Meteorol Atmos Phys 91, 247–265 (2006)DOI 10.1007/s00703-004-0082-y

1 Institut f€uur Meteorologie und Klimatologie, Universit€aat Hannover, Hannover, Germany2 Institut f€uur Physik der Atmosph€aare, DLR Oberpfaffenhofen, Oberpfaffenhofen, Germany

Aircraft icing research flights in embedded convection

T. Hauf1 and F. Schroder2

With 17 Figures

Received February 11, 2003; accepted December 1, 2003Published online: September 15, 2005 # Springer-Verlag 2005

Summary

Results from in-cloud measurements with an instrumentedaircraft from an icing research campaign in SouthernGermany in March 1997 are presented. Measurementswith conventional optical cloud probes and of the iceaccretion on a cylinder exposed to the flow show theexistence of supercooled large drops (SLD) in the sizerange up to 300 mm simultaneously with severe icingwith ice-accretion rates of up to 3.5 mm min�1. Nearly allperiods with icing, including the ones with severe icing,occurred in mixed-phase convective cells embedded insurrounding stratus clouds. The spatial scales of SLDoccurrence, respectively severe icing, ranged betweenseveral hundred meters and some kilometers and corre-spond to the length of the transects through the embeddedcells. SLD formed through the coalescence process andwere found through the whole cloud depth pointing toa source region near cloud top, in line with the argumentsof Rauber and Tokay (1991). No indication of ice-multiplication by the Hallet-Mossop process was found,despite of the favorable temperatures for that process.Comparisons of the measured amount of accreted icewith the observed cloud-particle size distributions quitesurprisingly indicate that ice accretion is mostly causedby 10–30 mm sized drops rather than by SLD. Thelatter, therefore, appear to be a by-product of ahypothesized liquid water accumulation zone near cloudtop which is also the primary cause of the observed severeice accretion. The results confirm the importance ofembedded convection and of mixed phase clouds withhigh amounts of liquid water and simultaneously occur-ring SLD.

1. Introduction

The existence of supercooled liquid water inclouds with temperatures ranging between 0 �Cand �40 �C represents a metastable thermody-namic state. Drops may freeze spontaneouslyupon conditions which are still not fully under-stood, despite of their fundamental relevance forthe precipitation formation process. One specialtype of spontaneous freezing occurs when an air-craft is flying through a supercooled cloud anddrops impinge onto the wing and fuselage andfreeze subsequently. The resulting ice accretionon the aircraft poses a severe safety problem asthe aircraft performance can be substantiallydegraded (e.g., Cooper et al, 1984). Ice-accretionrate depends primarily on the liquid water contentbut also on the drop-size distribution due to size-dependent collision efficiencies of drops and air-craft. As a result of several accidents in the past,the current world-wide scientific discussion in air-craft icing focuses on the role of supercooled largedrops (SLD) with diameters of 50 to 500 mm oreven more as aircraft are not certified for flightsin supercooled clouds having a drop size distribu-tion with a mean volume diameter (MVD) largerthan 50 mm. The Federal Aviation Regulation Part25 Appendix C (Federal Aviation Authorities,1999) defines probable maximum icing conditions

expected in winter storms and also gives testprocedures aircraft have to comply with for achiev-ing certification for flights in supercooled clouds.Drops with MVD greater 50 mm, however, werenot taken into account. Conventional icing occursmainly in zones of few centimeters width near astagnation point. Ice accretes there preferably inlayers but it may also form horns and fingers. Dueto their inertia large drops of usually more than50 mm in diameter may impact well off behindthese stagnation zones (Cooper et al, 1984). Solid-ification of supercooled water proceeds in twomajor stages, where in the first, with a time scaleof 10�5–10�4 s, only a small fraction of the waterfreezes, while in the second stage the exchange ofreleased latent heat of freezing to the environmentleads to a considerably longer time scale of 10�3

to 1 s (Pruppacher and Klett, 1997, p. 674). Dropsremain semi-liquid during that period of time andmay freeze completely first further downstream oftheir impact location and often well behind theprotected zones on wings, propellers and inlets.This process is referred to as run-back icing. Theaccreted ice not only increases the aerodynamicroughness but also may lead to substantial flowdistortion and even flow separation. The lattereffect may have been a primary physical causeof the ATR-72 crash near Chicago 1994 (Sandand Biter, 2000). One should also note that anyinitial substantial flow distortion on the otherwisesmooth surface of the wing or other parts of theaircraft, such as, e.g., by a frozen drizzle drop of afew hundred microns in diameter, causes a localstagnation point, which in turn increases the iceaccretion there and thus constitutes a positive feedback. Frozen drops grow subsequently to mm-sized roughness elements with corresponding de-crease of aircraft performance (cf. Fig. 10).

SLD are assumed to grow by a pure coales-cence process. This is referred to as the non-clas-sical formation of supercooled large drops, whilethe classical one denotes a situation where liquidcloud or rain drops fall into a layer with subzerotemperatures which results into what is usuallyknown as freezing drizzle, respectively, freezingrain. In this study, only the non-classical forma-tion process is considered. One of the not yet fullyunderstood key questions concerns the nature ofthe coalescence process itself and the favorableconditions for its occurrence (Vohl, 2000 andreferences ibid; Rasmussen et al, 2000). This

study reports on in-situ measurements under thespecial conditions of embedded convection wheresevere aircraft icing and SLD were observed. Thisatmospheric condition has, to our knowledge, notyet been recognized as favoring SLD formationand=or severe icing. The flights were part of theEuropean Community funded project EURICE onaircraft icing. Within the EURICE project, threeaircraft operated independently over the IberianPeninsula, the North Sea, and Southern Germany,respectively. This study reports on the SouthernGerman flight campaign with the DLR’s DO-228 research aircraft, near Munich, Germany.Results from the other flight campaigns in theEURICE project are presented elsewhere byAmendola et al (1998).

Most of the SLD observations so far arereported from North America, e.g. by Coberet al (1995; 1996a; 1996b; 2001a), Politovich(1989) or Rasmussen et al (1992). In 1992,Hoffmann and Demmel (DLR Oberpfaffenhofen)reported SLD and related ice accretion mea-surement during a flight campaign over SouthernGermany. The flights were performed in the samearea as those of the current paper and are the firstSLD documentation from outside North America.

