Lagrangian air-mass tracking with smart balloons during ACE-2

Randy JohnsonNational Oceanic and Atmospheric Administration, Air Resources Laboratory, Field Research Division, Idaho

Falls, Idaho 83402

Steven Businger and Annette BaermanUniversity of Hawaii, Honolulu, Hawaii 96822

ABSTRACT

A third-generation Smart Balloon designed at National Oceanic and Atmospheric Administration,Air Resources Laboratory Field Research Division, in collaboration with the University of Hawaii(UH), was released from ship-board during the recent Second Aerosol CharacterizationExperiment (ACE-2) to provide Lagrangian air-mass tracking data. ACE-2 is the third in a seriesof field experiments designed to study the chemical, physical, and radiative properties andprocesses of atmospheric aerosols and their role in climate and is organized by the InternationalGlobal Atmospheric Chemistry (IGAC) program. The adjective smart in the title of this paperrefers to the fact that the buoyancy of the balloon automatically adjusts through the act of pumpingair into or releasing air from the ballast portion of the balloon when it travels vertically outside abarometric pressure range set prior to release. The smart balloon design provides GPS location,barometric pressure, temperature, relative humidity, and other data via a transponder to a C130research aircraft flying in the vicinity of the balloon. The addition of two-way communicationallows interactive control of the balloon operating parameters by an observer. A total of threecloudy Lagrangian experiments were conducted during the ACE-2 field program which lastedfrom 16 June through 26 July 1997. This paper reviews the design and capability of the smartballoons and their performance during the ACE-2 Lagrangian experiments. Future developmentand applications of the technology are discussed.

1. Introduction

A Lagrangian frame of reference, one thatmoves with the flow of the atmosphere, has beenused for many years to track pollution and monitorthe evolution of a parcel of air in the atmosphere(Seinfeld et al. 1973; Zack 1983, Businger et al.1996). Some problems involving reactive tracespecies in the atmosphere are so complex that there isno substitute for observing their rates in the actualatmosphere. In Lagrangian experiments a volume ofair is tagged and tracked over time, allowingscientists the opportunity to repeatedly makemeasurements in the same volume of air as it moveswith the wind. These experiments are difficult toimplement, but it is a powerful approach and the datacollected in this way lend themselves to subsequentnumerical modeling (e.g., Suhre et al. 1998).

The mean concentration budget for the ascalar variable in well mixed boundary layer can bewritten

dS

dt

we S h

h

w S

hKi

S

xiQs= + + +

( ) ( )

∆ © ©02

∂

∂. (1)

where the horizontal advection contribution to thetotal derivative is minimized during a Lagrangianexperiment (Lenschow et al. 1981). Assumptions that

are being made are that the mixed layer is uniformwith height and the wind speed and the eddy transfercoefficient K are constants. If these quantities arevariable the terms in equation 1 become a little morecomplicated, contributing to uncertainty in quantitiesderived from equation 1.

To visualize the whole process it is useful todraw a box with the ocean surface as the bottom ofthe box and the top of the boundary layer as top (Fig.1). Assuming that the balloon moves from left toright during the period, the left hand side of the boxrepresents the height of the boundary layer h at time tand the right hand side, h at t + δ t. The 1st term onthe right-hand-side of equation (1) is the verticalturbulent flux at the top of the boundary layer(evaluated at the top of the boundary layer h = zi). Itis the result of entrainment of air from above into theboundary layer and is equal to minus the entrainmentvelocity we (positive upwards) times the gradient in Sacross the top of the boundary layer. This flux can bepositive or negative depending on the sign of ∆S.

The second term, the vertical turbulent fluxat the surface, is a source or sink of S at the air-earthinterface and is generally measured from a trailingship and/or estimated from satellite data. Previousfield experiments have demonstrated that dispersion,the third term, is small. The forth term is the netinternal source or sink of S.

The dispersion and advection terms must beexplicitly included in the error propagation analysis.Inert tracers are commonly used to estimate theimpact of dispersion. The contribution due tohorizontal advection (1st term) is minimized underone of two conditions: i) there is no mean verticalwind shear within the boundary layer; ii) there ismean vertical shear, but turbulent vertical circulationskeep the column of air advecting at a mean rate forthe mixed layer. In this latter case, the mass-weighted average horizontal velocity should be usedto advect the column of boundary layer air. For adecoupled boundary layer or one with substantialcumulus convection, the vertical mixing time scalemay be rather slow, perhaps of order 1 day,increasing the importance of any vertical wind shear.In timing the start of the Lagrangian experiments,care must be given in choosing synoptic conditionsthat optimize the success of the Lagrangian strategyand provide a trajectory accessible to aircraftsampling over an extended period.

