IASC-CWG workshop summary report“Workshop on observing and modelling meltwater retention processes
in snow and firn on ice sheets and glaciers”
confronting “the problem that won’t go away”
1-3 June 2016, Øster Voldgade 10, DK-1350 Copenhagen
Overview
Dr. Robert Fausto and Prof. Jason Box of the Glaciology and Climate department of theGeological Survey of Denmark and Greenland (GEUS) hosted an International Arctic
Scientific Committee “Workshop on observing and modelling meltwater retention
processes in snow and firn on ice sheets and glaciers”, 1-3 June 2016, Øster Voldgade
10, DK-1350 Copenhagen. There were 50 registered participants. A Workshop Agenda
including participants list is attached to this report.
Discussion framed how to approach a problem confronting this community for 40 years.
Scientific Highlights
We organized sessions by observations, modeling, synthesis and thus list the first three
Scientific Highlights in response to the questions:what observations are we missing?
lateral continuity of ice layers, unsaturated hydrological conductivity, and irreducible
water content topped a list that also included grain growth. Also on this list is havinggeo-statistical information of retained meltwater, i.e. spacing of conduits, lenses, andlayers in horizontal and vertical. Two retention regimes on the lower accumulation areaof the GrIS were highlighted: blocked percolation in the West and firn aquifer in theSoutheast. The transition between these two zones is still unmonitored.
1. Regarding modeling, how do we parameterize vertical transport in preferential
paths? An idea is to assume perfectly efficient vertical transfer allowing water to exit
one layer and reappear in entirety skipping some model layers below. Another was
to redistribute the meltwater from the surface to the underlying layers by using“percolation curves”. For alpine snowpack, advances have been made in modeling
wetting front instability after a fine to coarse grain horizon. The preferential pathways
enhanced by ice layers are still not being modeled. Simple model including
meltwater availability, depth and horizontal density of the preferential pathway andrefreezing rates should give a first estimate of how important the process is.
2. Regarding synthesis, we resolved it important to ranking of ‘what is important’, theoutcome of which suggested that with the expected future warming, accurate
treatment of meltwater retention in the upper accumulation area will be fundamental.
Actionable ideas how to expand much more realistically on the seminal work of
Pfeffer et al. (1991) include: repeat it with modern data and methods (topography,hypsometry, firn, depth, 2 models easy runoff model 2 obstacles to runoff model).
Regarding heterogeneous percolation of meltwater, it was agreed that no1

quantification of its extend neither of its importance for SMB processes wasavailable. Simple model experiments (water availability vs. percolation rate vs.
refreezing rate) were suggested to get an idea of the relevance heterogeneous
percolation modelling in polar firn.
3. The group discussed: To what accuracy do we need to know retention? We candesign simple questions to address the question. Several participants agreed that
Greenland ice sheet SMB calculation did not need to include microscopic modelingof percolation blocking or heterogeneities and that only the overall effect should beaccounted for.
4. To address the discussed question:”what constitutes a useful validation data set?”,we developed a list of Key observables and methods to accomplish the observation.
5. Accurate surface observations are needed to get the atmospheric forcing
parameters and computed melt correct. Multi-year cores and temperature records
that cover as much of the percolation area (~15-20m) as possible are very useful.
Key observables (method)
1. snowfall accumulation rate versus time
2. snowfall density (coring, snow pits)
3. firn ice content (stratigraphy from coring, snow pits)
4. firn air content (density and stratigraphy from coring, snow pits)
5. liquid water content (TDR, upward looking GPR)
6. irreducible water content (only from laboratory experiment?)
7. hydraulic conductivity (tomography?)
8. snow grain size (NIR photography)
9. thermal evolution (thermistor strings in top 25 m)
10. compaction rate (continuous logging strain gauges, a.k.a coffee cans)
11. 3d snow/firn mapping (multiple coring, Meier snow guillotine, large scaletomography)
12. stratigraphy, i.e. discontinuities (NIR photography)
Key model challenges (method)
1. vertical water motion in snow and firn within one grid cell, i.e. inhomogeneous
meltwater percolation (piping)
2. horizontal water motion in snow and firn between grid cells
3. formation and thickness of continuous ‘impermeable’ ice layers in firn
4. formation and magnitude/thickness of continuous perennial firn aquifers
5. formation of local discontinuities, grain growth and the influence on the hydraulicconductivity
6. Accounting for the different dominating processes depending on the scaleRecommendations
1. make no model intercomparison until adding something new in the model about
heterogeneity that is testable.