2. The flight campaign

In March 1997, a flight campaign was conductedin Southern Germany with a DO-228 instrument-ed research aircraft shown in Fig. 1. The aircraftwas equipped with two optical scattering spectro-meters (FSSP-100ER, diameter range 5–95 mm,FSSP-300, range 0.4–20 mm) to measure dropsizes and an optical array probe (OAP-2DC,range 20–750 mm) to determine shapes andphases of the particles, together with a tempera-ture sensor and a Johnson-Williams (JoWi) liquidwater probe. The instruments were installed in

Fig. 1. The DLR DO-228 research aircraft

248 T. Hauf and F. Schr€ooder

metal containers underneath both wings. Figure 2shows the left wing with the OAP-2DC and theFSSP-100ER. The cloud water content (CWC,water and ice) was compared with the Johnson-Williams-probe, which measures the liquid wa-ter content (LWC) up to drop diameters of40–50mm, and with the cloud-particle volumedistributions derived from the optical spec-trometers. Two cloud water content valuesCWC500 and CWC800 were determined, whereintegration over the particle volume distributionwas performed up to 500 and 800 mm, respec-tively, including ice and liquid particles. Noattempt was made to discriminate liquid fromsolid particles. Cober et al (2001b), in assessingcloud-phase conditions from various instruments,showed that liquid water content calculations,derived from FSSP based drop size distributionfor drops larger 35 mm in the presence of ice par-ticles with more than 1 L�1, may show substantialerrors. For both reasons calculated integral cloudwater contents CWC should be discussed morequalitatively rather than be considered as quanti-tative measures. Video cameras documented theice accretion on the wing and on an exposedcylinder of 4 cm diameter. The cylinder ismounted at the floor of the aircraft and is exposedto the flow with its axis perpendicular to the flowdirection. An onboard installed camera looksthrough a glass window in the floor along thecylinder on to a disc-shaped scale mounted atthe end of the cylinder. From the video recordingafter each flight, both ice accretion (in mm, reso-lution of 0.5 mm) and ice accretion rate (inmm=min) can visually be determined. Pleasenote that the video images (Fig. 3) do not only

show ice accretion on the cylinders’ flow facingside but also on its bend support (lower centralpart of Fig. 3). The measurements were madebetween 15 and 27 March 1997 near Munich,Germany to the north of the Alps. A ground-based Doppler and polarization radar (Hagenand Meischner, 2000), stationed close to theflight path at DLR, Oberpfaffenhofen, monitoredthe clouds during the flights.

The general weather situation during that pe-riod was governed by north-westerly flow, withadvection of cold and humid air masses from theNorth Sea into Central Europe, and by a highpressure system over the Southwest of Europe,which only temporarily expanded into CentralEurope. Several low-pressure systems over theNorth Atlantic moved eastwards and developedbranches with their respective frontal systemsover Central Europe and the north of the BritishIsles. Over most part of Central Europe low-levelclouds prevailed during the whole flight cam-paign. In some areas and at some times the cloud

Fig. 2. Cloud microphysical instrumentation mounted onthe left wing of the DLR Do-228

Fig. 3. Video view of the icing cylinder through the glasswindow in the aircraft floor, flow direction from top (front)to bottom (rear), March 15. Note the bent support in thelower central part. Upper panel: Individual frozen drops arerecognizable on the still clean cylinder (9:46 UTC). Lowerpanel: Ice accretion after a couple of minutes (9:57 UTC).Note the asymmetric ice shape in the lower panel

Aircraft icing research flights in embedded convection 249

field was broken. Usually, convective cells wereembedded within the stratus cloud (Fig. 4). Theywere vigorous and produced occasional rainshowers or light rain. The freezing level wasmostly in such a height below cloud base thatno solid precipitation reached the ground. Cloudfields ranged usually from the North Sea to theAlps. Cloud-top temperatures were rarely colderthan �11 �C. Despite the day-to-day variability inthe temperature structure and cloud top and baseheights, the convective unstable stratification dueto the advection of cold and humid air and low-level clouds with embedded and quite vigorousconvection were common basic features for thewhole period. This weather situation is quitetypical for March and April in Central Europe

and is in line with the earlier nine year icingstudies by Hoffmann and Demmel (1992), whereon average the largest number of days with air-craft icing was found in the month of March.

Due to the prevailing weather conditions, theemphasis of this paper is on low-level stratusclouds at sub-freezing, yet relatively high tem-peratures with embedded convective cells. InFig. 4 from 24 March 1997 a view from aboveon the stratiform cloud field is given. Embeddedconvective cells can be recognized by their topsovershooting the main cloud deck. Vertical tem-perature profiles from cloud samplings duringthe initial ascent and the final descent on 24 Marchare shown in Fig. 5. They reveal a small temper-ature inversion at the cloud-top altitude, whichprohibits further acceleration within the embed-ded cells and thereby limits the overshooting topsto a vertical extent of approximately 200 m. Anaircraft cruising directly above cloud top withinthis height range alternately encounters cloudyair and cloud-free regions. Occasionally, gravitywaves were observed within the upper cloud deckwhich could be identified by their regular un-dulations, respectively, the banded cloud struc-ture. This was especially found during flightno. 8 (not shown).

Altogether one test and seven research flightswere performed (for details see Table 1). Duringfive of the latter supercooled clouds weresampled and considerable icing was experienced.The focus of this paper is on the two cases with

Fig. 4. Upper stratus-cloud deck with an overshooting topof an embedded convective cell on 24 March 1997

Fig. 5. Vertical temperature pro-files on March 24, 1997. The thinsolid line represents measurementsfromthe initialascent, the thicksol-id line those from the final descent.Dotted lines represent mean wet-adiabatic temperature gradients.Average cloud-top and cloud-basealtitudes are also indicated

250 T. Hauf and F. Schr€ooder

moderate to severe icing, March 15 and 24, 1997,while the other days with icing were not deeplyevaluated. The results achieved for those daysdiffer in detail, but not with respect to the overallconclusions. The flight patterns consisted of ver-tically staggered straight and horizontal flightlegs approximately 50 km in length (Fig. 6).The number of legs per flight ranged between 5and 12. After an initial sounding through theentire depth of the cloud, the latter was surveyedusually from above with penetrations of over-shooting tops, if present. Then the cloud wasprobed level by level. As temperatures belowcloud base were always above zero, the aircraftcould descend and thaw off the accreted ice incase of severe performance degradation, whichhappened once. The flights were in the approacharea of the Munich International Airport. Theflight pattern was checked with air traffic control-lers and each flight level change had to be

Table 1. Overview of the eight flight missions

Flight number Date Time (UTC) Cloud description Weather Icing Special features

1 14 March 1997 1502–1530 Isolated cumuli Haze No icing Test flight; nomeasurement data

2 15 March 1997 0926–1107 Stratus withembeddedconvection,mixed phaseclouds

North-westerlyflow of moistarctic air, rain

Severe icing,SLD

No radar data

3 18 March 1997 1326–1424 Stratus, brokenstratus andstratocumulus

No icing,temperatures incloud >0 �C

4 19 March 1997 1040–1259 Stratus, embeddedas well asisolatedconvection

North-westerlyflow behindcold front,light rain

Icing No aircraftdata

5 24 March 1997 1030–1300 Stratus withembeddedconvection

North-westerlyflow of moistarctic air

Severe icing,SLD

6 25 March 1997 1317–1436 Cumulus andstratocumulusin two layers,mixed phaseclouds

North-westerlyflow behindocclusion front,temporarilylight rain

Icing No radar data

7 26 March 1997 1344–1509 Cumulus andstratocumulus,inversion above725 hPa

weak highpressuresystem

Icing No FSSP-300data

8 27 March 1997 1138–1254 Stratus, gravitywaves withformation ofdrizzle at wavecrests

after warmfront passagewith extendedrain field

No icing,temperatures incloud >0 �C

Fig. 6. Example of the flight pattern March 24, 1997. Theaircraft flew back and forth along a line approximately ori-ented NNW–SSE in the region between Munich (right) andAugsburg (upper left) in Southern Germany. The flight path issuperimposed on a horizontal radar image (ppi). Crosses markposition of aircraft for every minute. The radar is located half-way between Munich and lake Ammersee further to the west

Aircraft icing research flights in embedded convection 251

requested which led to nonuniform verticalspacing between consecutive flight legs. Due toair traffic requirements not all requested levelchanges were confirmed. No attempt was madeto select other than a straight level flight pattern.

