Mesoscale Atmospheric Systems Atmospheric moisture transport and stable water isotopes Stephan Pfahl 23 May 2017

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MAS Topics

Evaporation

Moisture transport

(Extreme) precipitation

Convection

Fronts

Fronts STE

Radar

Seasonal mean distribution of water vapour

Vertically integrated water vapour (IWV) Units: kg/m2

ERA-40 Atlas (2005)

DJF

JJA

Vertical distribution of humidity

Mean profiles in NH on 1 June 2001 12 UTC (ERA-Interim data)

Precipitation and evaporation over the ocean

Evaporation DJF Evaporation JJA

Precipitation JJA Precipitation DJF

Evaporation minus precipitation

DJF

JJA

Evaporation minus precipitation

DJF

JJA

Moisture flux divergence

= E-P (freshwater

flux)

(with w = IWV)

Water vapour fluxes

Column-integrated vector fluxes of water vapour and their convergence (in kg m-2s-1)

ERA-40 Atlas (2005)

DJF

JJA

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

Ralph et al. (2011) IWV

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

Cyclone-centred IWV (left) and IVT (right) for North Atlantic cyclone

fronts

Dacre et al. (2015)

Atmospheric rivers and cyclones Schematic view of an atmospheric river in the Northeast Pacific

Ralph et al. (2004)

along-front wind

specific humidity along-front moisture flux

Water vapour sources in an atmospheric river

Numerical water vapour tracers released from ocean surface in regional model simulation

Sodemann and Stohl (2013)

IWV, SLP, winds @ 700hPa water vapour tracers, 14mm IWV

Water vapour sources in an atmospheric river Strong AR event in southern Norway associated with a cyclone‘s

frontal system on 14 Dec 2006

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, atmosphere-vegetation feedbacks, microphysical processes in clouds, ...)

•  obtain information on past

climates (e.g., via their concentration in ice cores)

Stable heavy isotopes of O and H: 18O, 17O and 2H (or D)

→ stable water isotopes: H2

16O, H218O, HD16O, ...

Natural abundances of oxygen and hydrogen isotopes:

Stable water isotopes: species and molecules

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)

[‰]

18Rliq

18Rvap < 18Rliq

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)

Isotope fractionation

Mass difference causes slight changes in physical properties

18R0 18R1 = 18R0 18R0 18R1 < 18R0

Equilibrium fractionation during cloud formation

Continental isotope map interpolated from station measurements (Bowen and Wilkinson, 2002)

Isotope measurements in precipitation

Typical isotope ratios in natural reservoirsby definition: V-SMOV = 0‰

Mook, 2001

Lower diffusion velocities of heavy isotopes lead to additional, diffusion-controlled fractionation during transport under non-equilibrium conditions.

Non-equilibrium („kinetic“) fractionation

atmospheric example: evaporation from the sea

other examples: –  formation of ice clouds, when supersaturation occurs –  re-evaporation of rain drops under the cloud base in

unsaturated air

Non-equilibrium fractionation

zl

•  for equilibrium fractionation:

(only slight temperature dependence)

•  relative importance of non-equilibrium fractionation much larger for δ18O

•  deuterium excess is a measure for non-equilibrium effects

•  measurements in precipitation:

→ atmosphere and ocean are typically out of equilibrium

€

δDδ18O ≈ 8

d = δD−8 ⋅δ 18O

€

d =10 ‰

Deuterium excess

Kurita et al. (2005)

Global Meteoric Water Line

for equilibrium fractionation:

intercept of water line: mean deuterium excess

deviations from the GMWL: measure of non-equilibrium effects

€

δDδ18O ≈ 8

d = δD−8 ⋅δ 18O

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

δ18O in precipitation (‰) Six-hourly accumulated precipitation (mm)

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.

• 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.

Summary: Stable water isotopes