22. In many applications and considering the high complexity of meltwater movement
and refreeze in heterogeneous firn it seems worthwhile ‘don’t worry about thedetails’, i.e. add a physical based parameterization resolving heterogeneous
meltwater percolation in firn on grid scales from 500m to 5km
3. Track energy convergence/divergence in the vertical
4. have models track ice layer development of impermeable layers
5. local heating from the arrival of water is a valuable tracer
6. model intercomparison driven by same observational dataset
7. validation using observations, not between models.
8. ways to image snowpack 3d include excavation or as yet unavailable larger scale(than mm) tomography
9. do the grain by grain modeling then rewrite it elegantly and efficient computationally
10. deeper thermistor strings
11. cite/investigate older work, e.g. Braithwaite (1994)

3PROGRAM:	  Workshop	  on	  observing	  and	  modeling	  meltwater	  retention	  processes	  in	  snow	  and	  firn	  on	  ice	  sheets	  and	  glaciers	  	  Hosted	  by	  the	  Geological	  Survey	  of	  Denmark	  and	  Greenland	  (GEUS)	  1-­‐3	  June	  2016,	  Øster	  Voldgade	  10,	  DK-­‐1350	  Copenhagen	  Sponsorship:	  Danish	  Council	  for	  Independent	  Research	  (DFF)	  and	  International	  Arctic	  Scientific	  Committee	  (IASC)	  co-­‐sponsored	  	  Practical	  items:	  ·∙What	  will	  the	  workshop	  include:	  o Icebreaker	  on	  Wednesday
o Morning	  and	  afternoon	  coffee	  breaks.
o Lunch	  Thursday	  and	  Friday.	  
o Workshop	  dinner	  on	  Thursday	  (Please	  let	  me	  know,	  if	  you	  plan	  NOT	  to	  attend)••

Poster	  session:	  Thursday	  afternoon.	  Please	  use	  the	  prepared	  poster	  holders	  in	  the	  hallway	  	  •Please	  be	  on	  time	  for	  each	  session	  as	  the	  main	  entrance	  to	  GEUS	  will	  be	  locked	  most	  of	  the	  time	  during	  the	  workshop.	  

Discussion	  sessions:	  discuss	  observations	  and	  model	  development	  of	  meltwater	  retention	  processes	  in	  snow	  and	  firn.	  	  Need	  help?:	  • Robert	  work	  phone:	  Call	  +4591333838•Robert	  private	  phone:	  Call	  or	  text	  +4525132694

Publication:	  We	  plan	  a	  special	  issue	  on	  the	  workshop	  topic	  in	  Frontiers	  in	  Earth	  Sciences	  –	  Cryospheric	  sciences	  [link].	  	  Oral	  presentations	  Wednesday	  1	  June	  12.00-­‐13.00	  Convener:	  Jason	  Box	  13.00-­‐13.10	  13.10-­‐13.40	  13.40-­‐14.10	  	  Title	  Registration	  	  	  	  The	  little	  problem	  that	  won't	  go	  away:	  50+	  years	  of	  research	  on	  Greenland	  meltwater	  infiltration	  Measurement	  and	  modelling	  of	  meltwater	  retention	  on	  ice	  sheets	  and	  glaciers	  Introduction	  W.	  Tad	  Pfeffer	  Roger	  Braithwaite	  4

