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

AEJ

Cold Tongue

SAL

ITCZ

Heat Low

• C130 aircraft flew 4 legs over this region: 25-30 August 2000

• Dropsondes every 0.5-1 degree

• Low level flights on north-south transect

Met Office C-130At Niamey Airport, Niger

JET2000: Experimental Details

Zonal Wind 28th August 2000

Thermodynamic Profiles 28th August 2000

Thermodynamic Profiles 28th August 2000

Thermodynamic Profiles 28th August 2000

Height of AEJ

8 N

10 N

12 N

15 N

18 N

Static stability 28th August 2000 - 3 LAYERS

Saharan Air Layer

Mixed Layer

Monsoon Layer

Upper Troposphere

Lifting Condensation Level

Schematic of African Easterly Jet

θ

50oCθ

θe

90oC

θe

AEJ

20oC60oC

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

AEWs

MCSs

SAL

TC

Key weather systems that characterize the monsoon

(2) Mesoscale Convective Systems

TRMM based MCS climatology over Africa and tropical Atlantic for June-July-August

Rainfall Stratiform Rain Fraction

Percentage of MCSs with significant ice scattering

Average Lightning flash density

Schumacher and Houze (2006) QJRMS :Less stratiform rain over sub-Saharan Africa than Atlanticbut, 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

Berry, G. and Thorncroft, C.D. (2005) MWRBerry, G, Thorncroft, C.D. and Hewson, T. (2007) MWR

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.

OLR and 850 hPa Flow Regressed against TD-filtered OLR (scaled -20 W m2)

at 10N, 10W for June-September 1979-1993

Day 0Streamfunction (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 10N, 10W for June-September 1979-1993

Day-4Streamfunction (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 10N, 10W for June-September 1979-1993

Day-3Streamfunction (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 10N, 10W for June-September 1979-1993

Day-2Streamfunction (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 10N, 10W for June-September 1979-1993

Day-1Streamfunction (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 10N, 10W for June-September 1979-1993

Day 0Streamfunction (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 10N, 10W for June-September 1979-1993

Day+1Streamfunction (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 10N, 10W for June-September 1979-1993

Day+2Streamfunction (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 10N, 10W for June-September 1979-1993

Day+3Streamfunction (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 10N, 10W for June-September 1979-1993

Day+4Streamfunction (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 10N, 10W for June-September 1979-1993

Day+5Streamfunction (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