BOUNDARY LAYER BALLOONS IN THE MEDITERRANEAN DURING HYMEX
Alexis Doerenbecher1 , Clément Fesquet2 , Claude Basdevant3 , Philippe Cocquerez4 , and Nicolas Verdier5
1
Météo-France, CNRM-GAME, 42 av. G. Coriolis, 31057 Toulouse cedex, France, Email: [email protected]
2
IPSL/LMD, E. Polytechnique, 91128 Palaiseau, France
3
IPSL/LMD, E. Polytechnique, 91128 Palaiseau, France, Email: [email protected]
4
CNES, 18 av E.Belin, 31401 Toulouse cedex 4, France, Email: [email protected]
5
CNES, 18 av E.Belin, 31401 Toulouse cedex 4, France, Email: [email protected]
ABSTRACT
The international and multidisciplinary HyMeX
(Hydrological cYcle in the Mediterranean EXperiment)
project aims at a better understanding and quantification of the hydrological cycle and related processes
in the Mediterranean, with emphasis on high-impact
weather events, inter-annual to decadal variability of
the Mediterranean coupled system, and associated
trends in the context of global change. HyMeX will
focus on high impact weather events such as heavy
precipitation, wind-storms or regional strong winds
(e.g. Mistral) during special observing periods. The
purpose of BAMED (BAlloons in the MEDiterranean)
is to perform the deployment of drifting observing platforms on-board of pressurized balloons, during HyMeX
observing periods. Two platforms are developed. The
Boundary-Layer Pressurized Balloons (BLPBs) drifting
above the sea and the surface drifting Aeroclipper that
is a tethered balloon with a marine gondola dedicated to
air-sea flux estimates. Both aerostats will be deployed
upstream to heavy precipitation and during regional
wind events, collecting crucial data that are currently
lacking in operational weather and marine prediction
systems. These aerostats are planned to disseminate, in
near-real time, the in-situ measurements collected in the
boundary layer. The data, when assimilated in numerical
weather prediction systems, are expected to improve the
knowledge and the prediction of the events of interest.
Indeed, these aerostats could help controlling forecast
uncertainties. For instance, some aspects of the weather
phenomena are poorly predictable (e.g. location and
intensity of highest precipitations). This predictability
issue implies that it is crucial to consider aerostats as
adaptive observing platforms: for each event, targets will
be defined. Moreover, for both drifting platforms, the
Mediterranean basin is closed; as a consequence, the
trajectories will be short. The coastal location and, above
all, the date/time of the launch, are critical parameters to
guarantee the balloons’ trajectories towards the areas of
interest. Some possible launch sites in the North-West
Mediterranean have been evaluated on a sample of
typical HyMeX meteorological cases. To optimally
schedule the launches, a balloon’s observation simulator
developed at LMD/IPSL and a targeting guidance tool,
developed at the CNRM, will be used. The targeting tool
allows, in particular, the calculation (today) of an area
called sensitive where adding observations (in 1 or 2
days) would improve the prediction of a meteorological
event (3 or 4 days ahead).
Key words: balloons; boundary layer; Mediterranean
Sea; HyMeX.
1.
1.1.
THE MEDITERRANEAN CONTEXT AND
THE PROJECT
The Mediterranean context
The Mediterranean basin is an almost closed area, surrounded by steep orography, small-sized watersheds and
dense human settlements, especially along coastal areas.
The climate of the region allows intense events to occur such as heavy precipitation [8] (HPEs), wind storms,
flash floods for small scales, but also heat waves and
droughts at larger scales. All these environmental disruptions expose the populations to losses and damages.
The numerical weather prediction allows mitigating the
risks. But the low predictability of the scales and intensities of the dangerous phenomena render critical the use of
fine-scale models with frequent assimilation of observations. The Mediterranean environment is a strongly coupled ocean-atmosphere-continental interface (hydrology)
system in which the hydrological cycle has a predominant role. The Mediterranean Sea acts as a tank of heat
and moisture that contributes to the extreme character of
some weather events.
The Special Observing Periods (SOP) of the HyMeX
field campaign aims at collecting unprecedented dataset
thanks to a massive increase of the research observing
systems. The terrestrial routine observing system in the
basin is heterogeneous. The Northern part is fitted with
a dense network, but not the South, neither the sea itself.
