A talk of two halves:

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A talk of two halves 1. Drifting snow in
Antarctica modelling the interaction with
the boundary layer consequences for ice
cores. 2. Modelling denitrification of the Arctic
stratosphere issues of polar stratospheric
cloud nucleation and the controlling
influence of meteorology Graham
Mann Acknowledgements 1. Stephen Mobbs, Sarah
Dover, Michelle Smith (Univ of Leeds) John
King, Phil Anderson, Russ Ladkin, David Vaughan
(B.A.S.) 2. Ken Carslaw, Stewart Davies, Martyn
Chipperfield Funding NERC studentship, NERC
grants, EU Framework V initiative
2

• Why are we interested in blowing snow?
• Role in mass balance of ice sheets
  transport across grounding line evaporation
  (sublimation) during wind-blown transport
• Modification of atmospheric boundary layer
  evaporation cools and moistens surrounding air
  (conventional surface layer theory breaks
  down)
• Consequences for ice core interpretation
  long-term temperature trends are deduced from
  precipitation changes inferred from ice core
  accumulation histories. However, spatial
  variability in snow transport can
  introduce spurious features unconnected
  with any change in the paleoclimate.

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Blowing/drifting snow --- the basics
Two transport processes 1) saltation
(skipping), zsalt 10cm
2) suspension by turbulence
Increase in snow surface area exposed to air
during strong wind events --- sublimation during
transport can be important.
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Blowing snow particle size
Blowing snow particles are relatively fine
(r100 mm) Crystalline snow structure destroyed
by abrasion
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Winter seasonal mean blowing snow mass flux
Winter Antarctic wind climatology dominated by
drainage flow from Antarctic plateau. Boundary
layer stably stratified giving rise to katabatic
flow --- very strong downslope winds often
lasting several days. Blowing snow transport
large enough to be important in ice sheet mass
balance.
From Smith (1995), PhD thesis
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Winter seasonal mean blowing snow sublimation
Airborne Sublimation(mm water equivalent)
Total modelled winter season sublimation during
transport. At warmer, windier coastal sites
sublimation during transport is important.
From Smith (1995), PhD thesis
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STABLE2 experiment, Antarctic winter 1991
Halley
Suspended snow sampled 0.1-10 m Monitor
interaction with surface layer
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Blowing snow particle size distribution
STABLE2 experiment used Formvar resin-coated
slides in particle collectors to gain sample
of blowing snow particles. Examined using image
processing (Dover, PhD thesis, 1993) Size
distributions fit well to a 2-parameter gamma
distribution Particle numbers from counters
converted to mass fluxes
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Threshold wind speed required for blowing snow
Fallen snow becomes airborne once wind is strong
enough to overcome cohesive forces and provide
sufficient aerodynamic lift. Required surface
stress defines threshold friction velocity,
ut Because of cohesive forces and momentum of
already saltating particles, ut is higher at
beginning of strong wind event than at end.
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Parameterizing blowing snow transport
Figure shows four different blowing snow episodes
from the STABLE2 experiment. Different episodes
give very different transport rates for
equivalent wind strengths. Post-depositional snow
transport depends strongly on the value of ut
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Interaction of blowing snow with the boundary
layer
Lower temperatures mean lower saturation
humidities Source of moisture and latent
cooling from sublimating airborne snow raises
relative humidity (w.r.t. ice) easily. Stable
boundary layer inhibits vertical transport of
moisture and surface layer quickly saturates
w.r.t. ice. This negative feedback restricts
airborne snow sublimation
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Numerical modelling of interaction with boundary
layer (1)
Use simple 1D boundary layer model (1st order
mixing length closure) with source of moisture
of sink of heat (Mobbs Dover, 1993). Use
parameterization of snow particle number vs u
and assume particle size distribution at
lower boundary
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Numerical modelling of interaction with boundary
layer (2)
Introduce spectrum of snow particle sizes into
neutral PBL Near-surface relative humidity
rises quickly
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Numerical modelling of interaction with boundary
layer (3)
subl. off
subl. on
0.1
1
0.1
0.01
Sublimation of blowing snow also reduces mass
flux of blowing snow c.f. no-sublimation
run Modelled relative humidity profiles show
that the rh increase is due to effect of
blowing snow sublimation
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Numerical modelling of interaction with boundary
layer (4)
Total blowing snow transport strongly reduced
by blowing snow sublimation Column b.s.
sublimation restricted by negative feedback
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Blowing snow consequences for ice core
interpretation