14.10-­‐14.30	  Carl	  E.	  Bøggild	  Melt	  water	  in	  cold	  polar	  snow:	  a	  system	  of	  observational,	  understanding	  and	  modeling	  	  challenges	  14.30-­‐14.40	  Dirk	  van	  As	  Expert	  Survey	  Break	  	  	  Convener:	  Dirk	  van	  As	  	  15.00-­‐15.20	  Richard	  Forster	  Greenland	  firn	  aquifer	  investigations	  from	  remote	  sensing,	  geophysics,	  in	  situ	  measurements,	  and	  modeling	  15.20-­‐15.40	  Mike	  MacFerrin	  Compaction	  and	  firn	  cores	  from	  the	  Greenland	  ice	  sheet	  2012-­‐2016	  15.40-­‐16.00	  Paul	  Vallelonga	  Observations	  regarding	  the	  2012	  melt	  event	  in	  the	  Greenland	  accumulation	  zone	  16.00-­‐16.20	  Peter	  Kuipers	  Munneke	  Modelling	  elevation	  change	  of	  the	  Greenland	  Ice	  Sheet	  firn	  layer,	  1960-­‐2014	  16.20-­‐16.40	   Discussion	  points	  for	  the	  next	  Discussant:	  Jason	  Box	  days	  16.40-­‐18.30	  Ice	  Breaker	  Sponsored	  by	  International	  Arctic	  Science	  Committee	  (IASC)	  	  	  Thursday	  	  2	  June	  Convener:	  Babis	  Charalampidis	  	  09.10-­‐09.30	   Jens	  Hesselbjerg	  Christensen	   Sea	  ice	  –	  what	  does	  this	  has	  to	  do	  with	  the	  Greenland	  Ice	  Sheet	  SMB?	  
09.30-­‐09.50	  Konrad	  Steffen	  Greenland	  Swiss	  Camp	  Climatology:	  1990	  -­‐	  2016	  09.50-­‐10.10	  Horst	  Machguth	  Successive	  and	  intense	  melt	  rapidly	  decreases	  Greenland	  meltwater	  retention	  in	  firn	  10.10-­‐10.30	  Stef	  Lhermitte	  Clouds	  enhance	  Greenland	  ice	  sheet	  meltwater	  runoff	  Break	  	  	  11.00-­‐12.20	  Discussion	  Discussant:	  Koni	  Steffen	  Lunch	  	  Sponsored	  by	  International	  Arctic	  Science	  Committee	  (IASC)	  Convener:	  Baptiste	  Vandecrux	  	  13.30-­‐13.50	  Stefan	  Ligtenberg	  Modelling	  3-­‐D	  liquid	  water	  flow	  in	  the	  Greenland	  firn	  aquifer	  13.50-­‐14.10	  Sebastian	  Mernild	  Surface	  mass	  balance	  and	  runoff	  modeling	  for	  Greenland	  Ice	  Sheet	  based	  on	  SnowModel	  14.10-­‐14.30	  Peter	  Langen	  Introducing	  percolation	  physics	  in	  the	  regional	  climate	  model	  HIRHAM5	  14.30-­‐14.50	  Nander	  Wever	  Recent	  advances	  in	  liquid	  water	  flow	  modelling	  in	  the	  physics	  based	  SNOWPACK	  model	  Break	  	  	  15.20-­‐16.30	  Discussion	  Discussant:	  Michiel	  van	  den	  Broeke	  	  16.30-­‐18.30	  Poster	  Session	  	  19.00-­‐23.00	  Workshop	  dinner	  Café	  G	  [link]	  (Co-­‐sponsored	  by	  IASC)	  5