From 1991 to the present the NationalOceanic and Atmospheric Administration/AirResources Laboratory/Field Research Division

(NOAA/ARL/FRD), in collaboration with theUniversity of Hawaii (UH), has developed threegenerations of GPS balloon tracking systems forapplication in Lagrangian field experiments (Johnsonet al. 1998). Each of these systems incorporates aconstant-volume balloon in the design. Constant-volume balloons are alternatively referred to asconstant-altitude balloons or constant-densityballoons. For the appellation to be accurate thefollowing conditions must be met. (i) The balloonmust be filled to capacity by the time it reaches thedesired flying altitude. This means that the internalpressure of the balloon must be above ambient andsuch that balloon skin is tightened beyond any initialelasticity. (ii) The volume of the balloon is largeenough that once filled with helium it is capable oflifting the balloon and payload to the desired altitude.(iii) The strength of the external balloon shell iscapable of withstanding the force exerted by theinternal pressure when it reaches flying altitude. Thestrength of the shell must also be capable ofwithstanding the increased pressure due to daytimeheating.

Unlike a latex weather balloon that keepsexpanding as it is filled and rises in the atmosphere, aconstant volume balloon shell is made of a materialthat maintains a constant volume and shape. Inpractice, any material that can be used to constructthe shell has some elasticity and a thermal coefficientof expansion. Therefore, the balloon volume (anddensity) will change slightly with changes in

temperature and internal pressure. Filling theconstant volume balloon with helium causes theballoon to rise in the atmosphere until the mass of airdisplaced and the mass of the balloon payload reachequilibrium. As the temperature and barometricpressure of the surrounding atmosphere change withtime, the balloon will move up or down vertically tomaintain equilibrium. In addition, when the impactof precipitation, condensation, and small leaks in theballoon are considered, the value of lift controls toconstrain fluctuations in balloon altitude becomesclear.

The first generation NOAA/UH balloontracking system was deployed in the vicinity of theAzores in the Atlantic Ocean during the AtlanticStratocumulus Transition EXperiment and the MarineAerosol and Gas Exchange (ASTEX/MAGE) thattook place in June of 1992 (Businger et al. 1996).The ASTEX design included an inexpensivetetrahedral-shaped 1.6 cubic meter Mylar balloon(tetroon), chosen for ease of construction. TheASTEX tracking system was the first to apply GPStransponders that performed very well. However, theloss of tetroons due to loading from lightprecipitation during the first ASTEX Lagrangianexperiment reinforced the need for adding lift controlto the next generation constant-volume balloon.

The second generation NOAA/UH trackingsystem was a smart tetroon with an added ballast

bladder to provide adjustable lift. The adjectivesmart used here refers to the fact that the buoyancyprovided by the 4.25-cubic meter Mylar tetroonautomatically adjusts through the action of pumpingair into or releasing air from the ballast portion of thetetroon when it travels vertically outside a barometricpressure range set prior to release. The variable lifttetroon and transponder were designed early in 1995and deployed in unpolluted air south of Tasmania,Australia, during the first Aerosol CharacterizationExperiment (ACE-1) held in the fall of 1995 (Bates etal. 1998; Businger et al. 1998). The ACE-1 smarttetroon design provided GPS location, barometricpressure, temperature, relative humidity, and tetroonstatus data via a transponder to the NCAR C130research aircraft flying in the vicinity of the tetroons.

The ACE-1 Lagrangian experimentsinvestigated changes in the sulfur budget in a remotemarine boundary layer (MBL) over at least a day,providing data to estimate sources, sinks,entrainment, and conversion rates for sulfur gasesand aerosols (Clarke et al. 1996; Wang et al. 1998;Huebert et al. 1998). During two Lagrangianexperiments (LA and LB) data were retrieved via areceiver on board the NCAR C130 research aircraftfrom four smart tetroons. The first of these survivedmore than 33 hours and was tracked during twoflights of the instrumented C-130 aircraft. During LBa post-frontal air mass was tagged by three tetroons

over a 27-hour period and was sampled by asuccession of three flights. Results of ACE-1 smartballoon releases indicate that the balloons werecapable of adjusting lift to compensate for changes inconditions during the daytime when there was noprecipitation or condensation accumulation on thesurface of the balloon. However during the eveningwhen temperatures cooled enough to causecondensation on the balloon surface, the adjustmentrange of the smart tetroon was insufficient tocompensate for the resulting changes in tetroon-system lift.

