Boundary layer evolution over the central Himalayas from radio wind profiler and model simulations

14 Pages·2016·6.89 MB·English

Atmos.Chem.Phys.,16,10559–10572,2016
www.atmos-chem-phys.net/16/10559/2016/
doi:10.5194/acp-16-10559-2016
©Author(s)2016.CCAttribution3.0License.
Boundary layer evolution over the central Himalayas from radio
wind profiler and model simulations
NarendraSingh1,RamanSolanki1,2,NarendraOjha3,RuudH.H.Janssen3,AndreaPozzer3,and
SurendraK.Dhaka4
1AtmosphericScienceGroup,AryabhattaResearchInstituteofObservationalSciences,ManoraPeak,Nainital,India
2DepartmentofPhysics&Astrophysics,UniversityofDelhi,Delhi,India
3DepartmentofAtmosphericChemistry,MaxPlanckInstituteforChemistry,Mainz,Germany
4RadioandAtmosphericPhysicsLab.,RajdhaniCollege,UniversityofDelhi,Delhi,India
Correspondenceto:NarendraSingh([email protected])
Received:1February2016–PublishedinAtmos.Chem.Phys.Discuss.:11April2016
Revised:9July2016–Accepted:28July2016–Published:24August2016
Abstract. We investigate the time evolution of the Local of LBL would lead to a minor uncertainty in simulated
Boundary Layer (LBL) for the first time over a mountain surface ozone concentrations. However, it would lead to a
ridge at Nainital (79.5◦E, 29.4◦N, 1958ma.m.s.l.) in the significant overestimation of the dilution of black carbon
centralHimalayanregion,usingaradarwindprofiler(RWP) aerosols at this site. Our work fills a gap in observations
during November 2011 to March 2012, as a part of the of local boundary layer over this complex terrain in the Hi-
GangesValleyAerosolExperiment(GVAX).Werestrictour malayas,andhighlightstheneedforyear-longsimultaneous
analysistoclear–sunnydays,resultinginatotalof78daysof measurementsofboundarylayerdynamicsandairqualityto
observations.Thestandardcriterionofthepeakinthesignal- betterunderstandtheroleoflowertroposphericdynamicsin
to-noise ratio (S/N) profile was found to be inadequate in pollutiontransport.
the characterization of mixed layer (ML) top at this site.
Therefore,weimplementedacriterionofS/N>6dBforthe
characterizationoftheMLandtheresultingestimationsare
shown to be in agreement with radiosonde measurements 1 Introduction
over this site. The daytime average (05:00–10:00UTC) ob-
served boundary layer height ranges from 440±197m in Measurements on the diurnal evolution of the planetary
November(lateautumn)to766±317mabovegroundlevel boundary layer (PBL), the marine boundary layer (MBL),
(a.g.l.)inMarch(earlyspring).Theobservationsrevealeda andinparticularoverhigh-altitudecomplexterrainsthelocal
pronouncedimpactofmountaintopographyontheLBLdy- boundarylayer(LBL),areessentialtounderstandtheverti-
namicsduringMarch,whenstrongwinds(>5.6ms−1)lead cal transfer of momentum, energy and the mixing of pollu-
to LBL heights of 650m during nighttime. The measure- tants in the lower troposphere. The depth or height of the
ments are further utilized to evaluate simulations from the PBL is a fundamental parameter in numerical simulations
WeatherResearchandForecasting(WRF)model.WRFsim- of regional meteorology and air quality. The height of the
ulations captured the day-to-day variations up to an extent mixedlayer(ML)isameasureoftheeffectivenessoftheen-
(r2=0.5), as well as the mean diurnal variations (within ergytransferfromthesuntotheearth’ssurfaceandthereby
1σ variability). The mean biases in the daytime average to the lower atmosphere and is therefore important for un-
LBL height vary from −7% (January) to +30% (Febru- derstandingvariousatmosphericprocesses(Stull,1989;Gar-
ary) between model and observations, except during March ratt,1993).Despitebeingsimpletounderstandconceptually,
(+76%).Sensitivitysimulationsusingamixedlayermodel the measurement of ML height is rather difficult (Coulter
(MXL/MESSy)indicatedthatthespringtimeoverestimation and Holdridge, 1998). The most traditional method is the
analysisofpotentialtemperatureandspecifichumiditypro-
PublishedbyCopernicusPublicationsonbehalfoftheEuropeanGeosciencesUnion.
10560 N.Singhetal.:BoundarylayerevolutionoverthecentralHimalayas
files, which are obtained from radiosonde ascents (Hooper resolution(15min).Therefore,theRWPisthebestpossible
and Eloranta, 1986). However, in the past 3 decades sev- tooltodetermineMLevolutionoverthesite.TheRWPalso
eral remote-sensing devices such as the light detection and givesadeepinsightintotheverticalstructureoftheboundary
ranging (lidar), sound detection and ranging (SoDAR), ra- layer in general, which is crucial in understanding dynamic
dioacousticsoundingsystem(RASS),GPSoccultationmea- meteorology, pollutant transport and dispersion (B. Xie et
surements(BashaandRatnam,2009;F.Xieetal.,2012)and al., 2012). Studies on ABL dynamics over complex terrain
radiowindprofilers(RWP)havebecomeindispensabletools havebeenmadebyReddyetal.(2006)andKalapureddyet
toprobetheatmosphericboundarylayerandthetroposphere al. (2007) over a moderately hilly terrain in Gadanki valley
within the first few kilometers above the ground. A review regioninSouthernIndia.However,nosuchstudieshavebeen
anddetailedstudiescarriedoutearlieronPBLevolutionover made over the Himalayas for the highly contrasting winter
variouslocationsaregivenelsewhere(Friehe,1987;Garratt andspringseasons.Theimportanceofthisstudycanbeun-
andTaylor,1996). derstood by the fact that a strong diurnal cycle can appear
It is essential to mention that most of the studies cited in the LBL or ML over hilly terrain during the fair-weather
hereandthereferencesthereinareconfinedtomidandhigh- conditions(Reddyetal.,2002).
