During the months of June to September, the climate of southern West Africa (SWA) is dominated by the southwesterly flow of the West African Monsoon (WAM), which is principally driven by a north–south pressure gradient associated with the Saharan heat low and brings seasonal rains to the region (e.g. ). Clouds, through their diabatic effects, are known to exert an influence on the WAM circulation. For example, a number of studies have explored the role of moist convection in the Sahel (e.g. ), revealing that the diurnal cycle of latent heating and cloud radiative forcing affects the north–south pressure gradient and hence the northward advection of moisture.

Low-level clouds (LLCs) over SWA, with bases only a few hundred metres above ground level (a.g.l.), are also a common occurrence during the WAM season (e.g. ), yet it is only recently that their role has been considered in detail. LLCs typically form near the Guinea coast sometime after sunset following the initiation of the southwesterly nocturnal low-level jet. The jet is linked to the low-level pressure gradient, supplying moisture to the Sahel region where it is mixed as a result of convection during the day . The clouds then spread northwards inland during the night , and typically persist until the late morning, after which they transition to broken cumulus and dissipate. Through their impact on surface solar irradiance, the LLCs play an important role in the evolution of the atmospheric boundary layer (BL)  and the regional climate of West Africa (e.g. ).

The most comprehensive observational study of the atmospheric BL over SWA was conducted recently by  during the DACCIWA field campaign . Between 14 June and 30 July 2016, intensive observations were made at three ground sites – Savé (Benin), Kumasi (Ghana), and Ile-Ife (Nigeria) – using a variety of instrumentation including infrared cloud cameras, radiosondes and wind profilers , radars and ceilometers , and microwave radiometers . These ground-based observations were complemented by in situ measurements of aerosol and cloud properties from three European aircraft, which together conducted 50 research flights between 27 June and 16 July. The results presented in  reveal significant variability in the onset and dissolution of LLCs over SWA from day to day and from site to site. However the governing processes and mechanisms responsible are not fully understood. Furthermore, large-scale models struggle to represent these LLCs and their variability accurately.  found that many current general circulation models (GCMs) suffer a common bias in the form of insufficient low cloud cover over SWA, abundant solar radiation, and thus too large a diurnal cycle in temperature and relative humidity. They concluded that targeted model sensitivity experiments are needed to test possible feedback mechanisms among low clouds, radiation, BL dynamics, precipitation, and the WAM circulation.

Several studies have proposed specific mechanisms relevant for the formation and break-up of the cloud decks (e.g. ). Specifically, LLCs are believed to be sensitive to temperature and moisture advection from the south (controlled by the strength of the low-level jet), vertical mixing of heat and moisture arising due to shear-generated turbulence, radiative cooling at cloud top, condensational heating, sub-cloud evaporation, orographic lifting, and lifting induced by gravity wave propagation. In addition to each of these processes, it is important to also consider the role of microphysics in the evolution of LLCs and the potential modification of the cloud properties via the interaction with aerosols . The combination of ground-based and in situ measurements from DACCIWA offers a unique opportunity to explore the links among aerosols, microphysics, and the bulk cloud properties in SWA and to inform an appropriate level of parameterization for the representation of LLCs in regional and global models.

One particular microphysical process of interest is the role of droplet sedimentation – the gravitational settling of liquid droplets suspended within the cloud layer. Previous studies of non-drizzling marine stratocumulus have demonstrated that droplet sedimentation has a non-negligible impact on the evolution of liquid water path (LWP) (e.g. ). However the role of sedimentation in relation to low-level clouds over SWA has not yet been investigated in detail, despite the potential for changes in aerosol properties in the SWA region to modify the size distribution of cloud droplets (and in turn their sedimentation velocity). Hence the purpose of the present study is to perform large-eddy simulations of a selected DACCIWA case study to isolate the effects of droplet sedimentation and quantify its impact on the ability of the model to reproduce the observations. In doing so, the aim is to identify an optimum configuration for the parameterization of BL clouds over SWA. The rest of this paper is organised as follows. Section 2 presents details of the case study to be simulated; Sect. 3 describes the numerical model used to perform the simulations, along with details regarding model configuration and initialisation, and the results are presented in Sect. 4. Implications of the findings are discussed in Sect. 5, before the main conclusions are summarised in Sect. 6.

