Tethered balloon profiles in the nocturnal boundary

layer and CO2 fluxes in Sahelian Mali.

L Kergoat 1, F Guichard 2 F Baup 1 C Bain 3 E Ceschia 1 Y Traore 5 F Lohou 6 V Le Dantec 1 D. Epron 7 C. Lloyd 4 F Timouk 1 C Damesin 8 P de Rosnay 1 F Lavenu 1 and E Mougin 1.

1 CESBIO (cnrs/cnes/ird/ups) Toulouse France, 2 CNRM Toulouse France, 3 University of Leeds UK, 4 CEH Wallingford UK, 5 IRD Bamako, Mali, 6 L.A. Toulouse France, 7 INRA Nancy, France, 8 Université d'Orsay France

ECMWF analyses exhibit a well defined Low-Level Jet almost every night, a few hundreds m AGL, of magnitude a few to more than 10 m.s-1

ObjectivesEstimate meso-scale night-time CO2 fluxDocument night-time nocturnal boundary layer (monsoon flux)

Data : campaigns in 2004 and 2005 in Agoufou (Mali 15.3° N 1.5° W) in the Gourma, nortthernmost mesoscale window of AMMA.P, Tair, Rh, wind recorded with a balloon-borne Vaisala sonde. In 2004 air is pumped through a tube and flown through a LiCor IRGA. In 2005 a Vaisala GMP343 is pluged in the tethered sonde, CO2 data radiotransmitted.

RESULTS : Calm nights show a regular accumulation of CO2 near the ground allowing estimation of large-scale ecosystem respiration.

Strong fluxes ! Maxima 4.2 mol m-2s-1 in 2004, 5.2 mol m-2s-1 in 2005.

Nights with monsoon flow show significant vertical mixing. When wind > 7 m.s -1, few CO2 accumulates near the ground. Flux/gradient methods applicable ?

Stable layer most often quite thin in 2004, often < 100 m. thinner and more stable than suggested by ECMWF analyses. Low level jet seems to be « low ».

Strong consistency of CO2 , , rv and wind profiles. Advection apparently weak for CO2 (not for H20 and ) CO2/x is low

Significant inter-night variability in wind speed and direction in 08-2004 (dry year), strong monsoon flux prevailing in 08-2005 (wet year).

Conclusions

- Support hypothesis that Sahelian ecosystems display large CO2 respiratory fluxes during pulses.

- Large scale CO2 fluxes detectable with nocturnal boundary layer budget methods.

- In the context of AMMA, tethered balloons are suitable to scale-up surface CO2 fluxes and to document the low-level atmosphere by allowing high frequency soundings.

Perspectives- Comparison to tower fluxes, chambers, and modelling.- Investigating nocturnal mixing with CO2 and H2O as tracers- Daytime H, LE, CO2 budget in Benin with UHF/VHF in 2006 ?Support from ECCO-PNBC and French API is acknowledged. IER-Mali, Roselt, IRD Bamako, the people from Agoufou and Pangrou are greatly acknowledged as well.

CO2 flux estimations

JuneJuly

AugustSeptember

(°C)

(m.s-1)

diurnal composite

Surface met data: diurnal temperature range at the lowest in August but still ~ 8 K (monthly mean), diurnal cycle of wind speed, weak surface wind speed at night, but not above!

Pink : 2004 measurement nights (22, 24 wet)blue : soil moisture.

22

NDVI averaged over the balloon footprint and past heavy rainfall may explain higher CO2 fluxes on 22th than on 24th 08/2004.2005 : higher respiration fluxes are consistent with wetter soils + more vegetation.

24

Back trajectories

40 km

Nocturnal Boundary Layer Profiles

wind

direction

mixing ratio

21-08 2004 22-08 2004 24-08 2004 17-08 2005

Growth of a stable layer

day of Aug 2004

22/08/2004:Slope of CO2 accumulation gives a flux of 4.2 mol m-2s-1

22/08/2004 : 2.5 mol m-2s-1

17/08/2005 : 5.2 mol m-2s-1

Air sampled at the balloon site is tracked back. Green is for grasslands, orange for bare soil (MODIS image)

200 m

100 m

0 m

[CO2]