3. Measurements

3.1 The icing flight on March 15, 1997

After an initial sounding the aircraft performedan overview leg above the cloud layer, at 3400 m,and two in-cloud straight level flights at 3000 mbetween 9:52–9:57 and 10:13–10:19, separatedby a descent to below cloud base and a subse-quent flight leg at 2400 m, just above cloud baseat 2300 m. After the second leg at 3000 m theaircraft ascended further to 3300 m, just belowcloud top. A straight level flight was followed bysome maneuvers to test the aircraft performanceunder icing conditions which, however, will notbe discussed here. After altogether 32 minutes in3300 m a descent terminated the flight. Figure 7shows the time sequence of liquid water content(LWCJoWi), measured with the Johnson-Williamsprobe, and of ice accretion rate for the wholeflight period between 9:30 and 10:40. The formervalues were smoothed over 3 seconds. The domi-nant feature of the LWC signal is a relatively lowvalue of �0.1 g m�3 with short bursts of maxi-mum 0.28 g m�3 over time periods of minutes,corresponding to spatial scales ranging from sev-eral hundred meters to several kilometers. The

same feature can be found in the time series oftotal number of events given by the OAP-2DC(not shown). We will show in the next sectionthat these bursts correspond to passages throughconvective cells which were embedded in thestratus cloud.

3.2 Detailed studies of SLD-events – overview

We focus now on two time sequences of theflight at 3000 m and 3300 m, in the followingreferred to as Seq1 (Fig. 8; 9:52–9:57 UTC; leftpeaks in Fig. 7) and Seq2 (Fig. 9; 10:20–10:28UTC, middle of right peaks in Fig. 7), with aduration of 4–8 min each, where the aircraftpenetrated cloud sections with remarkable icingrates. The temperatures were �9 �C in Seq1 and�11 �C in Seq2 (see right upper scale of Figs. 8and 9). Please note that the flight level of 3000 mwas left for descent at about 9:57 UTC (¼ 9.95decimal hours) with a subsequent temperatureincrease. Similarly, the ascent to 3300 m inFig. 9 of Seq2 with the accompanying tempera-ture decrease was completed at 10:22 UTC(¼ 10.37 decimal hours). Maximum ice-accre-tion rates of 3.5 mm min�1 were experienced dur-ing Seq1, where the LWCJoWi typically rangedbetween 0.25 and 0.3 g m�3. Focusing now onthe two additionally calculated total water con-tent estimates CWC500 and CWC800 for Seq1 inthe upper panel for Fig. 8, we recognize first that,as required by their definition but despite of theuncertainties reported by Cober et al (2001b),

Fig. 7. Ice accretion rate (leftscale) and Johnson-Williams liq-uid water content (JoWi, rightscale) for the flight on March 15,9:30–10:40 UTC

252 T. Hauf and F. Schr€ooder

CWC500 is always smaller than CWC800. Thedominant feature, however, is that both quantitiespeak in small bursts of tens of seconds, respec-tively, spatial scales between 100 and 300 m, afact which was noticed already previously for theLWCJoWi. The photograph (Fig. 10) of the wingunderside was taken shortly after the exit fromSeq1. Besides ice accretion of more than 10 mmon the leading edge, a considerable amount of icewas observed on the black rubber panel of thepneumatic deicing boots about 30 cm down-stream of the active and floatable part of the lat-ter. Immediately behind the leading edge streaksof accreted ice can be identified which indicaterun-back ice. Further downstream ice accretes aslittle spots and humps with each of them verylikely results from frozen drizzle drops, respec-tively, SLD grown further by riming.

To emphasize the short scale fluctuations, wehave selected ten very short flight segments

Fig. 8. Icing parameters during flight sequence Seq1 (9:52–9:57) on March 15: Upper panel: JoWi-LWC (open circles),CWC < 500mm (solid line) and CWC < 800mm (dottedline, all left scale) and ambient temperature (step line, rightscale). Lower panel: Ice accretion on the cylinder (solidsquares, right scale) and icing rate (open circles, left scale).See also text for further details

Fig. 9. As in Fig. 8, but for flight sequence Seq2 (10:20–10:28) on March 15. The average JoWi-LWC during 10.35–10.50 decimal hrs is about 0.15 g m�3 which is 30–40% lesscompared to Seq1. Significant ice accretion was observedonly at the leading edge and on the boot protected regionsbelow the wing (not shown)

Fig. 10. Ice accretion shortly after Seq1 on the wing under-side, from the leading edge to behind the boot-protectedregion below the wing, between cabin and the turbopropengine (left). The boot protected region ends within thedark lined area along the wing span. The leading edgecarries at least a 10 mm thick, partly clear, ice cover.The boots were not activated yet. Ice streaks immediatelybehind the leading edge indicate back-running. Furtherdownstream and at least 30 cm behind the protected areapatchy ice fragments are observed

Aircraft icing research flights in embedded convection 253

within the time interval of Seq1, lasting 1–3 secwhere the OAP instruments gave significantcounts in the SLD range. Altogether the 10 flightsegments comprise �10% of the Seq1 time inter-val of 4 minutes and do represent the 10 strongestpeak events.

To contrast the two flight segments Seq1 andSeq2, a reference segment Seq3 (10:13–10:18)was chosen at a flight level of 3060 m. Seq3has only an insignificant large-particle-mode,significantly less SLD than both Seq1 and Seq2and also less liquid water in the cloud drop mode(cf. Fig. 13). The ten LWCJoWi peak segmentvalues are close to the average value for thewhole Seq1 of 0.25 g m�3 and do not exhibitmuch scatter (not shown). The average peakvalue is 0.26 g m�3 while for Seq2 we find with0.15 g m�3 less and for the reference sequenceSeq3 with 0.09 g m�3 significantly less liquidwater. The little scatter in the Johnson-Williamsdata is due to the low sensitivity for drop sizeslarger 30 mm (Biter et al, 1987), and the littlevariability of the cloud drop mode which willbe shown in the spectra (Fig. 11).

We have tested how sensitive the mean volumediameter (MVD) calculation is to the inclusionof larger particles and found that the averageMVD for Seq1 is shifted from a value some-where between 20 and 30 mm to the range 50 to70 mm if only the ten peak events within Seq1 are

considered. Thus during the peak events we haveonly a slightly increased cloud drop liquid watercontent but clearly enhanced MVD values.