latituderegions,andthatveryfeweffortshavebeenmadeto Themainobjectivesofourstudyarethefollowing:
characterizethePBLevolutionovertheIndiansubcontinent
1. To investigate the diurnal, and monthly variations in
andthecomplexterrainoftheHimalayanregion.Theavail-
LBL evolution over the central Himalayas using RWP
able studies using satellite and ground-based measurements
observations.
overtheIndianregionhaveshowninfluencesoftheboundary
layer evolution and associated mixing on the trapping, dis-
2. Toevaluateboundarylayerheightwithregionalmodel
persion, and transport of pollutants upwards to the pristine
(WRF)simulations,byusingasetupsimilartotheone
high-altitude locations. Some of these studies are connoted
evaluated and extensively used over the Indian region
subsequently.
(e.g.,Kumaretal.,2012a,b;Sarangietal.,2014;Ojha
The variations in boundary layer height are suggested to
etal.,2016).
influence the dispersion of air pollutants near the earth sur-
face (Guatam et al., 2007). Deeper boundary layers could 3. Toassesstheinfluenceofmodeluncertaintiesinbound-
alsomixresiduallayerairwithhigherozoneconcentrations ary layer height on air quality simulations, by con-
withtheairmassnearthesurface(Reddyetal.,2012).The ducting sensitivity runs with a mixed layer model
investigation of local boundary layer evolution and associ- (MXL/MESSy).
ated mixing is additionally important over the pristine Hi-
In this study we implemented a new criterion for the S/N
malayaslocatedjustnorthtothedenselypopulatedandpol-
that captures nearly systematic feature of LBL evolution
luted Indo-Gangetic Plain (IGP) in order to understand the
with time, since above a mountain peak, the structure of
role of regional pollution on the air quality above the Hi-
the LBL is not as prominent as that observed over high
malayas (Sarangiet al.,2014). Inprevious studies,the con-
pressure regions or flat terrain. The ML height estimation
vectivemixingofphotochemicalpollution(Ojhaetal.,2012)
methodusingtheRWPhasbeenvalidatedwithMLheights
and the potential influence of distantly transported and IGP
from radiosonde profiles. We investigate the mean diurnal
aerosolemissions,onairqualityinthecentralHimalayashas
variations of ML height during months covering the late
beendemonstrated(Solankietal.,2013;SolankiandSingh,
autumn (November), winter (December–January–February)
2014) with the measurements over the region. Continuous
and early spring (March). The impact of mountain topogra-
measurements of LBL evolution and mixing have not been
phyonLBLevolutioninthepresenceofstrongwindsduring
availableuntilarecentintensivefieldcampaign“GangesVal-
nighttime is also investigated. Finally, we compare the day-
leyAerosolExperiment(GVAX)”(Kotamarthi,2010;Kota-
to-day and diurnal variations in LBL height deduced from
marthiandSatheesh,2011;Manohanarnetal.,2014).How-
RWPmeasurementswithsimulationsfromaregionalmodel
ever, systematic and high-resolution measurements of the
(WRF).Theimplicationsofmodel-observationbiasesonair
PBL evolution over the IGP region and the LBL evolution
qualitystudiesarealsodiscussed.
overtheHimalayanregionforacompleteseasonalcycleare
Themanuscriptbeginswithabriefdescriptionoftheob-
stillmissing.
servational site, the RWP instrumentation and the mixed
During the GVAX campaign at ARIES, Nainital regular
layer determination methodology in Sect. 2. The Weather
radiosonde launches (four per day) were made, but a clear
ResearchandForecasting(WRF)modelthatisemployedto
pictureofLBLorMLevolutionwasnotpossiblesincefour
simulatethetemporalvariationsinboundarylayerheightand
radiosondeprofilesadayprovidedonlysnapshotsoftheat-
themixedlayermodel(MXL/MESSy)thatisusedforsensi-
mosphere. On the other hand, for the first time, RWP was
tivitysimulationsofairqualityaredescribedinSect.3.The
operatedforwindmeasurementsandcanalsobeutilizedto
results and discussion are presented in Sect. 4, followed by
deriveanestimateofboundarylayerheight(Angevineetal.,
thesummaryandkeyconclusionsinSect.5.
1994; Coulter and Holdridge, 1998) with a finer temporal
Atmos.Chem.Phys.,16,10559–10572,2016 www.atmos-chem-phys.net/16/10559/2016/
N.Singhetal.:BoundarylayerevolutionoverthecentralHimalayas 10561
2 Observationalsiteandmethodology Raghavan, 2003; Singh et al., 2008). The profiler S/N at a
givenrangeisdirectlyproportionaltoC2(Ottersten,1969).
n
2.1 Observationalsite The vertical structure of the PBL (with height Z ) con-
i
sistsofthreedifferentlayers(Angevineetal.,1994).Thefirst
The observational site is located at a mountain top called layeristhesurfacelayerstartingfromgroundto0.1(Z ),the
i
Manora Peak (79.5◦E, 29.4◦N and 1958ma.m.s.l.), near secondlayeristheMLextendingfrom0.1Z to0.8Z andthe
i i
Nainital,ahigh-altitudestationinthecentralHimalayas.The third layer is the entrainment zone from 0.8(Z ) to 1.2(Z ).
