The observed diurnal cycle of low-level stratus clouds over southern West Africa: a case study
<p>This study presents the first detailed observational analysis of the complete diurnal cycle of stratiform low-level clouds (LLC) and involved atmospheric processes over southern West Africa (SWA). The data used here were collected during the comprehensive DACCIWA (Dynamics-Aerosol-Chemistry...
| Main Authors: | , , , , , , , |
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| Format: | Article |
| Language: | English |
| Published: |
Copernicus Publications
2019-01-01
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| Series: | Atmospheric Chemistry and Physics |
| Online Access: | https://www.atmos-chem-phys.net/19/1281/2019/acp-19-1281-2019.pdf |
| Summary: | <p>This study presents the first detailed observational analysis of
the complete diurnal cycle of stratiform low-level clouds (LLC) and involved
atmospheric processes over southern West Africa (SWA). The data used here
were collected during the comprehensive DACCIWA
(Dynamics-Aerosol-Chemistry-Cloud-Interactions in West Africa) ground-based
campaign, which aimed at monitoring LLC characteristics and capturing the
wide range of atmospheric conditions related to the West African monsoon
flow. In this study, in situ and remote sensing measurements from the
supersite near Savè (Benin) collected during a typical day, which is
characterized by the onset of a nocturnal low-level jet (NLLJ) and the
formation of LLC, are analyzed. The associated dynamic and thermodynamic
conditions allow the identification of five different phases related to the
LLC diurnal cycle: the stable, jet, stratus I, stratus II, and convective
phases. The analysis of relative humidity tendency shows that cooling is a
dominant process for LLC formation, which leads to a continuous increase in
relative humidity at a maximum rate of 6 % h<span class="inline-formula"><sup>−1</sup></span>, until finally saturation is reached and LLC form with a
cloud-base height near the height of NLLJ maximum. Results of heat budget
analysis illustrate that horizontal cold-air advection, related to the
maritime inflow, which brings the cool maritime air mass and a prominent NLLJ
wind profile, has the dominant role in the observed strong cooling of
<span class="inline-formula">−</span>1.2 K h<span class="inline-formula"><sup>−1</sup></span> during the jet phase. The contribution from horizontal
cold advection is quantified to be up to 68 %, while radiative cooling
and sensible heat flux divergence both contribute 16 % to the observed heat
budget below the NLLJ maximum. After the LLC form (stratus phases I and II),
turbulent mixing is an important factor leading to the cooling below the
cloud base, while strong radiative cooling at the cloud top helps to maintain
thick stratus.</p> |
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| ISSN: | 1680-7316 1680-7324 |