Using the size distributions discussed below,we determined the number concentrations in var-ious intervals and found that the majority of theparticles in the SLD size range <500 mm residesin the range up to 150 mm. This is confirmed by theOAP shots (Fig. 12) where visually identifieddrops have diameters smaller 200 mm. There ismore than a factor 10 less SLD in Seq2 as com-pared with Seq1 but still a factor 10 more than inthe reference segment Seq3. The MVD values,therefore, tend to be shifted to smaller values fromSeq1 to Seq2 and to Seq3. These findings empha-size the occurrence of increased liquid water con-tent in the cloud drop mode and the existence ofSLD < 150 mm on short spatial scales.

3.3 Detailed study of spectraof SLD-event Seq1

Number and volume spectra of the cloud parti-cles during Seq1 were calculated for the tenflight segments and plotted together in Fig. 11.The cloud particle number distribution is shownin the left panel and the volume distribution inthe right one. Both were calculated from FSSP-100ER and OAP data for Seq1 (bold line andopen symbols) and also for ten peak events (thin

Fig. 11. Composite cloud particle number spectral distribution (left panel) and volume distribution determined from theFSSP-100 and OAP for Seq1 on March 15th (bold line and open symbols) and for ten peak events (thin lines with dots). Leftpanel: Spectra are plotted separately for both FSSP and OAP in the overlap size range 30–95 mm. The average of the peakevent spectra is displayed as a grey solid line without symbols. No continuous line in case of too few particles within thesampling time of 1–3 s. Right plate: Spectrum for Seq1 is highlighted by grey shading beneath. Ten peak segments with largenot-connected symbols, average spectrum grey solid line

254 T. Hauf and F. Schr€ooder

lines with dots) with their respective average(grey solid line without symbols). Please notethat spectra are plotted separately for both FSSPand OAP in the overlap size range 30–95mm.Differences between both instruments are animmanent feature due to specific differences inthe error characteristics. These differences, there-fore, indicate the general uncertainty in particlenumbers – an inherent feature in particle sizing.Also it should be kept in mind that the detectionof particles for size ranges over 300 mm isaffected by low numbers and, therefore, has apoor statistics, with related scatter in the calcu-lated distributions. The right plate shows thevolume distribution in a logarithmic-linear repre-sentation where the areas under the respectivecurves give exactly the total volume in that sizerange. The left plate gives the number distribu-tion in a log–log presentation.

The volume distribution shows quite clearlythe three discussed modes, the cloud drop mode,the SLD mode and the large-particle mode. Forbetter illustration the area under the Seq1 curve isshaded. The ten peak segments are displayedwith only large not-connected symbols to demon-strate the large scatter due to large individualparticles and their contribution to the total Seq1spectrum. The average over the peak events (greycurve) is always above the Seq1 curve. Thus,during 10% of flight time of Seq1 most of thewater is concentrated in short peak events and,therefore, much less in between. When discuss-ing the large-particle mode it has to be noted thatno algorithm with an automatic discriminationbetween liquid and solid particles was applied to

the data. The partition of total particle volumeinto liquid and solid is, therefore, not determined.From the OAP screen shots (cf. Fig. 12) we knowthat there were always during the flight mixedphase conditions but we did not find liquid dropslarger 500 mm. We, therefore, conclude that larg-er particles did not contribute to aircraft icing.This does not hold for particles smaller 500 mm.Here we have unambiguously identified a mix-ture of both liquid and solid particles.

Turning now to the number distribution (leftplate of Fig. 11), we find a well expressed clouddrop mode with one and only one maximum at20 mm. The SLD mode is at least 3 orders ofmagnitude lower than the cloud drop mode, andis only recognizable as a small hump in thedeclining part of the number spectrum at about150 mm. Comparing the Seq1 spectrum with thesegment spectra containing the individual bursts,we see that they differ significantly for the SLDmode with short term spectra having up to afactor ten more particles. The SLD mode is,therefore, mainly concentrated in the peaksrecognizable in the time series of liquid watercontent (Fig. 8). This is also confirmed whenthe video sequences of the ice accretion on theicing cylinder is viewed, as the impact of SLDcan be identified there. The detection of SLD byeye is not only possible on a deiced and cleancylinder as in Fig. 3, left panel, but also when icehas accreted already. From the volume distribu-tion (right plate of Fig. 11) we concluded that theSLD mode of the short term spectra may containmore liquid water than the cloud drop mode. Onthe average for Seq1, however, the cloud drop

Fig. 12. OAP-2DC images of cloud particles from March 24, taken near cloud top (right plate, 12:30 UTC, leg D in Fig. 14)and at 2200 m (left plate, 12:08 UTC, leg B in Fig. 14). Horizontal width of each of the three stripes corresponds to 0.8 mm inparticle size, the vertical scale is the same. Bright horizontal bars are separators for individual images

Aircraft icing research flights in embedded convection 255

mode is dominant in both number and volumespectrum. The third, large particles mode is neg-ligible in number. The previously discussed lowscatter of LWCJoWi values is also reflected inFig. 11, where the curves in the cloud drop modecoincide more or less.

3.4 Detailed study of SLD-event Seq2

Having focused mostly on Seq1 we turn now toSeq2. Figure 9 illustrates that during Seq2, at atemperature of �11 �C, no bursts as in Seq1 wereobserved. All three quantities LWCJoWi, CWC500

and CWC800 coincide nearly completely, pointingtowards an absence of larger particles, e.g., iceparticles. Figure 13 shows in a similar fashionas Fig. 11 the number and volume distributionfor Seq2 (triangles) and additionally for bettercomparison Seq1 (circles), Seq3 (squares), andfor the one peak segment within Seq1 at9:55:47–9:55:48 (dots). The volume distributionis shown in a log–log representation, in contrastto Fig. 11 where a lin–log one was preferred. Seq2was at �11 �C, thus nearly two degrees colderthan Seq1, and the LWC was 0.15 g m�3 andslightly lower than the 0.25 g m�3 value of Seq1.It should be noted that the segments were flown indifferent clouds having different cloud bases andvertical extents. The cloud with Seq1 ranged from2700 m (base) to 3050 m (top) with Seq1 at3040 m close to cloud top while the cloud with

Seq2 ranged from 3050–3350 m. Despite theirdifference in altitude, both Seq2 and Seq3 whereflown close to cloud top of the respective clouds.One should note that both sequences feature highicing rates with �3.5 mm=min in Seq1 and 1–2.5 mm=min in Seq2, while the reference segmentSeq3 had 0–0.5 mm=min (cf. Figs. 7 and 8).

