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Michael Tjernström

Department of Meteorology & Bolin Centre for Climate Research

Stockholm University, Sweden

Arctic atmospheric boundary layer

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Arctic atmospheric boundary layer

Michael Tjernström

Department of Meteorology & Bolin Centre for Climate Research

Stockholm University, Sweden

With help and support from many, to many to mention, and from organizations like IASC and the WWRPs PPP/YOPP, the Swedish Arctic Research Program and NCAR, funding agencies like NERC, Knut and Alice Wallenberg Foundation, Swedish Research Council, US National Science Foundation and the US Office of Naval Research.

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Who am I – really…?

  • I believe most important changes in life are - more or less - random bifurcations; possibilities open and close randomly and one cannot plan for them. I never had real a career plan, and if I did I constantly kept abandoning or changing it, but I’ve always been looking for exiting work
  • Long story made very short: I basically became a meteorologist to avoid being drafted as an army medic; an inspiring high-school physics teacher played a decisive role in that choice (#1)
  • I consequently spent some years as an air force forecast officer, but given the opportunity I left the service as a Captain to start a PhD at Uppsala University, Sweden (#2)
  • After 4+ years I defended a mesoscale modeling thesis, then did a postdoc on turbulence observations by aircraft (very much aided by people from NCAR/RAF), then did mesoscale modeling again – all at my alma mater Uppsala University

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Who am I – really…(cont.)?

  • Then ten years after the PhD, I applied for and got a special “Senior Scientist” position funded by the Swedish Research Council and moved to Stockholm University in 1998 (#3)
  • Almost immediately I was asked to spearhead a meteorological observation program on an Arctic icebreaker expedition in 2001; sounded fun, so I said “yes” (#4)
  • I firmly believed that would be a one-time-only experience. Now, 23 years, five expeditions and some ~150 papers later I’m about to retire with a load of Arctic ABL data to work on
  • So what is to be learned from this story? Find something exciting to work on, but do not be afraid to change track if a good opportunity arises, and most importantly: If its not “fun” its not worth the effort!

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~2.8 °C

~1.0 °C

}

3 times larger

Global and Arctic warming.

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Arctic amplification

Feedbacks:

  • Planck feedback
  • Ice-and-snow albedo feedback
  • Lapse rate feedback
  • Clouds and aerosols
  • Atmospheric (and ocean) circulation
  • …

Ratanen et al. 2022

Rantanen et al. 2022

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Large-scale atmospheric circulation; essentially resolved in models of weather or climate

Arctic climate:

Governed by many small scale processer that are not resolved in model of weather or climate

Courtesy of Matt Shupe

The atmospheric boundary layer is an integral part of this coupled system!

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What is so special about the Arctic?

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Temperature from soundings

# of obs per grid square ~1 month

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Jul-Aug 2001

Almost all process observations from the Arctic

Ocean are from summer

Aug 2008

ACSE (JAS 2014)

May – June 2023

Oct 97 – Nov 98

MOSAiC

Aug – Sept 2018

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The annual cycle

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Winter

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Winter conditions!

Courtesy of Ola Persson

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SHEBA polar night

Courtesy of Persson

Cloudy

~0 W m-2

Clear

~40 W m-2

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Spring

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Diurnal cycle (from SHEBA)

Tjernström 2019

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The surface energy budget

Cloudy←|→Clear

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Summer

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Summer temperatures

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July

August

Cold

events

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Clouds & radiation

From ASCOS

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Non-melt season: Variable Ts < 0° C

Available energy (W m-2)

Energy redistribution (W m-2)

Melt season: Fixed Ts ≈ 0° C

Available energy (W m-2)

Energy redistribution (W m-2)

Two Arctic seasons:

