Nonhydrostatic Icosahedral Atmospheric Model Research Articles

Comparison of observed and simulated tropical cumuliform clouds by CloudSat and NICAM

We use CloudSat observations of boreal summer tropical ocean cumuliform clouds to evaluate the behavior of the non‐parameterized cumuliform clouds in the Nonhydrostatic Icosahedral Atmospheric Model (NICAM), with a particular emphasis on deep convective clouds (DCCs). The CloudSat cloud mask and radar reflectivity profiles for cumuliform clouds are sorted by large‐scale environmental variables taken from the Aqua satellite and NCEP/NCAR reanalysis. The variables are total precipitable water (TPW), sea surface temperature (SST), and 500 hPa vertical velocity (W500), representing the dynamical and thermodynamical environment in which the clouds form. The sorted CloudSat profiles are then compared with NICAM profiles simulated with the Quickbeam CloudSat simulator. We first use the cloud mask to examine the transition between shallow clouds and deep clouds rooted in the planetary boundary layer. We find that NICAM simulates this transition fairly realistically. However, the transition occurs at slightly higher TPW and W500 values than the observations show. This may be indication of NICAM's inability to represent the formation of isolated narrow DCCs in marginally favorable environments. We then use simple metrics of the DCC‐only radar reflectivity profiles (cloud top height, cloud top reflectivity gradient, maximum reflectivity) to quantitatively compare the observations with NICAM. The results show that while the observed and simulated results agree generally, there are some disagreements in key respects. There is disagreement on the sensitivity of cloud top height to environmental conditions and on the transition between shallow and deep clouds in environments marginally suitable for deep convection.

Analysis of the tropical tropopause layer using the Nonhydrostatic Icosahedral Atmospheric Model (NICAM): 2. An experiment under the atmospheric conditions of December 2006 to January 2007

The roles of deep convection and generated waves in the Tropical Tropopause Layer (TTL) are investigated using a global nonhydrostatic model, the Nonhydrostatic Icosahedral Atmospheric Model (NICAM), which runs on the Earth Simulator with a horizontal spacing of 7 km. The model data, which successfully simulated a Madden‐Julian Oscillation (MJO) event for the period between 15 December 2006 and 15 January 2007, are analyzed. The frequency of deep convective clouds that reach the TTL is one of the key diagnostics for dehydration and transport. The present results revealed that the proportion of cumulus clouds that penetrate the lapse‐rate tropopause and the bottom boundary of the TTL (defined as the lapse rate minimum) is ∼0.5% and ∼20%, respectively, in the region between 5°S and 5°N. This result is reasonably consistent with atmospheric observations. Deep convective activity that reaches the TTL was observed over southern Africa, the Indian Ocean, the Indonesian maritime continent, the western Pacific, and southern America. Deep convection over the continents was most active during the local evening period. Over the oceans, high clouds reaching the tropopause were seen over the Indian Ocean and the seas around Java, where two tropical cyclones were generated. Prominent diurnal variations in tropopause temperature associated with deep convection occurred over the Indonesian maritime continent. These diurnal variations were superimposed on large, low‐frequency temperature variations associated with equatorial Kelvin waves generated by the MJO convection. Probably because of coarse vertical resolution, temperature variations simulated by the NICAM are larger than those in the real atmosphere. The two tropical cyclones caused relatively large tropopause temperature variations with a cyclone scale (∼500 km horizontally). The gravity waves generated by tropical cyclones cause small tropopause temperature variations that extend for 1000 km from the cyclone. We conclude that the Kelvin waves associated with the MJO convection cause the largest amplitude of temperature variations in the TTL and that tropical cyclones and diurnal variations of convective activity have large local impacts on temperature variations in the TTL.