• Snowfall
• Oxygen isotopes
• Other chemical and
• physical tracers

Addition of wind- borne snow
Removal of wind- borne snow
Ice core
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• Problems arising from snow transport
• Incorrect measurement of precipitation
• Dating problems if annual layers lost
• Biasing of annual means if snow lost/added
• preferentially during certain seasons
• Temporal changes in snow transport may
• introduce spurious trends

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Lyddan Ice Rise project 1998-2001
G.W. Mann, S.D. Mobbs and S.B. Vosper School of
the Environment, University of Leeds, UK.
J.C. King, P.S. Anderson, D.G. Vaughan and R.S.
Ladkin British Antarctic Survey,
NERC, Cambridge, UK
Hypothesis to be examined Wind-born
e snow transport can result in significant redistr
ibution of snowfall around relatively
gentle topographic features in Antarctica
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• Methodology of Lyddan Ice Rise project
• Make field measurements of snow accumulation
• and airflow round a topographic feature
• Model airflow using 3d Vosper Orographic Model
  (Vosper, 2003) and validate against observations
• Calculate snow transport using a simple
  parametrisation from STABLE2 measurements
  (Mann et al., 2000)
• Compare observed accumulation with computed
  horizontal snow transport divergence

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Study area - Lyddan Ice Rise
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Ground Penetrating Radar (25/50 MHz) and
kinematic GPS survey
Measure of accumulation and get accurate
orography info.
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Snow stake array and snow density cores
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Automatic Weather Stations
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Wind component across Lyddan ice rise
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Easterly
Westerly
N.B. Accelerating flow over hill (erosion) in
both cases
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Ground Penetrating Radar Transect
Depth (m)
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Annual accumulation from stakes
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Scaled accumulation from stakes and GPR
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Airflow modelling methodolgy
Radiosonde wind and temperatures recorded daily
at Halley. Initialise 3dVOM using radiosonde data
for selected strong wind cases Incorporate 1D
boundary layer model and match to daily mean AWS
wind speeds at Lyddan. Use this as upwind u,v,q
profile Use GPS measured Lyddan orography and
solve for surface stress transect across ice rise.
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Airflow modelling results
Airflow model predicts slowdown on upstream
side of ridge speed-up on lee slope of
ridge
Then apply blowing snow transport
parameterization to predict erosion/deposition
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Erosion/deposition from 3dVOM plus blowing snow
Strong wind event from Sept 2000 shows erosion
at summit at 2 points on upstream slope and
deposition on lee slope.
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Erosion/deposition for different upstream wind
strengths
To predict annual erosion, all Halley
radiosondes strong enough for blowing snow used
to gain erosion/deposition parameterization
dependent on near-surface wind speed. This
enables estimate of annual erosion/deposition
using Lyddan AWS data But erosion/deposition
also depends strongly on static stability profile
of troposphere(gravity waves strongly affect
speed-up across ridge)
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Annual erosion/deposition calculated from AWS data
Although erosion signal is largest in strongest
wind events, more modest strong-wind-events are
more frequent these dominate.
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Modelled and Observed Accumulation, 2000-01
3 erosion maxima
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Conclusions

• Measurements at Lyddan Ice Rise (LIR) show large
  accumulation
• variations associated with this very gentle
  topographic feature.
• AWS measurements show significant variations in
  wind speed across LIR. In particular, speeds
  on the lee slope are much greater than those
  at the summit when summit wind speeds are less
  than about 8 ms-1.
• Broad-scale annual average snow redistribution
  calculated from AWS data agrees well with
  stake measurements
• Pattern of redistribution calculated using a
  linear airflow model agrees well with stake
  and GPR measurements, although absolute values
  of annual average erosion are too small
• The LIR results suggest that caution should be
  exercised when interpreting ice core data
  obtained from regions of even quite gentle
  topography.

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HALF-TIME
And now for something completely different
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Denitrification of the Arctic stratosphere
Introduction

• Denitrification is the irreversible removal of
  HNO3 from the lower stratosphere by the
  sedimentation of HNO3-containing particles
  (nitric acid hydrates or ice)
• Removal of HNO3 reduces chlorine deactivation
  to ClONO2 and hence results in enhanced ozone
  loss
• Chemical Transport Model simulations have
  shown that denitrification can increase Arctic
  ozone loss by 30
• Denitrification is a ubiquitous feature of
  Antarctic winters and has been observed in
  the Arctic in the cold winters 1988/9, 1994/5,
  1995/6, 1996/7
• In a future colder Arctic stratosphere,
  denitrification could become more common,
  widespread and intense.