i i
observationalsitehas600mdeepvalleysontheeasternand Above the PBL is the free troposphere. Generally, over a
westernsidesandthevalleyaxesarealignedapproximately plain site the top of the ML is characterized by increased
intheSEandSWdirection,respectively.Tothenorthofthe turbulence, and strong temperature and moisture gradients,
peak the topography is very craggy and rising; towards the which are represented by a peak in the S/N profile, mark-
south,themountainsaregraduallyslopingintotheadjacent ingthetopoftheML(Simpsonetal.,2007).However,this
plains of Haldwani. The mountain peak has a slope of ap- study is carried out over a high-altitude site (2kma.m.s.l.)
proximately25◦ ontheeasternandwesternside,andiscov- andbecauseofthelackofprominentfeaturesoverthemoun-
ered with a dense forest on all sides, consisting mainly of tainoussitethegeneralmethodsofMLheightdetermination
trees and shrubs. All measurements presented in this study over plains cannot be directly applied. The complex moun-
weremadeonthemountainpeakwithnoobstructionssuch tainous topography generates its own LBL (e.g., Kossmann
as buildings or trees in any direction. The site (Sagar et al., et al., 1998), which is different from the PBL in some as-
2004) has been the subject of intensive studies on synoptic pects.OneofthoseisthattheS/N profile,insteadofexhibit-
wind patterns and trace gases (Sarangi et al., 2014), bal- ingapeakintheprofile,showsagradualdecreasewithalti-
loon soundings (Ojha et al., 2014) and aerosol extinction tude,asdepictedinFig.1;althoughatanotherhigh-altitude
profiles (Solanki and Singh, 2014). A detailed overview of research station Kleine Scheidegg (Ketterer et al., 2014) a
the measurement site and the atmospheric science research prominent peak in the S/N profile was always measured in
conducted there has been provided in a recent review paper convectivestateofboundarylayer.Itmayhoweverbenoted
(Sagaretal.,2015). that the station at Kleine Scheidegg is situated on a moun-
tainpass,whereasthemeasurementspresentedinthisstudy
2.2 RWPinstrumentationandmixedlayer were taken over a mountain peak; hence difference in char-
determination acteristicsofLBLcouldbeanticipatedduetodifferencesin
the topography. One possible explanation for this aspect of
Wind profilers are basically designed to make the continu- theLBLcouldbethedifferenceinthecharacteristicsofen-
ous measurements of wind components, i.e., zonal merid- trainmentzoneoverplainsandmountainousterrain,sincethe
ionalandvertical,atveryfinetemporalandspatialresolution entrainment zones basically form between the tops of high-
inallweatherconditions.Theseprofilersgenerallyoperateat est thermal plumes and deepest part of the sinking dry air
variousfrequencies(30–3000MHz)dependinguponthesci- penetratingthroughthefreetroposphere.Incontrasttoaflat
ence objectives. Such a wind profiler developed by DeTect, terrain, mountainous or complex terrain may not have that
Inc., which operates at 1290MHz was utilized at the obser- prominent formation of the entrainment zones due to com-
vational site. The system is capable of changing the beam plextopography,asconvectivemixingovertheridgemaybe
pointing angle on a pulse-by-pulse basis for five beam po- influenced by slope winds and occasional strong horizontal
sitions, and provided the backscattered signal and the wind flows. Therefore, in this study we have considered the re-
profilesbetween0.1and6km.Theprofilercollectedasam- gion of S/N>6 decibels (dB) as the LBL over the site and
pleevery30s,completingacycleofallfivebeampositions validated this method by comparing the results with those
every150s.Byselectingabeamelevationof90◦,S/N pro- deduced from the vertical profiles of potential temperature
files from the vertical beam were only incorporated in this andspecifichumidityfromradiosonde(VaisalaRS92-SGP)
analysis. The S/N data for the vertical beam are available ascents.IncaseofRWPS/N profilesshowingmultipletran-
witharesolutionof62.6m,withthefirstrangebinat124m. sitionsacrossthe6dBthreshold,themaximumaltitudewas
The ML height is inferred from the signal-to-noise ratio selectedastheMLheight,asshownbythe10:00UTCprofile
(S/N)recordedbytheRWP.Theunderlyingtheoryhasbeen in Fig. 1. Although averaging time of 30min is considered
discussedbyWhiteetal.(1991a,b).Theprincipalsourceof forparametersdeterminingtheevolutionofPBL,butforthe
scattered signal to RWP is moisture, which is the primary LBL study over the site we have considered the averaging
constituentofinhomogeneityintheradiorefractiveindexof time of15min (for mean S/N profiles) forbetter statistical
theair.Thesewindprofilersbasicallydetectthefluctuations averageandtakingintothemixingtimescale,sincetheLBL
in radio refractive index caused by vertical humidity and isinamuchmoredynamicalstate,i.e.,consideringthecon-
temperaturegradients.Theseinhomogeneitiesarecharacter- vective mixing and the topographic effects, as compared to
izedbytherefractiveindexstructureparameterC2(Tatarskii, PBL.ThecontourplotsofS/N werealsoanalyzedvisually
n
1971; Green et al., 1979; Gossard et al., 1982; Gage, 1990; in order to confirm the reliability of the ML height estima-
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10562 N.Singhetal.:BoundarylayerevolutionoverthecentralHimalayas
raphy of the site (a mountain peak) and the drift in the ra-
diosonde, which thus provides measurements of ML height
over the adjoining valleys or ridges at nighttime. Since the
ML top follows the topography over mountainous terrain,
and the degree to which ML top follows the topography
is minimum at noontime and afternoon hours (De Wekker
and Kossmann, 2015). Hence, a higher correlation between
radiosonde ML height estimation and RWP derived LBL
heightiscertainlyexpectedtobesignificantonlyduringthe
noontimetoeveninghours.
