Impact of Moisture Flux and Vertical Wind Shear on Forecasting Extreme Rainfall Events in Nigeria

Abstract

This chapter investigates extreme rainfall events that caused flood during summer months of June–September 2010–2014. The aim is to determine the impact of horizontal moisture flux divergence (HMFD) and vertical wind shear on forecasting extreme rainfall events over Nigeria. Wind divergence and convective available potential energy (CAPE) were also examined to ascertain their threshold values during the events. The data used include rainfall observation from 40 synoptic stations across Nigeria, reanalyzed datasets from ECMWF at 0.125° × 0.125° resolution and the Tropical Rainfall Measuring Mission (TRMM) dataset at resolution of 0.25° × 0.25°. The ECMWF datasets for the selected days were employed to derive the moisture flux divergence, wind shear, and wind convergence. The derived meteorological parameters and the CAPE were spatially analyzed and superimposed on the precipitation obtained from the satellite data. The mean moisture flux and CAPE for some northern Nigerian stations were also plotted for 3 days prior to and 3 days after the storm. The result showed that HMFD and CAPE increased few days before the storm and peak on the day of the storms, and then declined afterwards. HMFD values above 1.0 × 10−6 g kg−1 s−1 is capable of producing substantial amount of rainfall mostly above 50 mm while wind shear has a much weaker impact on higher rainfall amount than moisture availability. CAPE above 1000 Jkg−1 and 1500 Jk−1 are favorable for convection over the southern and northern Nigeria, respectively. The study recommends quantitative analysis of moisture flux as a valuable short-term severe storm predictor and should be considered in the prediction of extreme rainfall.

Extreme rainfall events in the West African Sahel: Understanding storm development over the Damergou gap using convection‐permitting simulations in the Weather Research and Forecasting model

“…For example, precipitable water is 10–30 mm above the climatological normal over the Niger, and 5–8 mm above normal over northern Nigeria for the August 20, 2019 and July 19, 2001 events respectively. Our results are consistent with other studies of West African extreme rainfall events (Knippertz and Martin, 2005; Engel et al ., 2017; Maranan et al ., 2019; Anash et al ., 2020; Fall et al ., 2020; Olaniyan et al ., 2021), and indicate that monitoring the atmospheric moisture content is a useful metric for determining the likelihood of a storm producing extreme rainfall totals. Conversely, environmental CAPE is not necessarily a good predictor of extreme rainfall. Our analysis demonstrates that it is possible for extreme rainfall to occur in a weakly unstable environment (e.g., <2,000 J·kg −1 ).…”

“…Extreme rainfall events are impactful over West Africa, including over the semi‐arid Sahel that lies between 12° N and 20° N. Past studies (Tarhule, 2005; Magami et al ., 2014; Zahiri et al ., 2016; Anash et al ., 2020; Elagib et al ., 2021; Olaniyan et al ., 2021) have noted that flooding from extreme events is associated with socio‐economic impacts including infrastructure, property, and agricultural damage, and the loss of life over this region. Furthermore, the storms responsible appear to be changing, becoming more frequent, better organized, and producing more intense rainfall (Taylor et al ., 2017) leading to an increase in flood‐related damage (Giorgi et al ., 2011; Panthou et al ., 2014).…”