2. The Third-Generation NOAA/UH SmartBalloon

Insights gained from the deployment of theACE-1 smart tetroons were used in the developmentof a third-generation smart balloon (Fig. 2). Theimproved smart balloon would need to have greaterdynamic lift range in order to overcome condensationor rain water loading. The increased ballast wouldrequire higher pressure inside the balloon andnecessitate stronger material to withstand the increasein force on the shell of the balloon. Either a thicker,stronger and therefore heavier Mylar film wouldneed to be used or another material would need to befound for the shell of the balloon. After researchingpossible high strength materials, a plain weaveSpectra fabric was selected as the best match forthe requirements of the new balloon shell. Thebenefits offered by Spectra fabric include (i) veryhigh ultimate tensile strength, (ii) very low elasticity(3% at ultimate tensile strength), (iii) light weight(0.97 gram/cc), and (iv) easy to handle. Spectra ,like any cloth fabric is easy to fold, cut and handlewithout damaging or puncturing. A disadvantage ofthe Spectra fabric was the requirement for separatebladders inside the shell to hold the helium and air.In previous balloon designs the Mylar outer balloonshell provided both strength and gas containment in asingle layer. However the advantage of the very highstrength, ease of handling, and greater durability ofthe Spectra outer fabric shell made up for therequired extra bladder. Standard 600-gm and 350-gmlatex weather balloons were used for the outer andinner bladders.

With significantly greater balloon shellstrength provided by the Spectra fabric, the ACE-2smart balloon is able to handle greater internalpressure and a much greater volume of air ballast.Air ballast was increased from ~160 grams for theACE-1 project to >1200 grams for ACE-2. Thisadded lift range proved to be essential formaintaining the balloon altitude during the ACE-2balloon flights.

New Transponder Hardware and Software Design

The larger balloon, with greater lift capacity,made it desirable to consider a totally newtransponder design. Although preliminary tests ofthe new balloon concepts were promising, only a fewmonths were available for hardware and softwaredevelopment, testing, and fabrication prior to thefield phase of ACE-2 in June of 1997.

Of the many design concerns that needed tobe addressed and resolved in just a few months,energy consumption for the tracking system wasparamount. Many of the new GPS receivers testedfor this project were able to acquire a new positionfrom a warm start in about 15 seconds most of thetime. Previous receivers had required 40 to 60seconds and required the same power supply current.Thus, the GPS receiver selected for this projectrequired only a fraction of the energy required in thepast.

With a larger balloon and a need to respondto conditions as quickly as possible, greater pumpingcapacity was needed. Two pumps were combined toprovide a pumping capacity of 20 to 25 liters perminute. Pumping at this rate provides a decrease inlift at a rate of ~ 20 to 25 grams per minute. Surfaceevaporation rate measurements made in the labindicated that evaporation would not take place anyfaster than 30 grams per minute. In a worst-casecondition the balloon could rise above the desiredcontrol elevation for a short period of time.

A larger valve than used during ACE-1 wasneeded to release ballast air at a sufficiently fast rateto compensate for surface condensation or lightprecipitation. Rather than using multiple valves toswitch the pumps over to pumping air out of theballast bladder, a single large pinch-off valve on alarge piece of latex rubber tubing was used. A smallgear motor was used to open and close the pinch-offvalve when increased lift was required. Thesuperpressure inside the balloon forces the ballast airout of the balloon. The ballast air pumps that wereused have one-way check valves that ensure there isno back flow of ballast air through the pumps. Therelease rate, even at low pressures, was several timesthe pumping rate. This allows the balloon tocompensate quickly for precipitation and/orcondensation that could accumulate on the balloonsurface.

