Shin-ichi Iga

The Non-hydrostatic Icosahedral Atmospheric Model: description and development

This article reviews the development of a global non-hydrostatic model, focusing on the pioneering research of the Non-hydrostatic Icosahedral Atmospheric Model (NICAM). Very high resolution global atmospheric circulation simulations with horizontal mesh spacing of approximately O (km) were conducted using recently developed supercomputers. These types of simulations were conducted with a specifically designed atmospheric global model based on a quasi-uniform grid mesh structure and a non-hydrostatic equation system. This review describes the development of each dynamical and physical component of NICAM, the assimilation strategy and its related models, and provides a scientific overview of NICAM studies conducted to date.

Multiscale organization of convection simulated with explicit cloud processes on an aquaplanet

Abstract This study investigated the multiscale organization of tropical convection on an aquaplanet in a model experiment with a horizontal mesh size of 3.5 km (for a 10-day simulation) and 7 km (for a 40-day simulation). The numerical experiment used the nonhydrostatic icosahedral atmospheric model (NICAM) with explicit cloud physics. The simulation realistically reproduced multiscale cloud systems: eastward-propagating super cloud clusters (SCCs) contained westward-propagating cloud clusters (CCs). SCCs (CCs) had zonal sizes of several thousand (hundred) kilometers; typical propagation speed was 17 (10) m s−1. Smaller convective structures such as mesoscale cloud systems (MCs) of O(10 km) and cloud-scale elements ( 16 km) of O(100 km) area was also reproduced. Planetary-scale equatorial waves (with wavelengths of 10 000 and 40 000 km) had a major influence on the eastward propagation of the simulated SCC; destabilization east of the...

Shear Instability in a Shallow Water Model with Implications for the Venus Atmosphere

Abstract The shear instability problem in the spherical shallow water system is investigated for three types of wind profiles that are observed at the upper cloud level in Venus. Destabilized Kelvin modes are obtained for all profiles, even when the wind profile is barotropically and inertially stable. The eigenfunctions of these unstable modes are a hybrid of Kelvin modes and continuous modes, which have singularity at the critical latitude. Destabilized Rossby–Kelvin modes are also obtained for the barotropically unstable profile with strong jets. When Lamb parameter ϵ = (2aΩ)2/gH is large, together with other destabilized gravity modes, these modes have the property of inertial instability modes, which are described by preceding studies on the tropical inertial instability. The destabilizing mechanism of unstable modes is described using resonance theory. It is found that the angular-momentum flux is equatorward for almost all growing modes obtained in this study; this result is consistent with what th...

Response of Upper Clouds in Global Warming Experiments Obtained Using a Global Nonhydrostatic Model with Explicit Cloud Processes

AbstractUsing a global nonhydrostatic model with explicit cloud processes, upper-cloud changes are investigated by comparing the present climate condition under the perpetual July setting and the global warming condition, in which the sea surface temperature (SST) is raised by 2°. The sensitivity of the upper-cloud cover and the ice water path (IWP) are investigated through a set of experiments. The responses of convective mass flux and convective areas are also examined, together with those of the large-scale subsidence and relative humidity in the subtropics. The responses of the IWP and the upper-cloud cover are found to be opposite; that is, as the SST increases, the IWP averaged over the tropics decreases, whereas the upper-cloud cover in the tropics increases. To clarify the IWP response, a simple conceptual model is constructed. The model consists of three columns of deep convective core, anvil, and environmental subsidence regions. The vertical profiles of hydrometers are predicted with cloud micr...

Convectively Coupled Equatorial Waves Simulated on an Aquaplanet in a Global Nonhydrostatic Experiment

Abstract Large-scale tropical convective disturbances simulated in a 7-km-mesh aquaplanet experiment are investigated. A 40-day simulation was executed using the Nonhydrostatic Icosahedral Atmospheric Model (NICAM). Two scales of eastward-propagating disturbances were analyzed. One was tightly coupled to a convective system resembling super–cloud clusters (SCCs) with a zonal scale of several thousand kilometers (SCC mode), whereas the other was characterized by a planetary-scale dynamical structure (40 000-km mode). The typical phase velocity was 17 (23) m s−1 for the SCC (40 000 km) mode. The SCC mode resembled convectively coupled Kelvin waves in the real atmosphere around the equator, but was accompanied by a pair of off-equatorial gyres. The 40 000-km mode maintained a Kelvin wave–like zonal structure, even poleward of the equatorial Rossby deformation radius. The equatorial structures in both modes matched neutral eastward-propagating gravity waves in the lower troposphere and unstable (growing) wave...

