Interactions of Diabatic Heating in Convective Superbursts with Energy Conversion Processes in the Genesis of Cape Verde Hurricanes from African Easterly Waves

Abstract

Abstract This paper defines a mechanism for the genesis of tropical cyclones from African easterly waves (AEWs) over the eastern Atlantic, the so-called Cape Verde storms. Convective “superbursts” produce strong diabatic heating, which then strengthens the African easterly jet (AEJ), leading to enhanced barotropic energy conversions, which occur at the critical developmental stages of the system. Diabatic heating is calculated using the Ertel isentropic potential vorticity (IPV) equation, while energy conversions are determined using energy equations first derived by Lorenz. The genesis mechanism is developed from studying Hurricane Bill (2009), as well as Tropical Storm Debby, Hurricane Helene, and a nondeveloping AEW, all from the 2006 NASA African Monsoon Multidisciplinary Analysis (NAMMA) field experiment, using the NCEP Final (FNL) analyses and the Advanced Research Weather Research and Forecasting model (WRF-ARW) simulations. A striking and singular maximum in the diabatic heating due to the convective superburst is shown to precede by 24–36 h a pronounced maximum in positive barotropic energy conversion, which is demonstrated to occur simultaneously with the strengthening of the AEJ. The maximum in barotropic energy conversion is documented to occur in the developmental stages of the system, typically in the depression or early storm stages. A physical mechanism is developed to explain how a mesoscale convective superburst can lead subsequently to an enhanced synoptic-scale AEJ over the eastern Atlantic, an enhanced jet that is critical to the genesis mechanism. The findings agree with cited idealized studies by other investigators who found that moist AEWs grow 3 times stronger than dry waves as a result of faster AEJ development and larger barotropic energy conversions.