AMTAtmospheric Measurement TechniquesAMTAtmos. Meas. Tech.1867-8548Copernicus PublicationsGöttingen, Germany10.5194/amt-9-4375-2016Turbulence fluxes and variances measured with a sonic anemometer mounted on a tethered balloonCanutGuylaineguylaine.canut@meteo.frCouvreuxFleurLothonMarieLegainDominiquePiguetBrunohttps://orcid.org/0000-0002-8070-7924LampertAstridMaurelWilliamMoulinEricCNRM-GAME (Météo-France and CNRS), Toulouse, FranceLaboratoire d'Aerologie, University of Toulouse, CNRS, Toulouse, FranceInstitute of Flight Guidance, TU Braunschweig,
Hermann-Blenk-Str. 27, 38108 Braunschweig, GermanyGuylaine Canut (guylaine.canut@meteo.fr)6September201699437543868December201518January201616August201617August2016This work is licensed under a Creative Commons Attribution 3.0 Unported License. To view a copy of this license, visit http://creativecommons.org/licenses/by/3.0/This article is available from https://amt.copernicus.org/articles/9/4375/2016/amt-9-4375-2016.htmlThe full text article is available as a PDF file from https://amt.copernicus.org/articles/9/4375/2016/amt-9-4375-2016.pdf
This study presents the first deployment in field campaigns of a
balloon-borne turbulence probe, developed with a sonic anemometer and an
inertial motion sensor suspended below a tethered balloon. This system
measures temperature and horizontal and vertical wind at high frequency and
allows the estimation of heat and momentum fluxes as well as turbulent
kinetic energy in the lower part of the boundary layer. The system was
validated during three field experiments with different convective
boundary-layer conditions, based on turbulent measurements from instrumented
towers and aircraft.
Introduction
The atmospheric boundary layer (ABL) is the lowest part of the atmosphere and
hosts turbulent processes responsible for the transfer of heat, moisture and
momentum between the surface and the free troposphere. This turbulence is
produced by dynamical instabilities due to wind shear or with convective
plumes generated by solar heating of the surface. Turbulence in the ABL and
its impact on mean thermodynamical variables such as temperature, water
vapour mixing ratio and wind can be estimated via the turbulent fluxes. In
this paper we focus on sensible heat fluxes and momentum fluxes as well as
variances and turbulent kinetic energy.
The observation of these turbulence processes raises specific problems
because the phenomena involve fine temporal (from a few tenths of a second to
a few minutes) and spatial scales (in the order of metres or tens of metres).
Rapid sensors may be available on instrumented towers for most variables
(temperature, humidity, wind), but at altitude high-frequency measurements
are limited, and the instruments to measure turbulence are mounted mainly on
research aircraft.
A large number of field campaigns (SHEBA: ; IHOP:
; AMMA: ; COPS: ) carried out in
recent years have provided observations that sample vertically the turbulent
fluxes in the boundary layer, such as heat and momentum fluxes. The two main platforms
used are the instrumented mast and research aircraft. These two very
different tools sample temporal and spatial variability concerning the flow.
The two points of view are complementary to the study of turbulent boundary-layer processes. Recently, studies with remotely piloted aircraft systems
(RPASs) () have shown the capability of this small and light
platform to measure turbulent heat fluxes at altitude.
Fixed-point tower measurements have largely been used to provide useful
characterisation of heat fluxes above the surface layer (e.g. ;
). This platform permits a temporal sampling of the turbulence.
However, these measurements are limited in height, with only a few towers
reaching more than 100 m. Towers with heights exceeding 50 m are practically
non-portable, which makes them inappropriate for deployment in a field
campaign. At altitude, previous studies (; ) used
instrumented aircraft to measure turbulent heat flux. Aircraft are costly and
have constraints: they have a minimal flight altitude. Their relative air
speed also imposes the use of fast-response instruments in order to measure
variations at physical scales equivalent to the ones measured from a fixed
location. However, there is no other platform which allows for in situ
sampling of the turbulence in such a way from aircraft, from 100 m a.g.l. to a height of several
kilometres, and covering large areas of
several tens or hundreds of kilometres. The use of RPASs adds a very interesting and
complementary approach, with, for example, lower costs, easier
operations, and low-altitude exploration. The range of the scaled probe is
however usually much lower than with an aircraft.
