5. Case study 2 – night of 18 th August

1. Thenocturnal boundary layer of the west African monsoon in observations and numerical prediction models • Caroline Bain1, Françoise Guichard2, Larent Kergoat3, • Doug Parker1, Chris Taylor4, Frédéric Baup3, Eric Mougin3 and Yakouba Traoré5 • Institute for Atmospheric Science, School of Earth and Environment, University of Leeds, UK • CNRM, Meteo-France, Toulouse, France (3) CESBIO (cnrs/cnes/ird/ups), Toulouse, France (4) Centre for Ecology and Hydrology, Wallingford, UK (5) IRD, Bamako, Mali However, the current literature on the nocturnal boundary layer in this Sahelian region (150N) is limited in its outlook as few observations of the boundary layer have been made in this region at night, despite the significant role the boundary layer plays in this moisture sensitive region. The current view is often based on a more general model of tropical boundary layer behaviour. In August 2005, profiles of the nocturnal boundary layer were taken using a tethered balloon. Details of the equipment and experimental method are given in section 2. The main interest in the profiles is to establish a realistic scientific picture of the nocturnal boundary layer, however profiles will also be used to study the characteristics of the monsoon at the lower levels in this region, and additionally to speculate on how the local boundary layer is effected by large scale wave dynamics and weather systems. • 2. Method • Location: Agoufou fieldsite near Hombori in Mali (15.1N, 1.3 W) • Dates: 6th – 22nd August 2005 • Time: Profiles conducted from sunset to sunrise if possible, though this varied • Equipment: • Tethered helium filled balloon, with a volume of 7 cubic meters • Simple PTU sonde, of TS-5A-SP Tethersonde type with tail (see diagram) measuring temperature, humidity, wind speed and direction • A separate sonde measuring CO2 was also attached on calm nights (contact L. Kergoat for further information) • Method: • The balloon was let out to 200 m in altitude and brought back down, taking continuous measurements approximately every 1.5 seconds • Profiles were conducted approximately every hour between sunset and midnight, every two hours there after • Stops were made for 30 seconds every 10 m until 100m was reached, then every 20 m till the full 200 m height. The data for the wind is then extracted only for the stops, reducing the affect of accent/decent of the balloon on results • Limitations: • Balloon was only able to fly in winds up to 10 ms-1, if winds were higher a tethered kite was used in its place • Unable to take measurements during squall lines, gust fronts or if at risk from lightning 1. Introduction Scientific understanding of the Meteorology of West Africa is limited by the lack of atmospheric observations made over the continent. Large scale weather systems associated with African Easterly wave activity are characterised by mesoscale convective thundestorm systems and squall lines, producing most of the rainfall for the Sahelian region. 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 convective, vertical air mass movements weakening the circulation at the surface during the day. Top photo: PTU sonde used in experiments Bottom photo: Sonde mounted below balloon platform as in experiments. 3. RESULTS – Profiles from the tethered balloon All profiles are shown each night, going from blues to red with time. The times of each profile are indicated next to each graph 9th August Weather: hot and no clouds in afternoon (some small cirrus at sunset) Evening profiles show a well mixed layer in late afternoon, with a strong, sudden inversion developing after sun set. 12th August: Overcast during preceding day with the passage of a brief squall line. Cold and humid overnight. Interesting warm dry layer seen around 950mb in early profiles 14th August: Calm night to begin with, wind speed increased rapidly to over 9ms-1 over 50m. Inversion development can be seen clearly and is coupled again with a pressure drop of almost 2mb at the surface 17th August: Weaker inversion, but general cooling of mixed layer during night. Winds change direction considerably during night, speed increases. (Please note: there are some problems with humidity) 20th August: Heavy rains the night before meant the ground was still wet. Stratus cleared in evening, some cirrus during night. Profiles show clear inversion after sunset (interesting double inversion). Layer mixing upwards during night. 6th August Blue skies with small cumulus during day preceding Profile with kite, only one profile as lightning seen to SSE 7th August Weather: fresh during day with cumulus, congestus forming in evening. Large squall line formed from east in evening, causing extreme rainfall (66mm) at Agoufou overnight 13th August: Calm evening with high cirrus observed Formation of weak inversion after sunset, coupled with a significant pressure drop 15th August: Unusual wind direction is due to strong outflow from a storm to the north (gust front observed on the horizon). Storms arrived from east at approximately midnight 18th August: Cumulus cells and congestus observed at sunset. Clear night – SEE CASE STUDY 2!!! 21st August: Really strong inversion in temperature and humidity fields. Direction of winds remains relatively constant, speed increases notably approaching sunset. 11th August: Ground wet from a storm the night before, blue skies during day – SEE CASE STUDY 1!! 