Eastward-Propagating Property of Large-Scale Precipitation Systems Simulated in the Coarse-Resolution NICAM and an Explanation of its Appearance

Using the Non-hydrostatic ICosahedral Atmospheric Model (NICAM) on an aqua-planet with a uniform longitudinal sea surface temperature, precipitation systems (PSs) with coarse horizontal resolutions are studied. An eastward-propagating (EP) property of the PSs is seen even though the horizontal resolution becomes coarser to 112 km, as long as the vertical stratification is kept conditionally unstable. The appearance or disappearance of the EP property is explained in comparison with mesoscale (100 km) and large-scale (1,000 km) PSs, a non-precipitation system (NPS), and a system simulated by a dry model with positive-only wave CISK heating (DP). By assuming that the height and the updraft intensity of convection are common in these systems, the turnover times of representative convections are estimated. Compared with the time scale of the equatorial beta (Tβ), where its effect essentially works and the distinct EP property appears, the turnover times of the large-scale PS and DP are longer than Tβ, producing the EP property, whereas those of the NPS and mesoscale PSs are shorter, inducing no EP property. These results are consistent with observations and model outputs (Nakazawa 1988; Nasuno et al. 2007, 2008).

The Intra-Seasonal Oscillation and its control of tropical cyclones simulated by high-resolution global atmospheric models

Project Athena is an international collaboration testing the efficacy of high-resolution global climate models. We compare results from 7-km mesh experiments of the Nonhydrostatic Icosahedral Atmospheric Model (NICAM) and 10-km mesh experiments of the Integrated Forecast System (IFS), focusing on the Intra-Seasonal Oscillation (ISO) and its relationship with tropical cyclones (TC) among the boreal summer period (21 May–31 Aug) of 8 years (2001–2002, 2004–2009). In the first month of simulation, both models capture the intra-seasonal oscillatory behavior of the Indian monsoon similar to the observed boreal summer ISO in approximately half of the 8-year samples. The IFS simulates the NW–SE-oriented rainband and the westerly location better, while NICAM marginally reproduces mesoscale organized convective systems and better simulates the northward migration of the westerly peak and precipitation, particularly in 2006. The reproducibility of the evolution of MJO depends on the given year; IFS simulates the MJO signal well for 2002, while NICAM simulates it well for 2006. An empirical orthogonal function analysis shows that both models statistically reproduce MJO signals similar to observations, with slightly better phase speed reproduced by NICAM. Stronger TCs are simulated in NICAM than in IFS, and NICAM shows a wind-pressure relation for TCs closer to observations. TC cyclogenesis is active during MJO phases 3 and 4 in NICAM as in observations. The results show the potential of high-resolution global atmospheric models in reproducing some aspects of the relationship between MJO and TCs and the statistical behavior of TCs.

Simulating the diurnal cycle of rainfall in global climate models: resolution versus parameterization

The effects of horizontal resolution and the treatment of convection on simulation of the diurnal cycle of precipitation during boreal summer are analyzed in several innovative weather and climate model integrations. The simulations include: season-long integrations of the Non-hydrostatic Icosahedral Atmospheric Model (NICAM) with explicit clouds and convection; year-long integrations of the operational Integrated Forecast System (IFS) from the European Centre for Medium-range Weather Forecasts at three resolutions (125, 39 and 16 km); seasonal simulations of the same model at 10 km resolution; and seasonal simulations of the National Center for Atmospheric Research (NCAR) low-resolution climate model with and without an embedded two-dimensional cloud-resolving model in each grid box. NICAM with explicit convection simulates best the phase of the diurnal cycle, as well as many regional features such as rainfall triggered by advancing sea breezes or high topography. However, NICAM greatly overestimates mean rainfall and the magnitude of the diurnal cycle. Introduction of an embedded cloud model within the NCAR model significantly improves global statistics of the seasonal mean and diurnal cycle of rainfall, as well as many regional features. However, errors often remain larger than for the other higher-resolution models. Increasing resolution alone has little impact on the timing of daily rainfall in IFS with parameterized convection, yet the amplitude of the diurnal cycle does improve along with the representation of mean rainfall. Variations during the day in atmospheric prognostic fields appear quite similar among models, suggesting that the distinctive treatments of model physics account for the differences in representing the diurnal cycle of precipitation.

