Classification of Intense Rainfall Days in Southern West Africa and Associated Atmospheric Circulation

Abstract

Daily rainfall in southern West Africa (4-8 • N, 7 • W-3 • E) is analyzed with the aim of documenting the intense rainfall events which occur in coastal Ivory Coast, Ghana, Togo, and Benin. The daily 99th percentile (P99) shows that the coastline experiences higher intensity rainfall than inland areas. Using Tropical Rainfall Measuring Mission (TRMM) rainfall data for 1998-2014, a novel way of classifying the intense events is proposed. We consider their space-time structure over a window of 8 • latitude-longitude and five days centered on the event. A total 39,680 events (62 at each location) are classified into three major types, mainly found over the oceanic regions south of 5 • N, the Bight of Benin, and the inland regions respectively. These types display quite distinct rainfall patterns, propagation features, and seasonal occurrence. Three inland subtypes are also defined. The atmospheric circulation anomalies associated with each type are examined from ERA-interim reanalysis data. Intense rainfall events over the continent are mainly a result of westward propagating disturbances. Over the Gulf of Guinea, many intense events occur as a combination of atmospheric disturbances propagating westward (mid-tropospheric easterly waves or cyclonic vortices) and eastward (lower tropospheric zonal wind and moisture anomalies hypothesized to reflect Kelvin waves). Along the coast, there is a mixture of different types of rainfall events, often associated with interacting eastward-and westward-moving disturbances, which complicates the monitoring of heavy precipitation.

Atmospheric circulation patterns that trigger heavy rainfall in West Africa

“…Moreover, the identified CPs of the subseasonal classification, associated with intense rainfall amounts (CP4 and CP6), are related to westward propagating wind systems at 700 hPa. This is in agreement with the study by Camberlin et al (2020). Thus, the classification can generate relatively reliable CP patterns for the study region of interest.…”

“… Note : a, Moron et al (2008a); b, Laux (2009); c, Guèye et al (2011); d, Moron et al (2018); e, Camberlin et al (2020). …”

“…Laux (2009) linked the onset of the rainy season in the Volta basin of West Africa to large‐scale atmospheric circulation patterns using a fuzzy rule‐based classification method. Camberlin et al (2020) described six different types of intense rainfall events and their associated atmospheric circulation at the daily scale over southern West Africa. A common feature of all types is the existence of westward‐propagating signals at the 700 hPa wind field.…”

Differential signal of change among multiple components of West African rainfall

“…As cluster 4 shows, most of the rainfall in this zone occurs in the boreal summer, with a peak in August (Nicholson et al, 2018). Cluster 2 and 3 have also featured prominently in previous literature as the other three regions described above, with the terms' Guinean' and 'Sudanian' used to describe them, respectively (Akumaga & Tarhule, 2018, Camberlin et al, 2020, possibly due to the vegetation class in these regions.…”

The Phenomenology of West African Coastal Rainfall Events Based on a New Rain Gauge Network over Abidjan (Côte d’Ivoire)

“…In North Benin, Lawin [53] found an average duration per event of about 5 h. In his PhD thesis, Mathon [6] showed that the convective systems south of 9 • N are less organized and, over the Côte d'Ivoire, these systems also have short lifetimes. Maranan et al [59] and Camberlin et al [60] also noted these characteristics of less-organized, smaller, and more short-lived systems along the West African coast. However, beyond meteorological considerations, the difference in the size of the three rain gauge networks could also partly explain the differences observed in the duration of rainfall events.…”