Melt and freeze

Courtesy of Ola Persson

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SHEBA specific humidity

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SHEBA relative humidity

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SHEBA relative humidity

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Clouds in the Arctic

Shupe et al. 2011

Shupe et al. 2011

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Clouds in the Arctic

Shupe et al. 2011

Shupe et al. 2011, Shupe 2011

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Clouds in the Arctic

Tjernström et al. 2012

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Summer

Spring

Tjernström et al. 2012

Courtesy of Sonja Murto

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Brooks et al. 2017

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Composite summer soundings

SHEBA

AOE1996

AOE2001

ASCOS2008

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Composite summer soundings

SHEBA

AOE1996

AOE2001

ASCOS2008

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Winter

Summer

Tjernström & Graversen 2009

A few more words about vertical structure

Across seasons

Capping inversion

Surface inversion

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Surface inversion

Elevated inversion

”Boundary layer”

​

Winter

Spring

Summer

Autumn

Surface

53%

15%

9%

61%

Elevated

47%

85%

91%

39%

Tjernström & Graversen 2009

Inversion base < 15 m

Inversion base > 15 m

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SHEBA inversion statistics – annual

Inversion base height

Inversion strength

Inversion thickness

Tjernström & Graversen 2009

Tjernström & Graversen 2009

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Near-surface stability (SHEBA)

Very stable

Nearly neutral

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Composite summer soundings

SHEBA

AOE1996

AOE2001

ASCOS2008

~1 km

~300-400 m

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Entrainment then often becomes a source of moisture

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Integrated water vapor

Air mass transformation�Moist and warm from south cools down when exposed to sea ice

Warm and moist air

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Air-mass transformation

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On-ice flow

(warm advection)

Tjernström et al. 2015

Temperature profiles from in-side ice edge but close to MIZ

Off-ice flow

(cold advection)

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So - the ”polar dome” concept is a myth!

Back to basics: Air Mass Transformation

Law et al. 2014

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Contributions to surface drag …

… by form drag from edges of sea-ice and by ridging

 

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Michael Tjernström, Stockholm University

2023-07-20

Brooks et al. 2017

PBL depth estimated from momentum flux

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Day of August 2008

Richardson number, Ri

Brooks et al. 2017

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Surface based

boundary layer

Cloud generated

turbulent layer

Partially decoupled layer

Brooks et al. 2017

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What is a cloud?

Mauritsen et al. 2011

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Can lack of aerosols prevent cloud formation?

Mauritsen et al. 2011

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Inner cities: 100-1000 times larger!

No haze!

Arctic summer air is very “clean”!

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Optically thin clouds

Sotiropulou et al. 2014

Sotiropoulou et al. 2016

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Dependences

in cloud

forcing…

Mauritsen et al. 2011

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Brooks et al. 2017

Models generally repre-sent the Arctic ABL rather poorly, in different ways for different models – including reanalysis (which is a model in this respect)!

​

I could have spent the hour talking just about this, so I made a choice to not include models here

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Main takehome messages…(1)

Same basic physical processes govern as elsewhere; nothing magical in the Arctic!

It is the environment that is special: e.g. small diurnal and large annual cycle, low temperature and (hence) low water vapor, very moist but pristine air (low aerosol count) and hence often tenuous clouds.

  • The Arctic ABL is not always stably stratified! It is more often neutrally stratified than anything else
  • In winter, which is when stable conditions occur, surface inversions can be very persistent; can last for days

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  • The Arctic ABL is most often shallow, often cloudy and capped by an inversion; this shallow structure is not captured by current satellite observations and other observations are rare
  • Arctic clouds contribute a substantial – maybe dominating – part of the ABL mixing by convective overturning from cloud-top cooling, and also plays largest part of the surface energy budget
  • Arctic ABL clouds are often decoupled from surface and moisture often increase with height across capping inversion
  • The Arctic ABL is interacting with the mid-latitudes; air mass transformation is strongest in warm-moist intrusions and in cold-air outbrakes but occurs on all scales

​

Main takehome messages…(2)

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Litterature

Tjernström, M., C. Leck, P. O. G. Persson, M. L. Jensen, S. P. Oncley and A. Targino, 2004: The summertime Arctic atmosphere: Meteorological measurements during the Arctic Ocean Experiment (AOE-2001). BAMS, 85, 1305 – 1321, https://doi.org/10.1175/BAMS-85-9-1305.

Tjernström, M., 2005: The summer Arctic boundary layer during the Arctic Ocean Experiment 2001 (AOE-2001). BLM, 117, 5–36, https://doi.org/10.1007/s10546-004-5641-8.