To reduce development time and risk to theproject schedule, a popular off-the-shelf data loggerwas selected. Many of the software applicationsneeded to gather data and control the operation of theballoon subsystems were a part of the data loggersystem functions, reducing the software developmentfor the project from weeks or months to days. Thedata logger also had most of the hardware monitoringand control functions built in, which minimized theneed for hardware development. Additionalhardware included pump and valve on/off and

open/close circuitry, the valve pinch-off system, andthe switching power supply.

In previous designs, the balloon transponderwas programmed or configured one time prior torelease to send data back every 5 minutes, even ifthere was no receiver in the area. Two-waycommunications now allow changes in balloonoperating parameters to be made after launch andtransmission of data to occur only when a good linkwas established. The radio communicationshardware was selected in conjunction with the datalogger. A modem designed to interface with the datalogger and a standard four-watt transceiver was used.This provided the transponder with two-waycommunications and reduced development time.

3. Performance of the NOAA/UH Smart Balloonduring ACE-2

The Second Aerosol CharacterizationExperiment (ACE-2) was held from 16 June through26 July 1997 over the Atlantic Ocean betweenPortugal and the Canary Islands (Raes et al. 1999).The third-generation smart balloon was deployedduring three cloudy Lagrangian experiments duringACE-2 to investigate the processes that change the

chemical composition of continental aerosol in thecloudy marine boundary layer as it advects over theNorth Atlantic Ocean (Johnson et al. 1999).

The 68-m long Ukrainian research ship, theProfessor Vodyanitskiy, was used as a launchplatform for the smart balloons. When we arrived atthe ship to set up and test equipment, the ship crewhelped build a retractable shelter over the aft portionof the ship deck. A large reinforced plastic tarp wasshipped with some of our other equipment to the portin Lisbon, Portugal. The balloon inflation shelterkept some of the wind and ocean spray off theballoon during balloon preparation in conditions ofenhanced winds and heavy seas.

After three weeks of waiting for apollution outbreak from Europe to affect theCanary Islands, a decision was made to conducta Lagrangian experiment in a relatively cleanmaritime air mass. Lagrangian 1 took placefrom 3 to 5 July 1997. The R/V Vodyanitskiywas moved off the coast of Portugal to a positionof 40˚ 12’ N and 11˚ 10’ W. Just beforemidnight on the 3 July 1997 a single smartballoon and a plume of PFC trace gas werereleased. Technical difficulties prevented moreballoons from being released. However, thesmart balloon (balloon 2) survived long enough

to give more than adequate direction for theaircraft to follow the parcel of air being studied(Fig. 3).

On release the smart balloon was lofted toreside in the middle of the boundary layer,estimated to be around 500 m altitude. Early inthe experiment, the balloon observer aboard theC-130 aircraft, adjusted the operating window ofthe smart balloon from 500-700 m to 600-700 m(Fig. 4a). The balloon transponder respondedcorrectly by adjusting the ballast to force theballoon to stay within the narrower window.The operating window was then relaxed back toits original 500-700 m altitude range to conservebattery charge.

Initially the air parcel was advectedsoutheast toward the African coast, but just aftermidday on 4 July it turned more southwesterlyand was heading just to the east of Tenerife. Atthat time one the engines failed on the UKMeteorological Research Flight (MRF) C-130and the aircraft had to divert to make anemergency landing at Tenerife South airport.

Contact with balloon 2 was maintainedthrough 0300 on 5 July and a complete data setfrom the balloon is available through that time.Balloon wetness and solar radiation showdiurnal cycles that are out of phase (Fig 4b). Airtemperature is a minimum at night and rises tomaxima in the morning and afternoon as a resultof direct solar radiation striking the sensor atthose times. Midday air temperatures are coolerin the shade of the balloon, more closely

representing the ambient air temperature. Toobtain more representative measurements infuture, balloon designs will need to includeaspirated temperature and humidity sensors thatare shielded from direct solar radiation.

The C-130 aircraft flew three sorties ofapproximately 9 hours each and separated byabout 3 hours ground time for refuelling andservicing. The last flight was somewhatforeshortened due to the engine failure. By thensufficient data had been collected to documentthe evolution of the aerosol in the air parcel overa 30-hour period. The CIRPAS Pelican aircraftcarried out a fourth flight in the vicinity ofTenerife but was unable to locate the balloon,leaving some uncertainty whether they weresampling in the same air parcel (Fig. 3).

