Convection–Kelvin Wave Coupling in a Global Convection-Permitting Model

Abstract

A convectively coupled equatorial Kelvin wave (CCKW) was observed over the equatorial Indian Ocean in early November 2011 during the DYNAMO field campaign. This study examines the structure of the CCKW event using two simulations made using the MPAS model: one with 3-km grid spacing without convective parameterization and another with a 15-km grid and parameterized convection.Both simulations qualitatively capture the observed structure of the CCKW, including its vertical tilt and progression of cloud/precipitation structures. The two simulations, however, differ substantially in the amplitude of the CCKW-associated precipitation. While the 3-km run realistically captures the observed modulation of precipitation by the CCKW, the 15-km simulation severely underestimates its magnitude. To understand the difference between the two MPAS simulations regarding wave-convection coupling within the CCKW, the relationship of precipitation with convective inhibition, saturation fraction, and surface turbulent fluxes is investigated. Results show that the 15-km simulation underestimates the magnitude of the CCKW precipitation peak in association with its unrealistically linear relationship between moisture and precipitation. Precipitation, both in observations and the 3-km run, is predominantly controlled by saturation fraction and this relationship is exponential. In contrast, the parameterized convection in the 15-km run is overly sensitive to convective inhibition and not sensitive enough to environmental moisture. The implications of these results on CCKW theories are discussed.

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

“…Perturbations to wave amplitude in these experiments also demonstrate that changes to rain rate within CCKWs are closely tied changes to CSF and DCIN (Figures 12d and 12f). The relationship with CSF was especially clear (Figure 12d), supporting previous work indicating that CSF partially controls CCKW rainfall (Weber et al, 2021;Wolding, Dias, Kiladis, Ahmed, et al, 2020;.…”

“…Simulated anomalies in rain rate, CSF, and DCIN are composited relative to tracked CCKWs and compared with IMERG and ERA5 data (Figure 12). As discussed by Weber et al (2021), longitude composites such as these can be interpreted as having time on the x-axis and increasing toward the left, due to the eastward propagation of CCKWs. There is a pronounced peak in IMERG-derived rain rate near the tracked center of both CCKWs (Figures 12a and 12b).…”

“…They found that CCKWs contributed nearly the same percentage of rainfall variance in coarse parametrized experiments (∼15 km, ∼120 km) as they did for storm-permitting runs (∼3.75 km) and observational data. Meanwhile, Weber et al (2021) used MPAS-A to simulate a single CCKW event over the Indian Ocean. While they found that storm-resolving resolutions (∼3 km) more completely captured the magnitude of the CCKW signal, a parameterized 15-km experiment was still able to simulate the main details of CCKW structure and evolution, including its vertical tilt and progression of cloud features.…”

“…Most studies have opted to look at how these waves are collectively represented in forecast systems across a large sample (e.g., Dias et al, 2023;Gehne et al, 2022;Judt & Rios-Berrios, 2021). Only a few recent studies have focused on high-impact CCKW events and their predictability (Doyle et al, 2023;Senior et al, 2023;Weber et al, 2021). This may serve as a limiting factor for our understanding of the representation and predictability of CCKWs in forecast models.…”

Response of convectively coupled Kelvin waves to surface temperature forcing in aquaplanet simulations

“…These coherence relationships are visualized in Figure S1. These criteria are motivated by the understanding that KW precipitation is modulated by interactions with both atmospheric moisture and temperature, as suggested by recent findings (Weber et al 2021). We posit that our tailored definition of the KW band is better suited for exploring the behavior of KWs under various climate scenarios.…”