Figure 13 shows that Seq2 differs substantiallyin both number concentration as well as volumedistribution in the SLD mode, with Seq1 by oneorder of magnitude, and even more with the onepeak segment of Seq1 (dots). All three segments,however, approach equal levels in both the clouddrop mode and the large-particle mode. The lat-ter fact is easily explained by the sufficiently dis-cussed presence of large ice particles. Theequality of the cloud drop spectra for Seq1 andSeq2 explains the nearly similar icing rates,which again is in line with the arguments thaticing is mainly caused by the cloud drops. Seq3has the same maximum value within the clouddrop mode, but the right-hand flank declines sig-nificantly earlier than for both Seq2 and Seq3.Thus the LWC is significantly lower for Seq3as well as the icing rate (cf. 10:13–10:18 inFig. 7). The occurrence of SLD seems to be cor-related with the LWC in the cloud drop mode.The difference in SLD number as well as volumecontent for Seq1 and Seq2, however, cannot berelated to such differences as spectra coincide till30 mm. The LWCJoWi differences between Seq1

Fig. 13. Composite cloud particle spectral number distribution (left panel; log–log presentation) and volume distribution (rightplate; log–log presentation) determined from the FSSP-100ER and OAP for Seq2 (triangles), Seq3 (squares), for better compar-ison also for Seq1 (circles), and for the one peak segment within Seq1 (9:55:47–9:55:48 UTC, dots) on March 15. Please notethat the volume distribution is shown in a log–log representation, in contrast to Fig. 11 where a lin–log one was preferred

256 T. Hauf and F. Schr€ooder

and Seq2 can only be explained by the lowervalues of number spectra larger 30 mm, despitethe declining sensitivity of the JoWi instrumentin that size range. In conclusion, Seq1 stands forrelatively high icing rates, high LWCJoWi andhigh SLD values, while Seq2 has also high, butslightly lower icing rates, slightly less LWCJoWi

and lower but still detectable SLD values. Thesingle peak segment (dots) exhibit almost fourorders of magnitude more SLD than in the refer-ence segment Seq3. Please note that for the sin-gle peak segment the existence of particles largerthan 400 mm is omitted due to poor statistics. Insummary, in the mixed phase embedded convec-tive clouds well expressed SLD cloud volumesexist on spatial scales of 100 to 200 m with tentimes more SLD than compared with the averageover the whole traverse through the cell. Thislarge variability within a cell represents a majorfinding of this study.

3.5 The nature of the particles

The question arises what cloud particles contri-bute to the three modes defined above andobserved in Seq1 and also Seq2. So far we haveassumed that particles in the SLD mode arepredominantly liquid. This, however, has to besupported by arguments (see Section 3.7). Eyeinspection of the OAP-2DC images for Seq1(not shown, cf. Fig. 12) reveal that no liquid dropslarger than �300 mm were present but only iceparticles of various shape. Thus particles largerthan that value can definitely be considered asice particles. On the other side of the size spec-trum the cloud drop mode centered at about 20 mmis caused by predominantly liquid drops. Theexistence of small ice crystals cannot be excluded,though there were no indications for that. Thequestion remains about the phase of the inter-mediate sized particles, respectively whetherthere are supercooled large particles (SLD) pre-sent. There are three strong arguments supportingthe existence of SLD. The first emerges from aninterpretation of the OAP-2DC images whereliquid particles in the size range <300 mm canbe identified (see Fig. 12). The topic of misinter-pretation of spherical particles is discussed furtherbelow in section 3.7. Second, SLD can also be iden-tified on video, but frequently also by eye, with ashort white flash during the time of impact when

they change phase. They can be observed on theicing cylinder (see Fig. 3, left plate), on the lead-ing edge and on the underside of the wing (seeFig. 10). Figure 10 also shows that particlesimpacted behind the protected parts of the wingand froze there. The impact in that area is onlyknown for liquid particles larger than �50 mm,especially for SLD (Politovich, 1996). We, there-fore, conclude that the SLD-mode in Seq1 is dueto liquid and supercooled drops. Studying thebimodal structure of the spectra (Fig. 11) fromSeq1 with the cloud drop and the SLD mode inmore detail, calculations were performed overvarious size ranges to determine the total watercontent, respectively the liquid water content inthat range. Quite surprisingly it was found that theSLD mode, on the average, constitutes not morethan about 20% of the total supercooled LWCwhere integration was performed up to cloud-par-ticle diameters of about 500 mm. This value mightbe even lower as the integration limit of 500 mmwill certainly include some ice particles. Even theaverage median-volume diameter (MVD) of thecomplete Seq1 with a length of �20 km stays wellbelow 40 mm and is, therefore, covered by theFAR=JAR Appendix C framework. MVD-valuesmay be larger than 50mm for peak events, but forshort time periods only, and shorter than requiredby Appendix C. Icing conditions of this study areconsistent with those of Appendix C.

3.6 The icing flight on March 24, 1997

The flight mission of March 24 allowed a moresystematic cloud sampling as compared withMarch, 15. The weather situation was very similarwith a stratiform cloud field having cloud base andtop at approximately 1800 and 3200 m, respec-tively. The flight track consisted of 6 parallel andvertically staggered legs. The legs were oriented ina NNW–SSE direction (see Fig. 6). The variousflight legs were flown along that direction at fivedifferent altitudes labeled A–E in Fig. 14.

The DLR weather-radar was active on thatday and provided continuously horizontal radarreflectivity scans. A detailed analysis of the flighttrack and the radar images was made by Stingl(1999). Figure 6 shows a radar image fromMarch 24 with the convective cells cloud fieldand the corresponding flight track superimposed.Quite surprisingly, the radar revealed individual

Aircraft icing research flights in embedded convection 257

clouds with spatial scales of several kilometers,rather than the visually apparent extensive stratuscloud as were seen by the aircraft. Having inmind that a C-band radar is sensitive only to par-ticles much larger than 100 mm and thus does notdetect the normal cloud drops centered around20 mm which were found on March 15 in thestratus cloud, it was concluded that the cloudfield consisted of a stratus cloud with embeddedconvective cells. This is supported by the photo-graph (Fig. 4) of the upper cloud deck where anovershooting top of such an embedded convec-tive cell is recognizable. This explains also theabove mentioned periods of vigorous turbulencenoted during the flights. Comparing the time ofthe passages through the convective cells withthe occurrence of significant icing rates andincreases in cloud particle number densities,Stingl (1999) found unambiguous evidence that

the icing events occurred when the aircraft tra-versed a convective cell. The horizontal scale ofsuch a convective cell is on the order of severalkilometers. As the aircraft does not always passthrough the center of the cell, aircraft traversescan be shorter, leading to the range of observedicing event durations. With a true airspeed of80 m s�1, the time spans of the aircraft’s exposureto icing ranged between 6 and 60 seconds. Againas on March 15, SLD were observed togetherwith high icing rates during the traverse of a con-vective cell (Fig. 14). No SLD were observedoutside a convective cell.

Stingl (1999) further found that the convectiveclouds moved with the mean wind across theflight path in a south-easterly direction, and,simultaneously grew in size, and later alsomerged with each other (not shown). As this allhappened during the flight, it became clear by theanalysis that the data from the various flightlevels originate from different clouds, from dif-ferent sections within one cloud and from differ-ent life stages of the respective clouds. We,therefore, cannot consider data from one leveland data from another level as snapshots of thesame one cloud at one instant of time. We ratherhave to compare them in a statistical sense.