Lagrangian 1 was carried out in a relativelyclean maritime air mass and provides a very gooddata set for comparison with the other twoLagrangians that were in polluted, continental airmasses. Lagrangian 1 produced the largest changesin aerosol characteristics of the three experimentsundertaken. See Johnson et al. (1999) for anoverview of the evolution of the marine boundarylayer and the cloud and aerosol microphysics duringthe three cloudy ACE-2 Lagrangian experiments.

The second Lagrangian was undertaken16-18 July 1997 within a polluted continental airmass advecting from over northern France withsignificant stratocumulus cloud coverexperienced throughout. Light drizzle wasexperienced at times. Two balloons (balloons 4and 5) were successfully released on 16 July

1997 at 2200z and 2300z from the deck of theR/V Vodyanitskiy located at 40 10'N, 11 04'W(Fig. 5). Three flights of the C-130 were carriedout in pursuit of the balloons. Despite changesin solar radiation and balloon wetness, theballoons tracked within the prescribed altitudelimits within the marine boundary layer until~2100 UTC on 17 July (Fig. 6). Time series oftemperature and humidity allowed equivalentpotential temperature to be retrieved (Fig. 6b).At this time both balloons began to rise into thefree troposphere because of slow leaks of ballastair from their bladders. Subsequent testsrevealed the leaks to be due to stretching of therubber bladder against the seam in the Spectrashell, under conditions of prolongedsuperpressure.

The third Lagrangian took place on 23 and24 July 1997 in a polluted continental air massthat originated over central northwest Europe.Three smart balloons were released at 0615z,0730z and 0815z on 23 July 1997 from the deckof the RV Vodyanitskiy located near 40˚ N, 12030'W (Fig. 7). Of the three balloons, the first(balloon 7) failed ~ 12 hours after release,perhaps because of a transponder failure. Thesecond (balloon 8) operated successfully until1000z on 24 July 1997 (Fig. 7a). The third failedon release (balloon 9) apparently because of apuncture in the ballast bladder that caused theballoon to rise into the free troposphere soonafter release. The balloons were each set up todrift at the estimated midpoint of the marineboundary layer (~300 m). The tracks of balloons7 and 8 remained very close to the prescribed

altitude and to each other (Fig. 7b), whereas thetrack of balloon 9, which quickly rose to analtitude of ~ 625 mb, diverged from the othertwo tracks. Balloon 8 maintained altitude andprovided a full suite of data (Fig. 8). As withprevious smart-balloon time series, solarradiation and balloon wetness are out of phase(Fig. 8b). The cloud conditions duringLagrangian 3 included a mixture of brokencumulus and stratocumulus clouds. It appearsthat the balloon entered cloud and remainedthere for ~12 hours (Fig. 8b). The structure ofthe boundary layer and pollution layer in thisexperiment were highly complex, in completecontrast to the aerosol characteristics evolution,which was rather simple (Johnson et al. 1999).Surprisingly, dramatic changes to thethermodynamic structure of the polluted lower

atmosphere were accompanied by only modestchanges in the aerosol characteristics.

Comparison of Balloon Data with Aircraft Data andModel Output

Balloon position data were used tocalculate the propagation velocity of the balloon.The resulting balloon velocity data werecompared with C-130 flight-level wind data toassess how well the NOAA/UH smart balloonstracked with the air in which they wereembedded (Fig. 9). To remove the potential biasassociated with vertical wind shear, flight leveldata within ±5 mb of the same pressure level asmeasured by the balloon were used in thecomparison. The close correlation between theballoon calculated and aircraft measuredvelocities is shown in Fig. 9. The RMS

difference between the velocity observed byaircraft and calculated from balloon motion is0.75 m s-1 for L1, 1.75 m s-1 for L2, and 0.96 m s-1 for L3, respectively. The slightly larger RMSvalue for L2 may be attributed to a slightlygreater difference in vertical position betweenthe C-130 and the balloon than observed in thedata for L1 and L3. Nevertheless, thesedifferences are smaller than the errors associated

with the wind measurements themselves, andlend confidence to the assertion that the balloonstravel with the speed of the air in which they areimbedded.

The balloon potential temperature datawere compared with observations taken by theC-130 (Fig. 10). The results reveal a radiationbias in the balloon temperature measurements,with a positive bias (balloon warmer than C-130)

during the day and a negative bias at night. Thelargest bias occurs during the afternoons whenthe balloon no longer shades the instrumentpackage and solar radiation directly strikes theinstruments (e.g., Figs. 4 and 10a). This findingsuggests the need for aspirated sensors on futuregenerations of the smart balloon.

