Scale Analysis of Moist Thermodynamics in a Simple Model and the Relationship between Moisture Modes and Gravity Waves

Abstract

Observations and theory of convectively coupled equatorial waves suggest that they can be categorized into two distinct groups. Moisture modes are waves whose thermodynamics are governed by moisture fluctuations. The thermodynamics of the gravity wave group, on the other hand, are rooted in buoyancy (temperature) fluctuations. On the basis of scale analysis, it is found that a simple nondimensional parameter—akin to the Rossby number—can explain the processes that lead to the existence of these two groups. This parameter, defined as Nmode, indicates that moisture modes arise when anomalous convection lasts sufficiently long so that dry gravity waves eliminate the temperature anomalies in the convective region, satisfying weak temperature gradient (WTG) balance. This process causes moisture anomalies to dominate the distribution of moist enthalpy (or moist static energy), and hence the evolution of the wave. Conversely, convectively coupled gravity waves arise when anomalous convection eliminates the moisture anomalies more rapidly than dry gravity waves can adjust the troposphere toward WTG balance, causing temperature to govern the moist enthalpy distribution and evolution. Spectral analysis of reanalysis data indicates that slowly propagating waves (cp ~ 3 m s−1) are likely to be moisture modes while fast waves (cp ~ 30 m s−1) exhibit gravity wave behavior, with “mixed moisture–gravity” waves existing in between. While these findings are obtained from a highly idealized framework, it is hypothesized that they can be extended to understand simulations of convectively coupled waves in GCMs and the thermodynamics of more complex phenomena.

Investigating the Role of Cloud‐Radiation Interactions in Subseasonal Tropical Disturbances

“…The shift in tropical wave activity when CRI is disabled in CESM2 (Figures c and f) also supports the recent work of Adames et al (). In their paper, a shallow water scaling analysis is used to identify processes that determine preferred development of disturbances across a continuum that spans moisture modes (e.g., the MJO) to gravity waves (e.g., Kelvin waves).…”

Investigating the impact of cloud-radiative feedbacks on tropical precipitation extremes

“…Color shading shows the percent change of the non-normalized spectral power of the tropical precipitation component symmetric about the equator 64,65 . Disabling CRE generally weakens disturbances within the so-called moisture mode group of tropical systems: tropical depression-like waves, equatorial Rossby waves, and the MJO 66 . These phenomena are the dominant drivers of extreme precipitation in the tropics and subtropics 67,68 .…”

The Unique Characteristics and Potential Mechanisms of the MJO‐QBO Relationship

“…A number of studies suggest that the MJO and CCEWs have distinct dynamics and growth mechanisms, which are supported by differences in their thermodynamic and dynamic profiles and relationship with moisture (e.g., Adames et al., 2019; Inoue et al., 2020; Raymond & Fuchs, 2007; Sakaeda et al., 2020; Sobel et al., 2001; Yasunaga & Mapes, 2011, 2013). Adames et al.…”

“…Adames et al. (2019) suggest that the more slowly propagating waves such as the MJO and equatorial Rossby (ER) waves behave more like moisture modes, where the weak temperature gradient (WTG) approximation (Sobel et al., 2001) is more applicable to its dynamics, the temperature fluctuations are more fundamental to faster‐propagating waves such as Kelvin and inertio gravity waves. Under the WTG assumption, the static stability impacts the sensitivity of vertical motions that arise in response to diabatic heating (Chikira, 2014; Wolding et al., 2016, 2017).…”