
larger for δ18O
•  deuterium excess
equilibrium effects
€
d = δ D − 8⋅ δ 18O
•  measurements in precipitation:
is a measure for non-
d = 10 ‰
→ atmosphere and ocean are typically out of equilibrium
€
Global Meteoric Water Line
for equilibrium
fractionation:
δD
18
δ O
intercept of water line:
mean €
deuterium excess
d = δ D − 8⋅ δ 18O
deviations from the
GMWL: measure of nonequilibrium effects
Kurita et al. (2005)
≈8
Stable water isotopes in weather systems
•  Case study of a winter storm in the US in January 1986
•  Simulation of the isotopic composition of precipitation with the
COSMOiso model
Six-hourly accumulated
precipitation (mm)
δ18O in precipitation (‰)
Pfahl et al. (2012)
Deuterium excess of marine vapor
Airplane measurements of the deuterium excess
of water vapor on two consecutive days during
the HYMEX field campaign in 2012.
Aemisegger (2013)
Deuterium excess of marine vapor
Surface latent
heat flux
Aemisegger (2013)
Deuterium excess of marine vapor
Deuterium excess of near-surface water vapor from a
COSMOiso simulation.
Summary: Stable water isotopes
•  Phase transitions leave distinct fingerprints in the isotopic
composition of water.
•  Thereby, stable water isotopes can provide information on
moisture sources and transport patterns.
•  Isotope observations provide independent means for
validating different aspects of the water cycle in weather
and climate models.