The tethered balloon is an intermediate platform between the fixed tower and
the aircraft. It allows the sampling of the atmosphere up to 800 m from
various locations. From the point of view of measurement it can be considered a tower which moves vertically, therefore providing a temporal view
of the atmospheric processes. However, to quantify turbulence with the
tethered balloon it is necessary to correct its motion with similar methods
to those used for aircraft or RPASs. This platform has been used in studies since the 1970s (; ; ; ), but it has
mainly been used to study mean thermodynamical measurements.
developed a system with a turbulence probe composed of a Gill propeller
anemometer attached to the tethering cable of a balloon. The authors used
inclinometers and magnetometers at a frequency of about 1 Hz in order to
determine the probe sensor orientation. The system weighed roughly 10 kg.
Recently, developed a tethered lifting system that could be mounted above a
kite or a balloon. The turbulence package embarked on this system record
high-frequency and low-frequency fluctuations of temperature and velocity
with the fast-response cold-wire temperature and hot-wire velocity sensors
respectively.
Today, 3-D, fast-response ultrasonic anemometers instruments are
commonly available, and technological advances have resulted in the availability of
compact and reasonably low priced attitude- and motion-measuring units from
commercial manufacturers. It has become second nature to combine both systems
in order to obtain wind measurements in new situations.
explore, for example, the performance of such devices mounted on a car for mean wind
and turbulence measurements. A team from the US Environmental Protection
Agency's Office of Research and Development used similar sensors mounted on a
tethered balloon in order to estimate mean wind speed and direction at
altitude, and the goal of present work is to prove that these devices are
fully able to measure turbulence in the ABL.
A detailed examination of the general applicability of an instrumented
balloon for measuring ABL turbulent fluxes has not been previously
undertaken. The objective of this study is to demonstrate that an
instrumented balloon can be used for measurements of heat flux, momentum flux
and the turbulent kinetic energy within the ABL. The major advantage of
tethered balloons is their potential to provide flux measurements at various
vertical heights covering a part of the vertical extent of the boundary
layer. In field campaigns, it is therefore a complementary tool for aircraft
and towers and, in the future, RPASs for the measurement of temporal and spatial
scales.
The turbulence tethered sonde presented here is designed to measure variance
of wind, heat and momentum fluxes. The paper is structured as follows.
Firstly, we will describe the general architecture of the system, the sensor
characteristics and the motion correction. Sections 3 and 4 are dedicated to
the validation close to the surface and within the boundary layer using
conventional data
from towers and aircraft. In Sect. 5, we will explore the capability of the
system to study the turbulence structure in the context of the transition of
late afternoon. Conclusions are provided in Sect. 6.
Overview of the system
The turbulence probe, denoted TS from now on, consists of a sonic anemometer
and an inertial motion sensor attached to the cable of a tethered balloon.
This part of the paper describes the general architecture of the system, i.e.
the balloon used and the turbulence probe. The aim is to develop a simple
device that can be easily deployed in different field campaigns. The platform
combines fast and slow sensors to quantify mean and turbulent processes.
Sensor characteristics
In this study we used the Vaisala 7 m3 tethered balloon (the model used
was the Vaisala TTB327 (L 4.6 m × H 1.84 m × W 1.84 m; 3.1 kg) inflated with
helium. The balloon is a zeppelin-shaped aerostat restrained by a cable
attached to the ground. The cable has an electric winch used to raise and
lower the balloon. The mass of the cable is 0.5 ×10-3 kg m-1 and
the maximum height of flights that can be reached depends on atmospheric
conditions (wind speed). We have never tested this system at an altitude higher than
1000 m.
The turbulence tethered sonde can be attached to a wide variety of balloons;
a specific balloon is not necessary for the purpose. The instrument package
consists of a slow measurement instrument, a 1 Hz Vaisala tethered
sonde (TTS111 model) mounted below the tethered line (at approximatively 6 m
below the balloon) and a fast measurement instrument, which is
suspended 8 m below the balloon in order to avoid wind flow distortion due
to the balloon. The TS is attached to the cable with a horizontal pivot. The
advantage is that yaw movements of the TS are limited. The Vaisala commercial
probe provides slow measurements of temperature, humidity, pressure, wind
speed and direction, and it is able to transmit 1 Hz data to the ground using
a radio link. This probe is mainly used to monitor the wind at flight
altitude in real time, so as to respect safety requirements. In our case, the
maximum permitted wind speed is 12 m s-1, following the balloon
manufacturer's specifications.