The vertical profiles of meteorological parameters ob-
tained from the radiosonde ascents are also utilized to pro-
videtheverticalstructureoftheatmosphereforcomparison
with RWP measurements. The hourly wind speed observa-
tions made through a collocated automatic weather station
(AWS) over the site are also incorporated to understand the
stateofthesurfacelayeroverthesite.
3 Modelsimulations
3.1 Weatherresearchandforecasting(WRF)model
This study uses the version 3.5.1 of the Weather Research
and Forecasting (WRF) model to simulate the temporal
Figure 1. The signal-to-noise ratio profiles (15min averaged) on variations of boundary layer height at Nainital during the
15 March 2012 during the peak sunshine hours. The estimated studyperiod.MeteorologicalfieldsfromNCEPFinalAnal-
mixedlayerheightforeachprofileismarkedbythehorizontalbar. ysis (FNL) data available at the spatial resolution of 1◦
and temporal resolution of 6h has been used to provide
the initial conditions and the lateral boundary conditions
tionsatthe6dBthreshold.However,theS/N isproportional in the model. Simulations were performed for two differ-
torefractiveindexstructureparameters,hencecanfurtherbe ent spatial resolutions. The simulations from the coarser
subjectedtoderivereflectivityandtheturbulenceparameters domain (15km×15km) were used to provide the initial
thatagainarethetracersofLBLorPBLevolution. and boundary conditions for the higher resolution domain
InordertodeterminetheMLheightfromradiosondemea- (5km×5km). Both model domains and the topography of
surements and thus authenticate the 6dB criterion, the bulk this region are shown in Fig. 2. The errors in the simulated
Richardson method, specific humidity gradient (maxima), meteorologyinthecoarserdomainwerelimitedbynudging
and virtual potential temperature profiles gradient method ofthetemperature,watervaporandhorizontalwindswitha
were tested; but as expected over a complex terrain, only nudgingcoefficientof6×10−4persecondatallthevertical
the specific humidity gradient (maxima) method provided levels(Kumaretal.,2012a,b;Ojhaetal.,2016).
promising results. A detailed correlation analysis was done The cloud microphysics was represented by the Thomp-
for ML derived from specific humidity gradient method vs sonmicrophysicsscheme(Thompsonetal.,2008).Thelong-
RWP (S/N method) and the maximum correlation (r2= wave radiation has been calculated using the rapid radia-
0.70) between the two is found to be at the S/N value of tivetransfermodel(RRTM)(Mlaweretal.,1997)andshort-
6dB,forthenoon-timeandevening(06:00and12:00UTC) wave radiation is calculated using the Goddard shortwave
profiles that closely correspond to the convective state of scheme(ChouandSuarez,1994).Thesurfacelayerhasbeen
boundary layer over the site. For the analysis, the cases parameterized using the Monin–Obukhov scheme (Janjic,
of minimum ML height estimation from RWP (RWP starts 1996). The Noah Land Surface Model, which utilizes the
measurementsbeyond124minthevertical)andradiosonde, UnifiedNCEP/NCAR/AFWAschemewithsoiltemperature
aswellasdaysofveryweak(<10gkg−1km−1)gradientin andmoistureinfourlayers,hasbeenusedtoparameterizethe
specifichumidityprofileswerediscarded. land surface processes (Chen and Dudhia, 2001). The plan-
The agreement between radiosonde and RWP estimated etary boundary layer dynamics were parameterized using
ML height for midnight and early morning profiles (18:00 the Eta operational Mellor–Yamada–Janjic (MYJ) scheme,
and00:00UTC)turnsouttobeverypoorforalloftheknown which is based on a one-dimensional prognostic Turbulent
methods. The disagreement can be attributed to the topog- Kinetic Energy (TKE) scheme with local vertical mixing
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N.Singhetal.:BoundarylayerevolutionoverthecentralHimalayas 10563
fore, scalars and reactants in the convective boundary layer
are characterized by a well-mixed vertical profile over the
whole depth of the ABL. In MXL, the transition between
the well-mixed BL and the free troposphere is marked by
an infinitesimally thin inversion layer. In this study, chem-
ical transformations are represented by the Mainz Isoprene
Mechanism 2 (MIM2; Taraborrelli et al., 2009). Black Car-
bon(BC)istreatedasapassivetracer,whichisappropriate
attheshorttimescale(6h)ofoursimulations.
4 Resultsanddiscussion
Figure 2. The WRF simulation domains used in the study are 4.1 EstimationofMLheight
shown.Thecoarserdomain(15km×15km)simulationsareused
todrivethesimulationoverthenesteddomainshownaswhitebox Figures 3 and 4 depict the diurnal variability (24h cy-
in panel (a) and separately in panel (b). The geographical topog- cle) of the RWP measured S/N profiles (15min averaged)
raphyoftheregionand thelocationofmeasurementsiteNainital
on two contrasting representative days in different months
(NTL)arealsoshown. (withsensibleheatfluxof17and50Wm−2 duringDecem-
ber and March). For each day, data from four radiosonde
launches (approximate launch time of 00:00, 06:00, 12:00
(Janjic, 2002). The cumulus parameterization was based on
and 18:00UTC) were also available, which are used to in-
the new Grell (G3) scheme for the coarser domain, while it
vestigate the profiles of potential temperature and specific
has been turned off for the nested domain as shown in the
humidityforcomparisonwithMLheightestimatedfromthe
Supplement (Fig. S1). Detailed discussions on simulations
RWP. A very clear smooth diurnal variation in S/N is ob-
using the WRF model, the chosen physics options and me-
served on 17 December 2011, as illustrated in Fig. 3; this
teorological nudging, including evaluations over the Indian
featureischaracteristicofamountainLBLinthewinterun-
region,canbefoundinrecentstudies(Kumaretal.,2012a,
dercalmwindconditions(windspeed<2ms−1),theevolu-
b;Sarangietal.,2014;Ojhaetal.,2016).Hourlymodelout-
putfromthenested5km×5kmsimulationhasbeenusedin tion is seenfrom 02:00 to 13:00UTC, i.e., 07:30 to18:30h
local time (LT=UTC+5.5h), attaining a peak of 500m
theanalysis.
