Congo Basin Hydrology, Climate, and Biogeochemistry. Группа авторов
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Название: Congo Basin Hydrology, Climate, and Biogeochemistry
Автор: Группа авторов
Издательство: John Wiley & Sons Limited
Жанр: География
isbn: 9781119656999
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Figure 3.22 Five‐year averages of (left) relative number of MCSs per year and (right) total volumetric rainfall (km2 mm/h × 104) from MCSs (from Jackson et al., 2009, based on TRMM data; © American Meteorological Society. Used with permission). All data are averaged for a 1° × 1 °grid box.
3.6. CONVECTIVE ACTIVITY
The Congo Basin is considered to be the site of the world‘s most intense thunderstorms. The thunderstorms are associated with MCSs. The precise definition of an MCS varies among authors, but a common one, from Nesbitt and Zipser (2003) is a precipitation feature with “at least 2000 km2 of contiguous area with a 85‐GHz polarization corrected temperature of ≤250 K and 185 km2 ≤ 225 K.” These systems are characterized by ice crystals and an anvil cloud in the upper levels. They develop in the late afternoon, producing primarily convective rainfall. At night, as the anvil spreads, a large proportion of the rainfall is stratiform.
Figure 3.22 shows the average number of MCSs per year over equatorial Africa. The number is termed “relative” because of the low sampling frequency of the TRMM satellite and the narrowness of the swath of its radar instrument. Over the central areas of the basin there is a distinct maximum in the number of MCSs per year and in the amount of rainfall from MCSs. Within that area are two local maxima at roughly 20°E and 28°E. Other areas with a distinct maximum include the mountainous region of Cameroon just north of the equator at roughly 8°E, over the Ethiopian highlands, and over Lake Victoria near the equator at 32°E. All MCS maxima correspond closely to the annual rainfall maxima described in Section 4.1 and evident in Figure 3.8a. Maxima in lightning frequency (Figure 3.23) also coincide with the maxima in MCS occurrence.
Figure 3.23 Five‐year averages of relative number of lightning flashes per year (from Jackson et al. 2009, based on TRMM data © American Meteorological Society. Used with permission). All data are averaged for a 1° × 1 °grid box, with a three‐point smoothing applied.
Figure 3.24 shows the diurnal cycle of MCS activity and associated rainfall. These are averages in two latitudinal spans across Africa: 0° to 10°N and 10°S to 0°. There is a minimum in MCS activity in the morning hours, then an increase to a maximum in late afternoon and early evening. The MCS count remains high through the night. The MCS count is notably lower in the southern latitude span than in the northern. The mean volumetric rainfall (total of stratiform plus convective rainfall) per MCS has a minimum in the afternoon, during the hours when the percent convective rainfall is highest. It is highest between midnight and 9 a.m., when convective rainfall is augmented by stratiform rainfall. The stratiform rainfall reaches 40 to 45% during this time period. The volume of rainfall per MCS during the nighttime is lower for the more northern areas.
Figure 3.24 Diurnal cycle of convection: annual average of the relative number of MCSs, the mean volumetric rainfall per MCS, and the percentage of convective rainfall in three‐hour intervals (from Jackson et al., 2009; © American Meteorological Society. Used with permission). Solid line represents averages across Africa in the latitudes 0° to 10°N. Dashed line represents averages for 10°S to 0°. Time indicated is local time. Volumetric rainfall is km2 mm/h × 104.
Hartman (2016, 2020) studied the interannual variability of MCS characteristics over the Congo Basin during the period 1984 to 2015. Some of the variables considered were total number, mean duration, minimum cloud‐top temperature (CTT, a measure of MCS intensity), and area. Hartman’s calculations for the latitudes of 0° to 5°N (region 1) and 0° to 5°S (region 2) are shown in Figure 3.25. For both regions there is a slight tendency for the number of MCSs to increase over time. The increase is largest in MAM in region 2, the area with strongest MCS activity. In region 1 there is a tendency for increasing/decreasing duration and area in MAM/SON. Minimum CTT decreases over time in both seasons, suggesting more intense MCSs. In region 2, the duration generally increases and the area generally decreases over time in both seasons. As in region 1, CCT decreases over time in both seasons. In both regions the decrease is greater in MAM than in SON.
The noted increase in MCS intensity over the Congo is consistent with the findings of Raghavendra et al. (2018), who found an increase in the intensity of thunderstorms over the Congo Basin during JJAS over the period 1982 to 2016. Taylor et al. (2018) also found an increase in MCS intensity over the Congo Basin during February. Hartman’s results for MAM, in particular the increased number and intensity of MCSs, appear to be at odds with the decrease in MAM rainfall. A paper by Hamada et al. (2015) might provide an explanation for this apparent paradox. They found that throughout the tropics the heaviest rain events are mostly associated with less intense convection. They also found contrasts in environmental conditions between the most extreme rainfall events and the most intense convection. The extreme rain events have lower moist static energy and temperatures (i.e., are less convectively unstable), higher relative humidity, and higher moisture flux convergence than the extreme convective events. Hamada et al. (2015) concluded that the contribution of warm‐rain processes might help to explain the weak relationship between extreme rainfall and extreme convection.
Figure 3.25 Interannual variability of select characteristics of MCSs over the Congo Basin during the period 1984 to 2015 (from Hartman, 2016). These include number, duration, minimum cloud‐top temperature, and area. Trend lines based on least squares regression are indicated. MAM and SON are shown separately. Region 1 extends from 0° to 5°N and region 2 extends from 0° to 5°S.
3.7. COMPARISON WITH THE AMAZON
As mentioned earlier, one of the open questions concerning the Congo Basin is why rainfall is relatively low compared to other equatorial areas, such as the Amazon. Here a brief comparison is made between the Congo Basin and the Amazon, in order to examine some aspects of that question.
Figure 3.26 presents mean annual rainfall over the Amazon Basin and mean rainfall for four months. TRMM 3B43 V7 is used here instead of CHIRPS2 because it has been more extensively validated over the Amazon (e.g., Zulkafli et al., 2014). Note that the means over the Congo Basin from TRMM are nearly identical to those from CHIRPS, so that the comparison with rainfall over the Congo Basin in СКАЧАТЬ