Weather Features Associated with Aircraft Icing Conditions: A Case Study

8/10/2019 Weather Features Associated with Aircraft Icing Conditions: A Case Study

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Research ArticleWeather Features Associated with Aircraft Icing Conditions:A Case Study

Sergio Fernndez-Gonzlez, Jos Luis Snchez, Estbaliz Gascn, Laura Lpez,

Eduardo Garca-Ortega, and Andrs Merino

Atmospheric Physics Group, IMA, University o Leon, Leon, Spain

Correspondence should be addressed to Jose Luis Sanchez; [email protected]

Received October ; Accepted December ; Published February

Academic Editors: S. Ghosh and Q. Wang

Copyright Sergio Fernandez-Gonzalez et al. Tis is an open access article distributed under the Creative CommonsAttribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original work isproperly cited.

In the context o aviation weather hazards, the study o aircraf icing is very important because o several accidents attributed toit over recent decades. On February , , an unusual meteorological situation caused severe icing o a C--, an aircrafused during winter - to study winter cloud systems in the Guadarrama Mountains o the central Iberian Peninsula.Observations in this case were rom a MP-A microwave radiometric proler, which acquired atmospheric temperature andhumidity proles continuously every . minutes. A Cloud Aerosol and Precipitation Spectrometer (CAPS) was also used to studycloud hydrometeors. Finally, ice nuclei concentration was measured in an isothermal cloud chamber, with the goal o calculatingconcentrations in the study area. Synoptic and mesoscale meteorological conditions were analysed using the Weather Researchand Forecasting (WRF) model. It was demonstrated that topography inuenced generation o a mesolow and gravity waves onthe lee side o the orographic barrier, in the region where the aircraf experienced icing. Other actors such as moisture, winddirection, temperature, atmospheric stability, and wind shear were decisive in the appearance o icing. Tis study indicates thaticing conditions may arise locally, even when the synoptic situation does not indicate any risk.

1. Introduction

Te main consequences o aircraf icing are unusual loss olif, such as a reduction in the rate o rise, an increase oriction, or rapid ice accumulation on windows, wings, ormeasurement instruments on the aircraf []. Te analysis o

aircraf icing is very important because o the numerous aircrashes it has caused over recent decades [].

Supercooled large drops (SLD) are drops o size greaterthan m in a liquid state at temperatures below Cand constitute the principal source o aircraf icing. Tis isbecause such drops can reeze on aircraf structures that areunprotected or inadequately protected by anti-icing systems[].

Tere are two possible mechanisms o SLD ormation.Te rst is when rozen hydrometeors melt upon transitingregions with temperatures above reezing (ofen called warmnoses) and reenter regions o subreezing temperatures(resupercooling). Te second mechanism is when liquid

droplets orm through a condensation process and grow intoSLD by collision-coalescence processes, a cycle in which tem-peratures are less than C [, ]. Te rst mechanism is ofenassociated with warm rontal passage[]. However, Strappet al. [] pointed out that approximately % o reezingprecipitation events are a result o the second mechanism.

High humidityand updrafs are essentialor thegrowth osupercooled water droplets, although there are other actorsinvolved []. For efficient production o SLD by collision-coalescence processes, a mixing process is necessary [].Korolev and Isaac [] claim that isobaric mixing producedby vertical air currents, which can produce supersaturation,may accelerate droplet growth to orm SLD. Tis mechanismis avoured by an inversion layer near cloud top. Wind shear isanother actor avouring SLD development because it inducesmixing, which accelerates droplet growth and reduces thetotal number o drops [].

At temperatures around C, efficiency o the nucle-ation process is very low, because o weak activity o reezing

Hindawi Publishing Corporatione Scientific World JournalVolume 2014, Article ID 279063, 18 pageshttp://dx.doi.org/10.1155/2014/279063
http://dx.doi.org/10.1155/2014/279063http://dx.doi.org/10.1155/2014/279063


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Te Scientic World Journal

nuclei at those temperatures []. Tus, this mechanism is notpronounced in clouds with tops at temperatures aboveC.Rauber and Grant [] indicated that supercooled liquidwater (SLW) layers are common in orographic cloud systemswith cloud tops at temperatures as low as C. Te causeo this phenomenon can be explained by imbalances between

liquid water content (LWC) input by condensation and thenucleation rate, which is very slow []. o this it must beadded that the reezing nuclei concentration is several orderso magnitude lower than that o condensation nuclei [].

Te average glaciation time in stratiorm clouds is around minutes []. Tereore, i the hydrometeor residence timeis much smaller than the characteristic glaciation time, it isexpected that virtually all hydrometeors will remain in theliquid phase. Tis is explained by Korolev and Isaac [], whoposit that SLD ormed in updrafs have an average lie o a ewtenths o a second.

Te processes discussed above are modied by terrain,making it necessary to perorm a comprehensive mesoscaleanalysis. Te importance o atmospheric ow modicationcaused by mountains depends on several parameters, suchas mountain height and shape, atmospheric stability, windspeed and direction []. On the lee side o mountains,an area o weaker wind or eddy shedding can appear [],and mountain waves can orm []. Various works[,]have shown the ormation o mesolows on the lee side oseveral mountain ranges o the Iberian Peninsula, a result oa window perpendicular to the orographic barrier.