at noon (07:00 to 08:00UTC). During nighttime (13:00 to
3.2 Mixedlayermodel(MXL/MESSy) 02:00UTC)theLBLisextremelyshallowindepth,andun-
der the RWP lower detection range (i.e., 124m). The spe-
The implications of biases in the WRF simulated bound- cific humidity and potential temperature profiles also show
arylayerdynamics,ascomparedtoRWPmeasurements,on a clear demarcation between the LBL and the free tropo-
air quality simulations have been investigated by conduct- sphere during daytime (06:22UTC), and the height of the
ing sensitivity simulations with the MiXed Layer/Modular inversion(∼300m)agreesquitewellwiththeRWP-derived
Earth Submodel System model (MXL/MESSy, version 1.0; LBL height at the same time. The other radiosonde profiles
Janssen and Pozzer, 2015). Although the BL dynamics are (23:50,11:48and17:41UTC)showstablepotentialtemper-
representedinadifferentwayinMXL/MESSythaninWRF, ature profiles, which is consistent with a stable (nocturnal)
wecanusetheformertoobtainaninsightintotheeffectsof boundarylayer.
anoverestimationofBLheightonmodeledchemicalspecies Figure 4 shows the ML height evolution on 15 March
concentrations. MXL/MESSy has been developed recently 2012, which is considered as representative of the early
as a column model in the MESSy framework (Jöckel et al., spring season over the site, based on the mean diurnal vari-
2010). Within MXL/MESSy, the MXL submodel accounts abilityoftheMLduringMarch.Twodistinctgrowthphases
for the dynamics of the convective boundary layer during oftheMLareobservedduringthe24hcycle,withonestart-
daytime, by explicitly calculating BL-free troposphere ex- ing at 05:00UTC and the other at 13:00UTC. The growth
changeofscalarsandchemicalspeciesthroughentrainment. and decay in ML from 05:00 to 12:00UTC is consistent
Through the coupling of MXL with other MESSy submod- withthediurnalcycleofincomingsolarradiation.Thepeak
elsforprocessesthatarerelevantforatmosphericchemistry, heightoftheMLisabove900m,andattainedfrom08:00to
MXL/MESSy can be used to evaluate the influence of BL 10:00UTC. A second growth phase in the ML depth starts
dynamics on atmospheric chemistry. The mixed layer the- from 13:00UTC, and the ML remains stable from 15:00 to
orystatesthatunder convectiveconditions,strongturbulent 20:30UTC with ML height reaching up to approximately
flowcausesperfectmixingofquantitiesovertheentiredepth 700m.TheminimumMLheightof300misobservedfrom
of the ABL (Vilà-Guerau de Arellano et al., 2015). There- 00:00 to 03:30UTC. The vertical profile of specific humid-
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10564 N.Singhetal.:BoundarylayerevolutionoverthecentralHimalayas
Figure 3. Mixed layer height for 17 December 2011. The upper Figure4.SameasFig.3butfor15March2012.
panel shows range time intensity (RTI) plot of S/N (15 min av-
eraged)measuredwithRWP,theverticalarrowsindicatethetime
of sunrise and sunset. The lower panel shows the vertical profiles
4.2 Diurnalvariations
of potential temperature and specific humidity (horizontal arrows
indicate the ML height estimated with specific humidity gradient
maximamethod)derivedfromthefourradiosondeflightsconducted The comparison of monthly average diurnal variations in
duringtheday(markedwithdifferentcolors). the ML height during the period from November 2011 to
March 2012 is presented in Fig. 5. The ML height was es-
timated only for the clear sunny days (without any large
ity shows a strong inversion between 500 to 550m, which mesoscale activity), which were selected for each month
isconsistentwiththeestimatedMLheightderivedfromthe throughtheskyconditionlogbookthatismaintainedatthe
RWPattherespectiveradiosondelaunchingtime(06:08and site and also by the visual inspection of sky camera videos.
11:39UTC).However,aclearinversioninthepotentialtem- Table1showsthenumberofclearskydaysforthedifferent
peratureprofileisseenonlyforthe11:39UTCascentanda months.
very weak gradient is also seen for the 06:08UTC profile, A clearly defined diurnal variation in ML height is ob-
forbothprofilestheinversionoccursatapproximately500m served from November to February with minimum ampli-
pointing at a convective ABL. It is also notable from 11:39 tudes during November and December, exhibiting a peak
and17:33UTCradiosondelaunches,thatinversioninthepo- value of 500m from 07:00 to 09:00UTC. From January to
tential temperatures and specific humidity are taking place March, the mean ML height shows overall a gradual in-
between 1500 and 2000m which could be due to advected crease,withpeakvaluesabove800m.Ingeneral,thenoctur-
residuallayers. nal boundary layer height remains below 300m from 16:00
to 01:00UTC. Towards the start of spring in the month of
March,theMLheightshowsadistinctbehaviorwithamix-
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N.Singhetal.:BoundarylayerevolutionoverthecentralHimalayas 10565
Figure 5. Monthly averaged mixed layer height determined from
Figure 6. The monthly average diurnal variation in surface wind
15minaveragedS/N profiles,measuredwithRWP.Forthesakeof
speedoverthesiteduringtheclearskydays,selectedforestimation
clarity,variabilityisonlyshownforNovemberandMarchmonths.
ofthemixedlayerheightasdepictedinFig.3.
