Download The Nocturnal Boundary Layer associated with the

The Nocturnal Boundary Layer associated with the West African Monsoon
Introduction
The West African monsoon (WAM) has been shown to be a significant dynamical feature during the summer
months (May to September) over the West African continent. The monsoon is caused by the northwards shift in the
Inter-Tropical Convergence Zone, due to the formation of a heat low of the Sahara during the summer when
heating is at a maximum. Many studies exist concerning the large scale dynamics of the WAM and the formation of
the African Easterly Jet and African Easterly Waves are well documented, both in observation and model
investigations. The seasonal scale of the monsoon is greatly important for annual rainfall amounts in West Africa,
but the day to day weather over the region is affected mainly by the diurnal cycle in the monsoon processes.
The cycle of the diurnal processes were discussed in Parker et. al. (2005), where model studies were combined with
dropsonde observations made during the JET2000 field experiment. This paper surmised that there was a coherent
diurnal cycle in the west African monsoon winds, with intense, predominantly south westerly winds overnight,
typifying the monsoon flux caused by the formation of the Saharan heat low, but with gentler winds during the day,
when the convective boundary layer is more turbulent and imposes greater drag on the horizontal winds, hence the
monsoon is unable to penetrate as far in land. Therefore the greatest moisture flux at the surface to the northern
latitudes is predominantly at night.
This emphasises the importance of understanding the nocturnal boundary layer (NBL), as the strongest monsoon
flux is observed during night time (? quote another paper?). Without a clear understanding of the processes that
characterise the NBL, the understanding of large scale dynamics of the monsoon is also compromised.
Observations of the NBL have been limited in the African region, but even early studies such as Hamilton45
included references to the diurnal cycle of winds and convection. McGarry78 used satellite and rainfall data from
the GATE field campaign to look more accurately the diurnal cycle of convection, and was the main beginning of
research into the significant role that diurnal variations play in the large scale mechanisms of the monsoon. More
recently, the diurnal variations in the low-level circulation was investigated in the BoDEx field campaign in Chad
{Washington06a}. The position and speed of the low-level jet was investigated, and the core of the jet was found to
peak in speed during the evening and be weakest during the day. This is in agreement with studies from other areas
of the globe, where observations have shown the formation of a nocturnal jet in reaction to the decrease in
turbulence with the absence of direct solar heating (e.g. Mahrt79).
LK: add Stensrud JCLIM 96 and african references therein (I have the Gouault and have ordered the Farquharson
papers)
These campaigns have provided valuable insight into the NBL mechanisms, but more detailed observations are
needed in order to fully understand the system as a whole as well as investigate variations in the processes. Poor
communication facilities and the environmental harshness of the area mean that few meteorological observations
are available from outside of the main city regions of Gao, Bamako (both Mali), Ouagadougou (Burkina Faso) and
Niamey (Niger). {Map?}
As part of the African Monsoon Multidisciplinary Analysis (AMMA) field campaign, tethered balloon soundings
were conducted in August 2005 in Agoufou, Mali, near Hombori (15.2N, 1.5W), on the AMMA Gourma super site.
This location was chosen for its position in the moisture sensitive Sahelian region, which should be ideal for
studying the diurnal cycle in monsoon winds. It is far enough in land that the large scale moisture flux is likely to
be obscured by convection during the day, but not so far that the monsoon cannot be felt at night. It is of particular
interest to study this area due to the well documented rainfall variability the local region has received, with the
recent famine in 2004/2005 being attributed to low rainfall amounts in the summer of 2004. In this study, the
experiments will be used to examine the structure of the Sahelian NBL for this period. Observations will be
combined with numerical model output and satellite images to ascertain whether the features identified are
representative of the NBL in a larger region.
Overview of experiments
The experiments that took place in August 2005 used a tethered balloon to profile the Sahelian boundary layer up to
200 m. A kite was used in high winds when the balloon was unable to fly. Soundings were made approximately
every hour from sunset to dawn. The measurements were taken by a simple Vaisala TS-5A-SP Tethersonde,
sampling every 1.5 seconds giving pressure, temperature and humidity. The windspeed and direction were
determined by a mounted anemometer on the sonde, and standard tailfins were added to give balance and align the
sonde with the wind to monitor wind direction. The balloon was a Vaisala Tethered Balloon TTB Series, filled with
helium and measured 3.8 m in length, 1.85 m in diameter.
