Abstract
AbstractThe representation of convection remains one of the most important sources of bias in global models, and evaluation methods are needed that show that models provide the correct mean state and variability, both for the correct reasons. Here we develop a novel approach for evaluating rainfall variability due to convectively coupled Kelvin waves (CCKWs) in this region. A phase cycle was defined for the CCKW cycle in OLR and used to composite rainfall anomalies. We characterize the observed (TRMM) rainfall response to CCKWs over tropical Africa in April and evaluate the performance of regional climate model (RCM) simulations: a parameterized convection simulation (P25) and the first pan-Africa convection-permitting simulation (CP4). TRMM mean rainfall is enhanced and suppressed by CCKW activity, and the occurrence of extreme rainfall and dry days is coupled with CCKW activity. Focusing on regional differences, we show for the first time that there is a dipole between West Africa and the Gulf of Guinea involving onshore/offshore shifts in rainfall, and the transition to enhanced rainfall over west equatorial Africa occurs one phase before the transition over east equatorial Africa. The global model used to drive the RCMs simulated CCKWs with mean amplitudes of 75%–82% of observations. The RCMs simulated coherent responses to the CCKWs and captured the large-scale spatial patterns and phase relationships in rainfall although the simulated rainfall response is weaker than observations and there are regional biases that are bigger away from the equator. P25 produced a closer match to TRMM mean rainfall anomalies than CP4 although the response in dry days was more closely simulated by CP4.
Highlights
The majority of rainfall over tropical Africa is associated with the tropical rain belt, a zone of heavy rainfall and deep convection that, in contrast to the oceanic intertropical convergence zone (ITCZ), is generally located equatorward of the maximum low-level wind convergence (Nicholson 2018)
The convectively coupled Kelvin waves (CCKWs) anomalies in the G25 simulation are weaker than the anomalies in the observations, varying between 75% and 82% of the observed mean in each phase—a result that is in agreement with the OLR anomalies shown in Figs. 1e and 1f
The amplitude of CCKWs in observed OLR anomalies is greatest in April and accounts for ;15% of the variance in Tropical Rainfall Measuring Mission (TRMM) daily rainfall
Summary
The majority of rainfall over tropical Africa is associated with the tropical rain belt, a zone of heavy rainfall and deep convection that, in contrast to the oceanic intertropical convergence zone (ITCZ), is generally located equatorward of the maximum low-level wind convergence (Nicholson 2018). Coupled Kelvin waves (CCKWs) are equatorially trapped envelopes of convection that travel. CCKWs were originally regarded as super cloud clusters (SCCs; Nakazawa 1988). The significant overlay between the Matsuno curves and the OLR spectral peaks yielded the convectively coupled equatorial waves (CCEWs) definition. The SCCs associated with the theoretical, nondispersive Kelvin mode were regarded as CCKWs. Recently, Blanco et al (2016) proposed a decoupling mechanism for idealized CCKWs
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