Читать книгу Congo Basin Hydrology, Climate, and Biogeochemistry - Группа авторов - Страница 29

Weather over central Africa involves mechanisms on wide spatiotemporal scales. These processes span from individual convective cells to organized mesoscale convective systems interacting with global systems such as tropical waves (Hartman, 2020; Nguyen et al., 2008). Precipitation occurring from moist convection makes it a crucial component of the water budget in the atmosphere. The Congo Basin appears as one of the major areas of convective activities in the world (Jackson et al., 2009; Nesbitt & Zipser, 2003; Taylor et al., 2018; Webster, 1983) and hotspot of high‐frequency lightning (Zipser et al., 2006). Predisposed large‐scale conditions for strong thunderstorms and lighting are not yet fully explained over the basin due to lack of observations. Analysis from reanalysis 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.