Reinking et al. [] indicated that prerontal ow isorced to ascend when it passes over an orographic barrier.Orographic lifing produces regions o LWC accumulation onthe windward slope. Afer passing the orographic barrier, airdescends abruptly and generates a cloudless area because othe Foehn effect. Subsequently, the ow rises suddenly, gen-erating the characteristic gravity waves. Gravity waves ormregions o short-lived but considerable amounts o LWC [].

Numerical weather orecasting models are unable to ore-cast the concentration o SLD accurately, because commonlyused parameterizations generally overestimate the amount orozen water content and underestimate the concentration osupercooled liquid water[].

Consequently, eld campaigns using research aircraf tomeasure in situ supercooled liquid water and rozen watercontent are very important to improve the accuracy onumerical models. Te vast majority o such eld projectshave been carriedout in the USA and Canada [, , ], with

some exceptions, as in Germany [].Underthe ECOAGUA project, a serieso ight plans has

been designed to collect scientic data rom in-cloud systemscapable o producingrainall during winter. Tese ights wereexecuted by a C-- aircraf, belonging to the NationalInstitute or Aerospace echnology (INA). One goal o thisproject was to y in an icing environment to understandicing conditions that are not inrequent near Madrid-Barajasairport.

As noted by Baumgardner et al. [], the Cloud Aerosoland Precipitation Spectrometer (CAPS) is suitable or mea-suring microphysical variables within clouds, so it wasdecided to install this instrument on the project aircraf

with the aim o measuring SLD concentration. Tis probeis capable o measuring hydrometeor concentration and size(and distinguishing theirphase), aerosols,LWC, temperature,relative humidity (RH), and vapour density, among other

variables.On February , , the aircraf took off rom the or-

rejon de Ardoz military base (Madrid) and began collectingvalid data at : (all times UC). Te aircraf ew northwith the goal o collecting data on the north side o theCentral System. Flying over the Lozoya Valley at altitudeabout msl, the aircraf penetrated a region with highSLD concentration and temperatures around C, andLWC as much as . g/m3. Tis caused ice accumulation onthe prole o the aircraf wings, orcing termination o theresearch ight.

Te objective o the present study is to analyse thesynoptic situation and mesoscale conditions during the dayo this aircraf icing. o do this, weather conditions weremeasured by the available instrumentation, and the synopticand mesoscale actors causing the icing were analyzed usingthe Weather Research and Forecasting (WRF) model.

2. Experimental Design and Methodology

.. Study Area. Icing o the C-- aircraf on February, , occurred while attempting to cross the GuadarramaMountains. Tis mountain range is between the provinceso Segovia and Madrid, in central Spain. Te orientation oridges and valleys is predominantly southwest-northeast. Tesouthwest end o the range is at N, W and itsnortheast end at N, W.

Elevations in these mountains are well in excess o

msl. Te mountains are separated in the middle, witha higher range to the north and lower one to the south. TeLozoyaValleyis betweenthese tworanges. Itis over this valley(Figure ) where the aircraf traversed the region o high SLDconcentration that caused rapid icing and ight termination.

.. Instrumentation

... MP-A Microwave Radiometric Proler. For the- winter eld project, a hyperspectral, multichannelmicrowaveradiometer(MMWR; MP-A) was installed at mslin the Guadarrama Mountains (location in Figure ).Te instrument continuously measured vertical proles o

temperature, humidity, LWC, and water vapour density (withtemporal resolution approximately . minutes) to kmheight. Te MP-A was manuactured by Radiometrics,Boulder, Colorado, USA. Characteristics o this instrumentand retrieved proles are described in Sanchez et al. [].

... CAPS. Te C-- aircraf carried the CAPS underthe lef wing during the eld project. Te CAPS consists ove probes. First is a Cloud and Aerosol Spectrometer (CAS),which measures aerosols and hydrometeor size distributionsbetween . and m. Second is a Cloud Imaging ProbeGrayscale (CIP-GS), which can measure hydrometeors rom to m and represent D images o hydrometeors.


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Madrid

Guadarrama mountains

(km)

Radiometer

SPAIN

Segovia

Torrejonairport

Cloud chamber

Lozoya valley

1

2

3

4

N

56

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High:2500

Low:500

0 10 205

F : Mapo Guadarrama Mountains, Madrid. Aircraf trajectory plus radiometer and cloud chamber locations have been superimposed.

Te advantage o the grey scale is that it gives additional

details on ice crystal characteristics and, more importantly,it better denes the depth o eld and permits more accu-rate hydrometeor identication. Tird is the LWD detector(LWC-Hotwire) probe, which estimates atmospheric LWCaccurately between. and g/m3. Finally, there wasa sensorto measure airspeed and another to measure temperature andRH. A more detailed explanation o the CAPS is ound inBaumgardner et al.[].

