Convection over Tropical Africa and the East Atlantic during the West African Monsoon: Regional and Diurnal Variability

Abstract

The geographic and diurnal variability of moist convection over tropical Africa and the east Atlantic is examined using the Tropical Rainfall Measuring Mission (TRMM) satellite and related to the variability of the convective environment. The stratiform rain fraction is highest within oceanic and continental regions just north of the equator. Both regions have high column relative humidity (CRH). In both monsoon and semiarid continental regions, stratiform rain fractions are significantly higher on days when the CRH is high, which suggests a relationship between these quantities. Large convective systems with high echo tops dominate the rainfall over the Sahel. The importance of CAPE and shear to the development of these types of systems is suggested by the fact these systems are especially common on days when the CAPE and shear are unusually high.Both deep convective and stratiform conditional rain rates increase with the size and echo-top height of convective systems. According to the TRMM Precipitation Radar (PR) near-surface rain rate, the highest deep convective and stratiform conditional rain rates occur off the coast of West Africa. However, comparisons between the PR near-surface rain rate and rain rates computed from Z-R relationships from the literature suggest that deep convective conditional rain rates over the Sahel are underestimated by the TRMM precipitation algorithm. Over the Sahel, small (large) convective systems produce most of the rainfall in the afternoon (early morning). This is associated with enhanced convective rainfall in the afternoon and stratiform in the early morning. The transition from small to large convective systems as convection propagates away from topographic features is also observed.

Objective Climatological Analysis of Extreme Weather Events in Arizona during the North American Monsoon

“…To account for atmospheric moisture content, we consider PWV as an additional thermodynamic parameter to CAPE. In monsoonal regions and in tropical continental regions in general, PWV provides a consistent positive correlation with precipitation (Bretherton et al 2004;Neelin et al 2009;Janiga and Thorncroft 2014) along with the onset and demise of the monsoon (Lu et al 2009). PWV and its temporal evolution have also been shown to have a strong positive relationship with precipitation in southern Arizona and Mexico, using both observations (Kursinski et al 2008a;Serra et al 2016) and sensitivity experiments within a regional atmospheric model: the Weather Research and Forecasting (WRF) Model (Kursinski et al 2008b).…”

Evaluation of historical and future simulations of precipitation and temperature in central Africa from CMIP5 climate models

“…They illustrate that some model simulations (e.g., M18 and M24) have substantially improved compared to their earlier versions and propose that modeling groups should intensify their efforts to The MCSs also play significant role in spatiotemporal variability of precipitation in CA. The importance of topographic (e.g., Rift Valley and Cameroon highlands) and environmental (e.g., relative humidity and convective available potential energy, remote forcings such as SSTs and wind stress) controls on the growth and propagation of MCSs, and the lack of their representation and subsequent poor simulation of precipitation in GCMs are evaluated in recent studies (e.g., see Farnsworth et al [2011], Jackson et al [2009], Janiga and Thorncroft [2014], and [Richter et al, 2014]). Therefore, we emphasize, for instance, that causal mechanisms related to the northward and southward phases of precipitation (Jackson et al [2009] and Hirst and Hastenrath [1983], see Figure 3) and their representation in models require separate consideration.…”

The interaction between moist diabatic processes and the atmospheric circulation in African Easterly Wave propagation

“…Despite the fact that there are rather few atmospheric observations over West Africa, the ERA‐Interim reanalysis fields generally show a good agreement with other observational products (Roberts et al , ). Reanalysis data have been employed in other studies of the climatological structure of African Easterly Waves (Kiladis et al , ; Berry and Thorncroft, ; Bain et al , ; Janiga and Thorncroft, ; Poan et al , ).…”