Table 1. RWP data set used for studying the mixed layer height
evolution. of surface layer, transforming into residual layer (Henne et
al., 2014). A decrease in wind speed is not observed from
Month Totalno.ofdays Clearsskydays 23:00 to 03:00UTC when the LBL decreases in depth, this
November2011 16 12 couldbeattributedtothecoolingofthesurfacebeyondacer-
December2011 31 26 tain extent, leading to the formation of a thin stable noctur-
January2012 31 10 nalboundarylayerdecoupledfromtheresiduallayerabove.
February2012 29 13 Highwindspeedsnearthesurfaceareoneofthecharacteris-
March2012 31 17 ticsofsuchstablelayers.
4.4 Comparisonwithmodelsimulations
ingdepthashighas650mduring14:00to22:00UTC,which
In this section, we utilize the ML height observations from
is attributed to the strong horizontal flow hitting the moun-
the RWP to evaluate the simulation from a regional model
tain (discussed in Sect. 4.3), which gives rise to the lifting
(WRF)thathasbeenusedinpreviousstudiesforsimulating
motions, and hence the rise of a few 100m in the mixing
the meteorology and regional air quality over Indian region
depth.
(e.g.,Kumaretal.,2012a,b;Sarangietal.,2014;Ojhaetal.,
4.3 Impactofmountaintopography 2016).Wefocusonthecapabilityofthemodel,withasetup
similartotheoneusedinaforementionedstudies,incaptur-
The impact of mountain topography on LBL dynamics can ing the diurnal and day-to-day variability in mixing depth.
be clearly identified by comparing the diurnal variability of Errors in simulating the mixing depth variations could lead
the ML height in December and March. Figure 6 shows toconsiderableuncertaintiesinthedispersionandmixingof
thediurnalvariabilityinwindspeedduringDecember2011 theairpollutantsoverthisregion.Forcomparison,themodel
and March 2012 measured by a collocated AWS at the site. output has been obtained for the same selected clear-sunny
In December, when the winds are calm (<2–3ms−1), the days as the RWP measurements. Since WRF output is in-
LBL growth starts around 03:00UTC. From then onwards, stantaneoushourlydata,instantaneoushourlydatahavealso
the ML height increases gradually and approaches a maxi- beenusedfromtheRWPforcomparison.
mum of about 700m in the afternoon, which appears to be Figure 7a shows the comparison of daytime (05:00–
inphasewiththeintensityofsunshine.MLheightdropsaf- 10:00UTC)averageboundarylayerheightfromRWPmea-
ter 10:00UTC, and in the evening from 13:00UTC onward surements and WRF simulations from November 2011 to
staysunchangedduringthenighttime(15:00to03:00UTC). March 2012. The day-to-day variations in the daytime
However,inMarchwhenthewindspeedsaremorethandou- boundary layer height, as observed from the RWP, are cap-
bled, reaching values as high as 6±3ms−1, the LBL set- turedbythemodelduringlate-autumnandthroughoutwin-
tling in nighttime hours is hindered since such high winds ter; however, the model shows significantly higher bound-
causesignificantwindshear,therebygeneratingturbulented- arylayerstowardsthestartofthespring(March).Figure7b
dies and increasing the vertical mixing of surface–layer air showsacorrelationanalysisbetweenmodelsimulationsand
(Solanki et al., 2016), leading to a second growth and de- observations of daytime boundary layer height. Overall the
cayphaseinLBLdepthfrom13:00to23:30UTC.Thissec- model and observations are in reasonable agreement dur-
ond growth phase can also be understood as the deepening ingthestudyperiod(r2=0.5).Themonthlystatisticsofthe
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10566 N.Singhetal.:BoundarylayerevolutionoverthecentralHimalayas
Table2.Daytime(05:00–10:00UTC)monthlymeanandpercentagebiasbetweenRWPmeasurementsandWRFsimulations.
Month Observationalmean(m) Modelmean(m) Meanbias(m) Percentbias(%)
November2011 440±197 381±90 –59 –13.4
December2011 500±245 415±112 −85 −17
January2012 624±289 577±149 −47 −7.5
February2012 686±268 890±439 +204 +29.7
March2012 766±317 1350±664 +584 +76.2
abilityandbiasesareobservedtochange.Themodeloveres-
timatestheboundarylayerheightby204m(30%)andshows
higher variability (439m) than the observations (268m), in
contrasttothemonthsofNovember–January.Modelisseen
tooverestimatethedaytimemeanboundarylayerheightsig-
nificantly(Table2)duringMarch(by76.2%),andthevari-
ability in the modeled boundary layer height is also seen to
bemorethanthatintheobservations(317m).
The mean diurnal variations in boundary layer height are
comparedbetweenmodelandobservationsforallthemonths
of the study period (Fig. 8). The model simulated diurnal
variationsareinagreementwiththemeasurements,asaver-
agedaytimeaswellasnighttimevaluesaregenerallywithin
1σ variationofeachother.Anappreciabledisagreementbe-
tween model and measurements is only seen towards the
evening hours (10:00 to 12:00UTC), when the boundary
layerheightshowsagradualdecreaseintheRWPmeasure-
ments, whereas the model simulations exhibit a rapid de-
crease in the boundary layer height. In contrast to the pe-
riodfromNovembertoFebruary,theboundarylayerheight
isoverestimatedthroughoutthedayinMarch,withnoontime
mixinglayerdepthabouttwotimeshigherthanintheobser-
vations(Table2).Theoverestimationofthenoontimemixing
layerdepthbythemodelcouldbeassociatedwiththeparam-
eterizationofboundarylayerandlandsurfaceprocesses,un-
certainties and/or errors in simulating other meteorological
Figure7.(a)Acomparisonofdaytime(05:00–10:00UTC)average
variables,inadditiontotheeffectsofunresolvedgeograph-
boundarylayerheightfromRWPmeasurementsandWRFsimula-
icaltopography.Theeffectsoftheseuncertaintiescouldbe-
tions at Nainital during the study period. Error bars represent 1σ
comemoreprominentduringthespringand/orpre-monsoon
standarddeviationduringdaytime.(b)Correlationanalysisbetween
duetointensesolarradiationcausingstrongerboundarylayer
observationalandmodeldaytimeboundarylayerheight.A1:1line
mixing,ascomparedtowinterandautumn(Ojhaetal.,2012;
isalsoshown.
