Turbulence measurements with a tethered balloon

This study presents the first deployment of a turbulence probe below a tethered balloon in field campaigns. This system allows to measure turbulent temperature fluxes, momentum fluxes as well as turbulent kinetic energy in the lower 5 part of the boundary layer. It is composed of a sonic thermoanemometer and inertial motion sensor. It has been validated during three campaigns with different convective boundarylayer conditions using turbulent measurements from atmospheric towers and aircraft. 10


Introduction
The atmospheric boundary layer (ABL) is the lowest part of the atmosphere and hosts turbulent processes responsible for 15 transfer of heat, moisture and momentum between the surface and the free troposphere.The time evolution of the parameters close to the surface is controlled by those turbulent processes.Also the coupling to the surface (either land or ocean) strongly depends on the boundary-layer processes.20 So, a precise understanding of those processes and in particular of the vertical profiles of turbulent fluxes is crucial to our ability to quantitatively describe and model the evolution of the lower part of the atmophere and the corresponding energy budget, which are necessery for numerical weather 25 and climate predictions.In recent years, dynamics and lower layers exchange of energy and trace species at the surface / atmosphere interface were studied during national and international programs (SHEBA (Utall and al. (2002)), IHOP (Weckwerth et al. (2004)), AMMA (Lebel et al. (2007)), COPS (Wulfmeyer et al. (2008))).
Understanding the turbulent processes in the ABL requires the knowledge of the evolution of the profile of the sensible heat flux.However, it remains difficult to measure.The observation of these processes raises specific problems because the phenomena involve fine temporal (a few tenths 10 of a second to a few minutes) and spatial scales (of the order of meters to tens of meters).If rapid sensors are available at the ground for most variables (temperature, humidity, wind), in altitude the high-frequency measurements are limited, and the turbulent instruments are mounted mainly on 15 research aircrafts.Previous studies (Lenschow and Stankov (1986), Saïd et al. (2010)) used instrumented aircraft to measure turbulent heat flux in altitude.This platform does not allow to obtain vertical profiles, but only provides some measurements at discrete vertical levels.Usually a linear interpo-20 lation of data is used to obtain a profile and estimate fluxes at surface and at the top of ABL.Another inconvenient of aircraft platform is the cost.Recently, studies with remotely piloted aircraft systems (RPAS) (Martin et al. (2014)) show the capability of these small and light platform to measure 25 turbulent heat fluxes in altitude.Fixed-point measurements on tall towers have provided significant insight into the heat fluxes characteristics well above the surface layer (Kaimal et al. (1976); Angevine et al. (1998)) but towers are limited in height with only a few towers worldwide reaching more than 100m.Towers with heights exceeding 50 m are practically non-portable, which makes them inappropriate for deployment in a field campaign.The logistical limitations of other platforms can partly be overcome by using tethered bal-2 from various locations.Past studies have used this platform since the 1970s.(Morris et al. (1975); Kaimal et al. (1976); Ogawa and Ohara (1982); Muschinski et al. (2001)) but this 10 platform has mainly be used to study mean thermodynamical measurements.Lapworth and Mason (1988) developed a system with a turbulence probe composed with a Gill propeller anemometer attached to the tethering cable of a balloon.The authors used inclinometers and magnetometers to determine 15 the probe sensor orientation.The system weighted around 10kg.
A detailed examination of the general applicability of an instrumented balloon for measuring ABL turbulent fluxes has not been undertaken previously.The objective of this study is 20 to demonstrate that an instrumented balloon can be used for measurements of the heat flux and turbulence structure of the ABL.The major advantage of tethered balloon is the potential to provide flux measurements at various vertical heights covering a part of the vertical extent of the boundary layer.

25
The turbulence tethered sonde presented here is designed to measure turbulence and sensible heat fluxes.The paper is structured as follows.First we describe the general architecture of the system, the sensor characteristics and the motion correction.Sections 3 and 4 are dedicated to the validation respectively close to the surface and within the boundary layer 5 using conventional data from towers and aircraft.In section 5, we explore the capability of the system to study the turbulence structure in the framework of the late afternoon transition.Conclusion ends the paper.
2 Overwiew of the system 10 This part describes the general architecture of the system, i.e. the balloon used and the turbulence sonde.The motivation is to develop a simple device that can be easily deployed in different field campaigns.The platform combines slow and fast sensors to quantify mean and turbulent processes.