The TS is based on a commercial sonic anemometer (Gill WindMaster Pro, Fig. a)
which provides measurements of three-dimensional wind and sonic temperature at 10 Hz. showed
that the sonic temperature can be used as a good proxy of the virtual temperature. The thermo-anemometer
allows the connection of other sensors to their own analogue inputs. A fast-response thin wire allows
the measurement of air temperature fluctuations and standard pressure and temperature sensors
provide slow reference measurements. An off-the-shelf coupled inertial-GPS motion and attitude
sensor (MTi-G at 10 Hz from Xsens, Fig. b) was added in order to correct
anemometer movements. The accuracy of the parameters provided in the MTi-G is given in
Table . The data acquisition system was built in our laboratory. It is
based on a PIC24F microcontroller, and it records data emerging from both sensors via
two RS232 connections onto two SD cards. The total mass of the system is 2 kg including
batteries (0.3 kg). The sonic anemometer represents half of the mass (1 kg), whereas the GPS-INS
weights only 0.15 kg.
The mean advantage of the instrumentation proposed here is the use of a sonic
anemometer instead of a propeller anemometer, which has a greater sampling
frequency and a lower weight (2 kg instead of the 10 kg for the Lapworth
platform; ). The decrease in weight was made possible by the
miniaturisation of sensors in recent years. The system can run for 4 h powered
by eight 1.2 V, 2700 mAh NiMH batteries.
Accuracy of the MTi-G instrument as given by the manufacturers.
ParameterGPS-INSPitch, roll0.8∘Heading0.5∘u,v0.1 m s-1w0.1 m s-1Position2.5 m
Image of the turbulence tethered sonde: (a) the sonic anemometer and
the electronic system and (b) the inertial motion sensor.
Motion correction
The off-the-shelf coupled inertial-GPS motion and attitude sensor is used to
measure the balloon's position and speed, as well as the orientation of the sonic
anemometer. It is fixed to the platform 40 cm above the sonic anemometer.
Linear and rotational speeds provided by the INS are used to compute 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 the simple
vector difference between the sonic and GPS-INS velocities:
Vplatform=Vsonic-VINS,
where Vplatform is the wind vector in the platform coordinate system,
VINS is the GPS-INS motion vector, and Vsonic is the
platform-relative flow vector measured by the sonic anemometer.
The INS measures angles of attitude (rolls, pitch and yaw angles), allowing us
to rotate the wind vector measured in the platform coordinate system in the
meteorological coordinate system. Geo-referenced u, v and w wind
components are then calculated from frequently adopted equations of
.
A series of tests were conducted in order to assess the capability of this
system to remove the motion of the sonic anemometer and to accurately compute
wind fluctuations at a frequency suitable for turbulence studies. In June 2010 the system was suspended below a gantry and left oscillating starting at
30∘ from the vertical. We verified that the oscillation was invisible
in the computed wind. Due to different processing times in both instruments,
the recorded data cannot be considered to be perfectly synchronous. We
empirically determined the delay as the value which minimises the variance of
the computed wind during the large oscillations described above. Data
recorded during real flights showed that the oscillation amplitude is smaller
than that which was measured on the ground: inclination very rarely exceeded
10∘ (whatever the axis). When the probe is suspended below the
balloon,
however, it is not strictly constrained on the vertical axis: we observed a
great variation in the heading (the angle of the horizontal plane between the
north and the first axis of the attitude and motion sensing system and the
north), with rotation rates reaching (and sometimes even exceeding) half a
revolution per second. An example of this behaviour is shown in Fig. , which demonstrates that the system is able to correct
this twisting motion: the heading changes rapidly, the rotation speed
magnitude reaches almost 1 rps (-6 rad s-1 at 15:33:33), the measured wind
component flips signs accordingly (panel 3, at 15:33:28; the second
component varies from 4 to -4 m s-1), and the fluctuation of the computed wind
remains limited, between 0.5 to 1 m s-1 (bottom panel).