... Isothermal Cloud Chamber. An isothermal cloudchamber was used to measure ice nuclei concentration o theair mass over the Guadarrama Mountains. Tis instrumentwas installed at ground level at Lozoya Reservoir (location in

Figure ) in the Lozoya Valley, where the aircraf experienced

icing. Tis cloud chamber has a tank with volume L. Teice nuclei concentration was measured at C, because othe low concentrations recorded at warmer temperatures inprevious Iberian Peninsula eld projects. Te procedure usedin the campaign was described in Castro et al. []. Tisinstrument is only capable o taking static measurements, soseveral measurements were taken during the study day toanalyze the evolution o ice nuclei concentration.

3. Observation

Te aircraf took off rom orrejon Airport at : onFebruary , . Te aim o this ight was to investigate


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expected icing conditions rom the approach o a cold ront tothe studyarea. Upon approach to the Guadarrama Mountains(:), the aircraf began to experience light icing. Icingconditions were worst afer reaching the Lozoya Valley (:)and, consequently, the aircraf experienced severe icing andthe pilot was orced to abort the mission and return to the

airport. Te aircraf continued to encounter severe icing until:, when it exited the valley. Light icing occurred near theGuadarrama Mountains until :, when the aircraf nallyexitedcloud. Te aircraf landed at orrejon at:. Te ightpath is shown inFigure . Te icing episode was recorded bythe instrumentation as ollows.

.. MP-A Radiometer. Atmospheric stability was con-tinuously monitored by the MP-A microwave radio-metric proler. Figure shows vertical proles at differenttimes. At : on February , it was observed that thenocturnal inversion had not dissipated at low levels. Tere

was practically neutral stability rom hPa to hPa,where there was a transition layer to greater stability at levels hPa and above. Tere was neutral static stability (nearlyunstable) between and hPa, above which was a strongstable layer.

At : when the aircraf neared the zone o interest,layers near the ground were saturated and the inversionhad weakened considerably. Vertical proles generally indi-cated an increasingly unstable atmosphere. Subsequently, themost unstable atmospheric layer developed rom hPa to hPa. Tere was a progressive increase o moisture in thislayer during preight (: to :). Tere were clouds inthis layer, and this was where the aircraf experienced icing(CAPS detected a region with high SLD concentration at hPa). From : to :, around which time the aircrafexperienced severe icing, the temperature and dew pointtemperature curves between and hPa were close,indicating cloudiness. Subsequent proles (::) indi-cated lesser instability and humidity, so we conclude that theaircraf crossed the Lozoya Valley during the most avourableconditions or aircraf icing all day.

Above the aorementioned unstable layer (just below hPa) was a more stable region that persisted the entireday, representing a weak thermal inversion. Tis layer pre-

vented updrafs in the Lozoya Valley rom reaching higherlevels and avoured ormation o vertical shear. Bernstein[] showed that stably stratied conditions are conducive to

ormation o regions o high SLD concentration. Tis wouldavour shear near cloud tops, which causes intense mixingand thereby efficient and rapid SLD ormation [].

Te thermodynamic prole revealed the absence o awarm layer, indicating that SLD causing aircraf icing wereormed by condensation ollowed by collision-coalescence,the entire process occurring at temperatures colder than C.Tese proles match type A described by Bernstein [], inwhich the entire atmosphere has subreezing temperatures,including the saturated layer.

Te MMWR data permit continuous calculation o sta-bility indices and determination o the presence or absenceo convection. Although there are many indices that can

be applied to this purpose in summer, there are ew suchor winter application. Most Unstable Convective AvailablePotential Energy (MUCAPE) has been widely used as a

variation on CAPE in studies o winter convection [].Tis index represents the total potential energy available tothe most unstable air parcel within the lowest mb, while

being lifed to its level o ree convection. o obtain thisindex, CAPE must rst be computed by lifing parcels romeach level in the humidity and temperature proles. Ten,MUCAPE is taken as the greatest CAPE encountered, that is,the most unstable parcel.

For continuous monitoring, the radiometer data wereused. Intermediate values o MUCAPE were recorded inthe hours beore the ight ( J/kg), but these valuesdeclined to J/kg at : (Figure ). Tese values are inad-equate or convective development [,].

.. CAPS. Hydrometeor size distributions rom data col-

lected by the CAPS during the ight o February wereanalyzed in the region o aircraf icing. Te gamma distribu-tion unction was chosen or these distributions, because itaithully represents droplet size distributions within clouds.Tis unction was dened by Ulbrich [] and was selectedbecause it better represents larger droplets. Te unctionhas three parameters dependent on time (and space in theEulerian case): the concentration o particles, their averagediameter, andspectrum width []. Te parameters were esti-mated ollowing the maximum likelihood method denedby Wilks []. Te gamma distribution unction has beenused or representation o the size distribution o cloudhydrometeors []. Te Lillieors test [] was used to ensuregoodness o t at the . level o signicance.

