Congo Basin Hydrology, Climate, and Biogeochemistry. Группа авторов

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СКАЧАТЬ shows that convergence uplift of low‐level wind associated with humid and warm air triggers intense thunderstorms. In addition, high convective potential available energy and low convective inhibition favor rising of air to easily reach the level of free convection. A recent study of Liu et al. (2020) shows that reinforcement of easterly wind is a bed for the development of intense thunderstorms. Skillful forecast of precipitation requires accurate representation of the development of convective systems. But in central Africa, mechanisms triggering and maintaining convection are less studied compared to what is investigated in West Africa, East Africa, or southern Africa. Past studies show that convection over the Congo Basin is influenced by many factors.

      

Figure 2.1 Climatology of rainfall regimes from CHIRPS (Funk et al., 2015) at 0.250 spatial resolution during the period 1981–2010 are represented with (a) spatial distribution of annual rainfall modes; (b) total annual length of rainy days during the wet seasons; and (c) climatological annual cycle of rainfall (black curve) and corresponding cumulative rain anomaly (green curve) at selected grid points over Central Africa along the transects 16°E and 25°E. Vertical lines, red and blue, delineate the rainfall seasons; time series with both sets of colors have a two‐season regime. The onset and cessation date of the season are materialized by dashed and solid lines, respectively. Annual rainfall modes and wet season dates (onset and cessation) are computed using the harmonic method introduced by Liebmann et al. (2001; 2012) and adapted by Dunning et al. (2016) for two‐season regions.

      The direct dynamical effect of topography remains a hot topic and needs to be investigated in detail to unravel the physical mechanism modulating convection and rainfall. The results of Laing et al. (2012) show that deep convection is collocated with maxima in the 925–600 hPa shear and propagating convection is closely associated with moderate low‐level shear, confirming the fact that vertical windshear significantly influences the life of convection. Laing et al. (2012) explored the effect of tropical waves on the propagation of convection. They showed that westward‐propagating convection is suppressed by the dry phase of convectively coupled Kelvin wave and active phases of Madden‐Julian oscillation limit spread of the propagation of convection. But in this region, there is no evidence that one type of wave mostly modulates convective activity (Berhane et al., 2015; Kamsu‐Tamo et al., 2014; Nguyen et al., 2008; Sinclaire et al., 2015). Over central Africa, convection depicts a strong diurnal cycle associated with intense thunderstorms most often in the afternoon due to intense heating of the land during the daytime (Jackson et al., 2009; Vondou et al., 2010). Unfortunately, models struggle to represent this important component. More observations are needed to explore the exact mechanisms that influence mesoscale convective systems to improve simulations of the diurnal cycle of precipitation (Mbienda et al., 2019; Nikulin et al., 2012; Vondou et al., 2017). A recent study by Raghavendra et al. (2016) shows that there is a change in the dynamics of mesoscale convective systems characterized by taller and wider thunderstorms in the Congo Basin, which impact evapotranspiration and moisture convergence.

      Gaps remain in the comprehension of mechanisms triggering convection in central Africa. The effect of mid‐level dry entrainment to preclude deep convection is well established (Holloway & Neelin, 2009). Entrainment of environmental dry air reduces cloud droplet number concentration (Guo et al., 2015) and inhibits deep convection. In the early stage of the convection process, boundary layer turbulence generates shallow clouds that can be diluted by mixing with environmental dry air through entrainment. This prevents deep cloud formation and in turn delays the transition to deep convection (Khairoutdinov & Randall, 2006). Henceforth, the location of dry subtropical deserts over southern Africa and North Africa and associated equatorward mid‐level dry air advection by shallow meridional circulation (Longandjo et al., submitted) impede the triggering or reduce the strength of regional convection over central Africa. Pivotal work in the future should focus on better understanding of the characteristics of rainfall‐producing systems.

2.4 CLIMATE MODELING

      There is an increasing number of recent studies using regional and global models to assess climate over central Africa (e.g., Aloysius et al.; 2016; Creese and Washington 2016, 2018; Dosio et al., 2019; Fotso‐Kamga et al., 2019; Haesnler et al., 2013; Sonkoué et al., 2018; Taguela et al., 2020; Tamoffo et al., 2019; Tchotchou and Mkankam, 2010; Vondou and Haensler 2017; Washington et al., 2013). Using CMIP5 global models, Aloysius et al. (2016) reveal that skills of simulated temperature is better than those of rainfall. There is an important discrepancy in the climatology of rainfall appearing in the seasonality, spatial patterns, and magnitude of precipitation. Tamoffo et al. (2020) highlight the importance of monitoring moisture variables and strength of low‐level flow that transports moisture toward the central African region. The findings of a large ensemble of climate models convey dissimilarity but possible outlines for the rainfall change are owing to the contrasts of climatology features across models. In their investigation, Creese and Washington (2016) demonstrated that simulated precipitation depends on the penetration of the moisture in the Congo Basin, where CMIP5 models strongly disagree. They also call for reinforcement of observations for a better description of the processes interacting and also required to represent convection explicitly in models in the Congo Basin.

Taguela et al. (2020) show that models involved in the CORDEX‐Africa project are able to capture basic features of temperature and precipitation despite the persistence of local biases. In contrast to what is obtained in other African regions, the model ensemble mean does not always depict better results than individual simulation. This interesting fact highlights the reliability of model output, as already mentioned by Washington et al. (2013). Vondou and Haensler (2017) show that increasing model resolution is not always the way to proceed for the enhancement of climate simulation over central Africa, consistent with analysis of Wu et al. (2019). It was pointed out that model formulation mostly impacts precipitation production rather than refinement of resolution. However it is important to note that the amplitude of the diurnal cycle of precipitation is influenced by spatial resolution of the model. For the improvement of the regional model RCA4 to mimic precipitation over the Congo Basin, Tamoffo et al. (2019) use an ensemble of experiments based on moisture transport and regional circulation analysis. Henceforth, they raise the question of the credence of models over central Africa and highlight the fact that much effort should be given to the improvement of boundary layer representation and land‐atmosphere coupling. Another big problem models encounter in central Africa is representation of land–sea contrast. Vondou and Haensler (2017), using regional climate REMO, show that the model tends to overestimate rainfall over the ocean while simulating less precipitation over the Congo Basin in accordance with recent results of Fotso‐Kamga et al. (2019), when they analyzed COSMO model output. Misunderstanding of the central African climate engine can lead to poor mimicking of key regional processes, discrepancies in simulated precipitation, and in turn important unreliable information in the guidelines for the future climate (Cook & Vizy, 2019; Creese et al., 2019; Tamoffo et al., 2020).

2.5 CONCLUSION

      The climate system in central Africa suffers from a lack of attention compared to other regions in Africa. A simplistic view of the annual rainfall regime over the region was adopted and associated with the north–south migration of the ITCZ, which suggests collocation of maximum temperature, low pressure, high cloudiness, and rainfall. This results from conjectures that were later found to be incorrect for the region. The wealth of regional studies over West Africa, East Africa, and southern Africa contributed to advances in the understanding of СКАЧАТЬ