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Jun-Ichi Yano

ABSTRACT

Objectively identifying a phenomenon from observation is often difficult. This essay reflects upon this problem from a philosophical perspective by taking the Madden–Julian oscillation (MJO) as an example. I argue that it can be considered as a problem of Gestalt. This concept is introduced by closely following Ludwig Wittgenstein’s two philosophical works, Philosophical Investigations (Philosophische Untersuchungen) and Remarks on the Philosophy of Psychology(Bemerkungen über die Philosophie der Psychologie). Reflections upon the concept of Gestalt suggest why an objective identification of a phenomenon is so difficult. Importantly, the problem should not be reduced to that of “pattern recognition.” Rather a given phenomenon must be considered as a whole, including a question of a driving mechanism.

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Jun-Ichi Yano

Abstract

Arakawa and Schubert’s cumulus parameterization is formulated under the two asymptotic limits: σ → 0 and τ c/τ L → 0, where σ is the fractional area occupied by cumulus convection, and τ c and τ L are the characteristic timescales for convection and large scales, respectively. The present note shows that the two asymptotic limits tend to contradict each other, so that the smallness of these two quantities is established only under a compromise. This contradiction is formally established by referring to the heat engine theories by Rennó and Ingersoll, and by Emanuel and Bister. The climatological estimate for σ indicates that the timescale separation is only marginally satisfied with respect to the tropical planetary-scale circulation.

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Jun-Ichi Yano
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Jun-Ichi Yano

Abstract

The basic idea of the maximum entropy principle is presented in a succinct, self-contained manner. The presentation points out some misunderstandings on this principle by Wu and McFarquhar. Namely, the principle does not suffer from the problem of a lack of invariance by change of the dependent variable; thus, it does not lead to a need to introduce the relative entropy as suggested by Wu and McFarquhar. The principle is valid only with a proper choice of a dependent variable, called a restriction variable, for a distribution. Although different results may be obtained with the other variables obtained by transforming the restriction variable, these results are simply meaningless. A relative entropy may be used instead of a standard entropy. However, the former does not lead to any new results unobtainable by the latter.

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Jun-Ichi Yano

Abstract

The present study shows by a linear hydrodynamic stability analysis that an unstable mixed-layer deep circulation can be generated in the dry convective well-mixed layer by the entrainment from the top. The newly identified instability arises under the two competing processes induced by the top entrainment: the destabilization by generating thermal perturbations and the damping by mechanical mixing. The former and the latter, respectively, dominate over the other in the limits of large and small scales. As a result, the instability is realized at the horizontal scales larger than the order of the mixed-layer depth (ca. 1 km), and the time scale for the growth is about 1 day. This study has been motivated from a question of whether the cloud-top entrainment instability (CTEI) can induce a transition of the stratocumulus-topped well-mixed boundary layer into trade cumulus. The present study intends to extend the previous studies based on the local parcel analyses to a full analysis based on the hydrodynamics. Unfortunately, being based on a dry formulation, the present result does not apply directly to the CTEI problem. Especially, the evaporative cooling is totally neglected. Nevertheless, the present result can still be applied to moist systems, to some extent, by redefining certain terms in the formulation.

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Jun-Ichi Yano and Marine Bonazzola

Abstract

A systematic scale analysis is performed for large-scale dynamics over the tropics. It is identified that two regimes are competing: 1) a dynamics characterized by balance between the vertical advection term and diabatic heating in the thermodynamic equation, realized at horizontal scales less than L ∼ 103 km given a velocity scale U ∼ 10 m s−1, and 2) a linear equatorial wave dynamics modulated by convective diabatic heating, realized at scales larger than L ∼ 3 × 103 km given U ∼ 3 m s−1. Under the first dynamic regime (balanced), the system may be approximated as nondivergent to leading order in asymptotic expansion, as originally pointed out by Charney.

