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An overview of the deployment of boundary layer balloons in HyMeX

by Alexis Doerenbecher

published in the Météo-France 2013 Research Report (ISSN : 2116-438X)


Some 30 CNES   (French National Center for Space Studies) instrumented boundary layer pressurized balloons were deployed during the HyMeX   (Hydrological cycle in the Mediterranean eXperiment) Special Observing Periods (SOP  ). During the autumn 2012 (SOP1), 19 balloons were launched from the Balearic Island of Minorca to document the moisturizing process of the lower atmospheric layers above the Mediterranean Sea that feeds convective systems. In February and March 2013 (SOP2), 16 more balloons were released during Mistral and Tramontane episodes from the Montpellier area, thus harvesting information on the structure of the wind over the sea.

On this occasion, the data assimilation systems at Météo-France used temperature, pressure and wind data in real time. Humidity measurements were added in the first HyMeX   reanalysis with the AROME   (Météo-France limited-area model, AROME  =Application of Research to Operational at Mesoscale) West Med model (AROME   western Mediterranean model) only during SOP1. Hence, some 400 messages (inc. 4 parameters) were assimilated over that period.

The quality of data collected by this new device has been carefully evaluated. In particular, the suspicion of radiative bias on the temperature data, collected on some flights in the autumn. Independent and co-localized measurements are almost non-existent : It has to be remembered that the main interest in using these balloons was to make measurements in regions where they are scarce. Hence, atmospheric model can be taken as reference, though the representation of the marine boundary layer in the model is far from being perfect. The quantification of these different sources of error (model and/or observation) was long enough under investigation.


Superimposed trajectories of balloons during both HyMeX   SOP1 (autumn 2012, left) and SOP2 (winter 2013, right). The colours show the altitude of the flights. To be noted that SOP1 flights were more stable and at lower altitude than SOP2 ones, which were particularly chaotic because of Mistral induced strong turbulence. The maritime area delineated in orange represents the region where the balloons are allowed to drift. Flights cease automatically as soon as this line is crossed over and balloons begin their slow descent to the sea.