	  Friday	  	  3	  June	  Convener:	  Xavier	  Fettweis	  09.10-­‐09.30	  Jan	  Leanerts	  09.30-­‐09.50	  09.50-­‐10.10	  Samantha	  Buzzard	  Christian	  Steger	  10.10-­‐10.30	  Kristin	  Poinar	  Break	  	  Convener:	  Ruth	  Mottram	   	  11.00-­‐11.20	  Sergey	  Marchenko	  11.20-­‐11.40	  Samira	  Samimi	  11.40-­‐12.00	  Achim	  Heilig	  12.00-­‐12.20	  Björn	  Saß	  Lunch	  	  13.30-­‐15.00	  Discussion	  	  	  Firn	  air	  depletion	  and	  high	  melt	  in	  grounding	  zone	  of	  an	  East	  Antarctic	  ice	  shelf	  The	  accumulation	  of	  surface	  meltwater	  on	  ice	  shelves	  Refreezing	  and	  liquid	  water	  storage	  on	  the	  Greenland	  ice	  sheet:	  a	  model	  comparison	  Model-­‐based	  constraints	  on	  the	  depths	  and	  thermal	  influence	  of	  water-­‐filled	  crevasses	  in	  western	  Greenland	  	  	  Measuring	  and	  simulating	  snow/firn	  temperature	  evolution	  at	  Lomonosovfonna	  affected	  by	  preferential	  water	  flow	  Meltwater	  Runoff	  and	  Storage	  Based	  on	  Dielectric	  Properties	  of	  the	  Supraglacial	  Snowpack	  on	  Haig	  Glacier,	  Canadian	  Rocky	  Mountains	  Continuous	  determination	  of	  liquid	  water	  retention	  in	  seasonal	  snowpacks	  and	  application	  to	  perennial	  firn.	  
Improvement	  of	  geodetic	  glacier	  mass	  balances	  with	  integrated	  firn	  elevation	  change	  modeling	  Sponsored	  by	  International	  Arctic	  Science	  Committee	  (IASC)	  Discussant:	  W.	  Tad	  Pfeffer	  	  Poster	  presentations	  	  1	  2	  3	  4	  5	  6	  7	  8	  9	  Posters:	  Title	  Patrick	  Alexander	  The	  impact	  of	  meltwater	  on	  modeled	  Greenland	  Ice	  Sheet	  density	  profiles	  Willem	  Jan	  van	  de	  Berg	  On	  the	  densification	  of	  snow	  Matthew	  Cooper	  What	  can	  supraglacial	  rivers	  on	  the	  Greenland	  Ice	  Sheet	  teach	  us	  about	  seasonal	  meltwater	  retention	  in	  snow	  and	  firn?	  
Alexandra	  Gossart	  The	  importance	  of	  wind	  on	  East	  Antarctic	  ice	  shelf	  stability	  Juliana	  Costi	  Surface	  meltwater	  production,	  retention	  and	  runoff	  trends	  on	  the	  Antarctic	  Peninsula	  (WITHDRAWN)	  Xavier	  Fettweis	  Sensibility	  of	  the	  Greenland	  ice	  sheet	  surface	  mass	  balance	  simulated	  by	  the	  regional	  model	  MAR	  to	  the	  irreducible	  water	  saturation	  in	  snow	  and	  the	  pore	  hole	  close	  off	  density	  Joel	  Harper	  Challenges	  and	  Limitations	  of	  Using	  In	  Situ	  Observations	  to	  Assess	  Modeled	  Infiltration	  and	  Refreezing	  in	  Firn	  Stan	  Jakobs	  Effects	  of	  snow	  layer	  initialisation	  and	  soot	  concentration	  on	  meltwater	  retention	  Leo	  Kampenhout	  Firn	  modelling	  in	  the	  Community	  Earth	  System	  Model	  (CESM)	  6

10	  11	  12	  13	  14	  15	  Horst	  Machguth	  Greenland	  surface	  mass	  balance	  observations	  from	  the	  ice	  sheet	  ablation	  area	  and	  local	  glaciers	  Ruth	  Mottram	  The	  importance	  of	  retention	  and	  refreezing	  to	  the	  surface	  mass	  balance	  of	  small	  Arctic	  glaciers	  Brice	  Noël	  A	  downscaled	  1km	  dataset	  of	  daily	  Greenland	  ice	  sheet	  surface	  mass	  balance	  components	  (1958-­‐2015)	  Ward	  van	  Pelt	  The	  changing	  role	  of	  snow	  conditions	  and	  refreezing	  on	  mass	  loss	  of	  Svalbard	  glaciers	  Baptiste	  Vandecrux	  Understanding	  non-­‐linear	  change	  in	  the	  permeability	  of	  firn	  on	  Greenland	  ice	  sheet	  Melchior	  van	  Wessem	   The	  simulated	  Surface	  Mass	  Balance	  of	  the	  Antarctic	  Peninsula	  	  7

Participant List
No.	  of	  	  participants:	  50	  Confirmed	  participants	  Country	   Affiliation	  Email	  Xavier	  Stefan	  Peter	  K.	  
Brice	  Christian	  	  Michiel	  xavier.fettweis@ulg.ac.be	  S.R.M.Ligtenberg@uu.nl	  P.KuipersMunneke@uu.nl	  B.P.Y.Noel@uu.nl	  C.R.Steger@uu.nl	  M.R.vandenBroeke@uu.nl	  Fettweis	  BE	  Ligtenberg	   NE	  Munneke	  NE	  Noël	  NE	  Steger	  NE	  van	  den	  NE	  Broeke	  Melchior	   van	  Wessem	   NE	  Stef	  Lhermitte	  BE	  Jan	  Leanerts	  NE	  Sebatian	   Mernild	  No	  Jens	  H.	  

