Mesoscale Convective Systems Modulated by Convectively Coupled Equatorial Waves

Abstract

Satellite observations of clouds in the tropics reveal a hierarchy of organized cloud clusters ranging from the planetary-scale Madden-Julian Oscillation (MJO), through synoptic-scale convectively coupled equatorial waves (CCEWs), to mesoscale convective systems (MCSs, S. S. Chen et al., 1996;Mapes & Houze, 1993;Nakazawa, 1988). These various types of convective organization are not necessarily independent from one another, and MCSs, with scales of 100s of kilometers, are often considered building blocks of those larger-scale tropical circulations (Mapes et al., 2006). The most intense MCSs often cause high-impact weather events. The frequency of these rainfall extremes has increased in the past few decades and is projected to rise with climate change (Z. Feng et al., 2016;Tabari, 2020;Taylor et al., 2017). While MCSs are difficult to predict in current global weather and climate models due to their smaller scales and dependence on subgrid-scale processes, larger-scale disturbances such as CCEWs can be predicted with higher skill beyond 1 week (

Impact of the Madden–Julian oscillation and equatorial waves on tracked mesoscale convective systems over southeast Asia

“…The enhanced MCS rainfall associated with the active phase of Kelvin waves corresponds with results seen in the literature (e.g., Cheng et al, 2023;Nakamura & Takayabu, 2022), but studies, including Nakamura and Takayabu (2022), suggest that enhanced MCS precipitation is also increased in the active phases of Rossby-1 waves. It should be noted, however, that their study focuses on convectively coupled waves specifically, identifying the active phases through filtering of an infrared radiation dataset, whereas our wave identification focuses on projected wind structures at 850 hPa, which are not always associated with the low brightness temperatures of low cloud.…”

“…On examining the vertical wind shear (200 hPa minus 850 hPa) in the different MJO phases we found that the wind shear is greatest in phases 4-7, which would enhance the size of the anvil. Cheng et al (2023) found an increase in frequency, size, and rain rate during the convectively active MJO compared with climatology but did not split the MJO into the eight different phases shown here. Our results show how these changes in different properties do not all occur in exactly the same MJO phases.…”

“…It has been suggested that, during the progression of an active MJO phase across the Maritime Continent, small individual MCSs become larger and deeper and aggregate to form MCS clusters with the proportion of stratiform rain increasing (Virtz & Houze, 2015;Zuluaga & Houze, 2013), thus mimicking the lifecycle of an individual MCS but over a much longer time-frame. Cheng et al (2023) found that frequency, rain rates, and areas of MCSs were enhanced during the convectively active part of the MJO compared with climatology. The enhanced convection over land and coastal regions precedes that over the ocean by approximately one MJO phase (Birch et al, 2016;Hagos et al, 2016;Oh et al, 2012;Peatman et al, 2014).…”

Environmental Controls on MCS Lifetime Rainfall Over Tropical Oceans

“…Additionally, weak negative correlations between the pre‐MCS moisture and MCS lifetime rainfall can be found in the middle‐to‐upper troposphere (Figure 1b). It is unclear why such a negative correlative relationship exists, but it may reflect the vertical structures of equatorial waves (e.g., equatorial Kelvin waves, Kiladis et al., 2009), since MCS activity over tropical oceans is strongly modulated by these waves (Cheng et al., 2023). The pre‐MCS environmental temperature above 775 hPa is positively correlated with MCS lifetime rainfall, and below that level the correlation is negative (Figure 1c).…”

“…Besides vertical wind shear, other dynamical factors (e.g., large‐scale vertical motion, low‐level convergence) also influence the initiation and organization of tropical oceanic MCSs. Also importantly, MCS precipitation over tropical oceans are often regulated by convectively coupled equatorial waves (Cheng et al., 2023), which can considerably modulate the local dynamical and thermodynamical conditions in which MCSs develop (Kiladis et al., 2009).…”