High cloud increase in a perturbed SST experiment with a global nonhydrostatic model including explicit convective processes

Results are presented from a series of sensitivity tests in idealized global warming experiments using the global nonhydrostatic model, NICAM, in which convection at scales of 7–14 km is explicitly resolved. All have a strong positive longwave cloud feedback larger than that seen in conventional GCMs with parameterized convection. Consequently, the global mean net outgoing radiation decreases in response to increased sea surface temperatures. Large increases in high clouds with tops between 180 and 50 hPa are found, and these changes contribute the most to this longwave cloud feedback. Relative humidity and upper tropospheric temperature also increases strongly, again more so than typically seen in conventional GCMs. The magnitude of the response varies considerably between different versions of NICAM. Most of the NICAM control simulations show large overestimates in cloud fraction between 180 and 50 hPa compared to observations. The changes in cloud fraction in the upper troposphere are strongly correlated with their control values. Versions of NICAM with stronger cloud feedbacks have large positive biases in high-top cloud amount and temperature in the free troposphere in their control simulations. The version which has the best agreement with the observations in this regard has the weakest longwave cloud feedback; however, this is still more strongly positive than that typically seen in conventional GCMs. These results demonstrate the potential for stronger high cloud fraction feedbacks in climate warming scenarios than currently predicted by conventional GCMs and highlight the potential relevance of deep convective processes.

Sensitivity of Hadley Circulation to Physical Parameters and Resolution through Changing Upper-Tropospheric Ice Clouds Using a Global Cloud-System Resolving Model

AbstractThe relationship between upper-tropospheric ice cloud properties and the Hadley circulation intensity is examined through parameter sensitivity studies of global cloud-system-resolving simulations with explicit cloud convection. Experiments under a perpetual July condition were performed by changing parameters in the boundary layer and cloud microphysics schemes, with a mesh size of approximately 14 km. One additional experiment with a mesh size of approximately 7 km was also conducted. These experiments produced a variety of upper-cloud coverage and outgoing longwave radiation (OLR) distributions. The authors found that, as the upper-cloud coverage increased, the total precipitation decreased and the intensity of the Hadley circulation weakened because of energy balance constraints that radiative cooling are balanced by adiabatic warming. Interestingly, the ice water path was not correlated with the upper ice-loud coverage or OLR, indicating that the spatial coverage of upper ice clouds, rather t...

Precipitation Statistics Comparison Between Global Cloud Resolving Simulation with NICAM and TRMM PR Data

A “global cloud resolving simulation” with horizontal grid interval of 3.5 km is conducted using a nonhydrostatic icosahedral atmospheric model (NICAM). NICAM is a cloud resolving model in the sense that updraft cores of deep cumulus that have a few km in horizontal size are marginally represented using explicit microphysical schemes. Results from the aqua-planet experiment of NICAM are compared with the TRMM PR (Tropical Rainfall Measurement Mission, Precipitation Radar) data that have a horizontal resolution close to the grid interval of NICAM.

Mountain-Wave-Like Spurious Waves Associated with Simulated Cold Fronts due to Inconsistencies between Horizontal and Vertical Resolutions

A newly developed global nonhydrostatic model is used for life cycle experiments (LCEs) of baroclinic waves, and the resolution dependency of frontal structures is examined. LCEs are integrated for 12 days with horizontal grid intervals ranging from 223 to 3.5 km in a global domain. In general, fronts become sharper and corresponding vertical flow strengthens as horizontal resolution increases. However, if the ratio of vertical and horizontal grid intervals is sufficiently small compared to the frontal slope s, the overall frontal structure remains unchanged. In contrast, when the ratio of horizontal and vertical grid intervals exceeds 2s 4s, spurious gravity waves are generated at the cold front. A linear model for mountain waves quantitatively explains the mechanism of the spurious waves. The distribution of the basic wind is the major factor that determines wave amplitude and propagation. The spurious waves propagate up to a critical level at which the basic wind speed normal to the front is equal to the propagation speed of the front. Results from the linear model suggest that an effective way to eliminate spurious waves is to choose a stretched grid with a smaller vertical grid interval in lower layers where strong horizontal winds exist.

Improved smoothness and homogeneity of icosahedral grids using the spring dynamics method

An icosahedral grid that has both high smoothness and homogeneity is proposed. The grid-generation method is based on the combination of the spring dynamics (SPR) method with zero natural spring length (SPR0) and transformation by a smooth analytic function around the 12 vertices of an icosahedron. As a preliminary step, we first showed that the grid interval of the grid generated by SPR0 was inversely proportional to a Lambert conformal conic projection map factor, with a map angle of 300^o around the vertices. Then, the transformation function was analytically determined, such that the resolution for the azimuthal direction became constant. In order to estimate cost-efficiency of numerical simulation with the newly proposed grid, we introduced an index defined as the ratio between the minimum grid interval and the squared maximum grid interval. It showed a 2.5% improvement from a recursive grid, and a 0.3-12% improvement from the best cases of the original SPR grid proposed by Tomita et al. (2002) [23] [hereafter, T02] dependent on global resolution. We also re-examined the original SPR method and found that the natural spring length proposed in T02 should be shortened to avoid instability when the global resolution is higher than grid-level 8. Finally, we examined the grids using advection/shallow water simulations.