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Masaki Satoh

Abstract

A conservative scheme for a compressible nonhydrostatic model including moist processes is formulated and is tested for experiments involving a squall line. The scheme is based on the flux form equations of total density, momentum, total energy, and the densities of water substance. The time-splitting scheme is used for the temporal scheme. In the small time step integration, the horizontal components of momentum are explicitly integrated, while the vertical components of momentum, density, and total energy are implicitly integrated. In particular, the flux form equation for the total energy is used to guarantee the conservation of the total energy. The internal energy is obtained by subtracting the kinetic energy and the potential energy from the total energy. This method is advantageous for the energy budget analysis. Only the warm rain cloud process is included for cloud physics. Using the squall-line experiments, the water budget and the energy budget are diagnosed and it is confirmed that the conservation of water and total energy is well satisfied.

As a quantitative improvement, more accurate formulas are used for the thermodynamics of the moist atmosphere by taking account of the effects of specific heats of the water substance and the temperature dependency of latent heat. These effects are generally neglected in most numerical models. If accurate moist thermodynamics are used, the total rain is reduced by more than 10% in comparison to the case when the simplified thermodynamics are used. The transportations of physical quantities due to rain are also appropriately introduced using a higher-order advection scheme. In the flux-form formulation, it is found that the change in energy due to the transportation of rain cannot be neglected in general, while that in momentum could be negligible.

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Masaki Satoh

Abstract

Hadley circulations in radiative–convective equilibrium are investigated using an idealistic axially symmetric model. Calculations show that the distribution of temperature in the Hadley cell is controlled by the moist process; the vertical profiles are close to the moist-adiabatic profile in the precipitating ascent branch, and the latitudinal distribution is nearly uniform. A sharp meridional temperature gradient exists within the poleward sloping boundary of the cell. Similar to Held and Hou, the latitudinal gradient of the vertically averaged temperature is determined by the cyclostrophic wind balance with the angular momentum–conserving flow in the upper layer.

The region where the Hadley cell exists can easily be predicted from the relationship between the profiles of the surface temperature and the vertically averaged temperature. Under the condition that the solar flux is specified, however, because of the interaction between the atmospheric circulation and the surface temperature, the behavior of the Hadley cell is a little more complicated. In particular, if the rotation rate is faster than or equal to the terrestrial value, two peaks of the upward motion exist on both sides of the equator.

Contrary to the traditional view of a steady indirect cell (the Ferrel cell), a systematic multicell structure exists in the middle and high latitudes. The horizontal scale of the cells is about 1000 km. They move equatorward at a speed of ∼1 m s−1.

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Masaki Satoh

Abstract

A new dynamical scheme with the conservative forms of the equations of density, momentum, and internal energy is proposed for the nonhydrostatic models. With this scheme, the conservations of the mass and the total energy are satisfied within round-off errors. In particular, methods for the integration of energy are discussed in detail, and three of the approaches are compared; one is in the form of the pressure equation, the second is the integration of internal energy with corrections on the transformation terms, and the third is use of the sum of internal energy and kinetic energy as a prognostic variable. This scheme is incorporated into a nonhydrostatic model with the horizontally explicit and vertically implicit time integration scheme for sound waves, and various numerical experiments for the dry atmosphere are performed. The numerical results show that the conservative properties are well satisfied.

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Yohei Yamada and Masaki Satoh

Abstract

Cloud feedback plays a key role in the future climate projection. Using global nonhydrostatic model (GNHM) simulation data for a present-day [control (CTL)] and a warmer [global warming (GW)] experiment, the authors estimate the contribution of tropical cyclones (TCs) to ice water paths (IWP) and liquid water paths (LWP) associated with TCs and their changes between CTL and GW experiments. They use GNHM with a 14-km horizontal mesh for explicitly calculating cloud microphysics without cumulus parameterization. This dataset shows that the cyclogenesis under GW conditions reduces to approximately 70% of that under CTL conditions, as shown in a previous study, and the tropical averaged IWP (LWP) is reduced by approximately 2.76% (0.86%). Horizontal distributions of IWP and LWP changes seem to be closely related to TC track changes. To isolate the contributions of IWP/LWP associated with TCs, the authors first examine the radial distributions of IWP/LWP from the TC center at their mature stages and find that they generally increase for more intense TCs. As the intense TC in GW increases, the IWP and LWP around the TC center in GW becomes larger than that in CTL. The authors next define the TC area as the region within 500 km from the TC center at its mature stages. They find that the TC’s contribution to the total tropical IWP (LWP) is 4.93% (3.00%) in CTL and 5.84% (3.69%) in GW. Although this indicates that the TC’s contributions to the tropical IWP/LWP are small, IWP/LWP changes in each basin behave in a manner similar to those of the cyclogenesis and track changes under GW.

