1. The Nocturnal Boundary Layer – Balloon Experiments 2005 Caroline Bain - University of Leeds Francoise Guichard (CNRM), Laurent Kergoat (CESBIO), Chris Taylor (CEH), Doug Parker (Leeds) AMMA UK Meeting

2. Introduction and Motivation Rainfall events in Sahel typified by large mesoscale thunderstorms during the monsoon season (May-September) The development of these individual systems, although modulated by the large scale vorticity anomalies, is ultimately dependant on surface and boundary layer processes. In general the south-west monsoon flux is stronger during the night time in the Sahelian region. This is due to the reduction of convective mixing which exerts a drag on the mean winds and weakens the monsoon circulation during the day. Boundary Layer literature limited by lack of observations In August 2005, profiles of the nocturnal boundary layer were taken using a tethered balloon

3. Method Location: Agoufou fieldsite (15.1N, 1.3W) Dates: 6 th -22 nd August 2005 Night time profiles Tethered helium filled balloon with simple PTU sonde Profiles up to 200m in boundary layer Hourly profiles, readings every 1.5 seconds Many experimental limitations

4. Raw Results 6th11th 7th 9th 12th 13th 14th 15th 17th 18th 20th 21st

5. 11 th /12 th Aug

6. Comparisons to ECMWF Model consistently underestimates speed of nocturnal low level jet Direction of winds generally good Theta profile good but some underestimates of inversion in later profiles (too much mixing in model) 18:22 model – purple real - pink 18:22 model – purple obs – pink 00:08 model – dark blue obs – light blue 04:05 model – light green obs – dark green

8. 18 th /19 th Aug

9. Comparison to ECMWF Again, model underestimates wind speeds, though the first diagram differences is due to time lag in comparison of model to observation Direction of winds good despite large change overnight (though as discussed this is likely to be due to large scale changes which models excel at) Small bias for model to have cooler temperatures 19:08 model (18:00) – purple obs - pink 00:02 model – dark blue obs – light blue 05:54 model – light green obs - turquoise

11. Unified Model TRMM/ Meteosat composite

12. Conclusions/ Further Work Aspects of the observed Nocturnal Boundary Layer are not fully understood Model underestimates speed of low level jet in comparison to observation case studies Ongoing research into boundary layer (and soil moisture) and how it effects AEWs More tethered balloon experiments planned for July/Aug 2006 caroline@env.leeds.ac.uk www.env.leeds.ac.uk/~caroline