Figure 14 shows in the upper plate the heighttime series together with the cloud water con-tents CWC500 and CWC800. During the traversesof convective cells very clearly peaks appear inboth quantities at all levels. These traverses areaccompanied by peaks in the ice accretion ratewith a maximum value of 3 mm min�1. It shouldbe noted that the ice accretion rate is increasingfrom level to level. After the aircraft had iceaccreted almost 40 mm on the icing cylinder dur-ing the flight and its deicing boots had beenactivated twice, it was for safety reason forcedto exit the upper cloud region at about 12:48UTC and made a descent. By that time theengine-thrust level had to be increased fromabout 45% to about 80% to maintain a true air-speed of 80 m s�1. The measurements on March24 confirm the findings of March 15 that icingoccurs in short bursts which now can unambigu-ously be attributed to the passage of convectivecells embedded in the stratus cloud.

Now we focus on mean vertical profiles. Foreach level A–E at least several minutes of cloudpenetration have been evaluated and the mean

Fig. 14. Icing parameters obtained during one hour cloudsampling on five different altitude levels, A–E, on March24 (cf. flight pattern Fig. 6). Upper panel: Integral CWC(<500mm, circles), CWC > 40 mm (OAP-derived, thinline) and pressure altitude (bold step line, right scale). Low-er panel: ice accretion onto the icing cylinder and icing rate(compare Figs. 8 and 9). The integrated ice accretion ex-ceeds 30 mm and the de-icing boots were activated twiceduring the entire period 11.7–12.9 decimal hrs

258 T. Hauf and F. Schr€ooder

values, the 25%-, 75%-, and 90%-percentiles ofliquid water content (JoWi), cylinder icing rateand the cloud water content derived from OAPdata for particles larger 42 mm were determined.These mean profiles of Fig. 15 very clearly show,that both the liquid water content and the icing rateincrease with height, while the cloud water con-tent CWC decreases. We also observe that thescatter of both liquid water content and icing rateincrease with height while the one of CWCdecreases. As the liquid water content was mea-sured with the Johnson Williams instrument,which is only sensitive to drops smaller 40 mm(Vali, 1997; see also Biter et al, 1987), theobserved liquid water content and its increase withheight must be caused by cloud drops. This findingis in line with the well-known and common struc-ture of a convective cloud. The liquid water con-tent amounts to approximately 10–20% of theadiabatic liquid water content. There is no changeof mean liquid water content between the twouppermost levels D and E. This is very likelycaused by entrainment from the cloud top as theflight level E was just 100 m beneath the cloud top.As the cloud did not show a sharp edge, rather afluctuating transition zone, parts of the flight pathcould even be closer to the dry atmosphere aloft,thus reducing the liquid water content. The scatterof the LWC values increases also with height up tolevel D. This is due to the fact that at the upperlevels, except the uppermost one E, the maximumliquid water content increases within the convec-tive cells while outside stratus values are lowerand furthermore do remain constant at one level.Thus the horizontal average must show an increas-ing scatter with height. As the liquid water contentincreases with height, ice accretion increases withheight as well. This correlation was already foundon March 15. The decreasing cloud water contentwith height for particles larger 42 mm can simplybe explained by ice particles growing in size andincreasing in number while falling. Thus the lowerpanel c reflects the falling motion with the cloud,while panel b indicates the upward motion ofcloud drops.

Do the spectra confirm this view? Figure 16shows on the left panel the number spectra andon the right panel the corresponding volume spec-tra. Spectra were calculated for various samplingtime intervals and compared with the Seq1 spec-trum from March 15 (solid line with circles).

Dotted spectra refer to flight levels A and B, whiledashed spectra refer to flight level D. Unfortu-nately, FSSP data are missing due to instrument

Fig. 15. Mean cloud profiles for March 24. Upper panel(a): cylinder icing rates (30 s averages). Middle panel (b):liquid water content (JoWi) with 25-, 75-, and 90% percen-tiles during the sequences A–E. Mean level temperaturesare indicated. Lower panel (c): cloud water content derivedfrom OAP-2DC data (diameter > 42 mm) with 25-, 75-, and90% percentiles. Time intervals are indicated. Note thatCWC is given in units of cm3 m�3

Aircraft icing research flights in embedded convection 259

failure causing a gap in the spectra in the clouddrop mode. The overall picture is the same as onMarch 15: (i) a cloud drop mode at about 15 mm,(ii) the SLD mode at about 150 mm, appearing asa hump in the declining part of the spectrum, and(iii) the large particle mode for diameters larger300 mm. There is only little scatter in the clouddrop mode for the various segments, in contrast tothe SLD-mode where a considerable scatter by afactor of �8 exists. Caused by the high variabilityin large ice particle concentration, we found againas for the March 15 flight a large scatter in thelarge particle volume distribution. Due to theinstrumental failure, no conclusions can be drawnfor the vertical profile of the spectra within thecloud drop size range.

Figure 17 was taken before the final descent andafter deicing. It shows that ice also accreted onparts of the wing which were not protected bythe pneumatic boots. The latter also failed toremove the ice close to the fuselage (upper right)and left a ridge near the leading edge. In summary,the flight on March 24 confirmed the results foundduring the previous flight on March 15, but, withthe help of the radar measurements, additionallyrelated the peaks and bursts in liquid water contentand ice accretion to embedded convective cells.Further results will be discussed below.

Fig. 16. Composite cloud spectral number distribution (left panel) based on FSSP-300 and OAP-2DC data, but due to failurenot with FSSP-100ER, and specific volume distribution (right panel) acquired over typically 6–20 s cloud sampling intervalson March 24. Dotted spectra were sampled between 11.65–12.15 decimal hrs on flight levels A and B near cloud base anddashed spectra between 12.5–12.6 decimal hrs on flight level D in the upper cloud part and close to cloud top. For comparisonreasons Seq1 spectra are included (cf. Fig. 11) as solid lines with symbols (squares FSSP-300, triangles FSSP-100ER, circlesOAP-2DC)

Fig. 17. View from below on the section of the wing be-tween engine (left) and fuselage (right) on March 24 takenafter deicing at about 3000 m and shortly before finaldescent. Note that parts of the accreted ice could not beremoved by the pneumatic boots

