The diurnal cycle of cloud cover over southern and central Africa

The current understanding of the temporal and spatial distribution of clouds over southern and central Africa is poor and the regional processes governing cloud occurrence is only weakly understood. This study seeks to improve the current understanding of cloud diurnal variability over this region by providing a base-line diurnal climatology of lowlevel, mid-level and high-level cloud cover. Diurnal variations of cloudiness are examined using ten years of cloud data from latest version of the International Satellite Cloud Climatology Project (ISCCP-D1). The broad seasonal average diurnal variability is explored across the region. Thereafter a more detailed analysis of regionally specific variability is made using a Self-Organising Map. The findings of this study are in broad agreement with previous work. Cloud over the southern and central African region shows clear spatial organisation, most significantly associated with the location of the Intertropical Convergence Zone. The diurnal variation of high-level cloud is large, closely correlated to its mean and is enhanced by orographic features. Minimum high-level cloud occurs at 1100 LST and maximum extent is reached during the evening around 1800 LST, except in locations experiencing deep convection which displayed a redevelopment of cloud in the early morning (0300 LST). This redevelopment of HLCA is hypothesised to be due to the destabilization of the upper troposphere through nighttime cloud radiative cooling. Mid-level cloud exhibits smaller diurnal variations, reaching maximum coverage at approximately 0300 LST. Clouds at this level are severely obscured by higher clouds and therefore the detected diurnal variation is due to both real and artificial signals and care needs to be taken in interpreting the results. Low-level cloud shows strong diurnal variations when not obscured by higher clouds, reaching a maximum just after midday. The results of this study are interpreted in terms of the life-cycle of deep convective cloudiness. A number of mechanisms are suggested to explain the regional differences in diurnal variations with land surface heating being the primary mechanism.

Grantor dc:publisher.institution

Department of Environmental and Geographical Science

Year dc:date.issued

2008

Author dc:creator

• Coop, Lisa Jane

Advisors dc:contributor.advisor

• Hewitson, Bruce
• Tadross, Mark

Language dc:language.iso

eng

Handle dc:identifier.uri

http://hdl.handle.net/11427/4838

OAI identifier oai:identifier

oai:open.uct.ac.za:11427/4838