Challenges and Limitations of regional climate model simulations in West

Africa for Present and Future studies Gregory S. Jenkins

Penn State University

Present and Future Climate issues in West Africa

• Present • Climate Variability• Land-Use Change

• Future • Climate Variability• Land-Use Change• Climate change

Issues in Regional Modeling for West Africa

• Model integration

• Regional Model parameterizations

• The state of knowledge in regional climate systems and observations for verification.

• Science topics

• Extension to other disciplines

Model integration

• Preprocessing

• Initial and Boundary conditions (observations and models)

• Grid Spacing

• Computing Power

• Storage

• Post-processing

Regional Model parameterizations

• Land Surface-Atmosphere transfer parameterization

• PBL parameterization

• Cloud parameterization

• Precipitation schemes

• Radiation schemes

The state of knowledge in regional climate systems and observations for

verification.

• West Africa – Precipitation Processes (trigger, characteristics)– Low level southwesterly winds– 600-700 hPa African Easterly Jet– African Easterly Waves (AEWs)– 200 hPa Tropical Easterly Jet – Meridional surface temperature gradients.

MCSs links to the Water Cycle

• MCSs (squall lines, MCCs, non-squall clusters) are responsible for most of West Africa’s rain.

• MCSs are found primarily in the Sahel region—downstream of mountains.

• A significant fraction of the MCSs are long-lived (> 24 hrs) –Mathon and Laurent, (2001).

MCSs links to the Water Cycle

• MCSs reach their maximum coverage between 1800-0000 UTC.

• The highest frequency of MCCs are over the Sahel.

Nesbitt et al. 2000 for Aug-Oct. 1998

• MCSs are a small fraction of precipitation features in Africa (< 3%) but account for more than 50% of rain.

MCSs are associated with high flash rates (Nesbitt et al. 2000)

Convective Rain Fraction

0.5

0.55

0.6

0.65

0.7

0.75

0.8

May June July August Sept.

1998-2000

Fra

ctio

n

Conv. Frac (Guinea)

Conv. Frac (Sahel)

Stratiform Rain Fractions

0.2

0.25

0.3

0.35

0.4

0.45

May June July August Sept.

1998-2000

Fra

ctio

n

Strat. Frac (Guinea)

Strat. Frac (Sahel)

Model Simulations

• CCM3.6 forced by Observed SSTs (AMIP)

• CCSM (A1B scenario) – SSTs computed from fully dynamic ocean

• RegCM 60km and 90 km runs forced by CCSM.

700 hPa AEJ simulations

CCM/CCSM/Observational Air Temps.

Sea Level Pressure comparison

ECMWF/CCM3 u,v spectrum

ECMWF/CCSM spectrum of u,v

CCM3/CCSM u,v spectrum

Regional Climate model driven by CCSM

Evidence of AEWs from Meridional winds at .688s

Evidence of AEWs from Rain rates

Evidence of AEWs from Water vapor at .668s

Science topics

• Climate Variability

• Land-use Change

• Climate change

• Sensitivity Studies

West African Precipitation Anomalies

“Africa is Highly Vulnerable to Climate Change.”

IPCC 2001

A1B scenario

21st century temp./prec. diff

Extension to other disciplines

• Agriculture

• Water Resources

• Health

• Economic Development

• Policy

Limitations of Regional climate modeling for West Africa

• Resource to complete science (computers, storage, boundary conditions)--especially for climate change integrations.

• Recognition that regional modeling results are very dependent on input of boundary conditions (poor results can be anticipated !).

• Critical mass of scientists to address various science questions.

• Observations to compare to model results.• Forum to discuss/publish results and issues

Challenges of Regional climate modeling for West Africa

• Formation of working groups to pool resources for examining scientific issues (climate variability, land-use, climate change, sensitivity tests, improvements in model parameterization).

• Recognition that regional climate models may need to undergoe significant modification to be a useful tool in a particular region.

• Develop tools that will ease the use of climate modeling (pre-processing, integration, post-processing) on multiple platforms (PCs, workstations, supercomputers).