Time series of sensor orientation and wind during 20 s of a
flight in the BLLAST campaign (5 July 2011). Panel's content, from top to bottom,
is
as follows: (top) heading – angle between the north and the first horizontal
axis of the MTi-G; (upper middle) rotation rate along the z axis of the MTi-G; (lower middle) two
components of the wind measured by the WindMaster anemometer along its x and
y axis; and (bottom) northward and eastward components of the computed wind.
As part of a routine monitoring, for each flight we compared the power
spectra density (PSD) of the raw and corrected wind components. Figure shows an example during the test flight in Lannemezan, on
31 August 2010 between 14:00 and 15:00 UTC. This example clearly illustrates
that the motion correction allows the removal of the peak linked to the modal
oscillation frequency of the system at 0.2 Hz.
Power spectra density for (a) the raw u, v and w provided by the
sonic anemometer and (b) the computed u, v and w after motion correction.
Validation close to the surface
In order to check the validity of the high-frequency measurements obtained by
the TS, measurements are compared with those of a three-dimensional sonic
anemometer fixed on masts and installed during three experimental campaigns
between 2010 and 2013. Note that our goal is to make sure that the turbulence
statistics are comparable, but not to make point-to-point comparisons.
Ideally, for comparison with fixed points on a tower, flying at constant
altitude close to the tower is desirable. The horizontal distance between TS
and the position of the towers was less than 200 m. The first two campaigns
took place in the summers of 2010 and 2011 at the BLLAST ()
experimental site using a tower equipped with a three-dimensional sonic
anemometer (CSAT, Campbell Scientific Inc, Logan, UT, USA) at 60 m. The third
took place at Bourges (France) at a French military site which was equipped
with a tower with three-dimensional sonic anemometer (GILL HS 3-Axis, Gill
Instruments Limited, Lymington, Hampshire, UK) at 30 m. For every day and
hours considered here, the atmospheric boundary layer was convective and
under clear skies. Only the campaign in August 2010 at the BLLAST site was
entirely dedicated to the validation of the TS. No scientific constraints
were therefore imposed. Indeed, for 2 days, the TS flew at a fixed height
corresponding to the instrumented level of the mast. Concerning the other two
campaigns, the TS did not remain at the same height for the whole of each day. Therefore, we
only selected measurement periods when the TS was at a similar
level to the fixed sonic anemometer.
Comparison of w′, v′, u′ and t′ measured by a
tethered-balloon probe (black) and a sonic anemometer (grey) fixed onto a
tower nearby for (a) 10 Hz time series for 30 min, (b) fluctuation distribution of a 2 h sample, and (c) power spectra density
corresponding to the same sample.
On average, the time series recorded during these different campaigns (after
motion correction was applied) exhibit a large coherence with each other,
despite the aforementioned spatial differences between the tower and the
TS. A degree of horizontal offset between the balloon and tower is
unavoidable due to the distance between both sites.
We hereafter denote u′, v′, w′ and t′ the fluctuations in longitudinal
wind, transverse wind, vertical 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 three-dimensional winds, and potential
temperature is shown in Fig. a for a 30 min
sample on 31 August 2010. The two records do not overlap perfectly, which is
expected with fast measurements made 200 m apart, but they do show the
same turbulence structures quite nicely. The comparison of the turbulence
statistics between both samples is based on the integral scales, the
distribution of the fluctuations, the density energy spectra and the
second-order moments. We calculated the integral length scales defined from
the autocorrelation function of longitudinal wind, transverse wind, vertical
wind and potential temperature. Table summarises the different
estimates. For w, we obtained 81 and 84 m respectively for the tower and
the tethered balloon. The values differ only slightly, indicating similar
turbulence sampled by both instruments.
However, the range of the fluctuations of u, v, w and t are similar
between the TS and the data from the fixed sonic anemometer. The distribution
of the fluctuations recorded during a 2 h period at midday is also
presented in Fig. b. Between both instruments a very
similar distribution of all the fluctuations is obtained with the same shape
and amplitude for all parameters here considered. Figure c presents a comparison of power spectra for 2 h
measurements at midday for wind components and the potential temperature
derived from both systems. The comparison between the TS spectra and the
tower spectrums is generally quite good, and both spectra show the expected
-5/3 slope at higher frequencies.