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Davies et al. (2002)
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New insight into Arctic denitrification

• Prior to 1999/2000, denitrification thought to be
  caused by ice coated with NAT.
• NAT particles assumed to be too small to sediment
  significantly (r1 mm, n0.1 cm-3)
• In-situ observations by Fahey et al. (2001)
  revealed the existence of large (r10 mm, n10-4
  cm-3) HNO3-containing particles capable of
  widespread denitrification
• NAT particles take 8 days to grow to 10 mm
• Consequently, an equilibrium based NAT scheme may
  be invalid.

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Denitrification by Lagrangian Particle
Sedimentation (DLAPSE) A 3D microphysical model
coupled to SLIMCAT

• non-equilibrium model forced by ECMWF analysed
  wind, T
• time-dependent growth and sedimentation of
  50,000 model NAT particles
• flexible nucleation scheme
• NAT particles grow in competition with STS
  particles
• full 41 tracer chemistry of SLIMCAT CTM
  included

Carslaw et al. (2002)

• Designed specifically to simulate
  denitrification by NAT particles alone and
  understand and test their evolution and formation

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DLAPSE/SLIMCAT denitrification in past cold
Arctic winters
Year Max. v. a. denit. Abs.max. denit.
1994/ 1995 50 92
1995/1996 52 78
1996/1997 44 85
1999/2000 66 97
Mann et al. (2003)
Intense and widespread denitrification in
1999/2000 Some denitrification in other cold
winters but not as strong
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Factors controlling Arctic denitrification

• Q. What was special about the 1999/2000 Arctic
  winter?
• Q. What conditions allow NAT particles to grow to
  rock sizes and cause strong
  denitrification?
• Several factors control intensity and extent of
  denitrification
• horizontal area vertical depth of NAT
  super-saturated region
• nitric acid and water vapour mixing ratios
• minimum temperature
• number concentrations of solid hydrate
  particles which form
• METEOROLOGY (proximity of vortex and cold pool
  centres)

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Idealised study of meteorology controlling
denitrification
Mann et al (2002)

• Use fixed ECMWF wind temperature from 23rd
  Dec 99 for 10 days
• 20o solid body rotation of temperature field
  relative to wind field

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Meteorology controls NAT-induced
denitrificationConcept of closed flow and
through flow
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The big unknownWhat is the nucleation process
for NAT rocks?

• Condensation onto ice followed by ice
  evaporation?
• Homogeneous freezing of ternary solutions?
• Heterogeneously on e.g. meteoritic debris, ion
  clusters (due to cosmic rays?), etc?
• Another mechanism?
• DLAPSE has a flexible nucleation scheme.
• Usually use slow nucleation rate everywhere
  TltTNAT.
• This was a pragmatic rather simplistic approach.
• Now done Arctic simulations with nucleation
  controlled by ice produced on mesoscale via
  mountain waves

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Mountain Wave induced ice PSCs produce mother
cloud which then rains out NAT
High resolution 1x1 DLAPSE/SLIMCAT run with
particles rained from base of mother clouds
(Fueglistaler et al., 2002) produced from
mesoscale ice regions using Mountain Wave
Forecast Model of Eckermann Preusse (1999)
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Mountain Waves
Mother clouds
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High resolution large-scale slow nucleation run
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High resolution mother cloud nucleation run
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Coverage of NAT rocks rained from mother clouds
Although mother clouds only cover 5 of NAT
region at maximum, sedimented NAT rocks cover
40 by mid-winter.
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Denitrification by NAT PSCs nucleated on small
and large scale
NAT from mother clouds
NAT from large-scale slow nucl.
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Comparing PSC properties and denitrification
NAT from mother clouds
NAT from large-scale slow nucl.
Denitrification in mother cloud run is
significant but is weaker and occurs later
than in large-scale slow nucleation run
Large-scale run has been shown to agree well with
timing and scale of denit observations (if
anything underestimates).
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• Conclusions
• Arctic denitrification is caused by
  slowly-growing nitric acid tri-hydrate (NAT)
  particles which sediment removing HNO3.
• Although nucleation mechanism remains uncertain,
  NAT rocks produced via mountain waves caused
  significant denitrification in the Arctic
  winter 1999/2000.
• Timing of denitrification in observations is
  consistent with the production of NAT via some
  large-scale nucleation mechanism.
• Whatever the mechanism for producing widespread
  NAT rocks during cold Arctic winters, the flow
  regime of the polar vortex-cold pool is the
  dominant controlling factor for
  denitrification.