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