Tjernström, M., and R.G. Graversen, 2009: The vertical structure of the lower Arctic troposphere analysed from observations and ERA-40 reanalysis. QJRMS, 135, 431-433, https://doi.org/10.1002/qj.380.

Mauritsen, T., J. Sedlar, M. Tjernström, C. Leck, M. Martin, M. Shupe, S. Sjogren, B. Sierau, P. O. G. Persson, I. M. Brooks, E. Swietlicki, 2011: An Arctic CCN-limited cloud-aerosol regime, ACP, 11, 165–173, https://doi.org/10.5194/acp-11-165-2011.

Shupe, M.D., V.P. Walden, E. Eloranta, T. Uttal, J.R. Campbell, S.M. Starkweather, and M. Shiobara, 2011: Clouds at Arctic Atmospheric Observatories, Part I: Occurrence and macrophysical properties. JAMC, 50, 626-644.

Shupe, M.D., 2011: Clouds at Arctic Atmospheric Observatories, Part II: Thermodynamic phase characteristics. JAMC, 50, 645-661.

Sedlar, J., M. Tjernström, T. Mauritsen, M. Shupe, I. Brooks, P. Persson, C. Birch, C. Leck, A. Sirevaag, and M. Nicolaus, 2011: A transitioning Arctic surface energy budget: the impacts of solar zenith angle, surface albedo and cloud radiative forcing. ClimDyn, 37, 1643–1660, https://doi.org/10.1007/s00382-010-0937-5.

Tjernström, M., C. E. Birch, I. M. Brooks, M. D. Shupe, P. O. G. Persson, J. Sedlar, T. Mauritsen, C. Leck, J. Paatero, M. Szczodrak and C. R. Wheeler, 2012: Meteorological conditions in the Central Arctic summer during the arctic summer cloud ocean study (ASCOS), ACP, 12, 6863–6889, https://doi.org/10.5194/acp-12-6863-2012.

Morrison1, H., G. de Boer, G. Feingold, J. Harrington, M. D. Shupe and K. Sulia, 2012: Resilience of persistent Arctic mixed-phase clouds, NatGeo, DOI: 10.1038/NGEO1332

Sotiropoulou, G., J. Sedlar, M. Tjernström, M. D. Shupe, I. M. Brooks and P. O. G. Persson, 2014: The thermodynamic structure of summer Arctic stratocumulus and the dynamic coupling to the surface. ACP, 14, 12573–12592, https://doi.org/10.5194/acp-14-12573-2014.

Tjernström, M., M. D. Shupe, I. M. Brooks, P. O. G. Persson, J. Prytherch, D. Salisbury, J. Sedlar, P. Achtert, B. J. Brooks, P. E. Johnston, G. Sotiropoulou and D. Wolfe, 2015: Warm-air advection, air mass transformation and fog causes rapid ice melt, GRL, 42, 5594–5602, https://doi.org/10.1002/2015GL064373.

Brooks, I. M., M. Tjernström, P. O. G. Persson, M. D. Shupe, R. A. Atkinson, G. Canut, C. E. Birch, T. Mauritsen, J. Sedlar, and B. J. Brooks, 2017: The turbulent structure of the Arctic summer boundary layer during ASCOS. JGR, 122, 9685 – 9704, https://doi.org/10.1002/2017JD027234.

Tjernström, M., 2019: The Arctic boundary layer. In A Century of Progress in Atmospheric and Related Sciences: Celebrating the American Meteorological Society Centennial. Boundary-layer meteorology, [Eds. Margaret A. LeMone and Wayne Angevine], Meteorological Monographs, AMS, 59, chapter 9, 84pp, https://doi.org/10.1175/AMSMONOGRAPHS-D-18-0013.1.

Rantanen, M., A. Y. Karpechko, A. Lipponen, K. Nordling O. Hyvärinen, K. Ruosteenoja, T. Vihma & A. Laaksonen, 2022: The Arctic has warmed nearly four times faster than the globe since 1979, Nature Communications Earth & Environment, 3, https://doi.org/10.1038/s43247-022-00498-3

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Thanks for listening