The Nocturnal Boundary Layer – Balloon Experiments 2005

Caroline Bain - University of LeedsFrancoise Guichard (CNRM), Laurent Kergoat (CESBIO),

Chris Taylor (CEH), Doug Parker (Leeds)

AMMA UK Meeting

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

MethodLocation: Agoufou fieldsite (15.1N, 1.3W)Dates: 6th-22nd 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

Raw Results6th 11th

7th

9th

12th

13th

14th

15th

17th

18th

20th

21st

11th/12th Aug

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)

18:22model – purple

obs – pink

00:08model – dark blue

obs – light blue

04:05model – light green

obs – dark green

18th/19th Aug

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

19:08model (18:00) – purple

obs - pink

00:02model – dark blue

obs – light blue

05:54model – light green

obs - turquoise

Unified Model TRMM/ Meteosat composite

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.ukwww.env.leeds.ac.uk/~caroline