The balloon was stopped every 10 m for 30 seconds during ascent/descent up to 100 m and every 20 m there after.
This was to remove the influence of the movement of the balloon from the wind readings, thus only the readings
within these stops are included in the results for wind. The temperature and humidity readings are monitored
continually as the movement of the balloon should have minimal effect on these. On average each profile took
approximately 20 minutes to complete (including stops). In total 12 nights were sampled, though the amount of
readings on each evening was limited by weather and operating conditions.
Overview of the season
The 2005 summer monsoon season was recorded as being a climatologically average year for rainfall in the
Sahelian region, with slightly above average rainfall in the study region of Hombori (source: NCEP Africa Rainfall
Estimate Climatology (CPC ARC) LK we shall provide real data). However this climate average is calculated from
the mean rainfall in the last nine years (1995-2004) and does not reflect completely the dry years experienced since
the 1970s. It should also be noted that the seasons of 2003 and 2004 were particularly dry, therefore 2005 could be
construed as being a relatively wet year in context. The influence of this enhanced convection over Africa could
also have connections with the record breaking Atlantic Hurricane season of 2005, with increased African wave
activity linking to tropical cyclone genesis {Landsea}.
In August, six rainfall events were recorded at the raingauge near Agoufou in the official record. Figure
{agoufou_rain} shows the rainfall recorded for the period. Significant rain events were recorded on seven
occasions, though there were at least two other smaller events recorded also. The readings must be analysed with
caution as many of the rain events were highly localised and so it is also likely that some rainfall could have been
received at the balloon field site, but not recorded by this raingauge (LK unlikely for Agoufou raingauge). The
meteorological variables shown in figure {agoufou_rain} can be referred to from section {results} to get an
overview of the weather activity for each experiment night.
epsfig{station_agoufou.eps}
General results and generic observations of NBL
The twelve dates on which measurements were taken were the 6th, 7th, 9th, 11th, 12th, 13th, 14th, 15th, 17th, 18th,
20th and 21st August 2005. Experiments on three out of the twelve nights had to be discontinued due to risk from
approaching storms. In total five full nights worth of data was collected, though data was also retrieved, and can be
used, from the remaining dates where fewer readings are available.
All the profiles collected are shown in Fig XX
Then something like:
Potential Temperature
The observations revealed some characteristic structures in the NBL evolution. On most of the nights the surface
layers showed a defined inversion in temperature and humidity fields after sunset due to rapid cooling at the
surface. In comparison to studies from the extratropics (e.g. ), the inversion is much faster developing, this can be
attributed to the greater role that solar heating at the surface will have in the tropics, and also the rate at which the
sun sets (LK not that obvious !), and therefore the greater affect the switching mechanism in fluxes will be between
day and night.
On some of the nights the temperature at the surface remained approximately the same through the night as it was
initially after sunset (11th, 20th), on the majority of nights the surface continued to reduce in temperature. On all
nights the steady erosion of the temperature inversion was observed, as the cooler temperatures were clearly mixed
upward from the lower to higher layers. (LK and also Tsurface cools and reduces LW cooling ?)
In the early profiles on the nights of the 12th and the 20th, a warm dry air mass is clearly present in the upper
levels. The transition is particularly notable in the 12th Aug 17:00-18:00 profiles with approximately a $4^\circ$C
increase in temperature above 960 hPa. On this night further profiles in the next two hours revealed that the
temperature in this layer gradually reduced but that higher temperatures were still present higher up. The warmer,
dryer layer is still present (though diminished) in the 23:30 profile. This may lead to the hypothesis that if a strong
enough inversion is present, cooling from the surface may not have time to affect the upper layers in the boundary
layer during the night. Unfortunately the profiles collected with this set of observations are insufficient to support
this theory, radiosonde soundings and numerical models must be consulted instead.