Trajectory simulations were constructedusing output from the National Center forEnvironmental Prediction - Global SpectralModel (NCEP GSM) (Kalnay et al. 1990) toevaluate the ability of the GSM to predict theobserved smart balloon tracks (Fig. 11). Resultsof the comparison of model and balloontrajectories are presented to evaluate the abilityof numerical weather prediction (NWP) modelsto predict the observed smart balloon tracks andassess the potential utility of model trajectorysimulations in planning future field experiments.Forward trajectories of the NCEP GlobalSpectral Model (GSM) are based on the aviationrun. The GSM has 28 sigma levels in thevertical and a horizontal resolution of ~111 km.The lowest pressure level is at ~995 mb.Trajectories were calculated using the HY-SPLIT (HYbrid Single Particle LagrangianIntegrated Trajectory) Model (Draxler 1992)applied to GSM hourly output over 16 sigmasurfaces at the midpoints of layers that vary inthickness from ~35mb at the surface to ~54mbthick at the model upper boundary. The threedimensional gridded GSM output values for uand v were input as initial velocities and verticalvelocity, w, was integrated using verticalintegration of horizontal velocity divergence.Using these variables, along with parcel latitude,longitude and height, z, above model terrain,trajectories were computed using a simple first-order approximation taking into accountcurvature in the wind field. Finally, applicationof a bilinear interpolation from the data griddefined meteorological variables at the parcelposition.

The model trajectories shown in thissection were initialized at a time near the releasetime for the balloons and are calculated for aconstant altitude of 500 m, corresponding to amodel level closest to that of the smart balloons.For flight-planning purposes, output from amodel run at least 12 hours previous to therelease time of marker balloons would beneeded. It is implicit that model resolution andquality and quantity of the data available for theinitial condition of the model runs are factorsthat need to be considered when makingtrajectory comparisons. ACE2 benefits from thefact that the Northern Hemisphere is relatively

data rich. The results shown here, therefore,may be considered a best-case GSM simulation.

A comparison of the tracks of the ACE-2smart balloons with GSM trajectories (Fig. 11)shows good agreement. For L1 the GSM windsmove the air parcel farther than is observed, withthe largest discrepancy (86 km) occurring duringthe evening of 4 July (04/07/1800 in Fig. 11a).At all times, the model trajectory error, whichranges from 24 to 86 km, is smaller than themodel resolution (~111 km). The bestperformance of the GSM is during L2, in whichthe model error ranges from 24 to 36 km andaverages 31 km (Fig. 11b). The model forecasttrajectory for L3 is also very good. In this casethe motion of the balloon is under forecast andthe error ranges from 43 to 60 km (Fig. 11c).The ability of the GSM to provide reasonablesimulations of the balloon trajectories benefitsfrom the good initial condition provided bysufficient upstream observational data in theNorthern Hemisphere. Relatively stableatmospheric conditions during the Lagrangianexperiments and the lack of divergence seen inthe trajectories of balloons 4 and 5 (Fig. 5) andballoons 7 and 8 (Fig. 7b), also improve theability of the GSM to correctly simulate thetrajectory. Comparisons of the balloontrajectories with trajectory forecasts frommesoscale NWP models with higher spatial andtemporal resolution will likely result in evensmaller forecast trajectory errors. Suchcomparisons are currently being undertaken andare beyond the scope of this paper.

4. Conclusions and Future DevelopmentA third-generation smart balloon was

designed at NOAA Air Resources Laboratory FieldResearch Division, in collaboration with theUniversity of Hawaii, for deployment during theACE-2 field program. Development of the improvedsmart balloon relied on insights gained from thedeployment of a first generation smart tetroon duringACE-1. All major components of the ACE-2Lagrangian tracking balloon were updated andimproved. Significant design improvements include(i) a stronger outer shell to significantly increasedynamic lift range, (ii) two way radio communicationwith the balloon to allow interactive control of theballoon operating parameters by an observer, and (iii)a spherical design to reduce exposure to precipitation.