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Proc. ‘20th ESA Symposium on European Rocket and Balloon Programmes and Related Research’
Hyère, France, 22–26 May 2011 (ESA SP-700, October 2011)
The satellite component of the system is not fully optimal. Despite the effort made to increase the contribution
of this segment to the improvement of forecast, some limitations remain: complex topography of the basin, presence of clouds, low vertical resolution of satellite data
close to the surface. Thus low-level observations above
the sea are missing and the use of boundary layer balloons
is a smart approach to collect additional useful observation at reasonable cost. The efficiency of such an adapted
drifting observing component depends on the control and
optimization aspects that are discussed on the present paper.
1.2.
HyMeX and BAMED
HyMeX stands for the Hydrological cYcle in the
Mediterranean EXperiment1 . HyMeX is an international
and multidisciplinary project, which includes a major
field campaign. Indeed, various environmental aspects,
spread over a large range of space-time scales and linked
to the water cycle, have to be studied in observing periods (OP). The decade-long observing period (LOP) focuses on climate and Mediterranean water budget aspects. The enhanced observing period (EOP) focuses
on seasonal aspects. The SOPs (Special Observing Periods) focus on small-scale but intense events (heavy precipitation, storms, deep ocean convection). The figure
1 depicts the organization of the various SOPs for the
NWMed-TA (North-West Mediterranean Target Area).
Indeed, HyMeX distinguishes two TAs in addition to the
NWMed-TA: the Adriatic TA and the Aegean TA. The
concept of “target” which is used anywhere else in the
paper do not refer to the HyMeX TA which correspond
large pieces of the Mediterranean basin. Hereafter, the
paper focuses on the North-West Mediterranean basin.
SOP1 are planned in September and October and SOP2
are planned in February and March. Each SOP will be divided in 4 to 5 intensive observing periods (IOPs) which
will be phased with the occurrence of the phenomena
of interest. SOP1 focuses on heavy precipitation events
that occur in the northwestern basin as described in [7]
and [8]. SOP2 focuses on strong regional wind events,
like Mistral and Tramontane that contribute to the conditioning of the surface stratification and to the triggering
of deep ocean convection (dense water formation) in the
same part of the Mediterranean.
During the SOP/IOP, low-level drifting balloons will
be deployed to collect an unprecedented in-situ dataset
above the sea. These balloons (or aerostats) are currently being developed in the BAMED project. BAMED
(BAlloons in the MEDiterranean) is funded by the
CNES-CSTB (Scientific and Technological Committee
of the joint French National Space Agency and National
Centre for Scientific Research). BAMED aims to develop the aerostats and some accompanying deployment
guidance for the SOP/IOP. The in-situ collected and nearreal time transmitted observations of air masses above the
1 http://www.hymex.org/
Figure 1. Diagram showing the agenda of special observing periods (SOP) of HyMeX, embedded in the 4-years
long Enhanced Observing Period (EOP), shown in red.
The LOP is shown in pink. The yellow parts are the two
occurrences of SOP1. The green shaded part is the first
occurrence of the SOP2. The second occurrence of SOP2
(SOP2.2) is shown with dashed green lines is likely to not
be implemented. The orange part enveloping the SOP1s
and SOP2.1 is labelled EOP+.
Mediterranean Sea are expected to benefit to the numerical weather prediction systems that will assimilate them.
Two kinds of drifting platforms are considered: the
Boundary Layer Pressurized Balloons (BLPB, see part
3.1) and the Aeroclippers (see part 3.2).
Past studies ([7] and [8]) have shown the importance of
air-sea interactions in the understanding and prediction
of intense precipitating events on the Cévennes region, in
France. However, observations at high spatial and temporal resolution and at low-level above the Mediterranean
Sea are lacking. Measurements made by BLPB and Aeroclippers would provide a better description of the low
level flow.
To optimize the use of drifting observing systems during
HyMeX, some numerical developments are carried out in
BAMED. Trajectory prediction and adaptive observation
(or targeting, see part 2) are coupled to help in predicting the best launch schedule for both types of aerostats.
Indeed, to help in a better quantitative prediction of the
phenomena of interest, the balloons trajectories have to
start from the right place and at the right time.
2.
ADAPTIVE OBSERVATION
Adaptive observation strategies are observing procedures
that aim to locally improve the weather forecast by managing some transient changes in the observing network.
The purpose is to design an optimal observation network
in a prognostic way: that is to decide now what modification of the observations of, say, tomorrow will improve
the forecast valid for, say, the day after.
This design may account for some dynamical aspects of
the atmosphere but also aspects about the data assimilation system that will process the observations of the modified observing network [1].