MODEL CASE STUDY (BRIEF) FOR 12TH PLUS RADIOSONDES
Humidity
In all sets of observations the drop in temperature is coupled with an increase in relative humidity due to an
increased condensation rate at the surface (e.g. Garret, other NBL papers). The behaviour of the humidity profiles is
a mirror image of the potential temperature profiles, with a considerable increase in humidity near the surface after
sunset, decreasing rapidly with height. In the later profiles this rate of decrease is reduced and the humidity
becomes more consistently moist throughout the sampled layer.
LK : Thoughts on mixing ratio profiles : RH variations are in line with a drop in Tair. However, mixing ratio has a
rather unexpected behaviour. There is a near surface maximum, which often coincides with inversion (and CO2
accumulation), that seems to result from a surface moisture flux. Indeed, horizontal advection would not create a
surface maximum I think, and buoyancy should not create accumulation of moist air at the surface. However, at
night, it is not that obvious that we should have a surface flux because: RH is high (evaporation should be low),
there is most often dew formation (so a loss of moisture for the air above the surface), and plants usually do not
transpire at night (but soil can evaporate). Despite dew formation, there is a possibility that moist air from the soil
pores is reaching the air and thus releasing moisture. I suggest we look in more details at the mixing ratio fluxes.
The Hapex paper by Dolman et al. (J Hydrology 97) shows one night of theta and q, with general moisture
advection but also a maximum near the surface (Stull says that qv profiles in NBL can show a near surface
minimum or maximum). Note also that there is sometimes a general increase of the mixing ratio at all levels,
expected from horizontal advection.
The response to these thermal and hydrological changes in the wind fields is not immediate. MOISTURE FLUXES
AND INTERESTING CASE OF SUDDEN WIND AT SUNSET/SUNRISE - Françoise
Later on and Winds
During the night a stable surface layer develops, eroding the inversion by reducing temperatures throughout the
profile. The wind speed would be expected to increase rapidly once the inversion is set up due to the reduction in
turbulence associated with a stably stratified profile. An increase in wind speed was recorded for all nights, the low
level jet maximum was reached on 6 nights between 100 and 200 m (on the 11th, 13th, 17th, 20th and 21st).
Mainly, this is due to there being a more complete set of observations for these nights, it is unlikely for the low
level nocturnal jet to extend far above 200 m (i.e. such as in the averaged plots of Mahrt79)
Why jet higher? Quantitative relationship between theta, RH, u and v
- Direction of winds from SW::::: what is the direction of winds settling at?
Possibly the most significant outcome is the variability in observations from one night to the next. In the next
section, two case studies are investigated to briefly examine the mechanisms which control boundary layer
behaviour.
Case study concentrations
‘Observations will be combined with numerical model output and satellite images to ascertain whether the features
identified are representative of the NBL in a larger region’
Two case study nights have been taken from the overall observation period to investigate closely the events that
determined the processes within the NBL. The 11th and the 18th were chosen for their volume of observations and
also because of the variety in observed features between the two nights.
\par
Night of 11th August 2005
The profiles from the 11th of August are shown in figure {fig11thprofile}. The weather throughout the day on the
11th was clear, with temperatures peaking around 34°. The first interesting feature is in the mixing ratio – there is a
large difference in values between 18Z and 19Z implying a significant and sudden increase in moisture. By
comparing to satellite pictures from a larger domain it can be seen that convection occurred to the south of the field
site at TIMEHERE. Therefore this increase in moisture is assumed to be associated to the outflow from this storm
and translates into approximately2.5g kg-1 increase in moisture per hour (for that two-hour period).
Profiles of night 11.
The later nocturnal profiles provide a more ‘classical’ example of the evolution of the NBL, with an initial clear
inversion after sunset due to rapid cooling, followed by an increase in wind speeds, assisting mixing and therefore
an erosion of the inversion, leading to a more uniform temperature and humidity profile in the later observations.
The wind direction changed little despite the increase in wind speed. The only notable large change in direction is
close to the surface, however this is accounted for by the lightness of the winds causing instability in the direction
of the sonde LK the Agoufou sonic displays wind direction fluctuations too. See fig below.