Sensor characteristics
In this study we have used the Vaisala 7 m 3 tethered balloon inflated with helium.The model is Vaisala TTB327 (L 4.6 m x H 1.84 m x l 1.84 m 3.1 kg).The balloon is a zeppelin shaped aerostat and it is restrained by a cable attached to the ground (the weight of the cable is 0.5 10 −3 kgm −1 ) with an electric winch which is used to raise and lower the 5 balloon.The maximum height of flights that can be reached depends on atmospheric conditions (wind speed).We have never tested an altitude higher 1000 m.The turbulence tethered sonde (denoted TS in the following) can be attached to a wide variety of balloon; a dedicated balloon is not necessary.

10
The instrument package consists of both a slow measurement instrument, a 1Hz vaisala tethered sonde (TTS111 model) mounted below the tethered line as well as fast measurement instrument, called the TS and suspended 8 m below the bal- loon to avoid wind flow distortion due to the balloon.The TS 15 is attached to the cable with an horizontal pivot.The advantage is to limit yaw movements of the TS.The 1Hz vaisala commercial probe provides slow measurements of temperature, humidity, pressure, wind speed and direction, and is able to transmit 1Hz data to the ground using a radio link.This probe is mainly used to monitor the wind in real-time at flight altitude.We have a security constraint given by the balloon manufacturer, in case of wind greater than 12 ms −1 , when the flight should be interrupted.

5
The TS is based on a commercial sonic anemometer (Gill windmasterpro model, fig.1(a)) which provides measurements of three-dimensional wind and sonic-temperature at 10 Hz.The thermo-anemometer allows to connect others sensors to own analog inputs.An off-the-shelf coupled inertial-10 GPS motion and attitude sensor (Mti-G at 10 Hz from Xsens, fig.1(b)) was added in order to correct the anemometer movements.A fast-response thin wire allows the measurement of air temperature fluctuations.Also a standard pressure and temperature sensors provide slow reference mea-15 surements.Data was logged aboard on two SD cards.A home made data acquisition system (micro controleur PIC24F) read, date, and log thermo-anemometer and inertial navigation system (INS) incoming numerical RS232 signals.The total mass of the system is 2 kg including batteries (0.3 kg).The sonic anemometer represents the half of the mass (1 kg) whereas the GPS-INS weighted only 0.15 kg.A first performance lies in the low weight of the system.Lapworth and Mason (1988) described a balloon borne turbulence probe system with a weight of 10 kg.The decrease in weight was possible by the miniaturization of sensors in recent years.The system can run for 4h powered by eigth 1.2V 2700 mA.h NiMH batteries.

2.2 Motion correction
The off-the-shelf coupled inertial-GPS motion and attitude sensor is essentially composed of two parts: (1) an inertial navigation system to measure the balloon's position, speed, and attitude relative to the Earth, and (2) a data acquisition 15 system to record all the incoming signals.
A miniature GPS-INS is attached to the platform 40 cm above the sonic anemometer to provide the position, speed, and orientation of the sonic anemometer.
Linear and rotational speeds provided by the INS are used 20 to calculate the speed of the platform in the coordinate system of the sonic anemometer.This means that the wind vector in the platform coordinate system is a simple vector difference between the sonic and GPS-INS velocities: where V platf orm is the wind vector in the platform coordi- is the GPS-INS motion vector, and V sonic is the platform-relative flow vector measured by the sonic anemometer.
The INS measured angles of attitude (rolls, pitch and yaw angles) allow us rotate the wind vector measured in the plat-10 form coordinate system to the meteorological coordinate system.Geo-referenced u, v, w wind components are then calculated from the well adopted equations of Lenschow (1986).