Average hydrometeor size distributions were calculatedevery seconds. CAPS measurements in cloud are shown inable . Images rom the CIP (part o CAPS) or each periodinable are shown inFigure . Tese nine measurementscorrespond to the highlighted circles along the trajectory inFigure , described previously inSection ..Correspondingdrop size distributions are shown in Figure , or which agamma distribution unction has been applied.

Initially, the predominant droplet size was


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F : Skew-/log-P diagram o radiometer data at :, :, :, :, :, and : on February , . emperature (red lines);dew point (blue dashed lines).


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T:Variablesmeasur

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(UTC)

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()

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()

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h

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(K)

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(g/m)

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F : Evolution o MUCAPE index during February , .

13:15:5113:14:2113:12:5513:11:55 13:18:02 13:18:32 13:19:45 13:21:00 13:22:27

Light icing Light icingServere icing1550m

F : Images captured by CIP when aircraf experienced icing.

steps oable . Te LWC increased to more than . g/m3,

with peaks . g/m3. Te table indicates the huge amountso SLD and hydrometeors over m. In the th, th, andth images oFigure , it is seen that SLD were larger thanthe small supercooled droplets in the other images. Figure shows how the th, th, and th curves adjusted to thegamma distribution, unlike the remaining times. Tis is veryimportant because it indicates that i the size distribution osupercooled liquid droplets ollows the gamma distribution,we can expect moderate to severe icing; i it does not do so,light icing is the maximum expected.

Afer exiting the Lozoya Valley and the cloud with SLDtoward the south, average drop size gradually decreased topredominant droplets smaller than m, with light icing.Tis period corresponds with the th, th, and th times oable ,the images inFigure , and curves in Figure . Te

images captured by the CIP, droplet size distributions andvalues inable or times , , and are similar to times , ,and , except that the temperature was slightly cooler becausethe aircraf ascended to msl.

Microphysical conditions observed over the Lozoya Val-ley by the aircraf during February are consistent with thosedescribed by Ellrod and Bailey []. Tey stated that icingis linked with temperatures between and C, liquid- ormixed-phase clouds, volume median diameter greater than

m, LWC > . g/m3, weak updrafs to replenish super-cooled liquid water, and clouds o considerable thickness.SLD can be very dangerous to aviation because they canaccumulate beyond the capabilities o current deicing boots.Tis signicantly reduces aircraf perormance, which cannotbe alleviated by ice protection devices such as pneumaticboots []. Tis occurred during the case study.


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25 50 75 100 125 150 175 200

123

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F : Gamma distribution curves or each o the nine times oable andFigure .

Cober et al. [] described avourable environmentsor SLD development, in which supercooled liquid waterdroplets greater than m orm ollowing melting andsubsequent resupercooling or via condensation processes ol-lowed by collision-coalescence, which occurred in our case.Severe icing is ormed by an updraf that provides sufficientwater vapour to maintain SLD growth, while existing shearnear the cloud top supports collision-coalescence processesresponsible or its ormation.

Tese results agree with those o Rauber and okay[]. Tey asserted that when the concentration o reezingnuclei is low, cloud top temperature is relatively warm (above

C) with weak updrafs, and thelikelihood o a LWC layerat cloud top is high. Based on data rom scientic ights ineld studies, Sand et al. [] ound that only % o icingreports were at temperatures belowC, with about %between C and C. Vidaurre and Hallet [] notedthat liquid-only clouds dominate at subreezing temperaturesclose to C, whereas ice-only clouds predominate belowC.

.. Isothermal Cloud Chamber Observations. wo measure-ments with the isothermal cloud chamber were taken onFebruary . Te rst was at :, resulting in a concentrationo IN/L (IN are ice nuclei). Te second was around ight

time at :, showing a decrease to IN/L (Figure ). Tethermal inversion during the morning, which disappearedafer midday, may have been responsible or the high icenuclei concentration o the rst measurement. Te breakingo the inversion layer at the surace allowed dispersiono IN to higher levels o the troposphere. Te surace INconcentration at : was representative o that during theight, because there was no inversion layer below hPa.

Tese values are extremely low compared with thosereported by other authors. Afer taking nearly measure-ments in the northwestern Iberian Peninsula, Castro et al. []obtained an average o IN/L active at C on days withmaritime air masses. Our relatively sparse IN represents an

F : Measurement o ice nuclei rom isothermal cloud cham-ber at :.

obstacle to glaciation, which was a determining actor or thelarge numbers o SLD during the ight and near absence ohydrometeors in the solid phase.

At hPa, near the cloud tops, the temperature wasslightly warmer thanC. Tis temperature was sufficientlywarm to prevent activation o most o the IN, hinderingoptimal glaciation.

4. Meteorological Analysis

WRFand other mesoscale models have been used oranalysiso aircraf icing episodes[,]. In this paper, weather con-ditions generating icing o the C-- aircraf on February were simulated by the WRF mesoscale model, version.. []. Initial and boundary conditions were urnished bythe National Centers or Environmental Prediction (NCEP)reanalysis, which has spatial resolution [].