260 T. Hauf and F. Schr€ooder

3.7 Cloud-particle types

In-cloud-particle images taken with the OAPcloud probe (Fig. 12) are shown for two flight seg-ments. The right plate of Fig. 12 was taken at2700 m near cloud top, the left plate at 2200 mMSL in the lower part of the cloud. The hori-zontal width of one image is 775mm. Horizontallines represent trigger pulses. The ‘‘zero-images’’in both panels are indicative for numerous trigger-ing events in the lowest OAP-channel (�20–42mm particle diameter). The diffraction pattern inthe center of a circular shaped image, the Poissonspot, is a necessary but not sufficient conditionfor the particle being a spherical drop. There isstill some ongoing discussion about problemsusing the Poisson spot as a criterion for identify-ing the liquid state of a particle (Korolev et al,1998). In the given case, however, we have evi-dence for the existence of liquid drops through thevideo documentation of impacting drops, and theimpact and secondary growth of frozen drops onthe wing underside. Some of the spherical shapedparticles may also be frozen. The images revealindividual ice crystals (hexagonal structure), grau-pel, needles, and spherical images of particles.All very large ice particles (>1 mm) seem to berimed and most likely do not contribute to iceaccretion on the airplane. We conclude that bothice particles and supercooled, liquid drops,respectively, SLD existed in the cloud. The sizeof the liquid drops is approximately 200 to300mm maximum, while partially imaged solidparticles may exceed 1 mm in size. Liquid parti-cles, therefore, seem to have been limited ingrowth. Please note that the slight elliptical defor-mation of the drop image is due to a not properlyset true air speed in the imaging software. In thelower part of the cloud, the ice particles are moreheavily rimed with a less pronounced though stillrecognizable crystal structure, and they contributemore to the observed cloud water content. This isin line with the common picture of precipitationformation in a convective cloud. In summary, theinvestigated convective cells contain both liquidand solid particles at all levels. While the clouddrop liquid water content increases with height,the opposite is true for the cloud water contentfor particles larger 42mm. The size of SLD seemsto be constant with height while their liquid water

content shows considerable scatter, but do insig-nificantly contribute to the total water content,respectively, to the accreted ice.

3.8 The further icing flights

Icing was observed on three more flights (seeTable 1: no. 4, March 19; no. 6, March 25; no. 7,March 26). On March 19, embedded convectionprevailed and moderate icing was experienced.SLD were identified on the video. The weathersituation was similar to March 24 with slight rainobserved at the ground. Aircraft data, however,was not available. On March 25, icing was foundunder mixed phase conditions. Unfortunately,radar data were missing on that day. On March26, light to moderate icing was observed withincumulus and stratocumulus clouds again undermixed phase conditions. In-cloud temperaturesranged between 0� and �5 �C. All these flightsconfirm the importance of embedded convectionwith mixed phase clouds for aircraft icing, thoughthe ice accretion rates were not as high as onMarch 15 and=or March 24. Because of the afore-mentioned and some other limitations, a deeperevaluation of this data was not performed.

4. Conclusions

This papers documents icing research flightswhich were performed in March 1997 in thepre-Alpine area near Munich, Germany underthe conditions of embedded convection. Twocases with significant aircraft icing were dis-cussed in detail. In this section, we discuss andsummarize the main findings.

4.1 Embedded convection and patchinessof aircraft icing

The two cases discussed in this paper and alsothe other flights with subzero temperatures hadsimilar weather conditions with an extended stra-tus cloud with embedded convective cells. Thecloud regime was found in the lower tropospherewith cloud tops at about �3500 m. Individualconvective clouds overshot the stratus cloud deckby approximately two hundred meters and couldbe visually identified from above. To the authors’knowledge, embedded convection so far wasnot considered as especially relevant for aircraft

Aircraft icing research flights in embedded convection 261

icing, in contrast to our results of substantialicing with a maximum icing rate of 3.5 mm=minmin in the convective cells but only marginalaccretion rates in the surrounding stratus. Thelength of an in-cloud passage with notable iceaccretion ranged in the reported cases betweenseveral hundred of meters and several kilometersand is determined by the random flight tracksegment length through the cell. This patchinessof aircraft icing was already observed in NorthAmerican East Coast storms (Cober et al, 1995).

Repeated traverses as they, e.g. may occur dur-ing holding patterns may result in an increasingamount of accreted ice. Such a scenario led to theRoselawn accident (Sand et al, 2000), though wedo not claim that embedded convection prevailedduring the time of that accident and did cause it.The hazard of embedded convection is enhancedby the fact that the pilot, while flying within thestratus cloud, does not recognize the embeddedconvective cells by eye.

4.2 Mixed phase conditions

The flights also emphasize the importance ofmixed phase conditions for aircraft icing. Acheck of OAP images reveals the existence ofdrops in all investigated clouds and at all flightlevels. Together with the cloud drop spectrometermeasurements we infer that liquid water was pre-sent, in both the cloud mode and the SLD mode,through the whole depth of all cells. Our findingsconfirm the relevance of coexisting phases foraircraft icing which has been documented in aseries of fine papers by Cober and his colleagueson research flights during the Canadian AtlanticStorms Program (Cober et al, 1995; 1996a;1996b) and the Canadian Freezing DrizzleExperiments (Cober et al, 2001a; 2001b). Coberet al (2001b) also worked out the technicalproblems in assessing cloud-phase conditions,especially in mixed phase clouds and usingcommon cloud particle sizing instrumentation.Despite of the documented importance of mixedphase conditions for aircraft icing, it has not beenmuch investigated yet (Riley, 1998), though theicing handbook (FAA, 1991) considers mixedphase conditions as very critical from an icingstandpoint. Mixed phase conditions are thoughtto be unstable, seldom and rapidly changing withtime. The reasoning behind is that the natural

glaciation of the cloud proceeds very rapidlyonce ice crystals have formed, leading to a com-plete depletion of liquid water. This is contrastedby the always found mixed-phase conditions inthe EURICE flights, and the 26% and 46% occur-rence during the first and third Canadian Freez-ing Drizzle Experiments (Cober et al, 2001b).Though ice particles grew by riming, there wasstill sufficient supercooled liquid water in thecloud drop mode to cause the observed ice ac-cretion. Although the temperature range [�5,�10 �C] was suitable for the Hallet-Mossop pro-cess of ice multiplication (Hallet and Mossop,1974), there was no indication found for that pro-cess. Two possible explanations for the absenceof complete glaciation can be given. First, therelatively warm temperatures >�11 �C do notsupport the activation of a sufficient number ofice nuclei. Second, vertical advection of humidair and subsequent condensation may lead tocontinuous and substantially new formation ofliquid water. Vertical wind speed was not mea-sured but may be guessed to be on the order of5 m s�1, as it is typical for continental spring timeshower clouds. Following the arguments byRauber and Tokay (1991), a possible reason forthe observed mixed phase conditions is that moreliquid water is produced by the combined actionof vertical advection and condensation than isdepleted by glaciation. Rauber and Tokay fo-cused on the uppermost layer of a cloud. Thisis also the region were we found the maximumof liquid water content of the cloud drop mode(cf. Fig. 15, panel b), respectively, measured withthe Johnson Williams instrument. Following thearguments by Rauber and Tokay, we concludethat the observed drizzle drops must have formedvia the coalescence process, very likely in theupper cloud layers, and then settled down tolower levels. Our conclusion is furthermore sup-ported by the fact that the convective cells wereembedded in a stratus cloud. Without this humidand cloudy environment, entrainment of ambientair would reduce the liquid water content in themain updraft, even more than in our case wherean already low value of 10% of the adiabaticwas found. One should note that the flight pathsdid not necessarily lead through the center ofthe cells. Thus the low value of 10% results al-ready from entrainment of ambient stratus airwith lower LWC values but still saturated air.