For the purpose of this study, we focus on a particular case on 4–5 July 2016, the seventh intensive observation period (IOP) from the DACCIWA field campaign . The satellite image in Fig.  reveals the extent of the cloud coverage over SWA at 10:12 UTC on 5 July 2016. The conditions were fairly typical of the campaign as a whole, with observations collected at the ground site at Savé (labelled “A” in Fig. ) revealing the onset of the low-level jet around 18:00 UTC on 4 July, followed by the formation of a low-level stratocumulus deck during the night. Cloud at the Savé ground site was first observed at 03:00 UTC on 5 July, which persisted until around mid-day local time, after which it began to break up into patchy cumulus (see , their Fig. 6). No precipitation was recorded at the Savé ground site for this case, consistent with the majority of days sampled during the campaign period. For an overview of the diurnal cycle of the atmospheric BL at Savé during DACCIWA, the reader is referred to .

The radiosonde data from IOP 7 provide more information on the structure and evolution of the BL on this day. Profiles of potential temperature and relative humidity from the 03:30 UTC sonde, launched approximately half an hour after the cloud was first detected at Savé, are shown in Fig. a, b. The relative humidity profile reveals a cloud layer approximately 200 m thick, with a cloud top height of 550 m capped by a temperature inversion of 1.5 K. The horizontal wind components, shown separately in Fig. c, d, reveal a low-level jet with a wind speed maximum at a height of 550 m a.g.l., and the cloud layer located directly beneath. Later sondes from 05:00, 06:28, 08:00, and 09:28 UTC (not shown) reveal that the cloud layer persisted throughout the morning, with the relative humidity occasionally peaking just below water saturation, suggesting the presence of some breaks in the cloud cover. This is consistent with images from the infrared camera at Savé (see Fig. ), further analysis of which is presented in  over the whole campaign period. The low-level jet persisted until around 11:00 UTC (Fig. a), by which time the depth of the BL had increased to 1 km due to solar heating of the surface, resulting in lifting of the cloud deck (as shown in Fig. a, b). Three research aircraft were also deployed in sequence on this day, taking in situ measurements along the transect between Lomé (labelled “B” in Fig. ) and Savé from 08:00 UTC through to 18:00 UTC in order to sample the microphysical evolution during the cloud life cycle .

To fulfill the needs of this study we utilise the Met Office NERC Cloud model (MONC; ). MONC is a rewrite of the original Met Office Large Eddy Model (LEM), which has been used extensively over the past 20 years to study cloud processes in a variety of regimes (e.g. ). MONC offers several key advantages over the original LEM, including code optimisations, bug fixes, and a new solver that enables simulations to be performed with relatively large domain sizes without having to compromise on the model resolution.

Radiation is represented in MONC by the Suite of Community RAdiative Transfer codes based on Edwards and Slingo (SOCRATES; ), the same as that used in the Met Office Unified Model, specifically the Global Atmosphere Model 6.0 . SOCRATES is called on a 3 min time step, allowing the effects of longwave cloud top cooling and shortwave absorption within the cloud layer to be captured in the model.

Regarding the treatment of cloud processes, MONC is coupled to the CASIM (Cloud AeroSol Interactive Microphysics) module, a newly developed user-configurable multi-moment scheme that represents five hydrometeor species (cloud, rain, ice, snow, and graupel) and multi-mode aerosols. CASIM has already been used within the Met Office Unified Model to study aerosol–cloud interactions in different meteorological contexts, e.g. , , and . Further details on the specific configuration of CASIM used in the present study are given in Sect. 3.2.

We begin with an initial inspection of results from the CASIM_NO_PROC experiment. Figure a shows a time–height plot of the domain-average cloud mass mixing ratio for the period 05:00–11:00 UTC. The presence of a cloud layer is revealed with an initial mean cloud base around 350 m and a cloud top of 600 m. Following sunrise at 05:37 UTC, the surface fluxes of sensible and latent heat increase sharply from around 07:00 UTC as shown in Fig. b, resulting in a deeper BL and lifting of the cloud layer from around 08:00 UTC. The general trend in the time series of cloud base height is well captured by the model, as seen in the comparison against the ceilometer measurements from the Savé ground site (Fig. b). Cloud top longwave radiative cooling was found to be crucial for the development and maintenance of the cloud, through the generation of an overturning circulation within the cloud layer. Indeed, without any longwave cooling, the model was unable to sustain the cloud layer, resulting in complete dissipation by the end of the spin-up period.

