Observed characteristics of the mean Sahel rainy season
This talk
(1) The basic state (some conclusions from the JET2000 field campaign)
(2) Mesoscale convective systems
(3) African easterly waves
(4) Final comments
SAL
Heat
Low
AEJ
ITCZ
Cold
Tongue
JET2000: Experimental Details
• C130 aircraft flew 4
legs over this region:
25-30 August 2000
• Dropsondes every 0.51 degree
• Low level flights on
north-south transect
Met Office C-130
At Niamey Airport, Niger
Zonal Wind 28th August 2000
Thermodynamic Profiles 28th August 2000
Thermodynamic Profiles 28th August 2000
Thermodynamic Profiles 28th August 2000
Height of
AEJ
8N
10 N
12 N
15 N
18 N
Static stability 28th August 2000 - 3 LAYERS
Upper Troposphere
Saharan Air Layer
Monsoon
Layer
Lifting
Condensation
Level
Mixed Layer
Schematic of African Easterly Jet
AEJ
90oC
θe
60oC
θe
θ
50oC
θ
20oC
Diurnal Cycle of heat low circulation
Sections based on Met Office
analyses/forecasts.
The monsoon flow is active at night
and in the morning.
Parker et al, (2005) QJRMS
Conclusions from JET2000 Field Campaign
(1) To first order the AEJ is balanced – with the temperature field strongly
linked to surface conditions and to variations in mean convection
Cook (1999), Nicholson (1998), Thorncroft and Blackburn (1999), Parker et al (2005a)
(2) The AEJ is characterised by distinctive layers with different thermodynamic
characteristics:
Monsoon layer, strongly impacted by surface features and
diurnal forcing
Saharan Air Layer, dry, dusty, low PV air that likely
influences the northern part of the rainy zone
Parker et al (2005a), Hamilton and Archbold (1945)
(3) The heat low circulation is characterised by a marked diurnal cycle that
impacts water vapour transport in the monsoon.
Parker et al (2005b), see also Racz and Smith (1999)
To what extent do climate models need to represent these
characteristics of the monsoon correctly?
Models can have problems with intensity of heat low
Intensity of heat low is likely impacted by radiative effects linked to presence of
aerosol (Jim Haywood, Met Office). Modeled heat low is sensitive to the way
aerosol is represented.
Data from SINERGEE project using 6Z, 12Z, 18Z, 24Z, July 2003
The +ve anomaly over desert is ~ -ve anomaly over ITCZ clouds
Key weather systems that characterize the monsoon
SAL
TC
AEWs
MCSs
(2) Mesoscale Convective Systems
TRMM based MCS climatology over Africa and tropical Atlantic for June-July-August
Rainfall
Percentage of MCSs with
significant ice scattering
Stratiform Rain Fraction
Average Lightning flash density
Schumacher and Houze (2006) QJRMS :
Less stratiform rain over sub-Saharan Africa than Atlantic
but, Stratiform rain increases in monsoon season compared to pre-monsoon season due
to (i) reduced upper-level shear?, (ii) reduced impact of dry SAL?, (iii) other?
(2) Mesoscale Convective Systems
• MCSs are the main rain producers in the region.
• Their structures vary regionally and during the season. Most importantly
this includes the relative contributions from convective and stratiform rain as
well as non-precipitating anvil.
How do these regional and seasonal variations impact the regional
radiation budget?
How do these variations impact the regional diabatic heating profiles
and associated circulations?
(3) African Easterly Waves
AEWs are synoptic structures associated with perturbations to the AEJ and
usually have embedded MCSs.
In climate studies we often filter fields to highlight just the synoptic scale
aspects of the waves. But intensity of waves is likely to depend on their
interactions with the MCSs.
