Introduction to African weather

Introduction to African weather Dr Caroline Bain Met Office, Exeter, UK

Overview Convection and general circulation A tour around the seasons: West African Monsoon, AEWs, Short rains East Africa, Low-level westerlies Central Africa, AEJ-S, Southern Africa interaction with extra-tropical troughs, tropical-temperate troughs, Long rains East Africa Challenges for models Africa Process Evaluation Group (Africa PEG) Summary

EEXT41 MSG Dust RGB 09/12/2014 1500 UTC

Hazards in squalls/ thunderstorms/ MCSs HAZARD: POTENTIAL OUTFLOW REGIONS, STRONG VERTICAL SHEAR HAZARD: LIGHTNING cool air downdraft HAZARD: INTENSE RAIN warm air updraft HAZARD: STRONG SHEAR ZONE Cold pool HAZARD: STRONG GUSTY WINDS HAZARD: SUDDEN WIND INCREASE AT GUST FRONT

The diurnal cycle of convection Late night/ Early morning Afternoon/ Late evening Composed from GridSat data (Knapp et al., 2011) Composed from remotely sensed ATD lightning data (Met Office)

The annual cycle of African weather - Intertropical convergence zone (ITCZ) Figure: Annual cycle of Average precip over Africa, created from a 12 year mean of GPCP (satellite+station) data

The annual cycle of African weather - SSTs warmest SSTs ITCZ over ocean in this region increases activity cold tongue forming along equator in JJA Aug: coolest SSTs ITCZ over ocean in this region suppressed Figure: Annual cycle of SSTs in the Gulf of Guinea and southern Atlantic, taken from a 30 year mean of OSTIA SST data

A tour around the seasons West African Monsoon, AEWs, Short rains East Africa, Low-level westerlies Central Africa, AEJ-S, Southern Africa interaction with extra- tropical troughs, tropical-temperate troughs, Long rains East Africa

J.-P. Lafore, C. Flamant, F. Guichard, D. J. Parker, D. Bouniol, A. H. Fink, V. Giraud, M. Gosset, N. Hall, H. Holler, S. C. Jones, A. Protat, R. Roca, F. Roux, F. Said and C. Thorncroft (2011): Progress in understanding of weather systems in West Africa. Atmos. Sci. Let. 12: p7 12 Sultan, B., S. Janicot, (2003): The West African Monsoon Dynamics. Part II: The Preonset and Onset of the Summer Monsoon. J. Climate, 16, 3407 3427.

African Easterly Waves 26th August 2010 DANIELLE EARL FIONA GASTON

Convection initiates at or behind the trough V-winds shaded 29Jun12Z African Easterly Waves 29Jun18Z 30Jun00Z Case study: June/ July 2013 30Jun06Z 30Jun12Z Convection moves ahead of the trough AEW trough moves from east to west over a few days

Cartoon of major synoptic weather influences in October-November-December very cool and moist after monsoon (which is now retreating!) African Easterly Jet - South AEJ-S at 600hPa (only present during OND) Low level westerlies at 950hPa Somali jet at 850hPa very hot and dry Namibian Desert (hasn t rained for 6 months)

Short rains and Long rains in East Africa LONG RAINS SHORT RAINS

Low level westerlies Central Africa As the monsoon retreats the centre of convective heating moves over the equator Latent heating in the core of the thunderstorms sets up a thermal and pressure gradient between the coast and the land Cool air flows onto the land supplying moisture to the storms heating cool air DRC/ Congo/ Cameroon West: Gulf of Guinea East: Central Highlands See Pokam et. al (2014)

Synoptic Influences in Southern Africa Low trough pushing south initiating convection Synoptic weather systems influencing from south

Tropical low Diabatic heating L L L Extra-tropical synoptic systems Southern Africa is affected by extra-tropical synoptic high and low pressure systems much more than anywhere else in Africa It is also impacted by localised/ tropical-style convection as well as tropical cyclones

Challenges for Models

Challenges for models Process Representation Predictability Evaluation Finding the truth! Convection Clouds/ radiation Land surface & vegetation Aerosols Orography Extratropical interactions Coasts, rivers, lakes Deserts & rainforests Unpredictability of convectively based weather! Few synoptic influences Weather present on scales of hours rather than days Not many in-situ data sets Historical records poor Difficult to know the truth! Have to be inventive Courtesy of South African Weather Service University of Wyoming

PEG research Blue-skies African Met/climate research Process evaluation from older models New science theories Themed long-standing research interests ACADEMIC PARTNERS AFRICA PEG PEG model development Parameterisation development Performance evaluation Testing and sensitivity tests Model configuration up-to-date MET OFFICE PEG operational Identification of current model deficiencies Evaluation in real-time Priorities of end users AFRICAN NMSs The Africa Process Evaluation Group (Africa PEG) is a UK Met Office initiative to bring together scientists and forecasters working on or using models for Africa for more information contact caroline.bain@metoffice.gov.uk and cathryn.birch@metoffice.gov.uk

Summary African weather is dominated by tropical convection which has a strong diurnal cycle The ITCZ moves north and southwards with the seasons and brings the rainy season to different parts of Africa In West Africa they call the rainy season the West African Monsoon because it is also associated with a shift in prevailing wind direction In East Africa they have two rainy seasons: the Short Rains in OND and the Long Rains in MAM (there may be regional variation on terminology) In Central Africa they do get a lot of rainfall all year round, but particularly in OND when the low level westerlies bring moisture from the Gulf of Guinea In Southern Africa rainy season tropical systems interact with extra-tropical influences creating complex weather situations Models must represent processes on fine scales and evaluation is a challenge The Africa Process Evaluation Group (Africa PEG) is a UK initiative to bring together scientists working on models for Africa

Thank you for listening, any questions? caroline.bain@metoffice.gov.uk