Masaki Ishiwatari

Coupled ocean‐atmospheric waves on the equatorial front

Striking 20–30-day sea surface temperature waves observed along the equatorial front in the later half of the year are generally believed to be of an oceanic origin. Here we report the detection of atmospheric waves that are unambiguously tied to these oceanic waves, using new satellite measurements of surface winds. A general circulation model simulation reveals that these atmospheric waves have a shallow vertical structure trapped in the planetary boundary layer (PBL), unlike El Nino/Southern Oscillation where changes in deep convection are the cause of anomalous winds. Vertical wave motion penetrates well above the PBL and is likely to impact the distribution and transport of climatically important gas species such as ozone and dimethyl sulfide.

Numerical modeling of Jupiter's moist convection layer

Moist convection of Jupiters atmosphere is examined using a large-domain two-dimensional fluid dynamical model with simplified cloud microphysics of water. The result shows that the water condensation level acts as a dynamical and compositional boundary. The convection below the condensation level is characterized by a steady regular cellular structure and a homogeneous distribution of water mixing ratio. Above the condensation level, cloud elements accompanied by the upward motion develop and disappear irregularly but successively, and water mixing ratio is highly inhomogeneous. The horizontal average of mixing ratio decreases rapidly with height just above the condensation level, resulting in a distinctive stable layer at 5 bar. The stable layer prevents the air masses above and below it from mixing with each other. As a result, the upper dry air does not reach 20 bar level, where the Galileo probe observed low humidity.

A Numerical Study on Appearance of the Runaway Greenhouse State of a Three-Dimensional Gray Atmosphere

Abstract A numerical study on the runaway greenhouse state is performed by using a general circulation model (GCM) with simplified hydrologic and radiative processes. Except for the inclusion of three-dimensional atmospheric motion, the system utilized is basically equivalent to the one-dimensional radiative–convective equilibrium model of Nakajima et al. in which the runaway greenhouse state is defined. The results of integrations with various values of solar constant show that there exists an upper limit of the solar constant with which the atmosphere can reach a statistical equilibrium state. When the value of solar constant exceeds the limit, 1600 W m−2, the atmosphere sets in a “thermally runaway” state. It is characterized by continuous increase of the amount of water vapor, continuous decrease of the outgoing longwave radiation, and continuous warming of the atmosphere and the ground surface. The upper-limit value of the solar constant obtained by the GCM experiments corresponds to the upper limit ...

Linear stability of thermal convection in rotating systems with fixed heat flux boundaries

Linear stability of rotating thermal convection in a horizontal layer of Boussinesq fluid under the fixed heat flux boundary condition is examined by the use of a vertically truncated system up to wavenumber one. When the rotation axis is in the vertical direction, the asymptotic behavior of the critical convection for large rotation rates is almost the same as that under the fixed temperature boundary condition. However, when the rotation axis is horizontal and the lateral boundaries are inclined, the mode with zero horizontal wavenumber remains as the critical mode regardless of the rotation rate. The neutral curve has another local minimum at a nonzero horizontal wavenumber, whose asymptotic behavior coincides with the critical mode under the fixed temperature condition. The difference of the critical horizontal wavenumber between those two geometries is qualitatively understood by the difference of wave characteristics; inertial waves and Rossby waves, respectively.

The effects of thermal conditions on the cell sizes of two-dimensional convection

The effects of thermal conditions on the patterns of two-dimensional Boussinesq convection are studied by numerical integration. The adopted thermal conditions are (i) the heat fluxes through both upper and lower boundaries are fixed, (ii) the same as (i) but with internal cooling, (iii) the temperature on the lower boundary and the heat flux through the upper boundary are fixed, (iv) the same as (iii) but with internal cooling, and (v) the temperatures on both upper and lower boundaries are fixed. The numerical integrations are performed with Ra = 104 and Pr = 1 over the region whose horizontal and vertical lengths are 8 and 1, respectively.The results confirm that convective cells with the larger horizontal sizes tend to form under the conditions where the temperature is not fixed on any boundaries. Regardless of the existence of internal cooling, one pair of cells spreading all over the region forms in the equilibrium states. On the other hand, three pairs of cells form and remain when the temperature on at least one boundary is fixed. The formation of single pairs of cells appearing under the fixed heat flux conditions shows different features with and without internal cooling. The difference emerges as the appearance of a phase change, whose existence can be suggested by the weak nonlinear equation derived by Chapman & Proctor (1980).

A numerical study of the Martian atmospheric convection with a two-dimensional anelastic model

Thermal convection of the Martian lower atmosphere is examined by the use of a two-dimensional anelastic model with a resolution fine enough to describe convection eddies.For a homogeneous radiative cooling of 50 K/day given in the layer below 5 km, a layer of time-dependent convection develops up to about 6 km in height. The intensity of realized vertical winds ranges up to 20 m/s. The dust, which is injected into the lowest layer and treated as a passive tracer, is transported immediately in the convection layer and mixed uniformly.The intensity of the horizontal winds near the surface reaches about 10 m/s, which, combined with large-scale motions, is expected to contribute to the dust injection into the atmosphere.