From those fluctuation measurements at 10 Hz, several second-order moments can
be determined for the use of the eddy correlation method. The following
subsection presents the validation of variances of the three components of
the wind, the temperature and the turbulent sensible heat flux.
Integral length scale of w, u, v and t in Lannemezan, on 31 August 2010 between 12:00 and 14:00 UTC.
Variance is commonly used for studying certain thermodynamical parameters in
the boundary layer as a measure of the intensity of the turbulence. Figure a and b present the comparison of the variance of
vertical velocity and temperature calculated every 20 min for 10 h
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
difference in altitude of the tethered balloon varies from a few metres to
tens of metres due to turbulent motions of the atmosphere. For this reason,
the variation in altitude around 60 m is greater in the middle of the
day, when convection is at its strongest. During the afternoon, when the
difference in altitude is often greater than 10 m, the values in
σw2 are higher for the TS, whilst the values in σt2 are
lower for the TS. This is consistent with the behaviour of a convective ABL
in which fluctuations of temperature are greater near the surface, whilst
fluctuations of vertical wind are greater in the middle of the ABL. The largest
discrepancy was obtained between TS and the tower when the altitude
difference between both was the greatest. Regarding the variances of the
horizontal components of the wind (not shown here), no trend is observed
between the two instruments. After 16:00 UTC for the day
represented in the figure, the values of the variances obtained are usually similar
between both instruments when the TS is positioned at exactly
the same level as the fixed sonic anemometer. Table summarises the correlation
coefficients computed for the different variances measured by both
instruments during the three field campaigns. For the 2010 campaign in
Lannemezan, these correlation coefficients are calculated based on the data
from 30 and 31 August 2010 between 08:00 and 20:00 UTC, i.e. more than 20 h of data. For σu2, σv2, σw2 and
σt2 the values are close to unity and confirm the coherence
between both instruments. For the two other campaigns the values are similar
to the values obtained in 2010 for all the variances.
Temporal evolution of turbulent moments measured by the tethered
balloon (black) and the tower (grey) on 31 August 2010: (a) vertical velocity
variance, (b) temperature variance and (c) buoyancy flux. The dashed line
represents the variation in the altitude of the tethered balloon relative to
the 60 m tower top.
The correlation coefficient between TS and sonic anemometer on the
mast for variances of the three components of wind, potential temperature,
sensible heat and momentum kinematic fluxes. r20102 concerns the
dedicated campaign in 2010 and rBLLAST2 and
rBOURGES2 corresponds to BLLAST and Bourges field campaigns
respectively.
Eddy covariance (; ) is a well established method
for the direct measurement of the vertical exchange of heat and momentum
fluxes in the atmosphere. The vertical turbulent flux (Fs) is provided by
the covariance between fluctuations of vertical wind velocity (w′) and
those of the tracer of interest (noted s below and represented in this
study by potential temperature t, or horizontal wind component u and v)
for the averaging period (Tm):
Fs=1Tm∫0Tmw′s′dx.Fu and Fv thus denote the momentum fluxes, and Ft the buoyancy flux.
Tm= 30 min and a measurement frequency of 10 Hz are generally
considered acceptable for tower-based instruments to be able to capture the
frequency bandwidth of eddy sizes contributing to the flux ().