Tree nested domains were dened. D covers south-western Europe, with grids in both the eastwest andnorthsouth directions. Tis domain has spatial resolution km and temporal resolution hours and was used or the

synoptic description.emporal resolution o domain Dis hour. It coversthe

entire Iberian Peninsula with grid points in the eastwestdirection and northsouth points, with spatial resolutionkm.

o analyze in detail the mesoscale actors that inuencedormation o icing conditions, domain D was used. Tisdomain acilitates accurate representation o weather con-ditions in the study area, because it has spatial resolution km andtemporal resolution hour. Figure shows the threedomains. D and D were used in mesoscale analysis, Dor synoptic analysis. Te cross section axis is perpendicularto Guadarrama Mountains. Mesoscale models are commonly


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Cross section D03

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8W 6W 4W 2W 0

F : Nested domains used in WRF simulation. Axis rom cross section rom N to S is overlaid on D and D.

used in orecasting and evaluation o in-cloud icing condi-tions [].

For parameterization o microphysical processes, theWRF New Tompson graupel scheme [] was chosen, since

it considers graupel and typical ice water concentrations inmountainous areas during winter. Further, we used the Noahland surace model [] and Eta surace layer scheme denedby Janjic []. For longwave radiation, the Rapid Radiative


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Geopotential (gpm) at500 hPa

Sea level pressure (hPa)

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ranser Model [] was used, along with the scheme oDudhia [] or shortwave radiation.

.. Synoptic Overview. From domain D, it was ound thatthesynoptic situationin Europe wasdominated by a powerulSiberian anticyclone (Figure ), orcing a dry and very coldnortheast wind into Central Europe. Northwest winds werepredominant during the morning over the Iberian Peninsula.

Figure depicts RH and wind at hPa. Progress oa dry intrusion induced the advection o moist air over thepeninsula, in a manner similar to the pattern shown byBrowning []. Tis dry intrusion is caused by a dynamictropopause anomaly, which is derived rom the jet stream.A weak cold ront associated with the anomaly crossed theIberian Peninsula rom north to south during the study

day. Tis synoptic pattern was associated with aircraf icingby Bernstein et al. []. Te situation coincided with thosereported by Bernstein et al. [], who related the leadingedges o arctic and cold ronts to in-ight icing episodescited in pilot reports. Tis also ts the Arctic Front synopticpattern described by Rauber et al. [], who indicated thatthis pattern is the most common in reezing precipitationepisodes.

Tere was another anomaly to the west o Lisbon at :.A weakened branch thereo penetrated the Iberian Peninsula,at latitudes slightly south o Madrid. Te two anomaliestended to associate, causing the accumulation and ascent omoist air in a strip between the two.

Relative humidity (%) at300 hPa

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20W 15W 10W 5W 0 5E

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F : Relative humidity and wind at hPa modeled by WRFin domain D.

Te ormation o dynamic tropopause anomalies is con-nected with the position o the jet stream. Its location can beidentied by the strong winds shown inFigure , depictedby wind barbs. Te incursion o a subtropical anticyclonenorth o the Azores displaced the jet stream northward,while the powerul Siberian anticyclone pushed it southward,producing strong jet curvature. Te jet stream was not clearlydened near the Iberian Peninsula but was separated into twobranches. One was west o Portugal, and the other crossedthe Pyrenees and moved toward the Mediterranean. Tebranches were associated with the two dynamic tropopauseanomalies described above.

.. Mesoscale. Te orographic orcing o the GuadarramaMountains can be seen more clearly by increasing the modelresolution. Tis orcing helped determine the generation

and modication o actors that triggered mesoscale weatherconditions supporting the icing. In the ollowing, the causeso the icing are analyzed.

... Dynamic ropopause Anomalies. From domain D(seeFigure ) the vertical cross section o potential vorticity(PV) and RH is shown in Figure , in whichthe two dynamictropopause anomalies are evident. A high PV region . kmsouth o the Guadarrama Mountains corresponds to theanomaly observed to the southwest o Lisbon. Tis anomalyappears responsible or the midtroposphere dry air mass overthe southern hal o the Iberian Peninsula. A deeper anomalywas north o the Guadarrama Mountains, corresponding to


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F : Cross section o relative humidity and potential vorticity in domain D.

the backside o the dry intrusion west o Italy. Te rear o that

anomaly moved southward, pushing an air mass with highhumidity ahead o it. Te convergence o the two anomaliesaccumulated moisture at the centre o the Iberian Peninsula.Substantial moisture is essential or SLD ormation [].

Nevertheless, updrafs over the Guadarrama Mountainscannot be attributed to the dynamic tropopause anomalies,because updrafs remained stationary on the lee side o themountains throughout the day, while the anomalies movedto the south. Tereore, we conclude that the anomalies werenot the main cause o the updraf that accumulated SLD overthe Lozoya Valley. Te backside o the anomaly at ight timeremained north o the Iberian Peninsula; its effects were notobserved in the mountains until afer :.