Overall the smart balloons performed verywell during the ACE-2 Lagrangian experiments. TheGPS receiver acquired position ~ 3 times faster thanprevious versions. The two-way communicationsworked well and provided operating flexibility notavailable during previous field experiments. Theballoon altitude control worked flawlessly to

overcome environmental impacts on balloonbuoyancy due to solar radiation and precipitation onthe balloons.

The ACE-2 smart balloons, however, didencounter problems with the external air ballastbladder. The ballast bladder developed slow leaksafter prolonged exposure to superpressure. The lossof ballast air caused the balloon to rise in altitudeabove the marine boundary layer after a day or longer, where they were no longer representative of theposition of the air parcel of interest. The source ofthe problem has been identified and a solution hassubsequently been developed and successfully tested.

Comparisons of flight-level data from the C-130 and balloon data show good agreement betweenthe observations, and help confirm that the balloonsare tracking the air in which they are embedded. Asimilar air temperature comparison shows a radiationbias in the balloon sensor, suggesting the need foraspiration in future generations of the smart balloon.Comparisons of the balloon trajectories with forecasttrajectories from the HY-SPLIT model, using GSMforecast wind data, show errors that were smallerthan the resolution of the GSM output. Results usingwinds output from a mesoscale NWP model willlikely further reduce the position errors, suggestingthat future Lagrangian experiments could perhaps beundertaken using forecast trajectories in conjunctionwith released tracers.

Work is progressing on several fronts toimprove the performance of smart balloons.Transponder design changes are being consideredthat will allow the transponder to fit inside theballoon. This will simplify balloon launch, reduceoverall weight of the transponder, offer greaterprotection from moisture and direct solar radiation,and will physically protect the transponder fromdamage during and after launch. The feasibility ofusing small-footprint inflation tubes is beinginvestigated. The tubes can be scattered around thedeck of a ship to allow near simultaneous release ofmultiple tracking balloons. The inflation tubes willprovide protection from the wind, rain, and salt spraywhile the balloon is inflated and lift is set. Afterinflation, the inflation tube top is pulled back and itbecomes a launch tube.

In the past pontoons attached to a long linehave been used to help protect the balloon andtransponder from going into the ocean if the balloonbecomes overloaded with precipitation. Experiencein ACE-2 suggests that such ballast pontoons can beeliminated, saving on weight and simplifying launch,with minimal risk of having the balloon impact theocean.

The Iridium Satellite phone system becameoperational in the fall of 1998. Cost estimates makethis an attractive option and costs will no doubtdecrease with time as the system gains participation.

With satellite telephone communication, smartballoons can be tracked and controlled over the mostremote locations even when no aircraft are in thearea, significantly increasing the flexibility of theballoon platform’s application.

In summary the NOAA/UH smart balloondesign provides the means to track a parcel of air atthe desired altitude by varying the density of theballoons as environmental conditions demand. Thedynamic range afforded by the high strengthspherical shell and the lift control in the ACE-2design allows the balloons to remain within desiredaltitude operating limits despite the impacts ofcondensation, rainfall, and radiative cooling. A suiteof lightweight instruments borne by future smartballoons can provide useful in situ and radiometricmeteorological data. The in situ data combined withthe balloon's dynamic range enable the balloontransponder to be programmed to remain at aconstant altitude, to follow an isobaric or isentropicsurface, or to perform vertical soundings. With theavailability of satellite data telemetry it becomesfeasible to remotely control the balloon flightanywhere on the globe in the absence of proximateaircraft. These capabilities and the economical cost ofthe design make the smart balloon an attractiveplatform for a broad range of future applications inthe atmosphere.

ACKNOWLEDGMENTSWe are grateful to Shane Beard and John Heyman forassistance in the field. We are indebted to RogerCarter who provided programming support for thetransponder design. Karsten Suhre was helpful inproviding needed C-130 data. We appreciate theconstructive comments of Dr. J. A. Businger and twoanonymous reviewers that contributed to the finalmanuscript. Nancy Hulbirt contributed to the finalgraphics. This research is a contribution to theInternational Global Atmospheric Chemistry (IGAC)Core project of the International Geosphere-Biosphere Programme (IGBP) and is part of theIGAC Aerosol Characterization Experiments (ACE).Mylar is a registered trademark of Dupont.Spectra is a registered trademark of Allied Signal.This work is supported by the National ScienceFoundation under grants ATM94-19536 and ATM96-10009 and ONR grant N00014-92-J-1285. SOESTcontribution xxxx.

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