A R C T E X Results of the Arctic Turbulence Experiments www.arctex.uni-bayreuth.de Long-term Monitoring of Heat Fluxes at a high Arctic Permafrost Site in Svalbard 1
A R C T E X Results of the Arctic Turbulence Experiments www.arctex.uni-bayreuth.de CONTENT Overview field experiments Meso-scale atmospheric circulation pattern - Kongsfjord Wind field and atmospheric stability Typical diurnal patterns spring time Disturbed temperature profile close to the surface Ratio of buoyancy / shear production - Free Convection conditions Surface energy budget of permafrost on Svalbard
Direct near surface measurements of sensible heat fluxes in the arctic tundra applying eddy-covariance and laser scintillometry (supported by the DFG) 1 st Measurement experiment May 02, 2006 to May 20, 2006 - Location: Ny-Ålesund, Spitzbergen (Svalbard), 79 North Latitude Lüers, J and Bareiss, J (2010): The effect of misleading surface temperature estimations on the sensible heat fluxes at a high Arctic site - The Arctic turbulence experiment 2006 on Svalbard (ARCTEX-2006), Atmospheric Chemistry and Physics, 10 (1), 157-168. Lüers, J and Bareiss, J: Direct near surface measurements of sensible heat fluxes in the arctic tundra applying eddy-covariance and laser scintillometry - The Arctic Turbulence Experiment 2006 on Svalbard (ARCTEX-2006), Theor Appl Climatol, in review 2010. Westermann, S; Lüers, J; Langer, M; Piel, K; Boike, J (2009): The annual surface energy budget of a higharctic permafrost site on Svalbard, Norway, The Cryosphere, 3 (2), 631-680. 3
Diurnal variability of near surface sensible heat fluxes during summer time over dry and wet high arctic Tundra using eddy-covariance and laser scintillometer techniques in a typical footprint area on Svalbard (DFG project LU 1400/2-1). 2 nd measurement experiment August 10, 2009 to August 20, 2009 - Location: Bayelva Catchment south-west of Ny-Ålesund, 79 North latitude. 4
Meso-scale atmospheric circulation pattern - Kongsfjord Ny-Alesund 5
Wind field and atmospheric stability (Lüers & Bareiss, TAC 2010) 360 Ny-Alesund NW SE NW SE NW SE SE NW May 2006 0.8 2.0 315 0.7 1.5 wind direction [ ] 270 225 180 135 90 45 0 0.0 07 08 09 10 11 12 13 14 15 16 17 18 19 20 Time [Day] 0.6 0.5 0.4 0.3 0.2 0.1 u * [m s -2 ] u [m s 1] 1.0 0.5 0.0-0.5-1.0-1.5-2.0 stability z/l z/l [-] [-] Interaction of micrometeorological atmospheric stability with change of wind direction down (SE) or up (NW) Kongsfjord valley Forced by change of thermal meso-scale atmospheric circulation pattern or synoptic weather situation around Spitsbergen Affected by mountainous Islands covered by glaciers and sea-water filled Fjords 6
z/l z [ ] z/l z [ ] Ny-Alesund 2.0 1.5 1.0 0.5 0.0-0.5-1.0-1.5-2.0 1.5 1.0 0.5 0.0-0.5-1.0-1.5-2.0 NW Typical diurnal patterns spring time (Lüers & Bareiss, TAC 2010) z/l QH ECV 00 01 02 03 03 04 04 05 05 06 06 07 07 08 08 09 09 10 10 11 12 11 13 12 14 13 1514 1615 1716 1817 19 18 20 19 21 20 22 23 21 00 22 23 00 Ny-Alesund 2.0 SE z/l SE QH ECV QH LC95 IR QH SLS CET [hour] SE SE NW QH LC95 IR QH SLS May 16, 11, 2006 10 20 00 01 02 03 03 04 04 05 05 06 06 07 07 08 08 09 09 10 10 11 12 11 13 12 14 13 1514 1615 1716 1817 19 18 20 19 21 20 22 23 21 00 22 23 00 CET [hour] SE NW 8 15 6 10 4 2 5 0-2 -5-4 -10-6 -8-15 -10-20 May 16, 2006 10 20 NW 8 15 6 10 4 2 5 0-2 -5-4 -10-6 -8-15 -10-20 QH [W m -2 ] QH [W m -2 ] 10 8 6 4 2 0-2 -4-6 -8-10 10 8 6 4 2 0-2 -4-6 -8-10 Vh [ m s -1 ] Vh [ m s -1 ] May 11 Day time sustainable neutral conditions with strong SE winds Night time stability weak or calm NW winds. May 16 vice versa Day time intermittent turbulent exchange at noon, calm winds during change of directions SE to NW. Night time sustainable neutral and windy conditions during low sun positions (Both days were sunny with some scattered alto-level clouds.) 7