Figure  provides further information about the evolution of the mixing state of the simulated BL, in terms of profiles of liquid water potential temperature, liquid water mixing ratio, and total water mixing ratio following the diagnostic analysis of . Domain-average profiles from 05:30 UTC (Fig. a) reveal a predominantly well-mixed cloud-topped BL capped by a temperature inversion at 600 m a.g.l. A stable layer exists from the surface up to 150 m a.g.l., consistent with longwave cooling of the surface during the night, with a thin fog layer which dissipates by 06:30 UTC. By 11:00 UTC (Fig. b), the increase in surface fluxes produces a deeper, convective BL with an unstable layer at the surface. A well-mixed layer exists between 50 and 400 m a.g.l. in the sub-cloud region, with a hint of a second shallower well-mixed layer directly below the top of the BL, where the values of liquid water mixing ratio are largest. These layers are separated by a transition region between 400 and 900 m a.g.l., where the liquid water potential temperature gradually increases with height. Figure a and b show that the model captures the general deepening of the BL as seen in the observations, with a simulated BL height of 1.1 km by 11:00 UTC compared with 1 km in the corresponding radiosonde profile. The vertical structure of both potential temperature and relative humidity is also reasonably well captured by the model.

Figure a compares the time series of simulated LWP from CASIM_NO_PROC with observations from the vertically pointing ground-based microwave radiometer at Savé . Because the radiometer measurements represent the time evolution at a single location, care must be taken when evaluating the model against this dataset to account for the difference in spatial sampling. Hence in Fig.  we plot both the simulated LWP time series taken from the centre of the model domain diagnosed at 1 min intervals, together with the variability in LWP across the whole domain. The model simulates the evolution of LWP in a manner that is broadly consistent with the measurements, with the observations for the most part lying within ±2 standard deviations of the simulated LWP values. Peak local values of LWP also occur at approximately the correct time in the model, i.e. after 08:00 UTC when the surface fluxes have started to rise sharply. No precipitation was produced by the model during the simulation period, consistent with the measurements at Savé.

Having validated the ability of CASIM_NO_PROC to capture the key features of the observations, we now consider the impact of disabling sedimentation by analysing results from the CASIM_NO_SED experiment. Figure b shows that CASIM_NO_SED underestimates the variability in LWP before 07:30 UTC compared to both the observations and CASIM_NO_PROC. Maps comparing the spatial distribution of LWP within the model domain for both simulations (Fig. ) confirm that the cloud is much more spatially homogeneous in CASIM_NO_SED initially, resembling a largely featureless sheet of stratus as opposed to the more lumpy stratocumulus seen in CASIM_NO_PROC. A comparison of the time series of mean LWP in the domain (solid lines in Fig. ) reveals that, although both simulations show a similar rise and fall pattern with a peak around mid-morning, there are still some notable differences despite neither simulation producing any precipitation. For instance, following completion of the spin-up phase at 05:00 UTC, the rate of LWP growth slows in CASIM_NO_SED relative to CASIM_NO_PROC such that by 07:00 UTC, CASIM_NO_PROC has the higher LWP. The peak LWP in CASIM_NO_SED occurs around the same time but persists for longer, before decreasing sharply around 10:00 UTC. The other difference between the two simulations is in the evolution of the domain mean cloud base height. Figure b shows that CASIM_NO_SED maintains an elevated cloud base height compared to CASIM_NO_PROC throughout the simulation period. Between 05:30 and 08:00 UTC, the mean cloud base height is 60 m higher in CASIM_NO_SED, increasing to an average of 140 m higher between 08:00 and 11:00 UTC.

The link between droplet sedimentation and LWP has been explored previously by   in the context of nocturnal non-drizzling marine stratocumulus layers in the subtropics. Sedimentation was found to ultimately increase LWP, caused by the removal of liquid water from the entrainment zone near cloud top. In turn this reduces the magnitudes of evaporative cooling and longwave radiative cooling, two processes which control the sinking of relatively dry air from the free troposphere into the cloud layer. Conversely, higher cloud condensation nuclei concentrations decrease the mean droplet size and fall speed, reducing sedimentation rates and thus making the cloud more susceptible to the effects of entrainment at the top of the BL. This results in a reduced LWP and a thinner cloud layer for more polluted conditions. We now conduct further analysis of the two MONC experiments to explore whether the results of the present study are consistent with the findings of .