Christensen	   DK	  Peter	  Lang	   Langen	  DK	  Horst	  	  Robert	  Jason	  Dirk	  Baptiste	  Tad	  Glen	  Joel	  Koni	  	  Machguth	  Fausto	  Box	  van	  As	  Vandecrux	  Pfeffer	  Liston	  Harper	  Steffen	  SW	  DK	  DK	  DK	  DK	  US	  US	  US	  SW	  Samira	  Samimi	  Roger	  Braithwaite	  Achim	  Heilig	  Bianca	  Perren	  Marco	  	  Tedesco	  Ward	  	  van	  Pelt	  Samantha	   Buzzard	  Sergey	  Marchenko	  Frank	  	  Rack	  CA	  UK	  GE	  UK	  US	  S	  UK	  S	  US	  Alexandra	   Gossart	  Richard	  Forster	  Nander	  Wever	  BE	  US	  FR	  University	  of	  Liege	  University	  of	  Utrecht	  University	  of	  Utrecht	  University	  of	  Utrecht	  University	  of	  Utrecht	  University	  of	  Utrecht	  University	  of	  Utrecht	  J.M.vanWessem@uu.nl	  KU	  Leuven	  stef.lhermitte@ees.kuleuven.be	  University	  of	  Utrecht	  j.lenaerts@uu.nl	  Sogn	  og	  Fjordane	  University	   smernild@gmail.com	  College	  Danish	  meteorological	  jhc@dmi.dk	  institute	  Danish	  meteorological	  pla@dmi.dk	  institute	  University	  of	  Zurich	  horst.machguth@geo.uzh.ch	  GEUS	  rsf@geus.dk	  GEUS	  jeb@geus.dk	  GEUS	  dva@geus.dk	  DTU	  b.vandecrux@gmail.com	  University	  of	  Colorado	  	  wtpfeffer@gmail.com	  Colorado	  State	  University	   Glen.Liston@colostate.edu	  University	  of	  Montana	  joel@mso.umt.edu	  Swiss	  Federal	  Research	  konrad.steffen@wsl.ch	  Institute	  	  University	  of	  Calgary	  samira.samimi@ucalgary.ca	  University	  of	  Manchester	  roger.braithwaite@manchester.ac.uk	  University	  of	  Munich	  heilig@r-­‐hm.de	  British	  Antarctic	  Survey	  biaper@bas.ac.uk	  Columbia	  University	  cryocity@gmail.com	  University	  of	  Uppsala	  ward.van.pelt@geo.uu.se	  University	  of	  Reading	  S.C.Buzzard@pgr.reading.ac.uk	  University	  of	  Uppsala	  sergey.marchenko@geo.uu.se	  University	  of	  Nebraska-­‐
frack2@unl.edu	  Lincoln	  KU	  Leuven	  alexandra.gossart@kuleuven.be	  University	  of	  Utah	  rick.forster@geog.utah.edu	  Institute	  for	  Snow	  and	  wever@slf.ch	  Avalanche	  Research	  	  8Matthew	   Cooper	  US	  Willem	  Jan	   van	  de	  Berg	   NE	  Leo	  Kampenhout	   NE	  Kristin	  Poinar	  US	  Babis	  Caralampidis	   DK	  Paul	  Vallelonga	   DK	  Björn	  Saß	  GE	  Sebastian	   Sastruzny	  Ruth	  	  Mottram	  DK	  DK	  Stan	  Mike	  Patrick	  Christine	  Nanna	  Carl	  Faezeh	  Bent	  	  NE	  US	  US	  DK	  DK	  DK	  	  NO	  DK	  Jakobs	  MacFerrin	  Alexander	  Hvidberg	  Karlsson	  Bøggild	  Nick	  Hasholt	  University	  of	  California	  University	  of	  Utrecht	  University	  of	  Utrecht	  NASA	  GEUS	  University	  of	  Copenhagen	  University	  of	  Erlangen-­‐Nuremberg	  University	  of	  Copenhagen	  Danish	  meteorological	  institute	  University	  of	  Utrecht	  University	  of	  Boulder	  NASA	  University	  of	  Copenhagen	  University	  of	  Copenhagen	  DTU	  UNIS	  University	  of	  Copenhagen	  mguycooper@gmail.com	  w.j.vandeberg@uu.nl	  L.vanKampenhout@uu.nl	  kristin.poinar@nasa.gov	  cc@geus.dk	  vallelonga@nbi.ku.dk	  bjoern.sass@fau.de	  qnb322@alumni.ku.dk	  rum@dmi.dk	  c.l.jakobs@uu.nl	  Michael.MacFerrin@Colorado.EDU	  tapat64@gmail.com	  wdk760@ku.dk	  nbkarlsson@nbi.ku.dk	  post@ceboggild.dk	  faezeh.nick@unis.no	  bh@ign.ku.dk	  9