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Woosub Roh and Masaki Satoh

Abstract

Cloud microphysics of deep convective systems over the tropical central Pacific simulated by a cloud system–resolving model using satellite simulators are evaluated in terms of the joint histogram of cloud-top temperature and precipitation echo-top heights. A control experiment shows an underestimation of stratiform precipitation and a higher frequency of precipitating deep clouds with top heights higher than 12 km when compared with data from the Tropical Rainfall Measuring Mission. The comparison shows good agreement for horizontal distribution and statistical cloud size distributions of deep convective systems. Biases in the joint histogram are improved by changing cloud microphysics parameters of a single-moment bulk microphysics scheme. The effects of size distribution of precipitating hydrometeors are examined. Modification of the particle size distributions of rain, snow, and graupel size distributions based on observed relationships improves cloud precipitation statistics. This study implies that a single-moment bulk cloud microphysics scheme can be improved by employing comparison of satellite observations and diagnostic relationships.

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Daisuke Takasuka and Masaki Satoh

Abstract

As one of the aspects of the diversity of the Madden–Julian oscillation (MJO), the modulation of initiation regions of the boreal-winter MJO is studied in terms of the relationship between intraseasonal and interannual variabilities. MJOs are categorized as those initiated in the Indian Ocean (IO), Maritime Continent (MC), and western Pacific (WP): referred to as IO-MJO, MC-MJO, and WP-MJO, respectively. The composite analyses for each MJO category using observational data reveal that the diversity of MJO initiation regions directly results from the modulation of areas where horizontal advective pre-moistening efficiently occurs via intraseasonal/synoptic-scale winds. This is supported by the difference in the zonal location of equatorial intraseasonal circulations established before MJO initiation, which is related to a spatial change in background convection and associated Walker circulations forced by interannual sea surface temperature (SST) variability. Compared to IO-MJOs (favored in the climatological background on average), MC-MJOs tend to be realized under the eastern-Pacific El Niño-like condition, as a result of eastward-shifted intraseasonal convection and circulation patterns induced by background suppressed convection in the eastern MC. WP-MJOs are frequently initiated under the central-Pacific El Niño-like and positive IO dipole-like conditions, in which the WP is selectively moistened with the aid of background enhanced (suppressed) convection over the WP (the south-eastern IO and the central-to-eastern Pacific). This major tendency derived from sample-limited observations is verified by a set of 15-year numerical experiments with a global nonhydrostatic MJO-permitting model under a perpetual boreal-winter condition where observation-based SSTs are prescribed.

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Tomoki Ohno and Masaki Satoh

Abstract

On the basis of numerical results of a three-dimensional model diagnosed using balance dynamics, a mechanism by which the upper-level warm core of tropical cyclones (TCs) forms is proposed. The numerical results reveal that an upper-level warm core develops when TCs intensify just prior to reaching the mature stage. Potential temperature budget analysis reveals that for the tendency of potential temperature, the azimuthal-mean component of advection is dominant at the upper level of the eye at the mature stage. Sawyer–Eliassen diagnosis shows that tendencies due to forced flow by diabatic heating and diffusion of tangential wind are dominant in the eye and are negatively correlated to each other. The distributions of the diabatic heating in the simulated TC are not peculiar. Therefore, it is unlikely that the heating distribution itself is the primary cause of the flow from the lower stratosphere. The analyses of forced circulations of idealized vortices show that the upper-level subsidence is enhanced in the eye when the vortex is sufficiently tall to penetrate the statically stable stratosphere. This result is deduced because the stronger inertial stability extends the response to the heating of the lower stratosphere and causes upper-level adiabatic warming. Therefore, the upper-level warm core emerges if angular momentum is transported into the lower stratosphere due to processes such as convective bursts. The present analysis suggests that TCs can be even stronger than that expected by theories in which the TC vortex is confined in the troposphere.

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Kazuyoshi Oouchi and Masaki Satoh

Abstract

This chapter proposes a working assumption as a way of conceptual simplification of the origin of Madden–Julian oscillation (MJO)-associated convection, or super cloud cluster (SCC). To develop the simplification, the importance of the synoptic-scale cold reservoir underlying the convection and its interaction with the accompanying zonal–vertical circulation is highlighted. The position of the convection with respect to that of climatological warm pool is postulated to determine the effectiveness of this framework. The authors introduce a prototype hypothesis to illustrate the usefulness of the above assumption based on a numerical simulation experiment with a global nonhydrostatic model for the boreal summer season.