Validation close to the surface
In order to check the validity of the high-frequency mea-15 surements obtained by the TS, the measurements are compared with those of a three-dimensional sonic anemometer fixed on masts and installed during three experimental campaigns between 2010 and 2013.Ideally, for direct comparison with fixed point on tower, flying at constant altitude close 20 to the tower is desirable.The horizontal distance between TS and the position of the towers was lower than 200 m.The two first campaigns took place in summer 2010 and 2011 in the BLLAST (Lothon et al. (2014)) experimental site with a tower equipped with three-dimensional sonic anemometers (CSAT, Campbell Scientific Inc, Logan, UT, USA) at 60 m and the third took place at Bourges (France) in a french military site which was equipped with a tower with threedimensional sonic anemometers (GILL HS 3-axis, Gill In-5 struments Limited, Lymington, Hampshire, UK) at 30 m.For all the days considered here, the atmospheric conditions were convective and clear sky.Only the campaign in August 2010 in the BLLAST site was entirely dedicated to the validation of the TS.No scientific constraints were therefore imposed.10 Indeed, during two days, the TS flew at fixed height corresponding to the instrumented level of the mast.For the other two campaigns, the TS did not remain the whole day at the same height.So we only selected measurement periods when the TS was at a similar level as the fixed sonic anemometer.
15 Globally, the time series recorded during these different campaigns, after motion correction applied, exhibit excellent agreement even with the aforementioned spatial differences between tower and TS.We hereafter denote u ,v ,w and θ the fluctuations in longitudinal wind, transverse wind, ver-20 tical wind and potential temperature respectively.Fluctuations x of a variable x are computed as: x = x − x where x is the mean over a chosen period.An example of the high-frequency measurements of fluctuations of the threedimensional winds, and potential temperature is shown in 25 figure 2(a) for a thirty-minute sample on 31 august 2010.The two records do not overlap perfectly but this is expected with fast measurements made 200 meters apart.However, the range of the fluctuations of u,v,w and θ are similar between the TS and the data from the fixed sonic anemometer.The distribution of the fluctuations recorded during 2-hour period at midday are also presented in figure 2(b).Between both instruments a very similar distribution of all the fluctuations is obtained with same shape and amplitude for all the parameters considered here.Figure 2(c) presents a comparison of smoothed power spectra between both systems for 2 hours measurements at midday for wind components and potential temperature.The comparison between the TS spectra and the 5 tower spectra is generally quite good and both spectra show the expected -5/3 slope at higher frequencies.
For those fluctuation measurements at 10 Hz, several 2nd order moments can be determined.The following subsection presents the validation of variances of the three components 10 of the wind, of the temperature and of the turbulent sensible heat flux.For all the data, the eddy correlation method is used.

Variance
The variance is commonly used for studying some thermody-15 namical parameters in the boundary layer because it allows to characterize the dispersion around mean values and can be linked to the intensity of the turbulence.Figures 3(a between the fixed sonic anemometer on the mast and the TS.The dashed line represents the difference in altitude between both instruments.Note that the position of the tethered balloon varies from a few meters to tens of meters because of turbulent motions of the atmosphere.That is why the varia-25 tion of altitude around 60 meters is greater in the middle of the day, when the convection is the strongest.During the afternoon, when the difference in altitude is often greater than 10 meters, the values in σ 2 w is higher for the TS while the values in σ 2 t is lower for the TS.This is consistent with the behaviour of a convective ABL in which the fluctuations of temperature are larger near the surface while fluctuations of vertical wind are more important in the middle of ABL.Re-  Table 1.The correlation coefficient between TS and sonic anemometer on the mast for the variances of the 3 components of the wind, the potentiel temperature, the sensible heat and the momentum kinematic fluxes.r 2 2010 is for the dedicated campaign in 2010 and r 2 BLLAST and r 2 BOU RGES correspond to BLLAST and Bourges fields campaigns respectively.