Berry, G. and Thorncroft, C.D. (2005) MWR
Berry, G, Thorncroft, C.D. and Hewson, T. (2007) MWR
OLR and 850 hPa Flow Regressed against TD-filtered OLR (scaled -20 W m2)
at 10N, 10W for June-September 1979-1993
Day 0
Streamfunction (contours 1 X 105 m2 s-1)
Wind (vectors, largest around 2 m s-1)
OLR (shading starts at +/- 6 W s-2), negative blue
Kiladis, G, Thorncroft, C.D. and Hall, N. (2006) JAS
OLR and 850 hPa Flow Regressed against TD-filtered OLR (scaled -20 W m2)
at 10N, 10W for June-September 1979-1993
Day-4
Streamfunction (contours 1 X 105 m2 s-1)
Wind (vectors, largest around 2 m s-1)
OLR (shading starts at +/- 6 W s-2), negative blue
OLR and 850 hPa Flow Regressed against TD-filtered OLR (scaled -20 W m2)
at 10N, 10W for June-September 1979-1993
Day-3
Streamfunction (contours 1 X 105 m2 s-1)
Wind (vectors, largest around 2 m s-1)
OLR (shading starts at +/- 6 W s-2), negative blue
OLR and 850 hPa Flow Regressed against TD-filtered OLR (scaled -20 W m2)
at 10N, 10W for June-September 1979-1993
Day-2
Streamfunction (contours 1 X 105 m2 s-1)
Wind (vectors, largest around 2 m s-1)
OLR (shading starts at +/- 6 W s-2), negative blue
OLR and 850 hPa Flow Regressed against TD-filtered OLR (scaled -20 W m2)
at 10N, 10W for June-September 1979-1993
Day-1
Streamfunction (contours 1 X 105 m2 s-1)
Wind (vectors, largest around 2 m s-1)
OLR (shading starts at +/- 6 W s-2), negative blue
OLR and 850 hPa Flow Regressed against TD-filtered OLR (scaled -20 W m2)
at 10N, 10W for June-September 1979-1993
Day 0
Streamfunction (contours 1 X 105 m2 s-1)
Wind (vectors, largest around 2 m s-1)
OLR (shading starts at +/- 6 W s-2), negative blue
OLR and 850 hPa Flow Regressed against TD-filtered OLR (scaled -20 W m2)
at 10N, 10W for June-September 1979-1993
Day+1
Streamfunction (contours 1 X 105 m2 s-1)
Wind (vectors, largest around 2 m s-1)
OLR (shading starts at +/- 6 W s-2), negative blue
OLR and 850 hPa Flow Regressed against TD-filtered OLR (scaled -20 W m2)
at 10N, 10W for June-September 1979-1993
Day+2
Streamfunction (contours 1 X 105 m2 s-1)
Wind (vectors, largest around 2 m s-1)
OLR (shading starts at +/- 6 W s-2), negative blue
OLR and 850 hPa Flow Regressed against TD-filtered OLR (scaled -20 W m2)
at 10N, 10W for June-September 1979-1993
Day+3
Streamfunction (contours 1 X 105 m2 s-1)
Wind (vectors, largest around 2 m s-1)
OLR (shading starts at +/- 6 W s-2), negative blue
OLR and 850 hPa Flow Regressed against TD-filtered OLR (scaled -20 W m2)
at 10N, 10W for June-September 1979-1993
Day+4
Streamfunction (contours 1 X 105 m2 s-1)
Wind (vectors, largest around 2 m s-1)
OLR (shading starts at +/- 6 W s-2), negative blue
OLR and 850 hPa Flow Regressed against TD-filtered OLR (scaled -20 W m2)
at 10N, 10W for June-September 1979-1993
Day+5
Streamfunction (contours 1 X 105 m2 s-1)
Wind (vectors, largest around 2 m s-1)
OLR (shading starts at +/- 6 W s-2), negative blue
(3) African easterly waves
• African easterly waves are the major synoptic perturbations to the African
easterly jet.
• They impact the transport of moist and dry air at the synoptic scale and hence
have a coherent relationship with convection (including MCSs).
• Rainfall tends to lead the AEJ-level trough over the land and follow it over the
ocean.
Do climate models need to accurately depict African easterly waves and
their relationship with clouds and rainfall?
(4) Final comments
• The West African Monsoon has a marked seasonal cycle often characterised by
a so-called “jump”. Models, in general, do not handle this subtlety of the
monsoon (see Janicot et al).
• The WAM is characterised by marked wet and dry spells but the mechanisms
that explain these are not well understood. Possible candidates include land
surface feedbacks and intraseasonal oscillations including MJO (see Sultan and
Janicot, 2007)
(4) Final comments
• This presentation has focused on observed characteristics of the West African
Monsoon that are clearly important for forecasting efforts at short to medium
range.
• To what extent must these be well represented in models used for climate
prediction, either explicitly or through parameterizations?
AMMA Field Programme
See http://www.amma-international.org