Fig. Wind direction (together with 10 times wind speed) from Agoufou sonic anemometer for night 11.
Maximum wind speeds in the jet are high compared to the other nights, this may be attributed to the heat of the
previous day which would lead to a greater rate of change in turbulence at sun set, hence a larger acceleration term
to achieve balance in the momentum equations.
LK : we can check that for all jets we have, with daytime sonic wind.
Data from the observations was entered into a pressure-time grid and the values were linearly interpolated to
produce a best fit impression of the development of the NBL. The results from each parameter field is shown in
figure {hovs11th} along with the track that the balloon took with respect to time (this allows quick comparisons of
interpolated data fields to the actual spacing of the observations). There are gaps in the field where there was not
enough data to interpret. The plots clearly show the formation of the nocturnal jet, with wind speeds increasing
steadily above 970 hPa throughout the night, decreasing to zero at the surface. There is a slight indication that the
jet reaches a maximum speed at 960 hPa (approximately 120 m above the ground), though lack of data in the later
profiles restricts the extrapolation both in pressure and time, which would confirm whether this is an accurate
assertion.
The next three parts of the figure should be studied simultaneously as they are all interlinked. Temperature,
humidity and wind direction all show development in agreement with the established view in boundary layer
literature. Temperature decreases, humidity increases and wind direction is steadily from the SW, as is suggested
by thermal wind balance and monsoon flow in the West African system. A slight turning of the winds to a more
southerly direction around midnight is coupled with an increase in humidity.
- relate to SOIL MOISTURE, recent rain events and synoptic systems
- compare with model
Night of 18th August 2005
The night of the 18th August is interesting as a case study as it displays some features, which either differ slightly
or were not observed on other nights. The most obvious one of these, seen clearly in the profiles shown in figure
{fig18thprofile}, is the bimodal direction of the winds, where there is a large change from SSE to SSW during the
night. In comparison to the profiles taken on the 11th, the temperature and humidity inversions on the 18th are also
less pronounced. This is likely to be mainly because the ground temperature on the 18th was initially much lower at
sunset than it was on the 11th, due to there being a larger amount of cloud cover during the day. The weather during
the day of the 18th was mostly overcast, with low cloud observed throughout the day and cumulus congestus with
rain shafts observed in the late afternoon/evening (0.5 mm of rain received at the rain gauge near the Agoufou field
site).
LK : For information, below are the met station data for upward LongWave radiation and temperature (air, skin and
ground at -5cm). Tskin, and Tair are in the lower part of the range over the 6 August 22 August period. The
difference 11 August versus 18 August is significant. If you like, I can either make a figure for the 11 th and 18th
only (because the whole period gives a rather ugly figure), or I can give you the data so that you can draw figures.
See also the temperature from the sonic anemometer (below). This is not really the air temperature (correction for
air density must be applied I think) but the relative differences are meaningful.
file = 0818th.eps
Again the observations were used to produce pressure-time plots for each variable (Fig.{hovs18th}), by
extrapolating results onto a simple pressure-time grid. The gaps between observations were filled by a simple linear
interpolation. This technique presents a clearer impression of events. It is notable using this method that there is a
warm anomaly (maximum of 28.0°C) in the temperature field at 20.30 at 971 hPa, which is closely followed in time
by a rapid cooling leading to a larger cool anomaly (minimum of 26.7°C). This pattern is followed in the humidity
field where the warm anomaly equates to a slight decrease in humidity and the cool anomaly is associated with
moister air. The direction of the wind shows no reaction to the warm anomaly, however the cool moist air is
coupled with a turning of the winds more towards the southwest.
Is this real? I must check anomaly against model.
LK : Yes this pattern is real, because it is seen by the flux station sonic anemometer in Agoufou. The Agoufou
sonic does see also the 22h 971 hPa increase in Tair. It is likely than this pattern (succession of warm-cool-warm
air at 20h30) is seen in Edgerit also, according to the preliminary figures that I have. However, I have problems
reading Edgerit sonic data , so I don't really want to use them before we solve that.