Physical interpretation of unstable modes of a linear shear flow in shallow water on an equatorial beta-plane

Unstable modes of a linear shear flow in shallow water on an equatorial β-plane are obtained over a wide range of values of a non-dimensional parameter and are interpreted in terms of resonance between neutral waves. The non-dimensional parameter in the system is E ≡ γ 4 /(gHβ 2 ) , where y, g, H and β are the meridional shear of basic zonal flow, gravitational constant, equivalent depth and the north-south gradient of the Coriolis parameter, respectively. The value of E is varied within the range -2.50 ≤ log E ≤ 7.50. The problem is solved numerically in a channel of width 5γ/β. The structures of the most unstable modes, and the combinations of resonating neutral waves that cause the instability, change according to the value of E as follows. For log E < 2.00, the most unstable modes have zonally non-symmetric structures; the most unstable modes for log E < 1.00 are caused by resonance between equatorial Kelvin modes and continuous modes, and those for 1.00≤ log E < 2.00 are caused by resonance between equatorial Kelvin modes and westward mixed Rossby-gravity modes. The most unstable modes for log E ≥ 2.00 have symmetric structures and are identical with inertially unstable modes. Examinations of dispersion curves suggest that non-symmetric unstable modes for 1.00 ≤ log E < 2.00 and inertially unstable modes for log E ≥ 2.00 are the same kind of instability.

The APE atlas

This Atlas presents statistical analyses of the simulations submitted to the Aqua-Planet Experiment (APE) data archive. The simulations are from global Atmospheric General Circulation Models (AGCM) applied to a water-covered earth. The AGCMs include ones actively used or being developed for numerical weather prediction or climate research. Some are mature, application models and others are more novel and thus less well tested in Earth-like applications. The experiment applies AGCMs with their complete parameterization package to an idealization of the planet Earth which has a greatly simplified lower boundary that consists of an ocean only. It has no land and its associated orography, and no sea ice. The ocean is represented by Sea Surface Temperatures (SST) which are specified everywhere with simple, idealized distributions. Thus in the hierarchy of tests available for AGCMs, APE falls between tests with simplified forcings such as those proposed by Held and Suarez (1994) and Boer and Denis (1997) and Earth-like simulations of the Atmospheric Modeling Intercomparison Project (AMIP, Gates et al., 1999). Blackburn and Hoskins (2013) summarize the APE and its aims. They discuss where the APE fits within a modeling hierarchy which has evolved to evaluate complete models and which provides a link between realistic simulation and conceptual models of atmospheric phenomena. The APE bridges a gap in the existing hierarchy. The goals of APE are to provide a benchmark of current model behaviors and to stimulate research to understand the cause of inter-model differences., APE is sponsored by the World Meteorological Organization (WMO) joint Commission on Atmospheric Science (CAS), World Climate Research Program (WCRP) Working Group on Numerical Experimentation (WGNE). Chapter 2 of this Atlas provides an overview of the specification of the eight APE experiments and of the data collected. Chapter 3 lists the participating models and includes brief descriptions of each. Chapters 4 through 7 present a wide variety of statistics from the 14 participating models for the eight different experiments. Additional intercomparison figures created by Dr. Yukiko Yamada in AGU group are available at http://www.gfd-dennou.org/library/ape/comparison/. This Atlas is intended to present and compare the statistics of the APE simulations but does not contain a discussion of interpretive analyses. Such analyses are left for journal papers such as those included in the Special Issue of the Journal of the Meteorological Society of Japan (2013, Vol. 91A) devoted to the APE. Two papers in that collection provide an overview of the simulations. One (Blackburn et al., 2013) concentrates on the CONTROL simulation and the other (Williamson et al., 2013) on the response to changes in the meridional SST profile. Additional papers provide more detailed analysis of the basic simulations, while others describe various sensitivities and applications. The APE experiment data base holds a wealth of data that is now publicly available from the APE web site: http://climate.ncas.ac.uk/ape/. We hope that this Atlas will stimulate future analyses and investigations to understand the large variation seen in the model behaviors.

Numerical Modeling for Venus Atmosphere Based on AFES (Atmospheric GCM for the Earth Simulator)

In order to elucidate phenomena of Venus atmosphere, an atmospheric general circulation model (AGCM) for Venus is being developed on the basis of AFES (AGCM For the Earth Simulator). As a first step toward high resolution numerical simulation with realistic physical processes, we investigate unstable modes on the condition of super-rotation by nonlinear numerical simulation with simplified physical processes. At initial state zonal super-rotation is assumed to exist. We use the relaxation forcing of the meridional temperature gradient to maintain the zonal flow. In the time evolution of this experimental setting, baroclinic modes grow in the cloud layer with small static stability. The structures of unstable modes are similar to those obtained in the linear stability analysis. We discuss resolution dependency of the results.

A Numerical Study of Convection in a Condensing CO2 Atmosphere under Early Mars-Like Conditions

AbstractCloud convection of a CO2 atmosphere where the major constituent condenses is numerically investigated under a setup idealizing a possible warm atmosphere of early Mars, utilizing a two-dimensional cloud-resolving model forced by a fixed cooling profile as a substitute for a radiative process. The authors compare two cases with different critical saturation ratios as condensation criteria and also examine sensitivity to number mixing ratio of condensed particles given externally.When supersaturation is not necessary for condensation, the entire horizontal domain above the condensation level is continuously covered by clouds irrespective of number mixing ratio of condensed particles. Horizontal-mean cloud mass density decreases exponentially with height. The circulations below and above the condensation level are dominated by dry cellular convection and buoyancy waves, respectively.When 1.35 is adopted as the critical saturation ratio, clouds appear exclusively as intense, short-lived, quasi-period...