Disturbed temperature profile close to the surface (Lüers & Bareiss, ACP 2010) Mean vertical profile of temperature and wind speed Case A reflects strong surface cooling and a sharp inversion within 1 or 2 meter (5 h to 8 h or 17 to 22 h CET). Case B occurs around noon (9 to 16 CET) indicating a disturbed profile caused by surface warming. 8
Ratio of buoyancy / shear production - Free Convection conditions Production of Turbulent Kinetic Energy TKE B = g θ v w θ v Buoyancy production: This term represents effect of thermal convection on the generation of TKE. S 2 = u v h z Shear production: This term represents mechanical production of TKE or of turbulent eddies. Interaction of the vertical wind profile with the kinematic stress against the surface. Ratio B/S (B/S = flux Richardson number Ri f ) TKE produced or consumed by the buoyancy term B versus TKE produced by the mechanical or shear stress term S Usable to assess the dynamical stability or the convective activity of the atmospheric boundary layer. 9
Ratio of buoyancy / shear production - Free Convection conditions Ny-Alesund May 2006 Ratio Buoyancy/Shear [ ] 12 9 6 3 0-3 -6-9 +1 or -3 Ratio B/S 12 9 6 3 0-3 -6-9 -12 07 08 09 10 11 12 13 14 15 16 17 18 19 20 Determination if a free convection event or extreme stability (Stull 2000) B/S threshold of -3 buoyancy is three times greater than wind shear B > 3 S high possibility to generate free convection. B/S threshold of +1 Time [Day] states dynamical stable, wave motion drive conditions. Most of the time in May 2006 relation thermal vs. mechanical produced TKE is near zero. Distinct, short periods of free convection or extreme stability (duration only 1 h to 2 h). Events at Ny-Ålesund appear mostly around noon or afternoon. -12 10
Surface energy budget of permafrost on Svalbard (Westermann, Lüers et al. TC 2009)
DS 18 DL +41 Q H 18 Q E +0.7 Light Winter 15 Mar to 15 Apr DS -41 DL +33 Q H 8 Pre melt 15 Apr to 31 May Q E +2.5 C ~0 Q G 5.9 Q G +3 C +10.5 DS 91 Q G +12 DL +43 Q Melt +27 Snow melt Jun Q H 6 Q E +11 C +3 Light winter: Net short-wave radiation increasing limited by high snow albedo. Energy loss by L compensated by sensible heat flux and short-wave radiation. Snow heat flux negative further cooling underlying soil column lowest soil temperatures. Pre melt: Net short-wave radiation dominant energy supply. Sensible heat flux add. melt energy. Net long-wave radiation main balancing factor. Latent heat remains insignificant. Snow and soil column start to warm (now positive snow heat flux). Snow melt: Warming of snow pack towards 0 C followed by snow melt. Energy consumed by melting snow is dominant component. Strong net short-wave radiation (albedo change) Compensated by net longwave radiation. Total net radiation is much stronger energy suppler compared to Q H. Snow melt almost entirely controlled by radiation.
