The 2005 Nocturnal Boundary Layer Experiment

The CO2 side of it.

Starring Caroline Bain, Françoise GuichardFrédéric Baup, Laurent Kergoat, Yacouba Traore and Chris ‘Bekham’ Taylor

with data from C. Lloyd, F. Lavenu, F. Timouck, V. Le DantecP de Rosnay E Mougin , P Hiernaux and others

Objectives

1) Obtain large scale surface fluxes of CO2

2) Document near surface atmosphere (mixing, monsoon flux, low level jet)

Method used in Brasil, Canada.

Method : on calm night, no wind, no turbulence, no vertical mixingCO2 from the surface accumulates in the first meters of air.

Using the atmosphere as a giant ‘cuvette’

How : mass balance of CO2 from CO2 vertical profiles during the same night

Material

Balloon

PTU sonde

Vaisala sonde plugged to the PTUdata radio-transmitted

Protocole : as many profiles as possible

Results

about 100 profiles from 7/08 to 22/08

a few nights suitable for CO2

(too windy, squall line arriving, technical failures)

CO2 data processed as of April 2006 :

14/0815/0817/08 CO2 Golden day18/08

Inversions, CO2, wind (jet)very consistent

Large accumulation over night

17/08/2006

Surface Flux = 5.1 mol/m2/s

u* around 0.01

120 km

MODIS NDVI. Orange = « bare » soil

16 08 2005 2h45 25.5 mm Kobou

120 km

14/08F=2.0

15/08F=4.6

18/08F=3.7

Conclusions

Vaisala carbocap+radio : great ! (Agoufou world record 700m)

Confirms 2004 high fluxes : order of 4-5 mol/m2/s

2005 = windy. Typical scenario : Jet starts, increases (speed and depth)see typical night og August 11th

On windy nights, shear driven turbulence creates ‘leaks’ of the ‘cuvette’.How much ? Sometime LLJ strong enough to mix the air completely. u* and richardson number filtering/parameterizing ?

Links with Low Level Jet : Is it a barrier for CO2 ? (some say yesour feeling is hum… interesting …)Would L.E.S help ? We’d like to try some.Some more processing to be done, and writing.

Perspectives 2006

No more night-time fluxes (habana !)

Field trip in Benin, Mali, (july/september).

Objective : try day-time Boundary Layer budgets

method : measure height of boudary layer + [CO2], T, Rv in the CBL.

Benin : Zi from profiler, UHF etc… [CO2] from kite (?)Mali : Zi from balloon (2.5 km !). Might be last balloon campaign