Near-surface turbulent fluxes at Kohnen Station on the East Antarctic Plateau

131 Pages·2017·24.97 MB·English

UNIVERSITY OF BREMEN
M ’ T
ASTER S HESIS
Near-surface turbulent fluxes at Kohnen Station
on the East Antarctic Plateau
Isabell Sonntag
Examiners:
Supervisor:
Prof. Dr. Thomas Jung
Dr. Christof Lüpkes
PD Dr. Ulrike Wacker
February 2017

Thismaster’sthesisissubmittedforthedegreeof
M.Sc. inEnvironmentalPhysics.
Theprogrammeisacooperationbetweenthe
InstituteforEnvironmentalPhysics(IUP)attheUniversityofBremen
andthe
AlfredWegenerInstitute(AWI),HelmholtzCentreforPolarandMarineResearch,
Bremerhaven.
ThethesishasbeenpreparedinthePolarMeteorologygroupoftheAWIBremerhaven.

v
Abstract
The longest ever obtained in-situ turbulence time series at Kohnen Station in Dronning
MaudLand,EastAntarcticaisanalysedundertheaspectofMonin-Obukhovsimilaritythe-
ory(MOST).Thelatterformsthebasisforthecommonlyusedparametrizationschemesfor
near-surface turbulent fluxes in numerical weather prediction and climate models. Based
ontheturbulencedataandmeasurementsofmeanmeteorologicalvariablesthathavebeen
obtainedduringthecampaignfrom10Dec2013to31Jan2014,thegeneralmeteorological
situation and daily evolution of the lower boundary layer is described. We find the mea-
surement period to be representative for summertime conditions at Kohnen Station with a
shallow but dynamic boundary layer which is moderately stable at night and slightly un-
stable during daytime. Average diurnal amplitudes of near-surface temperature and wind
speed amount to 10K and 3m/s. Different MOST stability functions are compared with
those based on the measured momentum and sensible heat fluxes under both stable and
unstable stratification. For this purpose, aerodynamic roughness length and temperature
roughness length are determined but show to be quite variable. To consider MOST stabil-
ity functions derived from our measurements we employ a new straight-forward method
thatusesonemeasurementlevelonly(insteadofusuallytwo). Wefindthatbythismethod
the obtained scatter is large so that a conclusion on the optimal function is not possible.
Nevertheless,weshowthatbulkapproachesbasedondragcoefficientincludingtraditional
stability functions can reproduce the measured momentum fluxes well under stable condi-
tions if an aerodynamic roughness length of the order of 10−5m is employed. However,
for friction velocities larger than 0.16m/s the momentum flux under stable conditions is
slightly underestimated. Another result of the present study is that the measured func-
tional dependence between the stability parameter z/L and the bulk Richardson number
agrees very well with the one shown by Grachev et al. (2007, SHEBA data) under stable
conditions. Since the relation between z/L and Ri also depends on MOST stability func-
B
tions, this result suggests the applicability of the Grachev stability functions under stable
conditionsforthelocationofKohnenStation.
The worse agreement for momentum flux parametrization under unstable conditions and
alsoforheatfluxparametrizationingeneralbasedontheinitiallyusedsingle-levelmethod
maybe dueto measurement uncertainties. However,it mayalsohint atlimitsof theappli-
cabilityofMOSTforthespecialandextremeconditionsonthehighAntarcticplateau.

vii
Acknowledgements
I want to thank the Polar Meteorology Group for their kind and engaged support while
working on this thesis. Each member contributed with their knowledge and experience to
progress in different parts of this very challenging work. Especially, I want to thank my
supervisor Dr. C. Lüpkes for his readiness and time for discussions that have been very
helpful on this foreign ground. Since I could not be at the AWI for most of the time, I
missed the pleasant working atmosphere of this institute very much. I also want to thank
the PHAROS group of the IUP for giving me the opportunity to gain very worthwhile
experience in Python programming prior to this thesis. Special thanks go to my friends
and family that have been very understanding for my lack of time and always supported
me in what I do. At last, my greatest appreciation must go with Stefan and our small
growingmiracleforstayingstrongandunderstandingevenduringthemostfrustratingand
difficulttimes. Youaremylight.