Returning to Fig. , following completion of the spin-up phase at 05:00 UTC, both simulations have the same value of LWP. As mentioned earlier, the initial development of the cloud layer during the spin-up phase is strongly dependent on the mechanism of longwave radiative cooling. By 05:30 UTC, the lack of droplet sedimentation in CASIM_NO_SED means that this experiment is able to maintain a slightly higher liquid water content at cloud top relative to CASIM_NO_PROC, with a more sharply defined peak value (see Fig. c compared to Fig. a). Over the following 1.5 h, the larger liquid water content within the entrainment zone in CASIM_NO_SED promotes stronger evaporative cooling and longwave radiative cooling relative to CASIM_NO_PROC (see Fig. ). This increases the downward heat flux at cloud top, reduces moisture fluxes, and reduces the circulation strength in the BL (Fig. ). The result is a slower rate of LWP growth with time relative to CASIM_NO_PROC, such that by 07:00 UTC, CASIM_NO_PROC has the higher LWP. Thus in CASIM_NO_PROC, the removal of liquid water mass from cloud top due to droplet sedimentation effectively acts to shield the cloud layer to some extent from the effects of entrainment, allowing LWP to grow faster with time.

A closer inspection of Fig.  reveals more information about the relative roles of radiative cooling and evaporative cooling in the evolution of the cloud layer. In both simulations, it is clear that radiative cooling is the dominant process, with peak rates that are typically an order of magnitude larger than those produced by evaporation near cloud top. The absence of sedimentation in CASIM_NO_SED results in larger cooling rates associated with both processes. However, the increase in longwave cooling rates is relatively modest – around 37 % by 07:00 UTC – whereas evaporative cooling rates increase by a factor of 2 within the same time period. Thus in relative terms, the effect of sedimentation appears to have the largest impact on rates of evaporative cooling.

It is important to remember that the present study is over land and the simulation period extends into the daytime, as opposed to the nocturnal marine BL simulated by . Thus it is no surprise that after 08:00 UTC, when the fluxes of sensible and latent heat dominate and the surface layer becomes unstable, the effect of sedimentation on LWP starts to break down. The convective vertical mixing associated with the prescribed sensible and latent heat fluxes coincides with the lifting of the cloud layer and a decrease in LWP, with a more rapid depletion evident in CASIM_NO_PROC (Fig. ). This is consistent with stronger evaporative cooling during mixing associated with the higher LWP around 07:30 UTC.

It is interesting to consider what would happen to the evolution of LWP in the absence of surface-driven mixing. This is important because, although the mean LLC onset time at Savé is 03:00 UTC (around 3 h before sunrise), it is notably earlier at other ground sites (e.g. 00:00 UTC in Kumasi, and 21:00 UTC at Ile-Ife; ). Assuming the sedimentation–entrainment feedback holds true, an earlier LLC onset would allow more time for the effects of sedimentation to impact LWP before sunrise. To explore this idea, both experiments were rerun with surface fluxes set to zero throughout and with shortwave radiation turned off for the duration of the simulation. The forcing of the low-level jet was left unchanged. The results are shown as dashed lines in Fig. . As anticipated, it can clearly be seen that when nocturnal conditions are maintained, CASIM_NO_PROC exhibits a higher LWP by around 33 % relative to CASIM_NO_SED by the end of the simulation period. Based on this analysis, we conclude that the response of the model is consistent with the reasoning of .

The numerical experiments performed in this study have shown that droplet sedimentation helps to promote a more heterogeneous cloud layer, with localised regions of both enhanced LWP and reduced LWP within the model domain relative to simulations without droplet sedimentation, whilst also lowering cloud base height. Whilst surface fluxes remain relatively small, in this case prior to 07:00 UTC, sedimentation also acts to increase the rate of mean LWP growth within the domain by buffering the cloud layer from the effects of cloud top entrainment driven by evaporative cooling and longwave radiative cooling.

