1. Anne Sophie Daloz Director : Colin Jones
The effects of increased horizontal resolution in simulating precipitation over Sub-Saharan Africa with GEM-LAM.
Source :
Anne Sophie Daloz
Director : Colin Jones

2. Why do we study Sub-Saharan Africa ?
1. Introduction
Why do we study Sub-Saharan Africa ?
Growing interests
Burpee, Lafore, Lebel,
Redelsperger, Chauvin …
Droughts caused catastrophic
period of starvation 70s et 80s
in Sahel.
Precipitation AEWs*
Tropical cyclogenesis AEWs*
Topography of Africa. In black, the Sub-Saharan region.
*AEW = African Easterly Wave

3. Precipitation mechanisms over Sub-Saharan Africa
1. Introduction
Position and extreme migration of the ITCZ in red .
Source :
AEWs
African Easterly Waves
MCS
Mesoscale Convective System
ITCZ
WAM
West African Monsoon
Topography of Africa. Source
ephotos.com/africa/afmaps/afscimaps/africaelevationmap.shtml
Alizés chargés d4humdité ,
West African Monsoon
Genesis : West Africa
Inter Tropical Convergence Zone
Area where easterlies of both hemisphere meet one another
Convergent Flux Intense convection
Important migration over Africa
African Easterly Wave
Easterly synoptic scale perturbations
Genesis : Eastern Africa
Mesoscale Convective Systems
Majors Precipitating systems
Regulated by AEWs
ITCZ migration Strong gradient of T° and humidity io n /

4. 1. Introduction
Objectives
Evaluate the effects of increased horizontal resolution of GEM-LAM* in simulating precipitation over Sub-Saharan Africa.
*GEM-LAM = Global Environmental Multiscale
Limited Area Model

5. 1.0° GEM-LAM grid ; Katja Winger (2009)
Data
2. Data & Methodology
1.0° GEM-LAM grid ; Katja Winger (2009)
GEM* (LAM*)
Horizontal resolution :
2º, 1º, 0.33°, 0.15°
- Convective scheme: KF*
- Vertical resolution : 53 levels
- Top : hPa
TRMM* (3B42)
Resolution : 0.25º
3 hourly
Satellite data
*GEM = Global Environmental Multiscale
*LAM = Limited Area Model
*KF = Kain-Fritsch
*TRMM = Tropical Rainfall Measuring Mission
Timesteps for the different runs :
0.15° = 450 sec
0. 33° = 900 sec
1.0° = 2700 sec
2.0° = 3600 sec
The TRMM data and GEM integrations, have been interpolated in a common 1° grid and averaged over 1 day for

6. Seasonal Mean (JAS*) of precipitation (mm/day) for 1979-2003.
Areas of interest
2. Data & Methodology
Seasonal Mean (JAS*) of precipitation (mm/day) for
Topography of Africa.
Zone I : Longitude 20°W – 40°E Latitude 5°N-15°N
Zone II : Longitude 20°W – 8°W Latitude 5°N-15°N
*JAS = July, August, September

7. Seasonal Mean (JAS*)
4 integrations are capable to detect the general patterns such as :
The center with less precipitation
Peaks of convection (Atlantic coast, Cameroon & Ethiopia)
3. Results
a) TRMM
b) GEM-LAM 2.0°
c) GEM-LAM 1.0°
d) GEM-LAM 0.33°
e) GEM-LAM 0.15°
Precipitation (mm/day) Zone I
mm/day
Latitude
Longitude
*JAS = July, August, September

8. Precipitation (Number of Very High Wet Days) 1999 – Zone II
Seasonal Mean (JAS*)
3. Results
Zone II (5°N – 15°N / 20°W – 8°W) Atlantic coast
Very high wet days = PR > 20 mm
a) TRMM
b) GEM-LAM 2.0°
c) GEM-LAM 1.0°
d) GEM-LAM 0.33°
e) GEM-LAM 0.15°
Precipitation (Number of Very High Wet Days) – Zone II
Latitude
Longitude
Maximum overestimate
= 50 days
= 70 % compared to TRMM

9. Time-Latitude diagrams of precipitation (mm/day) 1998 – 2003 Zone I
Migration of the ITCZ
The bias is decreasing with the augment of resolution in term of localization
In term of intensity, an important bias remains for all the integrations
3. Results
b) GEM-LAM 2.0°
c) GEM-LAM 1.0°
e) GEM-LAM 0.15°
d) GEM-LAM 0.33°
a) TRMM
Time-Latitude diagrams of precipitation (mm/day) – Zone I
Months
Latitude
Underestimate
Overestimate
Months
Months
Localization
Intensity
The ITCZ can be studied in term of
- localization
- intensity of the precipitation
Underestimate of GEM 2°
Overestimate of GEM 1° & GEM ST
The better simulation of the migration to North seems to be for the stretched integration of GEM.
Overestimate
Overestimate
Months
Months

10. Hovmöller analysis (Time-Longitude)
3. Results
Hovmöller = Study of easterly and westerly propagating systems ,
African wave Disturbances (AWD)
The bias is decreasing with the augment of resolution in term of localization and intensity
Time-Longitude diagrams of precipitation (mm/day) Zone I
a) TRMM
b) GEM-LAM 2.0°
c) GEM-LAM 1.0°
d) GEM-LAM 0.33°
e) GEM-LAM 0.15°
mm/day

11. Conclusion Benefits of increased resolution in :
- Representation of precip over the center of Sub-Saharan Africa
- Interannual Variability
could be a result of
African Wave Disturbance & migration of the ITCZ.
Significant bias in the areas of high convection (Atlantic & Ethiopia)
- Incorrect representation of intensity of precipitation in the ITCZ
- the convective trigger function of the KF scheme.
Future plans
- Changing the threshold in the convective trigger function of the KF scheme.

12. Thank you anne-sophie.daloz@cnrm.meteo.fr Colin Jones
Louis-Philippe Caron
Katja Winger
Marko Markovic

13. Fraction of precipitation ( MJJAS)

14. TRMM/GPCP 1998-2003 a) GPCP et b) TRMM.
Moyennes saisonnières (JAS) de la précipitation (mm/jour) pour pour 
a) GPCP et b) TRMM.