ix
Table of Contents
Abstract v
Acknowledgements vii
TableofContents ix
ListofFigures xiv
ListofTables xv
ListofAbbreviations xvii
1 Introduction 1
2 ConditionsatKohnenStation 5
2.1 Generalinformation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5
2.2 Climatology . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7
2.2.1 Near-surfacevariables . . . . . . . . . . . . . . . . . . . . . . . 7
2.2.2 BoundaryLayer . . . . . . . . . . . . . . . . . . . . . . . . . . 9
3 Instrumentation 13
3.1 Instrumentationanddatasets . . . . . . . . . . . . . . . . . . . . . . . . 13
3.2 TheSonic . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 15
3.2.1 Generalcomments . . . . . . . . . . . . . . . . . . . . . . . . . 15
3.2.2 Measuringprinciple . . . . . . . . . . . . . . . . . . . . . . . . 16
x
4 Theoreticalbackground 21
4.1 Surfacelayertheory . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 21
4.2 Monin-Obukhovsimilaritytheory . . . . . . . . . . . . . . . . . . . . . 23
4.3 Bulktransferrelations . . . . . . . . . . . . . . . . . . . . . . . . . . . . 27
5 Datapreparation 29
5.1 Derivationofsurfacetemperaturefromlongwaveradiation . . . . . . . . 29
5.2 QualitycontrolofAWSairtemperature . . . . . . . . . . . . . . . . . . 30
5.3 FromrawSonicdatatoturbulentfluxesusingtheeddycovariancemethod 35
5.3.1 Theeddycovariancemethod . . . . . . . . . . . . . . . . . . . . 35
5.3.2 DataprocessingofSonicrawdata . . . . . . . . . . . . . . . . . 37
5.3.3 QualitycontrolofSonicdata . . . . . . . . . . . . . . . . . . . . 44
5.3.4 OntheuseofThetainsteadofT . . . . . . . . . . . . . . . . . . 45
5.3.5 Sonicparametertest: Averaginginterval . . . . . . . . . . . . . . 45
6 Results 49
6.1 Meteorologicalconditions . . . . . . . . . . . . . . . . . . . . . . . . . 49
6.1.1 Near-surfacevariables . . . . . . . . . . . . . . . . . . . . . . . 49
6.1.2 Summerwarmingevent . . . . . . . . . . . . . . . . . . . . . . 52
6.1.3 Boundarylayerevolution . . . . . . . . . . . . . . . . . . . . . . 53
6.2 Near-surfaceturbulentfluxes . . . . . . . . . . . . . . . . . . . . . . . . 57
6.2.1 Dailycycle . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 57
6.2.2 NegativeCHvaluesandthermalemissivityofthesnowsurface . 59
6.2.3 Dataselectioncriteria . . . . . . . . . . . . . . . . . . . . . . . 63
6.2.4 Stabilityparameters . . . . . . . . . . . . . . . . . . . . . . . . 64
6.2.5 Roughnesslengthsandneutralbulktransfercoefficients . . . . . 65
6.2.6 Parametrizationofturbulentsurfacefluxes . . . . . . . . . . . . . 70
7 Conclusion 83
A Derivationofwindspeedandtemperaturefromsonictraveltimes 89
A.1 Derivationofwindspeedfromsonictraveltimes . . . . . . . . . . . . . 89
A.2 Derivationoftemperaturefromsonictraveltimes . . . . . . . . . . . . . 90
B StabilityfunctionsaccordingtoMOST 91
C TestofdifferentfilterfrequenciesforFFT(Sonichigh-passfilter) 95
D Timeseriesgraphs 99
E Lineardepictionofmeasuredψ 103
M