Peter Knippertz et al. – Continental stratus over summertime West Africa Continental Stratus over Summertime West Africa: Observations and Representation in Weather and Climate Models Peter Knippertz, Andreas H. Fink, Robert Schuster, Jörg Trentmann, Charles Yorke

Peter Knippertz et al. – Continental stratus over summertime West Africa  Example case  Climatology  Mechanism  Representation in – ERA-Interim re-analysis – IPCC climate models  Conclusions Outline

Peter Knippertz et al. – Continental stratus over summertime West Africa MSG RGB composite & SYNOP low-cloud cover 0130 UTC 20 Aug An example – Meteosat

Peter Knippertz et al. – Continental stratus over summertime West Africa CALIPSO lidar profile 0130 UTC 20 Aug An example – CALIPSO

Peter Knippertz et al. – Continental stratus over summertime West Africa ISCCP low-level cloud cover 0000 UTC 20 Aug An example – ISCCP

Peter Knippertz et al. – Continental stratus over summertime West Africa Ceilometer Nangatchori, central Benin, 19–20 Aug An example – Ceilometer

Peter Knippertz et al. – Continental stratus over summertime West Africa UHF wind profiler Djougou, central Benin, 19–20 Aug An example – Ceilometer

Peter Knippertz et al. – Continental stratus over summertime West Africa Diurnal cycle of mean summer (JAS) low-cloud cover Climatology – Cloud cover

Peter Knippertz et al. – Continental stratus over summertime West Africa Mean summer (JAS) vertical profile of wind speed from radiosondes Climatology – Wind 03 UTC

Peter Knippertz et al. – Continental stratus over summertime West Africa Mean summer (JAS) solar irradiance GEWEX (JAS) 1983–2007 Climatology – Solar radiation

Peter Knippertz et al. – Continental stratus over summertime West Africa (I) Around sunset mixing in the planetary boundary layer (PBL) ceases  minimum in cloudiness (II) Radiative cooling stabilizes shallow surface layer  winds slacken, moisture accumulates through evapotranspiration (III) Above inversion winds accelerate in response to strong monsoonal north–south pressure gradient  formation of low-level jet (IV) Shear-induced turbulence mixes moisture upwards  formation of ultra-low clouds (V) Sometimes several mixing cycles / intermittent turbulence until cloud deck fully established (VI) Midday / early afternoon solar heating erodes jet and cloud deck  often fair-weather cumuli Mechanism

Peter Knippertz et al. – Continental stratus over summertime West Africa Mean summer (JAS) low-cloud cover ERA-Interim 1989–2010 ERA-Interim – Low-cloud cover

Peter Knippertz et al. – Continental stratus over summertime West Africa Mean summer (JAS) diurnal cycle of cloud cover ERA-Interim 1989–2010 ERA-Interim – Cloud cover

Peter Knippertz et al. – Continental stratus over summertime West Africa Mean summer (JAS) diurnal cycle of wind speed ERA-Interim 1989–2010 ERA-Interim – Wind

Peter Knippertz et al. – Continental stratus over summertime West Africa Mean summer (JAS) solar irradiance ERA-Interim 1989–2010 ERA-Interim – Solar radiation

Peter Knippertz et al. – Continental stratus over summertime West Africa Mean summer (JAS) vertical profile of cloud cover IPCC models IPCC models – Cloud cover

Peter Knippertz et al. – Continental stratus over summertime West Africa Mean summer (JAS) vertical profile of wind speed IPCC models IPCC models – Wind

Peter Knippertz et al. – Continental stratus over summertime West Africa Mean summer (JAS) solar irradiance IPCC models IPCC models – Solar radiation

Peter Knippertz et al. – Continental stratus over summertime West Africa Standard deviation summer (JAS) solar irradiance IPCC models IPCC models – Solar radiation

Peter Knippertz et al. – Continental stratus over summertime West Africa  Southern West Africa is frequently affected by extensive decks of low stratus during the summer monsoon.  The stratus is connected to upward mixing of moisture by shear- induced turbulence underneath a nocturnal low-level jet.  Persistence into the day reduces surface solar radiation.  Observing these phenomena is a challenge for the existing network.  Representation in ERA-Interim reasonable.  IPCC models show massive biases towards too strong jets, too few clouds, too much solar radiation.  Future work: Improve monitoring, modelling and understanding of West African summer stratus decks! Conclusions