Tracking mesoscale convective systems in central equatorial Africa

Abstract

The Congo basin in central equatorial Africa is home to some of the most intense convection in the global tropics. Mesoscale convective systems (MCSs) provide much of the annual rainfall over this region during the March-April-May (MAM) and September-October-November (SON) rainy seasons. Features of these systems are essential to rainfall variability in this region and greatly impact human health, agriculture, livestock, and drought monitoring. Knowledge of variability is hindered by the lack of in-situ observations and meteorological stations. The present study identifies and tracks MCSs for the 33-year period 1983-2015 for MAM and SON. MCS and environmental parameters are calculated for the rainy seasons using satellite and reanalysis data. Spatial distributions of MCS parameters and diurnal cycles for select MCS parameters are compared to prior research. Statistical significance testing is performed to determine if there are meaningful differences between the seasons. Seasonal differences are briefly discussed. 650 hPa relative vorticity patterns suggest localized terrain effects may play a role near a local maximum in MCS initiation frequency in the lee of the Great Rift Valley. Spatial distributions of 33-year MCS counts, trajectories, speeds, sizes, maximum intensities, and durations, based on initiation locations, agree well with prior research. Differences between seasons are statistically significant and variable and latitude dependent. There is high interannual variability among all MCS and environmental parameters.

Relationships between subseasonal‐to‐seasonal predictability and spatial scales in tropical rainfall

“…This combination suggests weak control of daily rainfall events by the large‐ or regional‐scale dynamical factors, and the dominance of thermodynamical local‐scale factors related to the diurnal cycle and land surface conditions. While MCS (Hartman, 2021) and 2–9 day African easterly waves (Fortune, 1980; Galvin, 2010; Wu et al ., 2013) are pronounced, the results suggest relatively weak organization on S2S scales there (Figure 3b,c). Overall, in the case of equatorial and northern Tropical Africa, the results underline the challenge of achieving skillful S2S rainfall forecasts, even for the first week, because the structure of rainfall fields are dominated by small spatial scales with subweekly time scales. Considering rainfall frequency instead of the amount does not lead to larger spatial scales of weekly‐to‐seasonal anomalies over most of Africa (Figure 8).…”

A comprehensive framework for seasonal controls of leaf abscission and productivity in evergreen broadleaved tropical and subtropical forests

“…The expected latitudinal distributions are also evident in Asia/ Australia, where the synchronous regime predominates poleward of 19 N and 15 S and the asynchronous regime predominates between 10 and 15 N. The most important local effect concerns the limited rainfall seasonality and the resulting mix of all three climate-phenology regimes across the equatorial Maritime Continent (Figures 3C and 3F). 37 The expected latitudinal distribution breaks down in equatorial West Africa, dominated by a monsoonal climate, 29,38 where mean annual rainfall and the short duration of the wet season are insufficient to maintain dry-season productivity, 34,39,40 bringing the synchronous regime to unusually low latitudes (Figures 3B and 3E).…”

The Tropical Easterly Jet over Africa, its representation in six reanalysis products, and its association with Sahel rainfall

“…However, other studies suggest that MCS activity might not be the best proxy for rainfall. For example, over the Congo boreal spring rainfall is decreasing (Zhou et al, 2014;Hua et al, 2016) while convective activity (i.e., number and intensity of storms) is increasing (Hartman, 2016(Hartman, , 2020Raghavendra et al, 2018;Taylor et al, 2018). A paper by Hamada et al (2015) might provide the explanation for this apparent paradox.…”

“…They also found contrasts in environmental conditions between the most extreme rainfall events and the most intense convection. This is consistent with the facts that rainfall is much lower over the Congo Basin than over the Amazon and Indonesia, despite greater MCS activity and storm intensity over the Congo (Geerts and Dejene, 2005; Zipser et al ., 2006) and that MCS activity is greater during the March‐to‐May rainy season (the lesser rainy season over the Congo) than during the October–November season (Hartman, 2016, 2020).…”