4. Case Study 1 – night of 11th August 5. Case study 2 – night of 18th August • Profile description • Profiles with balloon and kite from 18.22 to 04.28 • Strong inversion in temperature and humidity fields after sunset, gradually eroding during the night • Significant shear in winds at surface in earlier profiles (particularly marked in the 19.06 and 19.29 profiles). However as wind speed increases during the night with the formation of a low level nocturnal jet, this strong shear layer increases in thickness, the maximum of the low level nocturnal jet possibly not being reached by the profile • Throughout the night the wind direction remains relatively constant from the direction of approximately 2300, from the SW, consistent with the direction of the monsoon flow • Profile description: • - Profiles with balloon from 1909 to 0554 • - Significant change in wind direction during night from S to SW • - Rise in wind speeds during night but no defined jet maximum reached • - Profile shows temp inversion mixing, getting less pronounced during night, humidity increasing through layer over night (note, there are some errors in humidity) • Synoptic patterns show a trough passed during the previous day so the profiles were taken behind the trough. • Significant dew formation observed in the morning, consistent with increase in humidity at the surface Above: Profiles obtained from the night of 18th August to the morning of the 19th Above: Profiles obtained from the night of 11th August to the morning of the 12th • Weather Situation • Shallow cumulus and some CuCg with isolated rain shafts on the 18th evening. Cloud based rose during the night. By dawn on 19th, high AuCu observed (see photos) • No significant rainfall since the 15th. Large system passed on 19th (night after this case). • Satellite pictures show Hombori in a convectively inactive region, with large MSCs located near the coast in Senegal and Mauritania, and also over Chad/Niger/Nigeria • Weather Situation • Heavy rain on the 10th => ground wet on 11th. Temperatures hot during day. Clear skies with cirrus. • Temperature cooled during night (see profile). Stratus bank well developed by dawn on the 12th. • Satellite pictures show small convective storms to the west, south and east during the late afternoon • A large Mesoscale Convective System (MCS) builds out to the east associated with a trough. The system moved SW, causing only slight rain in Hombori on the 12th. Above: IR satellite images from METEOSAT. The date and time are shown on each image Above: IR satellite images from METEOSAT. The date and time are shown on each image • Comparisons to model (ECMWF) • Figures below show model against balloon profiles • 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 • Comparisons to model (ECMWF) • Figures below show model against balloon profiles • ECMWF model consistently underestimates speed of nocturnal low level jet • Direction of winds generally good (with exception of early 1800 profile) • Theta profile good but some underestimation of inversion in later profiles (too much mixing in model) • Synoptic Situation from Models • The figures to the right show the operational analysis from the UK Met Office Unified Model (UM) and ECMWF for 12/08/05 at 00Z • The UM streamlines (top right) show a disturbance in the Hombori region at 850hPa, with winds flowing out of this region. Vorticity maximums are located to the east and west, suggesting Hombori is in a ridge in wave activity • ECMWF analysis of filtered meridional winds (bottom right) shows Hombori in an area of northerly flow at 850hPa. This is consistent with the profiles being taken in the ridge part of an African Easterly Wave (AEW) Photographs taken: Left 17:22 on 18th Aug. Right 05:22 on 19th Aug 19:08 model (18.00) – purple real (19.08) - pink • Synoptic Situation from Models • The figures to the right show the operational analysis from the UK Met Office Unified Model (UM) and ECMWF for 19/08/05 at 00Z • The UM streamlines (top right) show a strong trough to the west of Hombori, implying the observations were taken behind the trough • ECMWF analysis of filtered v winds (bottom right) shows Hombori in an area of northerly flow at 850hPa, with a significant vorticity maximum to the west. • The change in wind direction during the night is consistant with what would be implied from the large scale dynamics (from trough to behind a trough) 18:22 model – purple real - pink 00:02 model – dark blue real – light blue 00:08 model – dark blue real – light blue 05:54 model – light green real – turquoise 04:05 model – light green real – turquoise Above Right:Top UK Met Office Unified Model global operational analysis for 00Z 19/08/05, streamlines of u.v wind (not filtered) and vorticity (shaded – red/yellow colours represent positive vorticity) Bottom ECMWF analysis for 00Z 19/08/05, filtered v-wind (streamlines) and vorticity (black lines) Above:TopUK Met Office Unified Model global operational analysis for 00Z 12/08/05, streamlines of u.v wind (not filtered) and vorticity (shaded – red/yellow colours represent positive vorticity). BottomECMWF analysis for 00Z 12/08/05, filtered v-wind (streamlines) and vorticity (black lines) Acknowledgements: Volker Ermert (PhD student, Cologne, Germany) for his help in acquiring satellite images and ECMWF streamline and vorticity analysis • 6. Conclusions and Future Work • The observations have revealed some characteristic structures in the NBL evolution: • defined inversion in temperature and humidity fields after sunset • stable surface layer develops during night, eroding the inversion, reducing temperatures throughout the profile • formation of a low level nocturnal jet was also observed on several occasions. • But possibly the most significant outcome is the variability in observations from one night to the next - Further analysis is to be done on these results, to relate the changes seen in the boundary layer to land surface flux changes, as well as further relating boundary layer features to the effect this may have in the larger scale, using the UM and observations in parallel. Future plans are to revisit this theme, with more tethered balloon experiments taking place in summer 2006. These experiments will most likely occur in the Niamey region in Niger. For more information, contact Doug Parker at the University of Leeds. For further information: http://www.env.leeds.ac.uk/AMMA http://www.env.leeds.ac.uk/~caroline For more information about this poster please contact Caroline Bain, School of Earth and Environment, Environment, University of Leeds, Leeds, LS2 9JT Email: caroline@env.leeds.ac.uk Tel:(044) (0113) 3431635