On the cumulus diurnal cycle over the tropical warm pool

Abstract

An idealized cloud-resolving model experiment is executed to study the prominent cumulus diurnal cycle in suppressed regimes over the tropical warm pool. These regimes are characterized by daytime cumulus invigoration and cloud-layer moistening connected with enhanced diurnal cycles in shortwave radiative heating (SW) and sea surface temperature (SST). The relative roles of diurnally varying SW and SST in this cumulus diurnal cycle are assessed, wherein radiation is modeled and SST is prescribed. Large-scale subsidence is parameterized using the spectral weak temperature gradient (WTG) scheme, such that large-scale vertical motion (w wtg ), and hence subsidence drying, is modulated by diurnal changes in diabatic heating. A control simulation exhibits daytime cumulus invigoration that closely matches observations, including midday cloud-layer moistening. This cumulus invigoration is composed of two distinct modes: (1) a midday nonprecipitating (''forced'') mode of predominately shallow clouds, driven by the peak in SST and surface fluxes as the mixed layer deepens and dries; and (2) a precipitating late-afternoon (''active'') mode characterized by deeper clouds in connection with a more moist cloud layer. This cloudlayer moistening is driven by the daytime relaxation of w wtg subsidence, which is prompted by the midday peak in SW. The transition from the surface flux-driven forced mode to the active precipitating mode is accompanied by a transition from relatively small-scale boundary layer circulation cells to larger cells that are highly modulated by cold pools, consistent with observations. When the diurnal cycle is removed, clouds are persistently shallower with virtually no rainfall, emphasizing the inherent nonlinearity of the cumulus diurnal cycle.

Multiple and spin off initiation of atmospheric convectively coupled Kelvin waves

“…In the variable source simulations, the geopotential height anomaly, the wind anomaly and the wind convergence are stronger than in control simulation with the constant source. This link between the diurnally varying heat source and Kelvin waves and a relationship between the diurnal SST variability and development of atmospheric convection during the MJO suppressed phase, noticed by Chen et al (2015) and Ruppert and Johnson (2015), indicates that our simplified simulations are qualitatively agreeing with observations with the caveat that some of the incoming diurnal shortwave signal can be partitioned to directly heat the atmosphere and influence convection development (Ruppert 2016;Ruppert and Johnson 2016). Furthermore, shallow water model solutions suggest that the link between short-term variability of a source and a structure of Kelvin wave response is truly nonlinear and exemplifies interactions across many temporal and spatial scales.…”

“…In effect, the overall moistening is more vigorous than would occur without it. Subsequent modeling work (Ruppert 2016;Ruppert and Johnson 2016) emphasizes importance of the direct impact of the shortwave radiation on convection development and suggests that diurnal oceanic warm layers and the resulting diurnal cycle of SST is not the complete picture. Model studies suggest that in the absence of other forcing, the diurnal oscillation of precipitation responds to the diurnal variability of SST (Chen et al 2015;McLay et al 2012).…”

Diurnal timescale feedbacks in the tropical cumulus regime

“…Large-scale vertical motion (w wtg )-parameterized in this study using the WTG framework [Herman and Raymond, 2014]-exhibits a diurnal cycle with enhanced subsidence at night and relaxed subsidence during the day in response to the shortwave heating cycle [Ruppert and Johnson, 2016]. In addition to driving the diurnal cycle of moisture, the diurnal cycle in w wtg leads to a diurnal phase-shift of net warming in the midtroposphere relative to the boundary layer, in turn causing a cycle of static stability in the lower troposphere (Figure 9c).…”

The Two Diurnal Modes of Tropical Upward Motion

“…Sea surface temperature (SST) is fixed to this initial state, with no skin temperature parameterization. This simulation therefore presents a unique opportunity to assess the distinct role of direct cloud‐radiation interaction in the diurnal cycle, absent the diurnal variation of SST, which is known to modulate convection in suppressed regimes (Ruppert & Johnson, , ; Ruppert, ; Sui et al, ; Webster et al, ). A global animation of the entire simulation can be found at YouTube (https://www.youtube.com/watch?v=XEB4tGXiJzU).…”