Fig : a) Air temperature from the Agoufou sonic for night 11 and 18.
b) Wind data for night 18th, from Agoufou met station (AWS, cup anemometer 2.5 m agl), Agoufou sonic averaged
over 2 min or 30 min and Edgerit sonic averaged over 30 mn (Edgerit data to be checked)
Note : the Agoufou sonic is maybe 4 m high above ground level, and it is more on top of a hill, compared to the
balloon, which is in a middle slope position, whereas the Agoufou met station is more in a bottom. Therefore, I
would say the sonic sees air layers which are 'higher' than the met station, and correspond to maybe 10 m high (or
more ?) for the balloon. We should be able to find the precise elevation of the three sensors if needed.
Here are some additional comments on the nocturnal boundary layer: Night 18 :
- The Edgerit flux station (= bare soil, some 20 km north of Agoufou) shows the same change of wind direction
around midnight (see figure above with wind from all ground sensors, but again, consider Edgerit as tentative). It
does not show the easterly wind episode 22h - 24h though.
- At Agoufou, there seems to be a very low elevation 'circulation', or lack of circulation, according to many
profiles. Sometimes, there is air from the southwest flowing in the first 20 m or even 10 m, whereas south eastern
wind prevails above, e.g. between 22-24 h.
- The stability (profiles of thetav) is linked to these different air masses (eg 21h31 and 24h02 profiles). Richardson
numbers also reflect this phenomena, with sometimes 2 maxima at 20 m and 60 m for instance. This is different
from a one-D view of the formation of a stable boundary layer, because cooling and warming are not one-D in our
case.
- These near-surface circulations may be related to orography or microtopography (the dunes we climb to look at
arriving squall line for instance). However, the fact that the Agoufou sonic sees the moist and cold SW anomaly at
971 shows that at least this pattern is not a very local effect (e.g created by the very next dune). If it is confirmed
that the Edgerit data show the same event, that's even better. Would that be a meso-scale circulation ?
However, the first 10 m of the balloon profiles may still be affected by very local circulations (local drainage flow,
local subsidence).
- It terms of turbulence, the 22h-24h period is extremely calm (see plot of u* and tke below) the wind being mostly
laminar, with periods without wind at all. The comparison of wind speed and u* or tke (turbulent kinetic energy)
shows that most wind periods at the surface are not turbulent (eg wind of 2 m/s associated to very low tke), except
during mixing events at 19h30, 20h30, 24h30. The mixing of 24h30 is particularly striking, with a rather short peak
of tke or u* which mixes sup the CO2 and also thetav, It correspond to the time of change in wind direction (all
profile height). The second half of the night is more turbulent on average.
- The 22h30 NE wind event (Agoufou sonic) is not sampled by the balloon, because it occurs between two profiles.
Some additional comments on the nocturnal boundary layer: Night 11 :
I didn't look into the details of night 11 sofar. However, just a glimpse on the turbulence data shows that the
situation is very different from night 18, with significant turbulence, increasing with time.
Fig : Turbulence data from Agoufou sonic anemometer for night 11 (a) and 18 (b).
-
need SOIL MOISTURE
mesoscale/system analysis
relation to AEWs
Relation of profiles to larger synoptic events - Is the profile representative of larger scale events?
Hovmoeller of nocturnal jet formation – where is peak? Relate to other paper discoveries including Garrett
diagrams.
i) FACTS 11th August – formation of jet in clear ridge part of wave, little disturbances noted at ground
level. Compare to models, satellites
ii) FACTS 18th August – bimodial jet direction due to fluctuations in land surface OR mesoscale systems.
Comparison to models, satellites and details on where results came from
Plot model BL – observations
Plot model BL – synoptic
Plot other regional observations – radiosonde winds
Discussion
- What is the role of the NBL in the monsoon system?
- How much is it affected by AEWs? Evidence
- How much is it affected by local land surface? Evidence
- How does it differ from other boundary layers?
- How does it affect monsoon flux/ moisture flux to higher lats?
Summary and final comments