Tropical Cyclones and Equatorial Waves in a Convection‐Permitting Aquaplanet Simulation With Off‐Equatorial SST Maximum

Abstract

Tropical weather phenomena—including tropical cyclones (TCs) and equatorial waves—are influenced by planetary‐to‐convective‐scale processes; yet, existing data sets and tools can only capture a subset of those processes. This study introduces a convection‐permitting aquaplanet simulation that can be used as a laboratory to study TCs, equatorial waves, and their interactions. The simulation was produced with the Model for Prediction Across Scales‐Atmosphere (MPAS‐A) using a variable resolution mesh with convection‐permitting resolution (i.e., 3‐km cell spacing) between 10°S and 30°N. The underlying sea‐surface temperature is given by a zonally symmetric profile with a peak at 10°N, which allows for the formation of TCs. A comparison between the simulation and satellite, reanalysis, and airborne dropsonde data is presented to determine the realism of the simulated phenomena. The simulation captures a realistic TC intensity distribution, including major hurricanes, but their lifetime maximum intensities may be limited by the stronger vertical wind shear in the simulation compared to the observed tropical Pacific region. The simulation also captures convectively coupled equatorial waves, including Kelvin waves and easterly waves. Despite the idealization of the aquaplanet setup, the simulated three‐dimensional structure of both groups of waves is consistent with their observed structure as deduced from satellite and reanalysis data. Easterly waves, however, have peak rotation and meridional winds at a slightly higher altitude than in the reanalysis. Future studies may use this simulation to understand how convectively coupled equatorial waves influence the multi‐scale processes leading to tropical cyclogenesis.

The Representation of Convectively Coupled Kelvin Waves in Simulations With Modified Wave Amplitudes

“…In fact, the Atlantic CCKW was not just poorly predicted, but it was hardly represented in the modeling system at all. On the surface, this seems to contrast previous MPAS-A experiments demonstrating good skill in their representation of CCKWs (Judt, 2020;Rios-Berrios et al, 2020, 2022Rios-Berrios, Davis, & Martinez, 2023;Rios-Berrios, Judt, et al, 2023;Weber et al, 2021). However, other CCKWs in our simulations were still well captured, and the IFS system also struggled to capture the same CCKW event (Figure 16).…”

“…However, Weber et al (2021) also noted that these 15-km experiments underestimated the amplitude of the wave's rainfall, likely due to an inaccurate linear relationship between moisture and precipitation. Finally, Rios-Berrios, Davis, and Martinez (2023) found that MPAS-A aquaplanet simulations produced realistic CCKW structures at both parameterized and convection-permitting resolutions. The evidence showing that MPAS-A can accurately simulate CCKWs motivates our use of this model for this study.…”

“…The Model for Prediction Across Scales-Atmosphere (MPAS-A; Skamarock et al, 2012), is one modeling system that has shown relatively high skill in the representation of CCKWs. MPAS-A experiments using realistic input conditions (Judt & Rios-Berrios, 2021), aquaplanet configurations (Hagos et al, 2013;Rios-Berrios et al, 2020, 2022Rios-Berrios, Davis, & Martinez, 2023;Rios-Berrios, Judt, et al, 2023), and for an individual case study (Weber et al, 2021) have all demonstrated realistic CCKW structure and behavior. Furthermore, Judt (2020) found that CCKWs in stormpermitting MPAS-A simulations exhibited limited error growth over a 20-day period.…”