Numerical tools, called targeting techniques, allow to
predict regions where additional observations may significantly influence the forecast of some phenomenon: these
Figure 2. Diagram depicting the principle of the adaptive observation with drifting platforms. Real-time is the
diagonal axis. Forecast (or simulated) time is shown horizontally. The procedure starts with the future case detection in the forecast, which triggers various computations, including sensitive area prediction and balloons’
trajectories simulations (good trajectory in green, bad in
red). The numerical procedure yields a launch time (in
the future). The aerostats are prepared during the launch
delay and are released at the requested (near-real) time.
Subsequently, they drift toward the sensitive region and
the event of interest (shown as a thunderstorm on the left
hand side).
are called sensitive areas. Because additional observations introduce more information in the system, their assimilation should decrease the uncertainty on the forecast
and improve the latter on average. The idea for HyMeX
SOPs is to deploy BLPB and Aeroclippers within such
sensitive areas.
When the observing component considered for adaptive
observation is made of drifting platforms, one should also
account for the natural drift of these platforms to reach
the sensitive areas. Indeed, the sensitive area may vary
from one IOP to another, but not the balloons’ launch site.
As a consequence, the adaptive observation procedure (or
targeting guidance) should yield some favourable launch
schedule if this exists.
The figure 2 shows the principle of the adaptive observation with drifting platforms.
3.
Figure 4. Diagram showing various shapes of the Aeroclipper depending on the strength of the wind. Gondolas
are depicted in yellow.
is fixed at the top (northern pole) and the other one at the
bottom (southern pole) of the balloon. At the top, the scientific gondola is made of a miniaturized meteorological
shelter. Usual thermo-dynamical variables (temperature,
humidity, pressure) are collected. The wind is deduced
by means of the successive GPS positions of the balloon.
At the southern pole, the technical housekeeping gondola
includes remote and safety controls, transmission system
and batteries. The dialog between the two gondolas is
radio-based, which allows simplifying all the connectors.
Scientific and technical data are transmitted in near-real
time via Iridium. The figure 3 illustrates these aspects.
The BLPB can be flown individually or in cluster and are
designed to fly at most one month, however flights during
HyMeX are expected to be much shorter. For HyMeX,
the envisioned flight levels are between 925 and 850 hPa,
in order to collect atmospheric data above the ocean in
the converging low-level winds that feed the coastal convection leading to heavy precipitation events (HPEs). Despite the lightness of these aerostats, which render them
non-lethal, the safety rules require to terminate the flights
above inhabited land. Therefore, all the BLPBs are fitted with a depressurization valve that opens on demand
to progressively deflate the balloon prior it reaches the
shore.
DRIFTING BALLOONS
3.2.
3.1.
Boundary layer pressurized balloons
BLPBs are 2.5 m diameter balloons pressurized with helium. They drift with the atmospheric air masses (quasiLagrangian balloons) at a pre-set constant density level
in the boundary layer. The BLPBs have 2 gondolas, one
Aeroclipper
The Aeroclipper is a streamlined balloon vertically stabilized by a guide rope [5]. At the lower end of the guide
rope, an oceanic gondola measures sea surface temperature (SST). The figure 4 displays a diagram of the Aeroclipper.
The airship-shaped balloon with caudal fins makes the
Figure 3. Split diagram showing the shape and structure of a BLPB (Boundary Layer Pressurized Balloon). A specific
hydrophobic coating is applied on the envelope to avoid rain drops to significantly increase the weight (decreasing the
buoyancy) of the balloon.
wind to have a lift action on the balloon: the stronger
the wind, the higher the balloon. The marine gondola
and the guide rope maintain a contact with ocean surface and a surface drag. An atmospheric gondola contains instruments for measuring usual thermo-dynamical
variables (pressure, temperature, relative humidity). This
gondola is fixed on the guide rope, below the balloon. As
a consequence of the varying height of the gondola and
of the non-Lagrangian drift of the platform, both an altimeter and a sonic anemometer (relative wind speed) are
added to the set of sensors. A GPS position is collected
every minute and allows computing absolute wind speed
and direction. From those measurements one can derive
turbulent fluxes of moisture, heat and momentum.
The Aeroclipper is designed to provide at most 30 days
of data.
4.
