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

Journal Article · 25 November 2019 · Journal of the Atmospheric Sciences

DOI: https://doi.org/10.1175/jas-d-19-0121.1 · OSTI ID:1575795

Adames, Ángel F. ^[1]; Kim, Daehyun ^[2]; Clark, Spencer K. ^[3]; Ming, Yi ^[4]; Inoue, Kuniaki ^[5]

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1. Univ. of Michigan, Ann Arbor, MI (United States)
2. Univ. of Washington, Seattle, WA (United States)
3. Princeton Univ., NJ (United States)
4. National Oceanic and Atmospheric Administration (NOAA), Princeton, NJ (United States)
5. NASA Goddard Inst. for Space Studies (GISS), New York, NY (United States); Columbia Univ., New York, NY (United States)

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 N_mode, 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 (c_p ~ 3 m s^-1) are likely to be moisture modes while fast waves (c_p ~ 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.

• Research Organization: Univ. of Washington, Seattle, WA (United States)
• Sponsoring Organization: USDOE Office of Science (SC); National Science Foundation (NSF); National Aeronautics and Space Administration (NASA); Korea Meteorological Administration Research and Development Program; USDOE
• Grant/Contract Number: SC0019495; 80NSSC17K0227; AGS-1841559; KMI2018-03110
• OSTI ID: 1575795
• Alternate ID(s): OSTI ID: 1803863
• Journal Information: Journal of the Atmospheric Sciences, Vol. 76, Issue 12; ISSN 0022-4928
• Publisher: American Meteorological SocietyCopyright Statement
• Country of Publication: United States
• Language: English