Convective Coupling in Tropical-Depression-Type Waves. Part I: Rainfall Characteristics and Moisture Structure

Abstract

Tropical-depression (TD)-type waves are synoptic-scale disturbances embedded with deep convection over the western North Pacific. Studies of these disturbances began over six decades ago; however, some properties of these disturbances remain vague, e.g., the coupling mechanism between the deep convection and the waves. This two-part study aims to examine the rainfall progression in TD-type disturbances and associated tropospheric moisture controlling convective rainfall. Part I investigates the rainfall and moisture characteristics of TD-type waves using TRMM-derived rainfall products and the ERA-Interim data during the period of June–October 1998–2013. The rainfall features a north–south asymmetrical pattern with respect to a TD-type disturbance, with enhanced convective and stratiform rainfall occurring in the southern portion. Along with the northwestward propagation, deep convective and stratiform rainfall occur in phase with the TD-type disturbance without significant preceding shallow convective rainfall. Following the deepest convection, shallow convective rainfall increases in the anomalous southerlies. Such a rainfall progression differs from the paradigm from shallow to deep convection, then to stratiform rainfall, which is suggested in other convectively coupled equatorial waves. The rainfall progression and the atmospheric moisture anomaly are phase locked to the TD-type disturbances such that the relative displacements change little when the disturbances propagate northwestward. The latent heat release in deep convection, which is obtained from the TRMM 3G25 dataset, superposes with a broad warm anomaly in the mid- to upper troposphere, suggesting wave growth through the generation of available potential energy from diabatic heating.

Mesoscale Convective Systems Modulated by Convectively Coupled Equatorial Waves

“…On the contrary, TD frequency modulation is stronger over Africa and the Atlantic, as well as the Pacific. These regions are known for strong easterly wave activity during northern summer and fall (Cheng et al, 2019;T. Feng et al, 2020;Kiladis et al, 2006;Rydbeck & Maloney, 2014).…”

“…On the contrary, TD frequency modulation is stronger over Africa and the Atlantic, as well as the Pacific. These regions are known for strong easterly wave activity during northern summer and fall (Cheng et al., 2019; T. Feng et al., 2020; Kiladis et al., 2006; Rydbeck & Maloney, 2014). Lastly, the modulation by the MJO appears stronger over the Eastern than Western Hemisphere, consistent with the MJO's strong precipitation and circulation patterns over the warm pool (M. C. Wheeler & Hendon, 2004; Zhang, 2005).…”

Precipitation Characteristics of Easterly Waves Across the Global Tropics

“…The NH precipitation structure shares some similarities with other studies of TEWs in the North Pacific Ocean. Feng et al (2020) found that the greatest rain rates associated with TEWs in the western North Pacific Ocean were located equatorward of the cyclonic vorticity maximum. Huaman et al (2021) also showed the deepest convection south of the TEW trough center for TEWs in the eastern North Pacific.…”

Influences of central Pacific warming on synoptic-scale wave intensity over the northwest Pacific

“…Different types of SSWs may induce distinct moisture structures and modulate the characteristics of tropical rainfall. For example, the spatial structure of TD-type SSWs, in which the coupled in-phase vortex and deep convection slightly precede shallow convective rainfall, is different from the common progressive "shallow-to-deep-to-stratiform rainfall" paradigm (Kiladis 2009;Feng et al 2020a). Moreover, SSWs directly provide a common precursor disturbance for TC genesis and indirectly provide a favorable environment for TC genesis by enhancing local convection and the associated lower-level cyclonic winds (Frank and Roundy 2006).…”