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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. 2006
An example – Meteosat
Peter Knippertz et al. – Continental stratus over summertime West Africa
CALIPSO lidar profile 0130 UTC 20 Aug. 2006
An example – CALIPSO
Peter Knippertz et al. – Continental stratus over summertime West Africa
ISCCP low-level cloud cover 0000 UTC 20 Aug. 2006
An example – ISCCP
Peter Knippertz et al. – Continental stratus over summertime West Africa
Ceilometer Nangatchori, central Benin, 19–20 Aug. 2006
An example – Ceilometer
Peter Knippertz et al. – Continental stratus over summertime West Africa
UHF wind profiler Djougou, central Benin, 19–20 Aug. 2006
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
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