DETERMINING LAUNCH SITES
In order to be able to sample all the potential cases to occur in the HyMeX field phase and in order to optimize the
deployment of the balloons one would require ideally to
have several launch sites. However, this is not supportable because of the limited resources (multiplication of
costs). Accounting for the logistical constraints implies
to consider a single site per SOP. As a consequence of this
assumption, one of the objectives of the BAMED project
is to propose some possible launch sites that would allow
to maximize the success of the balloons and that match
the CNES operation requirements. The balloons’ success
may be expressed as a hit rate with respect to the sensi-
tive areas or with respect to the weather event themselves
(event-target).
To achieve this, two catalogues of typical weather cases
representative of either SOP1 or SOP2 have been collected. For each case, an event-related target has been
defined. For the SOP1 events (HPEs, see part 1.2), the
targets were fixed radius (200 km) circles centred on the
main convective events, the position of the centre changing from one case to another. For the SOP2 cases, a compact marine target region was defined following Candela
[4] in the Gulf of Lion as well as a large atmospheric
polygon. Figure 5 shows most of the sites that have been
tested. For SOP1 more remote sites have been considered, but they proved to not be effective in very first steps
of the evaluation.
To check the efficiency of each launch site, simulated
trajectories have been computed, considering continuous
launches of virtual balloons. The SOP1 possible sites
have been evaluated with respect to the BLPB trajectories. For SOP2 this was done with the Aeroclipper. This
distinction does not mean that only one kind of balloon
will be launched in either SOP1 or SOP2. The operations
will try to mix the platforms as much as possible. However, Aeroclipper is the key platform for SOP2 for which
their trajectories prevail.
In all cases, raw statistics have been computed on various flight characteristics: the hit rate (number of balloons
entering the target areas), the length of the flights, length
above the see, the rate of balloon loss. Afterwards, the
predictability of the hit rate was evaluated. Indeed, the
Figure 5. Map of the studied launch sites, as a function of
the HyMeX SOPs (1 or 2). The red dashed ellipse shows
the region where the SOP1-related target areas are located, on average. The orange circle shows the unique
marine SOP2-related target area. The turquoise polygon
depicts the unique atmospheric SOP2-related target area.
Table 1. Table showing the hit rate predictability for 3
sites and 2 flight levels. This applies to BLPBs and SOP1.
The sites are mapped on figure 5. The red cells show
the best cases. The grey-scale coloured cells depict cases
with no efficient drift.
launch schedule will be based on predictions, so it is critical to be able to predict the launch windows. For each
case and sufficiently good site, the predicted and the verified (as computed on a series of analyses) hit rate were
compared.
The table 1 shows the statistics obtained from 3 sites, for
two flight levels. Whatever the flight level and the site,
there are always case for which the tested site does not allow to reach the target, even for the site of Mao (Mahòn).
However, Mao appears as the best site. The CNES has
started searching the precise launch site on Minorca.
The figures 6 and 7 show 2 examples with simulated
BLPB trajectories. In the fig. 6, a case of October 2009
with heavy precipitations on the Cévennes, the site of
Mao works quite well. Most of the trajectories reach the
target and drift through the meteorological NWP-based
sensitive areas. The low-level flow is well oriented and
maintains sufficiently long time to allow several simulated balloons to collect data in the right places.
On the contrary, the fig. 7 illustrates a case of September
2008 with heavy precipitations on the Drôme, in which
the low-level flow is southwest oriented. The simulated
balloons reach neither the target nor the sensitive area,
which is located over the Iberic peninsula. The Barcelona
site does not produce a better solution on that case because most of the flights would be over land (including
mountains that may be barriers for BLPBs).
The same kind of selection procedure has been applied to
the Aeroclippers’ trajectories, but on winter wind cases.
Table 2. Table showing the length of the launch time window for each of the site tested in table 1. Black cells
correspond to cases for which the site and the flight level
do not allow the simulated BLPBs to drift into the target area. The longer the launch window, the easier the
operations and the expected mission success rate.
Figure 6. BLPBs simulated trajectories on a HPE case of October 2009 in France (Cévennes). The target is shown as a
circle, the NWP-based sensitive area (using an adjoint model) is shown in green shadings. Each trajectory represents the
drift of a single balloon with the coloured dots showing the valid date. The site of Mao is quite efficient on that case.
Figure 7. BLPBs simulated trajectories on a HPE case of September 2008 in France (Drôme). The target is shown as a
circle, the NWP-based sensitive area (using an adjoint model) is shown in green shadings. Each trajectory represents the
drift of a single balloon with the coloured dots showing the valid date. The site of Mao does not work on that case.
Table 3. Table showing the hit rate predictability for
3 sites, Aeroclipper and Mistral events. The sites are
mapped on figure 5. The sites are ranked from West to
East. The western the site, the more efficient the site.