Since droplet sedimentation rates are inversely proportional to number concentration, one would expect the effects of sedimentation on both LWP and cloud base height to become more prominent as cloud droplet number concentration (CDNC) reduces. In the case of CASIM_NO_PROC, predicted number concentrations lie in the range of 400–700 cm-3 at STP, which agrees well with in situ measurements with median values of around 500 cm-3 at STP (Jonathan Taylor, personal communication, 2018). In this section we perform some new experiments to explore the sensitivity to reducing CDNC. We introduce results from a new experiment, CASIM_200, which prescribes the initial CDNC to be 200 cm-3. This new simulation produces excessive variability in the LWP field and cloud bases that are too low (Figs.  and  respectively), confirming our hypothesis. This was found to be the case even with autoconversion switched off. The depth of the BL in CASIM_200 is also too shallow by the end of the simulation period, by virtue of the effect of increased droplet size and excessive sedimentation velocity on entrainment. However, mean LWP is slightly lower compared to CASIM_NO_PROC; this is because, around 08:30 UTC, cloud base becomes so low it touches the surface and liquid water is removed from the domain. At 200 cm-3, this removal of liquid is predominantly due to gravitational settling of cloud droplets as opposed to significant warm rain production. Further reductions in CDNC, down to 100 and 50 cm-3, respectively, deplete the LWP even more as a result of an increase in autoconversion. These results, as summarised in Table , suggest that the effects of droplet size on cloud top entrainment rates should not be ignored when considering the diurnal cycle of LLCs in the region.

In light of this result, it is pertinent to consider the potential implications of changes in CDNC in terms of cloud radiative effects. Any elevation of CDNC within urban plumes will increase cloud optical depth in a manner that is proportional to CDNC1/3 and LWP2/3 for shallow clouds. However, the reduced sedimentation associated with the increased CDNC would increase cloud top entrainment and therefore reduce LWP. Hence any effect of increased optical thickness arising from enhanced aerosol concentrations will to some extent be offset by the sedimentation–entrainment feedback and is likely to lessen any first-order indirect effect.

Our findings also have implications for the diagnosis of aerosol–cloud interactions from satellite data. An adiabatic cloud profile is typically assumed when estimating cloud properties from satellites, but a relevant issue here is the extent to which the adiabatic assumption holds in these low-level clouds . Since satellites view cloud top, it is conceivable that the sedimentation–entrainment effect may well bias retrievals significantly.

An important caveat in our results is the prescription of surface fluxes in our simulations; there is no feedback among changes in cloud cover, LWP, and the land surface radiation budget. What happens after sunrise in reality is likely to be dependent on such feedbacks, which the present model configuration is not able to capture due to the lack of an interactive land surface scheme. Coupling of MONC to an interactive land surface scheme is needed to be able to comment fully on the impacts of droplet sedimentation and cloud optical depth on the diurnal cycle of these low-level clouds.

In this study, large-eddy simulations of low-level clouds over southern West Africa have been performed with a focus on establishing the sensitivity of the cloud evolution to the treatment of droplet sedimentation. The simulations are constrained and validated using the unprecedented suite of measurements collected during the DACCIWA field campaign in 2016.

Our results reveal that, even for non-precipitating clouds, the evolution of low-level clouds over southern West Africa is sensitive to the effects of droplet sedimentation, suggesting that this mechanism should not be neglected when performing large-scale simulations of the region. Sedimentation of droplets acts to remove liquid water from the entrainment zone near cloud top, reducing the magnitude of evaporative cooling and longwave radiative cooling during entrainment mixing. This increases the rate of growth of liquid water path during the night-time and early morning period. For the conditions of prescribed subsidence and surface fluxes, the simulation best able to reproduce the observations was the one that came closest to matching the observed droplet number concentrations. Ignoring droplet sedimentation completely reduced variability in liquid water path by around a factor of 2 during the early morning and also elevated the mean cloud base height by an additional 200 m by the end of the simulation period. Conversely, overestimating sedimentation rates, by virtue of reducing the droplet number concentration by a factor of 2 or more relative to observed values, caused cloud base to lower to the surface by 08:30 UTC and liquid water path variability to increase by around a factor of 2. Both these changes degraded the realism of the model simulation with respect to the available observations. In all cases, cloud top longwave radiative cooling during the night was found to be crucial for the formation and maintenance of the clouds.

The link between sedimentation and liquid water path has been noted previously in relation to nocturnal non-drizzling marine BL clouds. But the clouds considered in the present study form over land and persist into the daytime, which means that the effect of sedimentation can potentially play an important role in regulating the surface radiation budget, with consequences for the diurnal cycle of the BL in southern West Africa and possibly the circulation of the West African Monsoon. The results of our study suggest the possibility of a complex feedback chain involving aerosols, sedimentation, entrainment, liquid water path, and surface energy fluxes. We recommend as part of future work that the experiments performed in this study be repeated using an interactive land surface scheme to determine the extent to which the sensitivities shown are modified due to feedbacks between cloud cover and the surface heat flux budget.