Flux
Eddy covariance (Kaimal and Businger (1963), Stull (1988)) is a well established method for the direct measurement of the vertical exchange of gases and/or particles in the atmosphere, suitable for use in a variety of environments.The turbulent flux (F x ) is given by the covariance between fluctu-5 ations of vertical wind velocity (w ) and those of the tracer of interest (noted x below and represented in this study by potential temperature t, or horizontal wind component u and v) for the averaging period (T m ) such that:  2012)).To ensure that the averaging period is long enough we calculate the ogive (not shown here) using the cumulative integral of the co-5 spectrum of the turbulent flux starting at the highest frequencies.From these plot, a period greater than 16 minutes is determined as sufficient to calculate the turbulent fluxes.Therefore, in the following, we chose 20 minutes for computing the fluxes with both tower and TS data.ments.The agreement is satisfactory even if F θ seems systematically larger for the tower data during the convective period.This is consistent with TS always positioned above the 10 tower (between ten and twenty meters) and a quasi-linearly vertically decrease of the sensible heat flux in these atmospherics conditions.For that day, correlation coefficients between both datasets is 0.92, 0.81 and 0.8 (Table 1) for F θ , F u and F v respectively.More differences are found for F u and F v than for F θ but one explanation could be that the In this section, we use data from aircraft and remotely piloted aircraft systems to look at the behaviour of the TS in altitude while the previous section concentrates on the validation of the turbulent data from TS close the surface with fixed sonic anemometers.Among the three campaigns of this study, only the BLLAST field campaign offers complementary data to validate the data of the TS above 60 m.The BLLAST field campaign has been described in details in Lothon et al. (2014).The aim was to understand the turbulent processes during the transition at the end of the afternoon, when the boundary layer turns from convec-

The turbulent kinetic energy
The turbulent kinetic energy, noted TKE, is one of the most 5 important variables used to study turbulent boundary layers since it quantifies the intensity of turbulence which controls vertical mixing (Lenschow andStankov (1974) André et al. (1978) Lenschow and Stephens (1980).It is defined as: and is one of the common parameters measured by TS, aircraft and M2AV.Depending on the different platforms the integrated times to compute TKE could vary.For TS we choose to take 20 minutes as determined by the ogive method (see section 3).For aircraft estimation the calculation is 15 made with data recorded along stacked legs of around 40 km (around 6 minutes).For M2AV we have an estimation for each straight leg of 1 km length (corresponding to 3 minutes).
To be consistent with the TS data, an averaging of 20 minutes is applied to the M2AV data.Figure 5 presents the compar-20 ison between the three platforms.We selected only the data when the difference in altitude between TS and aircraft or M2AV was smaller than 250 m.In convective conditions, the TKE parameter present quasi constant values in the middle of the ABL and it is the reason why we can compare the 25 TKE observed by the three platforms even if altitudes are not exactly identical.The data set consists of ten different IOPs.
The dataset presents a large range of values of TKE between 0 and 1.5 m 2 s −2 .Most of the time the altitude of the aircraft is above the TS and it is the contrary when we consider 30 the data from M2AV.Here again, the correlation coefficient is close to unity (r = 0.88) indicating a good agreement between the three platforms and confirming that the estimation of TKE in altitude by TS is reliable.

Heat flux 5
Unlike the TKE, the sensible heat flux observed in a convective boundary layer presents a linear decrease with height and becomes negative close to the boundary-layer top.This feature makes it difficult to compare data from TS and air-  and TS).However, after 1600UTC, when the decay starts, we can see that the value at 8 m remains larger than the TKE observed above until the end of the late afternoon transition.This result is consistent with the results obtained by

Anisotropy of the turbulence
One of the issues the BLLAST project focuses on is the ver-15 tical structure of the turbulence properties in the boundary layer.In particular, the anisotropy of turbulence is of interest during the afternoon transition (Darbieu et al. (2015), Couvreux et al. (2015)).
Here, to estimate anisotropy, we define the ratio It is based on the fact that when the turbulence is isotrope (u 2 = v 2 = w 2 ), T KE = 3/2w 2 and A = 1.The TKE can thus be estimated only from the variance of w .When A = 3, the horizontal contribution to the TKE is zero (u 2 = v 2 = 0)  and the turbulence is controlled by vertical motions.When A=0, the vertical contribution to the TKE is zero (w 2 = 0) and the turbulence is only created by horizontal wind fluctuations.
Figure 8 presents the time evolution of the ratio estimated 30 by equation 4 and calculated by the sonic anemometer at 8 m (Nilsson et al. (2015)) and by the TS higher in altitude (see table 2).For all the IOPs, values are larger at higher altitude Values of A show the anisotropy of turbulence in the middle of the ABL in these convective conditions.This is an important issue when for instance one wants to access to the 460 TKE while only w 2 is measured as for example with a vertically pointing doppler lidar (Gibert et al. ( 2011)).
This section demonstrates the interest of the observations made by TS which allows continuous exploration of the middle of the boundary layer during the transition phase.Syn-465 ergy with the other traditional tools (aircraft and tower) allow to study the turbulent processes between the surface and the top of the boundary layer as shown by Figure 6.

