Tatsuya Seiki

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.

Aerosol Effects of the Condensation Process on a Convective Cloud Simulation

AbstractUsing a nonhydrostatic model with a double-moment bulk cloud microphysics scheme, the authors introduce an aerosol effect on a convective cloud system by accelerating the condensation and evaporation processes (the aerosol condensational effect). To evaluate this effect, the authors use an explicit condensation scheme rather than the saturation adjustment method and propose a method to isolate the aerosol condensational effect. This study shows that the aerosol condensational effect not only accelerates growth rates but also increases cloud water, even though the degree of the acceleration of evaporation exceeds that of condensation. In the early developing stage of the convective system, increased cloud water is, in turn, linked to ice-phase processes and modifies the ice water path of anvil clouds and the ice cloud fraction. In the mature stage, although the aerosol condensational effect has a secondary role in dynamical feedbacks when combined with other aerosol effects, the degree of modulatio...

Improvement in Global Cloud-System-Resolving Simulations by Using a Double-Moment Bulk Cloud Microphysics Scheme

AbstractThis study examines the impact of an alteration of a cloud microphysics scheme on the representation of longwave cloud radiative forcing (LWCRF) and its impact on the atmosphere in global cloud-system-resolving simulations. A new double-moment bulk cloud microphysics scheme is used, and the simulated results are compared with those of a previous study. It is demonstrated that improvements within the new cloud microphysics scheme have the potential to substantially improve climate simulations. The new cloud microphysics scheme represents a realistic spatial distribution of the cloud fraction and LWCRF, particularly near the tropopause. The improvement in the cirrus cloud-top height by the new cloud microphysics scheme substantially reduces the warm bias in atmospheric temperature from the previous simulation via LWCRF by the cirrus clouds. The conversion rate of cloud ice to snow and gravitational sedimentation of cloud ice are the most important parameters for determining the strength of the radia...

Vertical grid spacing necessary for simulating tropical cirrus clouds with a high‐resolution atmospheric general circulation model

The distribution of simulated cirrus clouds over the tropics is affected by the particular models vertical grid spacing. To examine this effect, we use a high-resolution atmospheric general circulation model with 28 km and 14 km horizontal meshes. We show that a vertical grid spacing of 400 m or less is necessary to resolve the bulk structure of cirrus clouds. As one reduces the vertical grid spacing below about 1000 m, the visible cirrus cloud fraction decreases, the cloud thins (optically and geometrically), the cloud top height lowers, and consequently, the outgoing longwave radiation increases. These effects are stronger over the tropics. When using a vertical grid spacing of 400 m or less, the vertical profiles of effective radii and ice water content converge toward measurements (CloudSat satellite and Cloud-Aerosol Lidar and Infrared Pathfinder Satellite Observation).

Simultaneous evaluation of ice cloud microphysics and nonsphericity of the cloud optical properties using hydrometeor video sonde and radiometer sonde in situ observations

This study utilizes hydrometeor sonde and radiometer sonde in situ observations to simultaneously evaluate ice cloud microphysics and radiative fluxes. In addition, the impact of nonsphericity and heterogeneous ice nucleation schemes on radiative fluxes are examined using a double-moment bulk cloud microphysics scheme on a midlatitude frontal system. The distribution of simulated outgoing longwave radiation (OLR) is systematically reduced by assuming the presence of columnar ice crystals instead of planar ice crystals because of the difference in the effective radii (the projected area) between the two shapes. However, the choice of the heterogeneous ice nucleation schemes drastically changes the distribution of OLR by modifying the number concentration of the cloud ice (Ni) (more than tenfold). The observed shortwave fluxes are useful for evaluating the simulated number concentration of cloud ice when nonspherical single scattering properties are used instead of spherical single scattering properties. The dependence of the asymmetry factor on the effective radius is the key to quantitatively estimating the ice cloud radiative forcing and determining the aerosol indirect effect on ice clouds. Based on the comparison of shortwave fluxes, the cloud microphysics scheme was found to underestimate the Ni near the cloud base (a robust bias). A possible method of modifying the bias is discussed.