Dark Winter Oct to Mar Summer Jul & Aug DL +28 Q H 16 Q E +2.5 Q G 5 C 9 Dark winter: Long-wave radiation is main forcing Net long-wave radiation dominant energy loss L loss is compensated by negative sensible heat flux (warming of surface, cooling of atmosphere) Sig. positive latent heat fluxes (at high wind speed & neutral stratification) Sig. influence of other factors: wind speed & precipitation (rain on snow) Schematic diagram of the contributions of the surface energy budget for different seasons. Arrows pointing away from the surface indicate positive fluxes. The flux values are given in Wm - 2. DS -122 Q G +12 DL +43 Q H 22.5 Q E 22.5 Summer: strong forcing by short-wave radiation absence of snow cover. C +22 Net short-wave radiation S is compensated by: + net long-wave radiation L + sensible and latent heat flux + ground heat flux Thawing of the active layer.
Surface energy budget of permafrost on Svalbard Summer 1 Jul to 31 Aug 2008, Bayelva permafrost station Sensible (red), latent (blue) and ground (green) heat fluxes W m 2 (left axis). Soil water content θ w - Time Domain Reflectometry at 0.1 m depth (right axis). Average daily Bowen ratio vs. volumetric soil water content θ w Soil water content classes of widths of 0.02. Error bars represent the standard deviation of the Bowen ratio values within one class.
Surface energy budget of permafrost on Svalbard Snow melt - June Sensible (red), latent heat flux (blue), W m 2, left axis. Light gray : snow-covered Dark gray : snow-free fraction of the surface area, right axis. Intermediate gray : uncertainty in area fractions. Until large snow-free patches appear Air temperature between -1 C and +5 C snow surface temperature remains close to 0 C resulting temperature gradient is small weak Q H -flux positive Q E -flux, causing cooling of surface, becomes significant (presence of water). During snow melt Infiltrating melt water subsequent refreezing processes dominates snow pack, (isothermal close to 0 C). Underlying soil still colder resulting in a heat transport from snow-soil interface into the soil.
Surface energy budget (SEB) of permafrost on Svalbard Annual cycle During polar night - main components SEB: * IR-radiation *sensible heat flux *heat input refreezing active layer Determining factor surface temperature of snow: *incoming atmospheric IR-radiation and *downward sensible heat flux During spring - Apr/May - long-lasting snow cover - high albedo: Limitation of *short-wave radiation Dominant component: energy consumed by melting snow Albedo change by snow melt is of critical importance for annual SEB Marking transition point between two fundamentally different regimes During polar day - Jul/Aug - snow-free period - system governed by *short-wave radiation Main balancing factors in the surface energy budget *turbulent heat fluxes and *IR-radiation balance
Conclusions : Process understanding ARCTEX-2006 and 2009 + Understanding: variability and transport processes + Interaction: meso-scale atmospheric circulation pattern with micrometeorological near surface atmospheric exchange conditions. + Wind field change: short, developed unstable situations or even free-convection events (noon or early afternoon). + Positive feedback: snow and ice melt is supported by free-convection or short unstable periods + Disturbed temperature profile close to surface: Decoupling major error source to estimate heat fluxes and appropriate surface temperature + Comparison: heat-flux parameterizations with direct measurements. Hydrodynamic three-layer temperature-profile model reliably reproduces temporal variability of surface temperature 17
Conclusions : Practical recommendations Main technical error source drifting snow and precipitation effects through flux sensor pathways (over- or underestimation and misleading flux directions) Solution Measurement height should be recorded (variation of snow depth) and must be adjusted according to change of seasons, service of a present weather detector (weather code, visibility). Decoupling effect (disturbed temperature profiles) and the determination of the surface temperature Solution Additional meteorological data + near-surface vertical gradients of wind, temperature, humidity. Surf. temp.: IR-Thermometer or pyrgeometer or extrapolation applying a hydrodynamic three-layer temperatureprofile model (Lüers & Bareiss ACP 10/1). Measurement height of instrumentation (eddy-covariance systems or gradient towers) should be in appropriate layer within and above this disturbed wind and temperature profiles. General Adaptation to polar conditions of flux data corrections and quality assessment and quality control (QA/QC) techniques (rotation of coordinate system). Detection of intermittent turbulent conditions, of free-convection events, of wave motion. Investigation of meso-scale circulation pattern and micro-scale near surface profiles. Observation of variability of the snow and tundra soil surface conditions (e.g. Web-Cam). Finding a compromise between conflicting nature of the effect of the disturbed temperature profile and the search for an acceptable fetch and desired footprint area. 18
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