Associated with this anomaly was an advancing coldront. Tis ront swept across the peninsula during theafernoon o February , resulting in ascent o the warm andwet air mass above a wedge o cold and dry air. Reinking etal.[] stated that the ascent o a warm air mass over a coldone provides moisture and upwelling processes necessary orcollision-coalescence growth.

... Mesolow. By increasing model resolution in domainD, it was observed that surace winds were perpendicularto the Guadarrama Mountains during the ight. Tis ormeda mesolow on their lee side, caused by a phenomenon known

as an orographic dipole. Tis is a mesoscale structure causedby ow perpendicular to a mountain barrier, orming anoma-lous positive pressure on the windward side and a depressionon the lee side. Associated with the mesolow, surace windhad a cyclonic rotation on the lee side (Figure ), whichtriggeredwind convergence in thearea o severeaircraficing.

Orographic dipole ormation is explained by separationo the boundary layer, a well-known phenomenon in uiddynamics []. According to this theory, a steady streamencountering an obstacle generates a stagnation point anda pair o vortices; one is anticyclonic and upstream o theobstacle, and the other is cyclonic and downstream [].In such a D ow situation, additional effects should be

Aircraf trajectory

4130N

41N

4130

N

40N

3930N

5

30W 5W 430W 4W 330W 3W 230W

Terrain height (m)

Sea level pressure contours:990to1030by1

5 00 7 50 1000 125 0 15 00 175 0 2000

Terrain height (m)Wind (kts)

Sea level pressure (hPa)

1016

1016 1014

10141014

1014

F : errain height, sea level pressure, and surace wind indomain D. Aircraf trajectory has been superimposed.

considered, such as stratication, wave breaking, turbulence,and vertical wind shear[].

... Updrafs and Gravity Waves. In the same area asthe mesolow, an updraf appears in images produced bythe WRF. Tis coincides with ormation o the cloud bandresponsible or the aircraf icing. Geresdi et al. [] indicatedthat icing regions ofen are associated with mesoscale uplif

with vertical speeds around cm s1. In the vertical cross


12/18


z-wind component (m/s) at 650 hPa

Terrain height (m)Aircra trajectory

500 75 0 1000 125 0 15 00 175 0 2000

4130

N

41N

4130N

40N

3930N

5

30

W 5

W 4

30

W 4

W 3

30

W 3

W 2

30

W

(a)

Grid pointsNorthSouth

1.25 0.75 0.25 0.25 0.75 1.25

Height(km

)

Aircra trajectory

z-wind component (m/s1 )

z-wind component (m/s1 )

8.0

0 5 10 15 20 25

7.0

6.0

5.0

4.0

3.0

2.0

1.0

0.0

(b)

F : Vertical (z) wind component at hPa (a) and on a cross section (b) o domain D. Aircraf trajectory has been superimposed.

section oFigure , two updrafs are evident on the leeside othe Guadarrama Mountains. Tese updrafs are also seen inthe vertical (z) wind component at hPa, the level at whichthe aircraf experienced icing.

Coupled with the orographic dipole, mountain waves arecommon in regions o static stability (as shown by radiometerdata) when the wind is perpendicular to the orographicbarrier. Tis barrier offers resistance to passage o the ow.Unable to pass through the barrier, air tends to accumulate,resulting in loss o energy and wind speed reduction. Tisincreases pressure on the windward side by wind conver-gence. Air passing through the mountains descends into the

valley on the lee side and is then orced upward, generatingmountain waves. Away rom the mountains beyond a calmzone, the ow accelerates, producing divergence and a result-ing pressure decrease []. Te orographic dipole tends to

strengthen updrafs generated by gravity waves.Politovich [] noted that orographic orcing may trigger

convection embedded in stratiorm clouds, which acilitatesthe ascent o SLD and their accumulation at cloud tops. Ikedaet al.[] also asserted that strong ow perpendicular to amountain barrier amplied vertical motions (up to m/s)above local ridges, orming embedded convection.

Petersen et al. [] indicated that i mountains aresufficiently high to block the prevailing ow, a mesolowor eddy shedding on the lee side is likely. In the WRFsimulation, a mesolow was produced on the leeward sideo the Guadarrama Mountains, owing to partial blockage othe perpendicular ow. Te increase o PV caused by the

mountains and accumulated at the mesolow reduced geopo-tential height at midtropospheric levels []. Additional PVmay have come rom approach o the cold ront and dynamictropopause anomaly discussed above. Tis caused a stronggradient o geopotential height to the lee o the mountains, asdetected by an increase o windspeed andchange in directionat midtroposphere, generating strong shear. Tis is consistentwith Rauber [], who claimed that gravity wave appearancein orographic cloud systems is usually associated with sub-sidence and strong shear at cloud tops. Here we should alsonote the cyclonic gyre o surace wind on the lee side o theGuadarrama Mountains, associated with the mesolow.

... Atmospheric Stability. We also analyzed EquivalentPotential Vorticity (EPV) to determine the existence o

Conditional Symmetric Instability (CSI). Afer conrmingthat there was no negative EPV during the ight and nosignicant values o convective indices (analyzed using con-tinuous radiometer measurements), the presence o convec-tion was dismissed. We thereore conclude that the observedupdrafs were caused by mountain waves,strengthened by themesolow.