To ensure that the averaging period is long enough we calculate the ogive
(not shown here) using the cumulative integral of the co-spectrum of the
turbulent flux starting at the highest frequencies: a period greater than 16 min is determined as sufficient for the calculation of the turbulent
fluxes. Therefore, in the following, we chose 20 min for the computation
of the fluxes with both tower and TS data. Figure c
shows the comparison between the TS and the fixed sonic anemometer for the
sensible heat flux during 10 h of measurements. Agreement is satisfactory
even if Ft seems to be systematically larger for the tower data during the
convective period. This is consistent with TS always being positioned above
the tower (between 10 and 20 m) and a quasi-linearly vertical
decrease in the sensible heat flux in these atmospheric conditions. For the
day in question, correlation coefficients between both datasets are 0.92,
0.81 and 0.8 (Table ) for Ft, Fu and Fv
respectively. More differences are found for Fu and Fv than for Ft,
but one explanation could be that the flow is distorted by the tower
(), which may induce modifications of the fluctuations of zonal
and meridional winds. To summarise, Fig. presents the
comparison of the turbulent sensible heat fluxes between tower and TS
observations for the entire set of available data (63 segments of 20 min), including selected periods of BLLAST and BOURGES campaigns for
which the tethered balloon flight was located at a similar altitude to the
sonic anemometer on the tower for more than 20 min. The maximum altitude
difference is 30 m. The range of data is between 0 and 0.2 K m s-1. The coefficient correlation between TS and fixed sonic anemometer
for the sensible heat flux parameter is 0.85. This value indicates a
satisfactory agreement between TS and towers for different places and for
moments in the day.
Correlation plot between sensible heat flux obtained by TS and data
from towers during three different campaigns at two different places in the
summers of 2010, 2011 and 2013. The colour corresponds to the altitude difference
between TS and the sonic anemometer on the tower.
Validation throughout the planetary boundary layer (PBL) depth
In this section, we use data from aircraft and remotely piloted aircraft
systems to look at the behaviour of the TS at altitude while the previous
section concentrated on the validation of the turbulent data from TS close to
the surface with fixed sonic anemometers. Of the three campaigns in this
study, only the BLLAST field campaign offers complementary data for the
validation of the TS data above 60 m. The BLLAST field campaign has been
described in detail in . The aim was to comprehend the turbulent
processes during the transition at the end of the afternoon, when the
boundary layer turns from convective to residual. This campaign brought
together many complementary observation devices including remotely piloted
airplane systems (RPASs), aircraft, wind profilers, sodar, lidars, tethered
balloons and balloon soundings among others, with the objective of achieving
an exhaustive description of the dynamical processes in the boundary layer.
The campaign documented 11 days with systematic intensification of the
observations during the afternoon. It is in this context that the TS was
deployed during the 11 intensive observation periods (IOPs). Table summarises the duration of the flights of the TS. Generally,
flights began at the beginning of the afternoon and ended before 20:00 UTC.
Battery life was not long enough to cover the entire period and the flights
were divided into two parts. The altitude of the TS varied according to the
different IOPs but remained between 150 and 500 m, corresponding to the first
half of the ABL. The French Piper Aztec aircraft from SAFIRE mainly flew in
the middle to late afternoon and measured pressure, temperature, moisture,
CO2 concentration and 3-D wind at 10 Hz with a spatial resolution of 3 m
within the ABL (10.6096/BLLAST.PiperAztec.Turbulence). Flights generally
included stacked level runs in vertical planes within the ABL in the region
of the instrumental site. M2AV (; ) remotely piloted
aircraft systems were deployed for four IOPs with an intensification in
flights in the middle to late afternoon. M2AV measured temperature at 100 Hz, 3-D wind
and humidity at 1 Hz. Flights included straight legs of 1 km in length at
around 300 m of altitude. In this paper, only the M2AV data from the IOP on
2 July 2011 are used.
The turbulent kinetic energy
Turbulent kinetic energy (TKE) quantifies the intensity of turbulence which
controls vertical mixing (; ; ). It is
defined as
TKE=0.5(σw2+σu2+σv2),
and is one of the common parameters measured by TS, aircraft and M2AV.
Depending on the different platforms, integrated times with which to compute
TKE can vary. For TS we chose to take 20 min as determined by the ogive
method (see Sect. 3). For aircraft estimation the calculation is made with
data recorded along stacked legs of around 40 km (around 6 min). For M2AV
we have an estimation for each straight leg of 1 km length (corresponding to
3 min). In order to be consistent with the TS data, an averaging of 20 min is applied to the M2AV data. Figure presents the
comparison 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, TKE presents quasi-constant values in the middle
of the ABL, and it is the reason why we are able to compare the TKE observed
by the three platforms even if altitudes are not exactly identical. The data
set consists of ten different IOPs and presents a large range of values of
TKE between 0 and 1.5 m2 s-2. Mostly, the altitude of the aircraft is
above the TS and the contrary when we consider the data from M2AV. The
correlation coefficient is close to unity (r=0.88) between the three
platforms. This confirms that the estimation of TKE at altitude by TS is
reliable.