Conclusions
In this paper, a new system to estimate turbulent transfer in 470 the boundary layer as well as the associated first masurements have been presented.It consists in a turbulence probe mounted on a tethered balloon.Those measurements have been evaluated by comparison to turbulent measurements derived from tower, aircraft and remotely piloted aircraft sys-475 tem and show very good consistency with those more traditional turbulence measurements.This new system presents several advantages: the turbulence is estimated in the lower part of the PBL at altitudes where the research aircrafts encounter some 480 difficulties to fly.
with this TS system, measurements in the boundary layer can be made frequently and inexpensively.
The only limitation for the deployment of this platform is that moderate wind (<12 ms −1 ) conditions are required.We 485 demonstrated here that the turbulence sonde is capable of measuring heat and momentum fluxes using the direct eddycovariance method.For the first time, this new instrumental platform was used to measure heat flux and TKE.It was shown that it is possible to characterize different kinds of ver-490 tical motions occurring at the middle of PBL.After this first validation we are considering to explore the possibility to estimate continuous vertical profiles of the dissipation rate of TKE by maintaining a slow descending rate during the profile and using a moving average over a given time period.Also 495 we would like to load off the system to add a fast humidity sensor such as a krypton KH2O (Campbell Scientific Ltd) to measure turbulent latent heat flux simultaneously with the turbulent sensible heat flux.Another advantage is to deploy the system simultaneously with other instruments (particles 500 counter, O3-CO2 probes, droplets, ...) to better understand the link between microphysics and atmospheric turbulence like for example in fog.The TS can also be used to validate remote sensing turbulence measurement (lidar, radar, sodar). Acknowledgements. 15

Figure 1 .
Figure 1.Image of the turbulence tethered sonde: (a) The sonic anemometer and the electronic system; (b) the inertial motion sensor.
) and (b) present the comparison of the variance of vertical velocity and temperature calculated every 20 minutes during 10 hours 20

5
garding the variances of the horizontal components of the wind (not shown here) no trend is observed between the two Atmos.Meas.Tech.Discuss., doi:10.5194/amt-2015-386,2016   Manuscript under review for journal Atmos.Meas.Tech.Published: 18 January 2016 c Author(s) 2016.CC-BY 3.0 License.

Figure 2 .
Figure 2. Comparison of w , v , u and θ measured by a tethered-balloon probe (black) and a sonic anemometer (gray) fixed on a tower nearby for: (left) 10 Hz time series during 30 minute; (middle) fluctuation distribution of a 2-hour sample; (right) power spectra density corresponding to the same sample.

2) 10 F
u and F v thus denote the momentum fluxes, F θ the buoyancy flux.A measurement frequency of 10Hz and T m = 30 minutes are generally considered acceptable for tower based instruments to capture the frequency bandwidth of eddy sizes contributing to the flux(Aubinet et al. ( Figure 3(c) shows the 5 comparison between the TS and the fixed sonic anemometer for the sensible heat flux during 10 hours of measure-Atmos.Meas.Tech.Discuss., doi:10.5194/amt-2015-386,2016 Manuscript under review for journal Atmos.Meas.Tech.Published: 18 January 2016 c Author(s) 2016.CC-BY 3.0 License.

Figure 3 .
Figure 3. Temporal evolution of turbulent moments measured by the tethered ballon (black) and the tower (gray) on 31 August 2010: (a) vertical velocity variance, (b) temperature variance and (c) buoyancy flux.The dashed line represents the variation of the altitude of the tethered ballon) around 60 meters.