As seen in Figure depicting differential equivalentpotential temperature (deth), there was strong stability nearthe surace, with a thermal inversion in various areas oD. Immediately above that inversion was a stable layer,which reached about msl. Most notable is the neutralstability layer located between and km, which allowed


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deth (C/km)

Grid pointsSouth North

Height(km

)

Atmospheric stabilityUnstable Stable

Aircraf trajectory

8.0

7.0

6.0

5.0

4.0

3.0

2.0

1.0

0.0

0 5 10 15 20 25

0 1.5 3 4.5

F : Cross section o deth in D.

gravity wave development. Above this, there was anotherstable layer that prevented vertical development o the gravitywaves above km. Moreover, ripples caused by gravity wavescan be discerned.Figure portrays a small unstable region,coincident with the mesolow. Te situation observed by theradiometer matches that modeled by the WRF, because iwe represent deth in cross section, around .N (wherethe aircraf experienced icing) there was a neutral layer(almost unstable) rom to msl. Immediately above msl, there was a stable layer.

Pobanz et al. [] argued that shear above cloud top ina thermodynamically stable atmosphere can orm a dynam-ically unstable layer and, thereby, turbulence and risk oKelvin-Helmholtz waves as well as entrainment o subsatu-rated air and mixing, which avour SLD ormation. Marwitz[] had a similar theory. He affirmed that wind shear

induced dynamical instability in gravity waves. However, thestable layer above hPa blocked urther ascent o air andthereby that o SLD. Tereore, the layer with greatest SLDaccumulation ormed immediately below this layer.

Te nearly neutral static stability, together with weakupdrafs associated with mountains, may be associated withproduction o SLW in this layer as pointed out by Pobanzet al. []. Tey claimed that a neutral or weakly unstableatmosphere promotes SLD ormation. A static stability layerpromotes generation o mountain waves. Orographic lifingis greater in a neutral atmosphere than in a stable one,ostering greater accumulation o SLD and LWC near cloudtop. Furthermore, within a neutral stability region, cloud

tops do not reach high altitudes. Tis is conducive to a lowconcentration o ice crystals, which acilitates the presence oSLD [].

On the morning o February , , there was a stableatmosphere with a strong inversion layer near the surace, asexplained in the observation section. However, the conditions

were increasingly unstable on the lee side o the GuadarramaMountains, which were associated with ormation o themesolow. In addition, the cold ront linked with the dynamictropopause anomaly was approaching. As was the case inthe present study, several authors have indicated that atransition rom stable to more unstable conditions supportsSLD ormation[,].

... emperature. Figure represents temperature at hPa, near the aircraf icing altitude. emperature therewas about C, an optimal value or such icing. Tis isbecause most reezing nuclei are not activeand the nucleationprocess is thereore inefficient [,].

Te gure shows that over the Lozoya Valley where therst mountain wave was located, there was a band with lowertemperatures than adjacent regions. A ew miles south, therewas another cold band collocated with a second mountainwave. Contiguous with these two regions, there were threebands with warmer temperatures than expected at this alti-tude; these correspond to subsidence regions produced by themountain waves.A wave cloud occurs between the maximumand minimum vertical velocity, where temperatures are lessthan the undisturbed mean value [].

... Liquid Water Content. Te cloud band over the Lozoya

Valley shown in Figure was responsible or the severeicing o the C--. Te aircraf was at msl (pressure hPa), heading north. Arriving about .N, the aircrafentered cloud (mesolow area andsecond mountain wave) andexperienced light to moderate icing (supercooled droplets

smaller than m and LWC about . g/m3)through.N.Upon reaching .N the aircraf reached the rst mountainwave and a region o severe icing (supercooled large droplets

greater than m and LWC about . g/m3), orcing themission abort.

Figure explains why the aircraf experienced light icingin the rst cloud, because LWC was low. Later, upon enteringthe zone o greater LWC, there was severe icing, coincident

with the region o high SLD concentration. Just beore theaircraf turnaround, a region with smaller droplets and lowLWC was evident in images produced by the CIP-GS probe.Tis region corresponded with the northernmost mountainwave. Back to the south, the aircraf reentered the region ohigher LWC and SLD, again enduring severe icing. Te pilotascended rom to msl in an attempt to get abovecloud but, as seen by the WRF output, cloud tops clearlyexceeded this altitude.

During the return ight, the icing changed rom moder-ate tolight south o.N, andaround .N the craf exitedthe cloud. Tere the pilot activated the anti-icing systems thatdetached the ice accumulated on the wing prole.


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41

30N

41N

40

30N

40N

39 30N

530 W 5W 430W 4W 330W 3W 2 30W

Terrain height contours:0 to2000 by250

Aircraf trajectory

12.5 12 11.5 11 10.5 10 9.5

1000

1000

1000

15001000

1000

1000

1500

500

500

500

) at650 hPa

Temperature (C)

Temperature (C

F : emperature at hPa in D. Aircraf trajectory has been superimposed.

