African Easterly Waves (AEWs) are the primary precursor for Atlantic tropical cyclones (TCs). We update the statistics on this relationship using reports from the U.S. National Hurricane Center. Sixty‐one percent of TCs originate directly from AEWs. Indirectly, AEWs are implicated in the formation of an additional 11% of TCs. AEW activity is quantified by eddy kinetic energy (EKE). The correlation between seasonal mean EKE and TC genesis is maximized in the lower troposphere below the southern AEW storm track, instead of where the canonical AEW is maximized. Therefore, midlevel AEW activity is a poor predictor of TC genesis, whereas its lower tropospheric circulation exerts stronger control. In most seasons, AEW activity is supercritical, and therefore, EKE is only a controlling factor in seasons when the low‐level EKE is weak. Predicting 1000–800 hPa EKE below the southern AEW track may be useful for seasonal TC prediction.

The dynamics of African easterly waves (AEWs) are investigated from the perspective of potential vorticity (PV) using data from global reanalysis projects. To a leading order, AEW evolution is governed by four processes: advection of the wave-scale PV by background flow, advection of background PV by the AEW, diabatic forcing due to wave-scale moist convection, and coupling between the wave and background diabatic forcing. Moist convection contributes significantly to the growth of AEWs in the midtroposphere, and to both growth and propagation of AEWs near the surface. The former is associated with stratiform clouds while the latter with deep convection. Moist convection helps maintain a more upright AEW PV column against the background shear, which makes the wave structure conducive for tropical cyclogenesis. It is also argued that—contrary to the hypothesis in some prior studies—the canonical diabatic Rossby wave model is likely not applicable to AEWs.

We examine the upscale effect of moist convection on African easterly waves (AEWs) by limiting condensational heating and initial ambient moisture in convection-permitting simulations. Moist convection is fundamental in maintaining and destabilizing AEWs. The contribution from barotropic-baroclinic instability, albeit important, is relatively smaller. Mesoscale convective systems (MCSs) are initiated downstream of the AEW troughs and are associated with extensive trailing stratiform cloud regions. Using a potential vorticity (PV) budget, we show that the attendant diabatic heating profile reinforces the AEW. A model for destabilization is proposed that relies on the phasing of stratiform heating and the PV anomaly of the AEW. It qualitatively resembles stratiform instability and stretched building blocks hypotheses introduced in previous studies. The generation of PV by deep moist convection in the vicinity of the trough counters the shearing effect of the background flow. This helps maintain an upright PV column, which is conducive for formation of tropical cyclones. AEW propagation is dominated by advective processes and intermittently modified by moist convection when large MCSs move ahead of the AEW. Plain Language SummaryAfrican easterly waves (AEWs) are weather systems that impact a broad region of Africa and the tropical Atlantic. The interaction between AEWs and cloud systems is not well understood though. We investigate this problem by simulating two AEWs and then controlling the extent of clouds within these simulations. We find that cloud systems are essential for the growth and maintenance of the AEWs. Multiple cloud systems combine together to produce regions of high and expansive clouds coinciding with the trough of the AEWs. Precipitation formation within those cloud regions leads to a release of heat which in turn leads to new regions of rotation. Because these new regions of rotation coincide with the AEW trough, they maintain and enhance the AEW. Further, new regions of rotation generated in the same manner, but close to the surface by regions of deep cloud at the head of the cloud systems, leads to the deepening of the AEW circulation. The latter point, has implications for the downwind initiation of tropical cyclones in association with AEWs over the tropical Atlantic.

The Integrated Multi-Satellite Retrievals for Global Precipitation Measurement Mission (IMERG) is a global precipitation product that uses precipitation retrievals from the virtual constellation of satellites with passive microwave (PMW) sensors, as available. In the absence of PMW observations, IMERG uses a Kalman filter scheme to morph precipitation from one PMW observation to the next. In this study, an analysis of convective systems observed during the Convective Process Experiment (CPEX) suggests that IMERG precipitation depends more strongly on the availability of PMW observations than previously suspected. Following this evidence, we explore systematic biases in IMERG through bulk statistics.In two CPEX case studies, cloud photographs, pilot’s radar, and infrared imagery suggest that IMERG represents the spatial extent of precipitation relatively well when there is a PMW observation but sometimes produces spurious precipitation areas in the absence of PMW observations. Also, considering an observed convective system as a precipitation object in IMERG, the maximum rain rate peaked during PMW overpasses, with lower values between them. Bulk statistics reveal that these biases occur throughout IMERG Version 06. We find that locations and times without PMW observations have a higher frequency of light precipitation rates and a lower frequency of heavy precipitation rates due to retrieval artifacts. These results reveal deficiencies in the IMERG Kalman Filter scheme, which have led to the development of the Scheme for Histogram Adjustment with Ranked Precipitation Estimates in the Neighborhood (SHARPEN; described in a companion paper) that will be applied in the next version of IMERG.

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