Low-Level Cloud Development and Diurnal Cycle in southern West Africa
during the DACCIWA Field Campaign: Case Study of Kumasi Supersite, Ghana
The presence, spatial extent and persistence of low-level clouds (LLCs)
largely impact on the diurnal surface radiation and energy balance, as
well as, the regional climate. Notwithstanding, there is limited
understanding on their evolution and processes, particularly in southern
West Africa. This paper assesses the development of LLCs and their
dominant formative factors, as well as, their relationship with
radiation and energy balance. Firstly, ceilometer and radiosondes
deployed during the DACCIWA (Dynamics-Aerosol-Chemistry-Cloud
Interactions in West Africa) field campaign were used in identifying the
LLC. Afterwards, the cloud fraction was employed to characterize
different LLC phases. Averagely, break-up, dissipation and build-up of
LLC were marked at 0900 GMT, 1200 GMT and 2200 GMT respectively.
Moreover, composites of LLC diurnal evolution and their relationship
with net radiation, energy storage and surface stability showed that
LLCs significantly impact on net radiation flux, by reducing downwelling
shortwave radiation. Additionally, LLC onsets were characterized by a
near-steady state in net radiation flux, whereas the rate of energy
storage within the lower layers marginally oscillated about equilibrium.
Finally, with observations from selected intensive observation periods
(IOPs), the dominant factors influencing LLC development were evaluated.
Horizontal cold air advection, with enhancement by nocturnal low-level
jets, was observed to primarily influence the development of LLCs for
the study period. Findings of this paper are necessary for improving the
understanding of LLC characteristics, formation and interactions with
surface properties, particularly over southern West Africa.