262 T. Hauf and F. Schr€ooder

The hypothesized reduced mixing effect, thoughtheoretically obvious, might, therefore, probablybe small. Results are in agreement with the pre-sent picture of cloud-particle spectra in mixed-phase clouds (see, e.g., Strapp et al (1996), andHobbs and Rangno, 1985) dividing the spectruminto a cloud drop mode centered at about 15–20 mm and an ice particle mode related to graupeland other ice crystals larger 200 mm. Both modesare separated by a gap at �100 mm, the level ofwhich varies according to the existence of a thirdmode with drizzle drops, respectively SLD. Inthe volume spectra these drops can be identifiedby an additional relative maximum within thegap, respectively, in the number spectrum as aslight hump in the declining flank of the clouddrop mode (see Figs. 11, 13 and 16).

4.3 Existence of supercooled largedrops SLD

The previous discussion provided convincing evi-dence for the existence of liquid drops in the sizerange 50–300 mm. This SLD mode is predomi-nantly found during embedded cell traverses inshort bursts of few seconds, respectively, a coupleof hundred meters. The increase of ice particlesize towards cloud base seems to be caused byriming. Whether SLD contribute significantly tothe observed riming could not be documented.We only note that there seems to be an upperbound for the SLD size at about 200–300 mm,the reason for that is not understood. Riming mightbe one explanation, contact freezing another.

4.4 Ice accretion rates and limitationsof liquid water content measurements

We should note that for a true air speed of 80 m=san ice accretion rate of I mm=min requires atleast a supercooled liquid water content SLWCin g m�3 of �I=4.8. Thus the measured maxi-mum value of 3.5 mm=min corresponds to aSLWC of �0.7 g m�3. How consistent is theobserved liquid water content with the measuredice accretion rate? Figures 8, 9 and 14 show thatliquid water content values of 0.25–0.3 g m�3

correspond with icing rates of 2–3.5 mm min�1.The latter, however, must have been caused,according to our above estimate of I=4.8, bya SLWC of 0.5–0.7 g m�3, twice as high as

observed. The discrepancy is likely due to thesensitivity of the Johnson-Williams instrumentwhich declines sharply for liquid particles larger30 mm (Gayet, 1986), implying that more SLWCwas present than detected by the Johnson-Williams instrument and, second, that the SLDcontribution to the observed icing rates, thoughsmall, was underestimated, if not completelyignored. Checking the correlation of LWCJoWi

and ice accretion rate during the two sequencesSeq1 and Seq2, we found (not shown) a roughcorrelation with, however, considerable scatter.Large values of LWC may exist, as for exampleon March 24, but are not correlated with equiva-lent fluctuations in the icing rate. This can becaused by drops smaller 10 mm which did notimpinge on the icing cylinder. Vice versa, allicing rate values above a line given by I¼ 4.8LWC must have been caused by drops not cap-tured by the Johnson-Williams. Figure 9 empha-sizes again the problem of underestimatedSLWC. Not only the LWCJoWi but also the twoother liquid water content measures reveal valueshalf as large as required. One further reasonmight be that the icing cylinder is too close tothe fuselage and within the aircraft aerodynamicboundary and thus collects more SLWC than thecloud probes beneath the wing. The problemneeds further clarification with a first step follow-ing Cober et al (2001b) to correct for LWC mea-surements in the presence of ice particles withnumber concentration exceeding 1 L�1.

4.5 Aircraft performance degradation

The observed aircraft performance degradation ofabout 30% on March 24 and the required safetydescent are without doubt due to the accreted ice.We observed several factors contributing to thisperformance degradation: (1) substantial iceaccretion on the wing leading edge (see Fig. 17),(2) parts of the ice could not be removed by thepneumatic boots, especially close to the fuselageand on the adjacent upper and lower section of thewing as can be seen in Fig. 17, (3) run-back icesimilar as for March 15 (Fig. 10), and (4) humpsup to 40 cm downstream the cord. The relativecontribution of the various ice accretion forms tothe performance degradation, however, could notbe determined. The joint occurrence of high liquidwater content in the cloud drop mode and of SLD

Aircraft icing research flights in embedded convection 263

makes this discrimination difficult. Our flightssuggest that SLD seem to contribute only with20% to the total liquid water content and may beconsidered as a by-product of and correlated withhigh liquid water content. The impact of both fac-tors on performance, however, is quite differentand has to be determined by other means. Mostof the accreted ice on the aircraft wings stemsfrom normal cloud drops, while SLD causedrun-back ice and impacted downstream the pro-tected regions of the wings. The relative contribu-tions of the cloud droplet and SLD modes to theobserved 30% degradation loss have to beexplored further.

4.6 Summary

In summary, the EURICE flights demonstratedthe existence of SLD under conditions ofembedded convection which so far where notconsidered as hazardous. The flights furthershowed the importance of mixed phase condi-tions. This has to be considered when thecertification procedures will be revised. Thecoexistence of SLD and high liquid water contentvalues in convective clouds has to be exploredfurther. Our flights indicate that SLD might bea by-product of liquid water content maxima.

Acknowledgements

The aircraft campaign and a great part of the data evaluationwere performed while both authors were affiliated with theInstitut f€uur Physik der Atmosph€aare at the German Aerospaceand Research Center (DLR), Oberpfaffenhofen. To performicing research flights with the DO-228 aircraft, which has notyet been used for this purpose, and to investigate unknownand potentially hazardous atmospheric conditions was a hugechallenge for pilots, technicians and researchers. The authorsare grateful to all colleagues who made this piece of researchpossible. The flights were part of the EC-funded projectEURICE. Funding is gratefully acknowledged. The fruitfulcooperation with many European partners on the improve-ment of safety in aviation was an encouraging experience.

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Authors’ addresses: T. Hauf, Institut f€uur Meteorologieund Klimatologie, Universit€aat Hannover, Hannover, Germany(E-mail: hauf@muk.uni-hannover.de); F. Schr€ooder, Institutf€uur Physik der Atmosph€aare, DLR Oberpfaffenhofen, Ober-pfaffenhofen, Germany

Verleger: Springer-Verlag GmbH, Sachsenplatz 4–6, 1201 Wien, Austria. – Herausgeber: Prof. Dr. Reinhold Steinacker, Institut fur Meteorologie und Geophysik,Universitat Wien, Althanstraße 14, 1090 Wien, Austria. – Redaktion: Althanstraße 14, 1090 Wien, Austria. – Satz und Umbruch: Thomson Press (India) Ltd., Chennai. –

Druck und Bindung: Grasl Druck&Neue Medien, 2540 Bad Voslau, Austria. – Verlagsort: Wien. – Herstellungsort: Bad Voslau. – Printed in Austria.

Aircraft icing research flights in embedded convection 265