Turbulent kinetic energy measured from tethered balloon (y axis) as
opposed to 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 colour corresponds to the altitude difference
between TS and aircraft or M2AV.
Heat flux
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 aircraft when their altitudes differ. In particular, aircraft flight
altitude is often located close to the level where the sign of the heat flux
changes making uncertain the flux determination by aircraft. When comparing
data from the TS during BLLAST with fixed mast data we systematically observe
a decrease in the sensible heat flux with altitude. Figure shows an example of the profile of the sensible heat flux
obtained in the ABL on 2 July 2011. At two different times, the combination
of data from the fixed tower, TS and aircraft shows a decrease in the
sensible heat flux with altitude. This does not allow us to directly validate
the measurement of sensible heat fluxes, but it at least indicates consistency
among the different datasets. In fact, one of the interests here regarding the combination of those complementary platforms is to obtain the vertical
structure of the turbulence throughout the PBL.
Sensible heat flux profiles obtained with (triangle) aircraft,
(circle) TS and (square) sonic anemometer on the 60 m tower on 2 July 2011
during the BLLAST field campaign. The colour corresponds to the time of day.
Time evolution of the turbulent kinetic energy measured from
(□) the turbulence tethered sonde (∘), the 8 m tower and the
(≺) aircraft during 10 IOPs. The colour is function of z/zi.
Time evolution of the anisotropy ratio measured by tethered balloon
(cyan squares) and 8 m tower (red dots). The altitude of the tethered
balloon is between 200 and 400 m.
The turbulence tethered sonde in the framework of the BLLAST study
As seen in Sect. 4, during the BLLAST campaign, the TS data have been added
to very rich datasets with several levels of instrumented measurements on the
60 m tower, aircraft flights and RPASs. In this section we will focus more on
the evolution of the TKE during the afternoon obtained with the TS.
Characteristics of the different TS flights; z/zi is the ratio
between the altitude and the top of the boundary layer.
DayStartEndAltitude ofz∗timetimeflight (m)15 June 201114:5616:522000.218:1518:504000.419 June 201113:1515:501500.117:2020105000.520 June 201113:0016:001500.116:4519:505000.524 June 201117:1519:002500.425 June 201112:4516:253000.417:0019:452500.526 June 201112:3017:003500.417:1020:002500.527 June 201113:3016:004000.416:4519:453500.930 June 201112:4015:253000.216:2019:504000.21 July 201113:1014:302000.114:5015:302000.22 July 201112:3015:304000.416:3019:453500.35 July 201113:4516:453000.317:3020:003000.3Turbulent kinetic energy
Figure presents the afternoon evolution of the TKE during the
10 IOPs obtained with aircraft, TS and the tower. Concerning the timing of
the decrease in the TKE, we observe a similar behaviour for each day. Up
until 16:00 UTC, before TKE starts to decrease, it remains close to
1 m2 s-2 at the surface (the sonic anemometer is on a tower at 8 m) and in
the middle of the ABL (between 0.2 and 0.6 z/zi with aircraft and TS).
However, after 16:00 UTC, when the decrease begins, 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 and . The authors have shown the
existence of a “pre-residual layer” at altitude characterised by a decay of
the TKE which is initiated first at altitude.
Anisotropy of the turbulence
One of the focuses of the BLLAST project is the vertical structure of the
turbulence properties in the boundary layer. The anisotropy of turbulence is
of particular interest during the afternoon transition (;
).
Here, in order to estimate anisotropy, we define the ratio as
A=32w′2‾TKE.
This is based on the fact that when turbulence is isotropic (u′2‾=v′2‾=w′2‾), TKE =3/2w′2‾ and A=1.
The TKE can thus only be estimated from the variance of w. When A=3, the
contribution of horizontal wind fluctuation to the TKE is zero
(u′2‾=v′2‾=0) and the turbulence is mainly due to
vertical wind fluctuations. When A=0, the vertical contribution to the TKE is
zero (w′2=0) and the horizontal wind fluctuations are the main
contributor to the TKE.