Table 4. Same as table 3, but for Tramontane cases with
two sites dedicated to that type of event.
Tables 3 and 4 show some hit rate predictability results,
with dedicated sites on the French coast and with Aeroclippers. These are potential sites for SOP2. The Montpellier site appears clearly to be the best and it works for
both kind of event (Mistral and Tramontane). Some trajectories are illustrated on figure 8, with a case of Tramontane in January 2009.
5.
LAUNCH STRATEGY
The technical details and schedule of the launch strategy
are not known yet. These will be defined in coordination
with the HyMeX Operation Centre (HOC). Even though
the balloons have to be launched prior to other aerial research platforms such as an aircraft, some coordination
is necessary to get the same cases sampled and to possibly manage co-located observations. In parallel, the early
observations collected by the balloons along their trajectories should enter the operational data assimilation systems, possibly influencing the forecasts and in that way,
contribute to the planning of the deployment of other observing platforms during HyMeX IOPs.
The figure 9 sketches the general scheme we plan to implement. This procedure may be seen as a kind of targeting guidance applied to drifting platforms.
In relation with the HOC, a constant weather watch
should allow to detect the possible occurrence of some
weather events of interest, several days in advance. A
targeting case definition procedure may start at that point.
Figure 8. Simulated Aeroclipper trajectories on a case
of January 2009. The launch site is on the French
coast, close to Montpellier. 25 simulated Aeroclipper are
launched, one every 2 hours starting on the 6th of January. On this case the drifts are quite efficient in sampling the marine target area (for dense water formation)
depicted as an orange circle at the South-East limit of the
Gulf of Lyon [4].
Forecast time
Early detection of
likely HPE
Select cases
or targets
Computation
of sensitive areas
Identify broad
launch time windows
back-traj
Proba
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ili s t
ic t
al
Re
ra je
ct o
e
tim
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v a lu
ri
ecto
es
numerical weather prediction context. Indeed, the balloon (vehicle) and the scientific gondola make an integrated platform, which is quite new. In addition to the
sensors, the drift of the balloon will give some integrated
information about the wind field. All will be assimilated
within operational data assimilation systems. Being able
to deploy these platforms in adaptive way is another challenge. This preparation to HyMeX is also an opportunity
to renew the CNES platforms, having further objectives
in mind, such as tropical cyclones...
a t io n
ACKNOWLEDGMENTS
Fine launch
time window
Launch /
No launch
Figure 9. Simplified scheme of the planned targeting
guidance procedure to be implemented during HyMeX
SOPs, in close collaboration with the HyMeX Operation
Centre.
The cases are identified with a date of occurrence, a domain of occurrence (or verification, that is also the eventtarget) together with a sensitivity date and a date of detection. The sensitive areas associated with detected cases
and valid for the sensitivity dates will be computed. Depending on the results, the field teams may be put into
alert in order to prepare the balloons. Back-trajectories
from event-target and sensitive areas will allow sketching a preferable launch time schedule. Once the rough
time schedule is available, favourable time windows are
selected. From these, finer scale simulated trajectories
will allow to refine the time schedule. Some probabilistic ingredients may enter the system at this stage, by either perturbing the trajectory prediction model or using
meso-scale ensemble wind fields. The targeting techniques used to compute the sensitive areas may be of
various kinds. The initial plan was to implement the
KFS (Kalman Filter Sensitivity), which is developed at
the CNRM ([2]). However, it may be of great interest
to also implement other techniques that may suggest to
sample other, but contiguous, regions of the atmosphere.
Thus, the ETKF (Ensemble Transform Kalman Filter,
[3]) from NRL and the Ensemble Sensitivity [6] from
UIB are among the possible targeting techniques to be
considered in the HOC.
At the end of the process, the targeting guidance yields
a proposal for a launch schedule. This will be crosschecked with the CNES operation centre (i.e. checking
any further local constraint that would prevent the balloons to be launched), prior to the launch (or not) of the
balloons.
6.
CONCLUSIONS
The HyMeX campaign is an opportunity to deploy
boundary layer balloons within the Mediterranean in an
The authors are grateful to the colleagues working for
both the BAMED and HyMeX projects who helped in
carrying out these studies. We acknowledge for the
support of A. Vargas at CNES, P. Durand at OMP/LA
(Toulouse III University) and P. Drobinski at LMD/IPSL.
A special thank to F. Duffourg at Météo-France CNRMGAME for the support with high-resolution simulations.
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