4130

N

41N

4130N

40N

3930N

530

W 5W 430W 4W 330W 3W 230 W

Terrain height (m)

250 500 750 1000 1250 1500 1750 2000

Liquid water content (g/kg) at 650 hPa

Aircraf trajectory

(a)

5.0

4.0

3.0

2.0

1.0

0.00 5 10 15 20 25

Grid pointsSouth North

Cloud water mixing ratio (g/kg)

Cloud water mixing ratio (g/kg)

0.025 0.1 0.175 0.25 0.325 0.4

Height(km

)

Aircraf trajectory

(b)

F : Liquid water content at hPa (a) and on a cross section (b) o domain D. Aircraf trajectory has been superimposed.


15/18


A cloudless band just downwind o the GuadarramaMountains is evident in the WRF output. Tis band wascaused by Foehn wind. Tese winds cause descending air andheating by compression on the lee side, dissipating the clouds.Subsequently, the air is orced to rise by mountain waves(with additional ascent caused by the mesolow in our case).

Tis rising air cools by expansion, orming clouds associatedwith mountain waves afer reaching the dew point level [].Tese authors showed that gravity waves produce signicantamounts o supercooled LWC.

Te region o aircraf icing had stratiorm clouds withweak updrafs. Politovich [] stated that when such cloudshave maritime characteristics (as in this case, because thedominant air mass was maritime arctic), such as highhumidity andvery low IN concentration hindering ice crystalormation, they create an environment conducive to icing.

Te updraf in the Guadarrama Mountains was themain cause o SLD accumulation in the region o aircraficing, because it provided liquid water, mixing, and timeor collision-coalescence processes. Te updraf also allowedSLD accumulation, because it obstructed precipitation[].Te updraf was generated by mountain waves and strength-ened by the mesolow in the lee o the mountains.

Afer analyzing the D output o WRF, we concludedthat the severe icing o the C-- aircraf was caused bymountain waves. Te aircraf rst crossed the second (south-ernmost) mountain wave, experiencing light to moderateicing since the updraf there was less intense, and there wasless LWC. However, the aircraf reached the northernmostmountain wave over the Lozoya Valley, experiencing severeicing caused by updrafs greater than m/s and LWC in

excess o . g/m3. In addition, the high SLD concentrationcaused ice accretion in areas unprotected by anti-icing sys-tems, orcing ight termination. Stationary gravity waves arecommonly dominated by cloud droplets smaller than mbut, in m/s updrafs, drop sizes between and m canprevail, generating the greatest risk o aircraf icing[].

5. Conclusions

In summary, severe icing o a C-- aircraf during anapproach to the Guadarrama Mountains on February , occurred as a result o several actors.

(i) emperatures encountered by the aircraf at mslwere optimal or icing, around C. Tis act,

together with a low IN concentration (measured atthe surace o Lozoya Valley by an isothermal cloudchamber during the ight), hindered the nucleationprocess. Furthermore, the radiometer registered astable layer just below hPa. Tis weak ther-mal inversion avoured ormation o vertical shearincreasing collision-coalescence process efficiency.LWC presence was detected by CAPS observation,which demonstrates that most o the hydrometeors inthe gravity wave over the Lozoya Valley were liquid.

(ii) In the WRF simulation, a dynamic tropopauseanomaly approaching rom north o the IberianPeninsula pushed a warm and moist air mass ahead o

it. Secondarily, another dynamic tropopause anomalysouthwest o the peninsula supported moisture accu-mulation in the centralpeninsula. Airow perpendic-ular to the Guadarrama Mountains during the hoursbeore the ight ormed a downwind mesolow, iden-tied by an area o low pressure and a cyclonic gyre.

Tis mesolow avoured convergence and updrafsin the region o aircraf icing. ogether with themesolow, mountain waves were the main cause oupdrafs in the lee o the Guadarrama Mountains.Tese updrafs provided LWC, sufficient time ormixing, and SLD accumulation. A neutral atmo-sphere below km altitude permitted the ormationo gravity waves. Further, a stable layer above thisaltitude blocked development o these gravity waves,so a layer o high concentration o SLD and LWCappeared just beneath cloud top.

Te combination o all these actors created an optimalenvironment or aircraf icing in a small region a ew kilome-

tres downwind o the Guadarrama Mountains, between and msl.

Conflict of Interests

Te authors declare that there is no conict o interestsregarding the publication o this paper.

Acknowledgments

Tis paper was supported by the ollowing Grants: ECO-AGUA, Micrometeo.com, and Granimetro (CGL-)

and LEA- and LEB awarded by the Junta deCastilla y Leon and MINECO. Special thanks are due toRoberto Weigand, Angel Guerrero, Steven Hunter, AnalisaWeston and Lauren Giera. Te authors would like to thankCanal de Isabel II Gestion, INA, and CLAEX or theacilities. Sergio Fernandez-Gonzalez acknowledges the grantsupported rom the FPU program.

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