Figure presents the time evolution of the ratio estimated by
Eq. calculated by the sonic anemometer at 8 m
() and by the TS at a higher altitude (see Table ).
For all the IOPs, values are larger than 1 at higher altitude than close to
the surface, which means that the contribution of the horizontal motion is
small. At low altitude (8 m in the figure), but also at 30, 45 and 60 m (not
shown here), the contribution between horizontal and vertical motion is more
equivalent. At the end of the day, the values are similar between
measurements at altitude and close to the surface. The evolution of the
anisotropy ratio obtained with TS is in agreement with the results from
obtained with large eddy simulation models for one IOP of the BLLAST campaign.
The authors also show 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 small vertical velocity variance close to the
surface and in the entrainment zone and larger shear at the interfaces.
Values of A show the anisotropy of turbulence in the middle of the ABL in
convective conditions. This is an important issue when, for instance, one wants
to estimate the TKE while only w′2‾ is measured (for example
with a vertically pointing doppler lidar; ).
This section demonstrates the interest in the observations made by TS, which
allows continuous exploration of the middle of the boundary layer during the
transition phase. Synergy with other traditional tools (aircraft and tower)
allows the study of turbulent processes between the surface and the top of the
boundary layer as shown in Fig. .
Conclusions
In this paper, we presented a new system for the estimation of turbulent
transfer in the boundary layer as well as the associated first measurements.
The system consists of a lightweight (< 2 kg) turbulence probe based on a
three-dimensional sonic anemometer suspended below a tethered balloon and
coupled to an inertial motion sensor. These measurements have been evaluated
by comparing turbulent measurements derived from a tower, aircraft and
remotely piloted aircraft systems. Large correlation coefficients are
obtained (systematically higher than 0.8) between the tower and TS
measurements despite an unavoidable horizontal offset and an altitude
difference during the convective period during the day. This innovative
sensor has several advantages compared to more traditional turbulence
measurements:
The turbulence is estimated in the lower part of the PBL at altitudes where research aircraft encounter some difficulties in terms of flying but which is also higher than towers.
With this TS system, measurements in the boundary layer can be made frequently and inexpensively.
It complements a fixed-point tower, aircraft and/or new sensors embedded in RPASs in the framework of future field campaigns.
The only limitation for the deployment of this platform is that moderate wind
(< 12 ms-1) conditions are required. We have demonstrated that the
turbulence sonde is capable of measuring heat and momentum fluxes using the
direct eddy-covariance 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 characterise different sorts of vertical motion occurring in the
middle of PBL. After this first validation we are considering exploring the
possibility of estimating continuous vertical profiles of the dissipation
rate of the TKE by maintaining a slow descending rate during the profile and
using a moving average over a given time period. We would also like to load
off the system to add a fast humidity sensor such as a KH2O Krypton (Campbell
Scientific Ltd) in order to simultaneously measure turbulent latent heat flux
with the turbulent sensible heat flux. Another option worth exploring would
be the simultaneous deployment of the system with other instruments (e.g. particle
counter, O3–CO2 probes, droplets) to better understand the link between
microphysics and atmospheric turbulence such as in fog.
Data availability
The data used in this study are freely available from the database:
http://bllast.sedoo.fr/database.
Acknowledgements
The authors would like to thank F. Said for providing the tower measurements.
The BLLAST field experiment was made possible thanks to the contribution of
several institutions and sources of support: 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 Toulouse), EUFAR (European Facility for Airborne Research),
BLLATE-1 & 2, and COST ES0802 (European Cooperation in the field of Scientific
and Technical). The field experiment could not have been made possible without
the contribution of all participating European and American research groups,
all of whom have contributed to a significant degree. The Piper Aztec
research airplane was operated by SAFIRE, a unit supported by INSU-CNRS,
Météo-France and the French Spatial Agency (CNES). The BLLAST field
experiment and the 2010 campaign were hosted at the Centre de Recherches
Atmosphériques, on the Pyrenean Platform for Observation of the
Atmosphere P2OA (http://p2oa.aero.obs-mip.fr). P2OA facilities and staff are
funded and supported by the University Paul Sabatier, Toulouse, France, and
CNRS (Centre National de la Recherche Scientifique).
Edited by: E. Pardyjak
Reviewed by: two anonymous referees
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