Link between Low Level Clouds and Nocturnal Jet within the West African Monsoon System

During the monsoon season, the coastal region of West Africa is frequently affected by an extensive coverage of nocturnal low stratus clouds, which dissipate during the following morning. These low-level clouds (LLC) are typically associated with a nocturnal low-level jet. The meteorological and climate models have difficulties to correctly represent those clouds and the jet, although they may play a significant role in the Monsoon system through their impact on the surface energy budget, and their interaction with aerosols and deep convection, among others. Many uncertainties about the relevant factors and processes which control their formation, evolution and dissolution remain. Up to know it is still unclear what the main relevant characteristic features are and how far the model representation differs from reality.

The DACCIWA (Dynamics-Aerosol-Chemistry-Cloud-Interactions over West Africa) field experiment, which took place in summer 2016 at three supersites in Ghana, Benin and Nigeria, brought up a very rich and comprehensive observational dataset to cover this issue.

Based on this 7-week dataset, especially on the observations of frequent radiosoundings, an infra-red cloud camera, a UHF wind profiler, a ceilometer and on a cloud radar all based at Savé, Benin, we analyzed the occurrence and features of the nocturnal low level clouds and of the nocturnal jet. We will present a quantified overview of those characteristics, the representative diurnal cycle of the low troposphere, and of the link between the jet and the clouds.

Additionaly, we estimated the relative contribution of the predominant processes that may generate those clouds: radiative cooling, horizontal advection, and low level jet shear-generated turbulence. For this, we consider the cooling close to surface and at the height of the jet, their relative variability, and link them to stability at the surface and with a so-called jet Richardson number. We will show that although those relative contributions and the complexity vary a lot, the characteristics and occurrences of the clouds and the jet are very consistent.