The Nocturnal Boundary Layer – Balloon Experiments 2005 Caroline Bain - University of Leeds Francoise Guichard, Laurent Kergoat, Chris Taylor, Frédéric Baup, Yacouba Traoré Doug Parker, Andreas Fink, Volker Ermet and many others... Toulouse Meeting 11 th April 2006

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

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

Idealised Nocturnal Profile

6th 7th9th11th 12th 13th14th15th 17th 18th20th21st Results – Balloon Profiles

Nocturnal Profile Features 11 th /12 th Aug (time-pressure plots) Formation of nocturnal jet around 960hPa Jet max speed ~ 12ms -1 Initial significant temperature inversion Mixing during night Stably stratified by the end of the night wind direction generally from SW, turning more westerly during night Inversion at sunset, becoming more mixed and moister during night

Comparisons to ECMWF (Francoise Guichard) 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 obs – pink 00:08 model – dark blue obs – light blue 04:05 model – light green obs – dark green

Comparisons to UM (Operational Model) 18:22 obs - pink model (18:00) – purple 00:08 obs – light blue model – dark blue 04:05 obs - turquoise model – light green - UM slightly underestimating wind speed - UM underestimating theta in later profiles (too cold) - Direction of winds not as good as ECMWF

6th 7th9th11th 12th 13th14th15th 17th 18th20th21st Results – Balloon Profiles

Nocturnal Profiles 18 th /19 th Aug Jet does not reach a clear maximum Max speed = 7ms -1 Clear turning of the winds during night 21:00 Warm anomaly => wind direction anomaly (westerly) => humidity anomaly (moister?)

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 obs - pink model (18:00) – purple (Francoise Guichard) 00:02 obs – light blue model – dark blue 05:54 obs - turquoise model – light green

Comparisons to UM (Operational Model) 19:08 obs - pink model (18:00) – purple model (20:00) – brown 00:02 obs – light blue model – dark blue 05:54 obs - turquoise model – light green Better representation of wind speed PT strange at ground reading? Temp again underestimated later, overestimated early Direction not subtle enough for small scale changes

Unified Model (18 hr forecast) TRMM/ Meteosat composite Soil Moisture from 18 th Aug 05 x

Wavetracking and Synoptic Events ECMWF data used to produce a hovmöller to track wave patterns N and S track more synchronised towards the end of the month 5 waves in August identified to go on and contribute to tropical cyclone development Time (date in August) Figure: Hovmöller V-winds Lines: South track 700hPa Shading: North track 850hPa L H H IRENE TD 10 H KATRINA TS LEE H MARIA H NATE Fieldsite

Conclusions/ Further Work Obs show formation of a jet, strong initial inversion and mixing during night in accordance with theory ECMWF model underestimates speed of low level jet in comparison to observation case studies More to be done on wind direction changes on the 18 th and links to AEWs and soil moisture More tethered balloon experiments planned for July/Aug 2006 in Niamey