5
flow is distorted by the tower(Miller et al. (1999)) and may induce modifications of the fluctuations of zonal and meridional wind.To summarize, Figure4presents the comparison of the turbulent sensible heat fluxes between tower and TS observations for the entire set of available data (63 segments 10 of 20 minutes), including selected periods of BLLAST and BOURGES campaigns for which the tethered balloon flight was located at a similar altitude of a sonic anemometer on the tower which for more than 20 minutes.The maximum of altitude difference is 30 meters.The range of data is between 15 0 and 0.2 Kms −1 .The coefficient correlation between TS and fixed sonic anemometer for the sensible heat flux parameter is 0.85, indicating a good agreement for different places, moments of the days.4Validation within the PBL 20

Figure 4 .
Figure 4. Correlation plot between sensible heat flux obtained by TS and data from towers during three different campaign at two different place in summer 2010, 2011 and 2013.The color corresponds to the altitude difference between TS and sonic anemometer on tower.

Figure 5 .Figure 7
Figure 5. Turbulent kinetic energy measured from tethered ballon (y-axis) vs. the one measured by aircraft or M2AV (x-axis) for 10 IOPs during the BLLAST campaign in june/july 2011.Small symbols are used for aircraft and bigger ones are used for M2AV (for 2 july).The color corresponds to the altitude difference between TS and aircraft or M2AV.

Figure 6 .
Figure 6.Sensible heat flux profiles obtained with (triangle) aircraft, (circle) TS and (square) sonic anemometer on the 60 m tower on 02 july 2011 during the BLLAST field campaign.The color corresponds to the time of the day.

10
Darbieu et al. (2015).The authors have shown the existence of 'pre-residual layer' in altitude characterized by a decay of the TKE which is initiated first in altitude.

Figure 7 .
Figure 7. Time evolution of the turbulent kinetic energy measured from ( ) the turbulence tethered sonde (•), the 8 m tower and the (≺) aircraft during 10 IOPs.The color is function of z * .
Atmos.Meas.Tech.Discuss., doi:10.5194/amt-2015-386,2016   Manuscript under review for journal Atmos.Meas.Tech.Published: 18 January 2016 c Author(s) 2016.CC-BY 3.0 License.thanclose the surface.The ratio is larger than 1 which means that the contribution of the horizontal motion is small.At low altitude (8 m on the figure) but also at 30, 45 and 60 m (not shown here) the contribution between horizontal and vertical motion are more equivalent.At the end of the day, the values are similar between measurements in altitude and close the surface.The evolution of the anisotropy ratio obtained with 450 TS is in agreement with the results fromDarbieu et al. (2015) obtained with LES models for one IOP of the BLLAST campaign.The authors show also that the contribution of the vertical velocity variance contribute to the TKE is larger in the middle than in the upper and lower parts of the PBL, due to 455 small vertical velocity variance close to the surface and in the entrainment zone and larger shear at the interfaces.

Table 2 .
Characteristics of the different TS flights; z * is the ratio between the altitude and the top of the boundary layer.
The authors thank F. Said for providing the 505 tower measurements.The BLLAST field experiment was made possible thanks to the contribution of several institutions and supports: INSU-CNRS (Institut National des Sciences de l'Univers, Centre National de la Recherche Scientifique, LEFE-IMAGO program), Météo-France, Observatoire Midi-Pyrénées (University of 510 Toulouse), EUFAR (European Facility for Airborne Research), BLLATE-1& 2, COST ES0802 (European Cooperation in the field of Scientific and Technical).The field experiment would not have occurred without the contribution of all participating European and American research groups, which all have contributed in a signif-515 icant amount.The Piper Aztec research airplane is operated by SAFIRE, which a unit supported by INSU-CNRS, Météo-France and the French Spatial Agency (CNES).BLLAST field experiment was hosted by the instrumented site of Centre de Recherches Atmosphériques, Lannemezan, France (Observatoire Midi-Pyrénées, 520 Laboratoire d'Aérologie).