target="_blank" rel="nofollow" href="#ulink_0c452155-f875-5142-9f4d-71052b99b9b1">Figure 3.7 Maps of rainfall (mm/mo) for March, based on nine satellite products and three gauge products (GPCC, individual stations, NIC131‐gridded) (from Nicholson et al., 2019; © American Meteorological Society. Used with permission).
Figure 3.8 Interannual variability of rainfall (mm/yr) over the central Congo Basin and over a large portion of the Central African Republic (modified from Nicholson et al., 2019). Rainfall is averaged for March–April (left) and for October–November (right). The number of available gauge stations in each year is indicated below each graph, with the top graph indicate the number in GPCC and the bottom indicating the numbers in the NIC131 archive.
Source: Jackson et al., 2009. © American Meteorological Society. Used with permission.
Figure 3.8 shows the interannual variability of March/April and October/November rainfall over the Congo Basin and over the Central African Republic (CAR) to the north. The gauge network is dense over CAR and the satellite products are in good agreement with each other and with the NIC131‐gridded data set. For the Congo Basin, a fair amount of gauge data was available until the mid‐1990s, after which time there is wide disparity among the estimates and little agreement with the NIC131‐gridded data set. These results suggest that the reason for the poor performance of satellite products in this region is the paucity of gauge data.
When the products shown above were validated against gauge data (Camberlin et al., 2019; Nicholson et al., 2019), the best performing products appeared to be CHIRPS for mean rainfall, TRMM for daily rainfall, and CHIRPS2 and TRMM for interannual variability. Both products show generally good agreement with gauge data over the Congo on monthly time scales and thus are selected for use in this study.
Most rainfall analyses in this chapter are based on the CHIRPS2 satellite product (Funk et al., 2015). It has a spatial resolution of 0.05° × 0.05° and a daily temporal resolution. CHIRPS2 begins in 1981 and extends through 2019. However, TRMM 3B43 Version 7 is used to evaluate rainfall over the Amazon. It runs from 1998 to 2014 and has a spatial resolution of 0.25 degrees of latitude/longitude. Its temporal resolution is monthly. Both CHIRPS2 and TRMM 3B43 Version 7 have been extensively validated over equatorial Africa and show a close relationship to gauge rainfall (e.g., Camberlin et al., 2019; Nicholson et al., 2019). TRMM 3B42 V7 is used here to ascertain the diurnal cycle of rainfall. Its successor from the global precipitation measurement mission (GPM), IMERG, has much higher temporal and spatial resolution and is available since 2014. However, it has not been validated over the Congo Basin and for that reason TRMM is used instead.
3.4. MEAN RAINFALL
3.4.1. Annual Rainfall
Figure 3.9 shows mean annual rainfall based on two products, CHIRPS2 and raw gauge data in the NIC131 archive. The raw gauge data are averaged for the period 1945 to 1984, the period with the greatest number of gauges in the Congo Basin. CHIRPS2 was averaged over the time period 1981 to 2019. A comparison with gauge data during that time period would be desirable, but the station density over the Congo Basin is too low. In that the averaging period for both data sets is roughly four decades, the long‐term means derived from the two should be comparable. In fact, there is excellent agreement between the two analyses, despite the much lower resolution of the gauge data. There is, likewise, excellent agreement with the analysis of Bultot (1971) for 1930–1959, which is based on some 500 stations in the Congo (Camberlin et al., 2019).
Figure 3.9 (a) Mean annual rainfall (mm) based on gauge data for the period 1945 to 1984 and CHIRPS2 data for the period 1981 to 2019. (b) Mean rainfall for four individual months from CHIRPS2 for the period 1981 to 2019.
Throughout the basin mean annual rainfall exceeds 1250 mm, but it exceeds 1500 mm over most of the region. In the core of the basin, annual rainfall exceeds 1750 mm. Within that area are three maxima, located at roughly the equator and 20°E, 2°S, and 28°E, and 3°N and 28°E. CHIRPS2 shows the maximum at 2°S and 28°E to be the strongest. Mean annual rainfall exceeds 2000 mm in at least some areas. Rainfall maxima are also evident along the Atlantic coast between roughly the equator and 7°N, over the Ethiopian highlands, over the western portion of Lake Victoria, and in a small sector of western Kenya (northeast of Lake Victoria). The aridity of eastern Africa contrasts sharply with the high rainfall over the Congo Basin.
3.4.2. Monthly Rainfall
Figure 3.9 also shows mean rainfall during four individual months based on CHIRPS2: January, April, July, and October. The gauge analysis indicates similar results and hence is not shown. The seasonal shift in the tropical rainbelt over the region is clearly evident. In January peak rainfall is south of the equator, with monthly rainfall on the order of 100 to 200 mm. It exceeds 150 mm over most of that area. North of the equator monthly rainfall progressively decreases from 100 mm near the equator to less than 10 mm at around 6°N. In April the rainbelt is displaced northward. Mean rainfall is on the order of 100 to 200 mm throughout the latitude span of 10°S to 7°N, with rainfall decreasing rapidly further north. Further displacement is evident in July, with high rainfall north of the equator and a rapid decrease southward from the equator. July rainfall decreases from roughly 50 mm near the equator to less than 10 mm in the southernmost regions. In October the rain belt retreats southward and high rainfall extends throughout the basin. The rainbelt is most extensive and strongest during this month. Vast areas receive 200 to 300 mm of rainfall.
3.4.3. The Seasonal Cycle
Compared to other regions of Africa, the seasonality of rainfall over the Congo Basin is relatively weak. Over most of the region only 30 to 40% of the rainfall falls within the wettest quarter of the year (Figure 3.10). In semi‐arid regions to the north and south, that figure is generally 60% to more than 90%. Traditionally, the seasonal pattern over the Congo Basin is considered to be bimodal, with two rainy seasons and two dry seasons. The two rainy seasons are considered to be MAM and SON (e.g., Dyer et al., 2017; Pokam et al. 2014; Washington et al. 2013,). The latter season is generally assumed to be the wettest season, June to August (JJA) the driest. These generalizations are most applicable in the central basin. Beyond that area there is considerable diversity in the seasonal cycle.
Figure 3.11 shows the seasonal cycle averaged for 5‐degree grid cells. The map on the left is based on gauge data for the period 1945 to 1984 and the map on the right is based on CHIRPS2 for the period 1981 to 2019. The Congo Basin lies roughly within the latitude span 10°N to 10°S. Just north of the equator (latitudes 0° to 5°N) a bimodal pattern is evident across the region to roughly 30°E (roughly where the East African highlands begin). However, within that sector it is well‐developed (i.e., with very