Inherent subtleties of scale analysis at large scales for the tropical atmosphere are emphasized. The subtleties chiefly arise from a strong sensitivity of the nondimensional β parameter to the horizontal scale. This amounts to qualitatively different dynamic regimes for scales differing only by a factor of 3, as summarized above. Because any regime under asymptotic expansion may have a wider applicability than a formal scale analysis would suggest, the question of which one of the two identified regimes dominates can be answered only after extensive modeling and observational studies. Preliminary data analysis suggests that the balanced dynamics, originally proposed by Sobel, Nilsson, and Polvani, is relevant for a wider range than the strict scale analysis suggests.

A rather surprising conclusion from the present analysis is a likely persistence of balanced dynamics toward scales as small as the mesoscale L ∼ 102 km. Leading-order nondivergence also becomes more likely the case for the smaller scales because otherwise the required diabatic heating rate becomes excessive compared to observations by increasing inversely proportionally with decreasing horizontal scales.

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Jun-Ichi Yano and Robert Plant

Abstract

The present paper presents a simple theory for the transformation of nonprecipitating, shallow convection into precipitating, deep convective clouds. To make the pertinent point a much idealized system is considered, consisting only of shallow and deep convection without large-scale forcing. The transformation is described by an explicit coupling between these two types of convection. Shallow convection moistens and cools the atmosphere, whereas deep convection dries and warms the atmosphere, leading to destabilization and stabilization, respectively. Consequently, in their own stand-alone modes, shallow convection perpetually grows, whereas deep convection simply damps: the former never reaches equilibrium, and the latter is never spontaneously generated. Coupling the modes together is the only way to reconcile these undesirable separate tendencies, so that the convective system as a whole can remain in a stable periodic state under this idealized setting. Such coupling is a key missing element in current global atmospheric models. The energy cycle description used herein is fully consistent with the original formulation by Arakawa and Schubert, and is suitable for direct implementation into models using a mass flux parameterization. The coupling would alleviate current problems with the representation of these two types of convection in numerical models. The present theory also provides a pertinent framework for analyzing large-eddy simulations and cloud-resolving modeling.

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Jun-Ichi Yano and Kerry Emanuel

Abstract

The Wind-Induced Sea–Air Heat Exchange (WISHE) Model of the 30–60-day oscillation developed by Emanuel is improved by adding downdrafts to the representation of convection and by coupling the troposphere to a passive stratosphere into which equatorial waves may propagate. The downdrafts are associated with a precipitation efficiency that is less than unity; this means that not all of the adiabatic cooling due to ascent in a wave disturbance can be countered by condensation heating, and the wave therefore “feels” a stable stratification as in the work of Neelin et al. As in the latter's model, growth rates of eastward-propagating Kelvin-like modes asymptote to a constant at large zonal wavenumber.

The presence of the stratosphere is shown to have a profound effect on the unstable tropospheric modes. As the upward group velocity is larger for smaller zonal wavelengths, short waves in the troposphere are strongly damped and the most unstable mode shifts to low wavenumbers.

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Jun-Ichi Yano and Agostino Manzato

Abstract

It is typically interpreted that more moisture in the atmosphere leads to more intense rains. This notion may be supported, for example, by taking a scatterplot between rain and column precipitable water. The present paper suggests, however, that the main consequence of intense rains with more moisture in the atmosphere is that there is a higher chance of occurrence rather than an increase in the expected magnitude. This tendency equally applies to any rains above 1 mm (6 h)−1, but otherwise to a lesser extent. The result is derived from an analysis of 33 local rain gauge station data and a shared sounding over Friuli Venezia Giulia, northeast Italy.

Significance Statement

Moisture is the source of clouds. Clouds, in turn, are source of rain. So we may expect that more moisture in the atmosphere causes more intense rains. We may further speculate that with more moisture in the atmosphere as a consequence of the global warning, we must face more catastrophic rain events and floods. However, this paper, by analyzing data over Friuli Venezia Giulia, northeast Italy, suggests otherwise: more moisture indeed increases frequencies of intense rains, but not their magnitudes as much.

Open access