Sarangi et al., 2014). We suggest that a mixing layer depth
abouttwotimeshigherthaninthemeasurementscouldlead
tosignificantdilutionorleadtoadditionalentrainmentwhen
modelobservationcomparisonaregivenintheTable2.The
usedinairqualitymodels.Inthenextsubsection,weexplore
meanbiasistheaveragedifference(WRF−RWP)forthese-
thepossibleimplicationofmodelbiasesinMarchabovethis
lecteddaysofeachmonth,andsimilarlythepercentagebias
siteonairqualitysimulationsusinga1-Dmixedlayermodel.
isthepercentagedifference(WRF−RWP)normalizedwith
respecttothemeanRWPmeasurements. 4.5 Effectsofboundarylayerheightoverestimationon
Onaverage,thenoontimeboundarylayerheightisslightly airqualitysimulations
underestimatedfromNovembertoJanuary,by47to85m(7–
17%).Thevariability(1σ standarddeviation)inthemodeled In this section, we use MXL/MESSy (Sect. 3.2) to investi-
boundary layer height is also lower (90–149m) than in the gatetheinfluencesofuncertaintiesinboundarylayerheight
observations(197–289m)duringthisperiod.However,dur- simulatedbyregionalmodelsonairqualitysimulations.For
ingthetransitionfromwintertospring(February),thevari- thispurpose,wecombinedtheeffectsoftheboundarylayer
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N.Singhetal.:BoundarylayerevolutionoverthecentralHimalayas 10567
Table 3. The initial and boundary conditions in the atmospheric
boundary layer (ABL) and free troposphere (FT) as used in
MXL/MESSy. All initial conditions are imposed at 03:00UTC. t
isthetimeelapsedsincethestartofthesimulation(s)andtd the
lengthofthesimulation(s).Thesubscriptssandeindicatevalues
atthesurfaceandtheentrainmentzone,respectively.
Property Value
InitialABLheight 280
h(m)
Subsidencerate 2×10−5
ω(s−1)
Surfacesensibleheatflux 0.24sin(πt/td)
w(cid:48)θ(cid:48)(Kms−1)
s
Entrainment/surfaceheatfluxratio 0.2
β=w(cid:48)θ(cid:48)/w(cid:48)θ(cid:48)(dimensionless)
e s
InitialABLpotentialtemperature 303.8
<θ>(K)
InitialFTpotentialtemperature 304.0
<θ>FT(K)
PotentialtemperaturelapserateFT 0.0065
γθ (Km−1)
Surfacelatentheatflux 0.11sin(πt/td)
w(cid:48)q(cid:48) (gkg−1ms−1)
s
InitialABLspecifichumidity 2.0
<q>(gkg−1)
InitialFTspecifichumidity 1.9
Figure 8. Comparison of the monthly averaged diurnal variations qFT(gkg−1)
intheboundarylayerheightdeterminedfromRWPmeasurements SpecifichumiditylapserateFT −0.0010
andWRFsimulationsatNainitalduringthestudyperiod. q(gkg−1m−1)
overestimationduringMarchwiththeavailableinformation
Figure9showstheresultsfrom4simulations.Thesimu-
on vertical gradients and emissions at Nainital to assess the
lation which is closest to the observations (CTRL) includes
impactontwochemicaltracers:Ozone(O )andBlackCar-
3
bothlarge-scalesubsidenceandadvectionofcoolandmoist
bon(BC).
air.Thesimulationwithoutanysubsidenceleadstoanover-
First, we set up MXL/MESSy to reproduce the observa-
estimation (by ∼250m) of the maximum boundary layer
tions of boundary layer height, potential temperature, spe-
height. The third simulation, in which there is no advec-
cifichumidityandwindspeed(Supplement,Fig.S3)forthe
tion, overestimates LBL height and temperature and under-
representativedayof15March2012.Heatfluxeswerepre-
estimatesspecificmoisture.Thefinalsimulation,withoutad-
scribed to the model using typical values for March, based
vectionandsubsidence,yieldsanoverestimationoftheLBL
onourobservationsatthissiteusingsonicanemometermea-
heightby400m.
surements (Solanki et al., 2016). Initial and boundary layer
The ozone gradient between the boundary layer and free
conditionsaregiveninTable3.Asetofsimulationswasper-
troposphere is assumed to be 5ppbv, as reported for the
formed (Fig. 9) to identify the simulation that reproduces
spring (MAM) season over this site based on ozonesonde
the observations best, which show a rapid boundary layer
observations (Ojha et al., 2014). Further, ozone-poor air is
growthinthemorningandthesimultaneousincreaseofpo-
transportedupwardsfromthevalleyduringdaytime(Ojhaet
tential temperature and specific moisture. In addition to the
al.,2012;Sarangietal.,2014).MXL/MESSyconsistsoftwo
surfacefluxesofsensibleandlatentheat,whicharethemain
boxesandcannotexplicitlyaccountforupslopeflows.There-
driversofboundarylayergrowth,large-scalesubsidenceand
fore,wemimicitseffectbyassumingaconstantozoneloss
advection of cool and moist air were required to reproduce
at the bottom of the lowest box. The effect of variations in
the observations. The assumption of large-scale subsidence
the boundary layer height on O concentrations is found to
in MXL/MESSy is consistent with the vertical downward 3
wind speed of a few cms−1 in the WRF simulations over be relatively small for the different simulations. The ozone
mixing ratios differ by less than 2ppb when the maximum
theregion.