Responses of Tropical and Subtropical High-Cloud Statistics to Global Warming

AbstractData from global high-resolution, nonhydrostatic simulations, covering a 1-yr period and with horizontal grid sizes of 7 and 14 km, were analyzed to evaluate the response of high cloud to global warming. The results indicate that, in a warmer atmosphere, high-cloud cover increases robustly and associated longwave (LW) cloud radiative forcing (CRF) increases on average. To develop a better understanding of high-cloud responses to climate change, the geographical distribution of high-cloud size obtained from the model was analyzed and compared with observations. In warmer atmospheres, the contribution per cloud to CRF decreases for both the LW and shortwave (SW) components. However, because of significant increases in the numbers of high clouds in almost all cloud size categories, the magnitude of both LW and SW CRF increases in the simulations. In particular, the contribution from an increase in the number of smaller clouds has more effect on the CRF change. It was also found that the ice and liqui...

Scalable rank-mapping algorithm for an icosahedral grid system on the massive parallel computer with a 3-D torus network

Abstract In this paper, we develop a rank-mapping algorithm for an icosahedral grid system on a massive parallel computer with the 3-D torus network topology, specifically on the K computer. Our aim is to improve the weak scaling performance of the point-to-point communications for exchanging grid-point values between adjacent grid regions on a sphere. We formulate a new rank-mapping algorithm to reduce the maximum number of hops for the point-to-point communications. We evaluate both the new algorithm and the standard ones on the K computer, using the communication kernel of the Nonhydrostatic Icosahedral Atmospheric Model (NICAM), a global atmospheric model with an icosahedral grid system. We confirm that, unlike the standard algorithms, the new one achieves almost perfect performance in the weak scaling on the K computer, even for 10,240 nodes. Results of additional experiments imply that the high scalability of the new rank-mapping algorithm on the K computer is achieved by reducing network congestion in the links between adjacent nodes.

High Cloud Responses to Global Warming Simulated by Two Different Cloud Microphysics Schemes Implemented in the Nonhydrostatic Icosahedral Atmospheric Model (NICAM)

AbstractThis study examines cloud responses to global warming using a global nonhydrostatic model with two different cloud microphysics schemes. The cloud microphysics schemes tested here are the single- and double-moment schemes with six water categories: these schemes are referred to as NSW6 and NDW6, respectively. Simulations of one year for NSW6 and one boreal summer for NDW6 are performed using the nonhydrostatic icosahedral atmospheric model with a mesh size of approximately 14 km. NSW6 and NDW6 exhibit similar changes in the visible cloud fraction under conditions of global warming. The longwave (LW) cloud radiative feedbacks in NSW6 and NDW6 are within the upper half of the phase 5 of the Coupled Model Intercomparison Project (CMIP5)–Cloud Feedback Model Intercomparison Project 2 (CFMIP2) range. The LW cloud radiative feedbacks are mainly attributed to cirrus clouds, which prevail more in the tropics under global warming conditions. For NDW6, the LW cloud radiative feedbacks from cirrus clouds als...

High cloud size dependency in the applicability of the fixed anvil temperature hypothesis using global nonhydrostatic simulations

The applicability of the fixed anvil temperature (FAT) hypothesis is examined using data of a global nonhydrostatic model, focusing particularly on high cloud size dependency. Decomposition of outgoing-longwave radiation (OLR) into three components, including cloud top temperature (TCT), upward cloud emissivity (e), and clear-sky OLR (FCLR), reveals that the relative contributions of these three components to changes of OLR are highly dependent on cloud size. That is, the FAT hypothesis is applicable only for smaller clouds, because the contribution of TCT by those clouds is small, and e is more important. In contrast, for larger clouds, the contribution of e is comparable to that of TCT, and thus, both components are equally important. FCLR slightly reduces OLR but shows dependence on cloud size.

Numerical Experiments to Analyze Cloud Microphysical Processes Depicted in Vertical Profiles of Radar Reflectivity of Warm Clouds

AbstractInformation about microphysical processes in warm clouds embedded in satellite measurements must be untangled to be used to improve the parameterization in global models. In this paper, the relationship between vertical profiles of horizontally averaged radar reflectivity Zm and cloud optical depth from cloud top τd was investigated using a hybrid cloud microphysical model and a forward simulator of satellite measurements. The particle size distributions were explicitly simulated using a bin method in a kinematic framework. In contrast to previous interpretations of satellite-observed data, three patterns of the Zm–τd relationship related to microphysical processes were identified. The first is related to the autoconversion process, which causes Zm to increase upward with decreasing τd. Before the initiation of surface precipitation, Zm increases downward with τd in the upper part of the cloud, which is considered to be a second characteristic pattern and is caused by the accretion process. The ap...