Abstract
AbstractThis second part of a two‐part study uses Weather Research and Forecasting simulations with aquachannel and aquapatch domains to investigate the time evolution of convectively coupled Kelvin waves (CCKWs). Power spectra, filtering, and compositing are combined with object‐tracking methods to assess the structure and phase speed propagation of CCKWs during their strengthening, mature, and decaying phases. In this regard, we introduce an innovative approach to more closely investigate the wave (Kelvin) versus entity (super cloud cluster or “SCC”) dualism. In general, the composite CCKW structures represent a dynamical response to the organized convective activity. However, pressure and thermodynamic fields in the boundary layer behave differently. Further analysis of the time evolution of pressure and low‐level moist static energy finds that these fields propagate eastward as a “moist” Kelvin wave (MKW), faster than the envelope of organized convection or SCC. When the separation is sufficiently large the SCC dissipates, and a new SCC generates to the east, in the region of strongest negative pressure perturbations. We revisit the concept itself of the “coupling” between convection and dynamics, and we also propose a conceptual model for CCKWs, with a clear distinction between the SCC and the MKW components.
Highlights
The close correspondence between Matsuno’s 1966 modes and the OLR spectral peaks associated with the ITCZ variability, as seen on a wavenumber-frequency diagram, was found by Takayabu (1994) and Wheeler and Kiladis (1999, hereinafter WK99) and since the concept of Convectively Coupled Equatorial Waves (CCEWs) has been widely used
The generally small lag between OLR, divergence at 850 mb (DIV850) and qtotal at 500 mb (Q500) indicates that these 3 variables are highly correlated with the super cloud clusters” (SCCs) life cycle
We introduce the concept of Moist Kelvin Wave (MKW), to characterize a phenomenon that is physically different than the SCCs: the latter are shorter-lived entities that can coexist in time, and propagate somewhat slower
Summary
The close correspondence between Matsuno’s 1966 modes and the OLR spectral peaks associated with the ITCZ variability, as seen on a wavenumber-frequency diagram, was found by Takayabu (1994) and Wheeler and Kiladis (1999, hereinafter WK99) and since the concept of Convectively Coupled Equatorial Waves (CCEWs) has been widely used. Some of the more widely accepted ideas are: Wave-Conditional Instability of the Second Kind (wave-CISK; Hayashi 1970; Lindzen 1974), Wind-Induced Surface Heat Exchange (WISHE) or evaporation-wind feedback (Neelin et al 1987; Emanuel 1987), and the stratiform-instability/vertical-mode theory (Mapes 2000; Khouider and Majda 2006; Kuang 2008) This last theory provides an alternative explanation for the slower propagation of CCKWs compared to their dry counterparts. The major finding of these works was the hierarchy (in terms of different spatial and temporal scales) of both leftward and rightward propagating waves, with great resemblance to more realistic CCEWs, as seen by Hovmoller diagrams and power spectra One of these tropical waves corresponds to the Kelvin mode, in absence of planetary rotation effects the waves are referred to as convectively coupled gravity waves (CCGWs).
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