boundarylayerheightis∼1350m,ascomparedtothemore
realistic∼1000m.Thisisduetotherelativelysmallgradi-
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10568 N.Singhetal.:BoundarylayerevolutionoverthecentralHimalayas
of BC were initially set at their values in the nearby val-
ley, which were in the order of 10−12kgm−2s−1, based on
HTAPinventory(Janssens-Maenhoutetal.,2015).Theemis-
sionswerethentunedtoobtainthebestcomparisonwiththe
observations and we found that a sinusoidal emission pro-
filewithamaximumof5.5×10−12kgm−2s−1gavethebest
results.
ModeledBCconcentrationsaremoresensitivetothevari-
ationsinboundarylayerheightthanthoseofozone,because
ofalargerverticalconcentrationgradient.BCconcentrations
aresimulatedtobelowerby∼300ngm−3 inthecasewhen
theboundarylayerheightis∼350mhigher,duetomissing
effects of subsidence and advection. Note that with advec-
tion,weonlymeanadvectionofcoolandmoistair,andnot
advectionofBC.Thecoolandmoistairleadstoadecreaseof
theBLtemperatureandanincreaseofthehumidity.Conse-
quently,thepotentialtemperatureandspecifichumiditygra-
dientbetweentheBLandthefreetroposphereincrease,and
therefore entrainment decreases (Janssen et al., 2013). For
thesameBCemissions,thismeansthatmoreBCistrapped
in a shallower BL and that it is diluted less with BC-poor
airfromthefreetroposphere.Subsidenceactstooppressthe
BLgrowth,butenhancesentrainment(Janssenetal.,2013).
Therefore,theBCconcentrationisdilutedalittlelessinthe
simulationwithoutsubsidence.
Figure9.DiurnalvariationsinMXL/MESSysimulated(a)bound- Besides subsidence and advection, the conditions in the
arylayerheight,(b)surfaceozone,and(c)BlackCarbon(BC)on freetropospherecanaffecttheBLdynamicsaswell.There-
15 March 2012 for four different simulations. The CTRL simula- fore,theeffectofboundarylayervariationsonBCatNaini-
tionincludesbothsubsidenceandadvectionofcoolandmoistair. talarefurtherexploredbyconductingsimulationsfordiffer-
Inthenosubs.simulationsubsidenceisturnedoff,inthenoadv. entgradientsofpotentialtemperaturefromboundarylayerto
simulationadvectionisturnedoff,andinthenosubs.+adv.simu-
freetroposphereandfordifferentlapseratesofthepotential
lationbothsubsidenceandadvectionareturnedoff.Ineachpanel,
temperaturein thefree troposphere(Fig.10). Initialmixing
theblackdotsshowobservationsmadeatthesurface.Inaddition,
ratioandsurfaceemissionfluxesaregiveninTable4.Wefind
thereddotintheupperpanelshowstheboundarylayerheightfrom
thatwhentheinitialpotentialtemperaturegradient((cid:49)θ )is
aradiosondemeasurement. 0
increasedfrom0.2to2.0K,theBLgrowthissuppressedin
thefirsthoursofthesimulation.Consequently,theBCemis-
sionsareconcentratedinashallowerBLandthereforeover-
ent in ozone mixing ratios across the LBL-free troposphere estimatedcomparedtotheobservationsby350ngm−3(max-
interface,whichmakessurethatentrainmentofairfromthe imum).However,duringthecourseoftheday,theinitialtem-
free troposphere only dilutes the LBL ozone concentrations peraturebarrierisovercomeandtheeffectonthesimulated
byasmallfraction. BC concentration is reduced to 200ngm−3 at 09:00UTC.
Unfortunately vertical profiles of black carbon are not Finally,thepotentialtemperaturelapserate(γ )determines
θ
available over Nainital site, and therefore measurements thegrowthrateoftheBL:ifitissmall,thepotentialtemper-
at another high-altitude site Hanle (78.96◦E, 32.78◦N; aturedifferencebetweenBLandfreetropospheregrowsless
∼4.5kmabovesealevel)intheHimalayanregionwereused withincreasingBLheightthanwhenitislarge.Whenγ is
θ
toestimatetheverticalgradientofBC.AverageBCconcen- set to 0.002Km−1 instead of 0.0065Km−1, as in the con-
trationsarereportedtobe110ngm−3 atHanleascompared trolexperiment,theBLgrowthismuchstrongerandtheBL
to 1340ngm−3 for our site (2km above sea level) during height reaches 1650m at 09:00UTC, which is an overesti-
spring (Dumka et al., 2010; Babu et al., 2011). The mea- mationofabout700m,andcomparabletotheoverestimation
surements show that BC concentrations increase during the byWRF.Consequently,BCconcentrationsaredilutedmuch
day, which is attributed to the upward mixing of air masses moreandareunderestimatedbyupto700ngm−3.Foraγ
θ
from the nearby valley in the polluted Indo-Gangetic plain of 0.009Km−1, the BL height is underestimated by about
region(e.g.Dumkaetal.,2010).Thisleadstoanetincrease 150matmaximum.TheresultingerrorintheBCconcentra-
in BC concentration during daytime, although entrainment tioncomparedtothecontrolexperimentis300ngm−3.
of BC-poor free tropospheric air is also active. Emissions
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