Premises for the hypothesis include 1) that the cloud cluster (CC) is a basic building block of tropical convection accompanying the precipitation-generated cold reservoir in its subcloud layer and 2) that a warm-pool-induced quasi-persistent zonal circulation is key for the upscale organization of CCs. The theory of squall-line structure by Rotunno, Klemp, and Weisman (hereafter RKW) is employed for the interpretation. No account is taken regarding the influences of equatorial waves as a first-order approximation. Given the premises, an SCC of O(1000) km scale is interpretable as a gigantic analog of a multicellular squall line embedded in the quasi-stationary westerly shear branch of the zonal circulation east of the warm water pool. A CC corresponds to the “cell,” and its successive formation to the east and westward movement represents an upshear-tilting core of intense updraft. The upshear-tilted SCC is favorably maintained with the precipitating area being separated from the gust front boundary between the cold reservoir and a low-level easterly, which is supported in the realm of the RKW theory where two horizontal vortices associated with the cold reservoir and vertical shear are opposite in sign but cold reservoir’s vorticity can be inferred to be larger, leading to upshear-tilted and multicellular behavior. As a counterexample, CCs to the west of the warm pool (Indian Ocean and Arabian Sea) are embedded in the easterly shear and organized into a less coherent cloud cluster complex (CCC) given the situation of RKW where two horizontal vortices associated with the cold reservoir and vertical shear are still opposite in sign, but the smaller vertical shear west of the warm pool causes even more suboptimal vorticity imbalance in the western flank of cold reservoir, leading to larger tilt with height and intermittent, less viable storm situations.

A cold pool or cold reservoir, having been prevalent in mesoscale convection research, is argued to be important for the MJO as pointed out by the emerging evidence in the international field campaign for the MJO called Cooperative Indian Ocean Experiment on Intraseasonal Variability (CINDY)/DYNAMO. The simplified and idealistic hypothesis proposed here does not cover all aspects of MJO and its validation awaits further modeling and observational studies, but it can offer a framework for characterizing a fundamental aspect of the origin of MJO-associated convection.

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Masaki Satoh and Yuya Matsuda

Abstract

Statistics on high-altitude cloud areas associated with deep cumulus clouds and their sensitivities to cloud microphysics are studied in the framework of single-cloud experiments with an explicit cloud system–resolving model. A comprehensive six-category single-moment bulk cloud microphysics scheme is used to investigate parameter dependency. High-cloud areas are defined by the threshold values of the outgoing longwave radiation, and probability distribution functions of high-cloud areas are obtained. First, resolution dependencies on grid sizes of approximately 3.5, 7, and 14 km are examined. It is found that although quantitative differences are confirmed, diurnal variations in high-cloud covers are similarly captured by all three experiments conducted. The main focus of the sensitivity experiments of cloud microphysics is on the fall speed and number concentration, or mean radius, of ice particles. The results clearly show that the sum of snow and cloud ice amounts is closely related to high-cloud covers. Among the number of experiments conducted, one interesting result is that the intercept parameters of snow and graupel have opposite effects on high-cloud covers. As the intercept parameter of graupel increases, the graupel amount increases because of an increase in the accretion rate of cloud water by graupel, which results in a decrease in the amount of snow and hence a decrease in high-cloud covers. This implies that a greater production of graupel leads to an increase in precipitation efficiency.

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Daisuke Takasuka and Masaki Satoh

Abstract

As one of the aspects of the diversity of the Madden–Julian oscillation (MJO), the modulation of initiation regions of the boreal-winter MJO is studied in terms of the relationship between intraseasonal and interannual variabilities. MJOs are categorized as those initiating in the Indian Ocean (IO), Maritime Continent (MC), and western Pacific (WP), referred to herein as IO-MJOs, MC-MJOs, and WP-MJOs, respectively. The composite analyses for each MJO category using observational data reveal that the diversity of MJO initiation regions directly results from the modulation of areas where horizontal advective premoistening efficiently occurs via intraseasonal/synoptic-scale winds. This is supported by the difference in the zonal location of equatorial intraseasonal circulations established before MJO initiation, which is related to a spatial change in background convection and associated Walker circulations forced by interannual sea surface temperature (SST) variability. Compared to IO-MJOs (favored in the climatological background on average), MC-MJOs tend to be realized under the eastern-Pacific El Niño–like condition, as a result of eastward-shifted intraseasonal convection and circulation patterns induced by background suppressed convection in the eastern MC. WP-MJOs are frequently initiated under the central-Pacific El Niño–like and positive IO dipole–like conditions, in which the WP is selectively moistened with the aid of background enhanced (suppressed) convection over the WP (the southeastern IO and the central-to-eastern Pacific). This major tendency derived from sample-limited observations is verified by a set of 15-yr numerical experiments with a global nonhydrostatic MJO-permitting model under a perpetual boreal-winter condition where observation-based SSTs are prescribed.

Open access