Comparison between Satellite Water Vapour Observations and Atmospheric Models’ Predictions of the Upper Tropospheric Thermal Radiation

Atmospheric profiles (temperature, pressure, and humidity) are commonly used parameters for aerosols and cloud properties retrievals. In preparation of the launch of the Global Change Observation Mission-Climate/Second-Generation GLobal Imager (GCOM-C/SGLI) satellite, an evaluation study on the sensitivity of atmospheric models to variations of atmospheric conditions is conducted. In this evaluation, clear sky and above low clouds water vapour radiances of the upper troposphere obtained from satellite observations and those simulated by atmospheric models are compared. The models studied are the Nonhydrostatic ICosahedral Atmospheric Model (NICAM) and the National Center for Environmental Protection/Department Of Energy (NCEP/DOE). The satellite observations are from the Terra/Moderate Resolution Imaging Spectroradiometer (Terra/MODIS) satellite. The simulations performed are obtained through a forward radiative transfer calculation procedure. The resulting radiances are transformed into the upper tropospheric brightness temperature (UTBT) and relative humidity (UTRH). The discrepancies between the simulated data and the observations are analyzed. These analyses show that both the NICAM and the NCEP/DOE simulated UTBT and UTRH have comparable distribution patterns. However the simulations’ differences with the observations are generally lower with the NCEP/DOE than with the NICAM. The NCEP/DOE model outputs very often overestimate the UTBT and therefore present a drier upper troposphere. The impact of the lower troposphere instability (dry convection) on the upper tropospheric moisture and the consequences on the models’ results are evaluated through a thunderstorm and moisture predictor (the K-stability index). The results obtained show a positive relation between the instability and the root mean square error (RMSE: observation versus models). The study of the impact of convective clouds shows that the area covered by these clouds increases with the humidity of the upper troposphere in clear sky and above low clouds, and at the same time, the error between the observations and the models also increases. The impact of the above low clouds heat distribution on the models is studied through the relation between the low clouds cover and their effective emissivity. The models’ error appears to be high at midrange effective emissivity clouds.

Diurnal Convection Peaks over the Eastern Indian Ocean off Sumatra during Different MJO Phases

The authors investigated diurnal convection peak characteristics over the eastern Indian Ocean off the island of Sumatra during different phases of the Madden-Julian oscillation (MJO). During MJO phases 2 to 3 (P2 and P3) defined by Wheeler and Hendon (2004), prominent diurnal variation in convection was observed by satellites when moderate low-level westerly winds were dominant over the eastern Indian Ocean. The diurnal convection peaks were prominent over the island of Sumatra in the evening, while migrations of the convection toward the Indian Ocean were observed in the early morning. By using the Global Positioning System around the western region offshore of Sumatra, a significant reduction in water vapor was observed from evening until midnight, compensating for the upward motion over the island. During midnight to early morning, the water vapor in-creased in the western offshore region as the convections migrated from the island. This prominent diurnal varia-tion confirmed the result from a numerical experiment by Miura et al. (2007) using the Nonhydrostatic ICosahe-dral Atmospheric Model (NICAM).During P2 to P3, the atmosphere over the eastern Indian Ocean contains abundant water vapor, while the Maritime Continent is fairly well heated by solar radiation under calm conditions. This situation should be favor-able for the development of two diurnal convection peaks: the evening convection over the land induced by solar radiative heating and the midnight convection over the ocean triggered by convergence of the low-level westerly wind and the land breeze.

A Three-Dimensional Icosahedral Grid Advection Scheme Preserving Monotonicity and Consistency with Continuity for Atmospheric Tracer Transport

A tracer advection scheme for a three-dimensional icosahedral grid model is developed. Then, its tracer transport performance is evaluated using realistic flow fields. The consistency with continuity (CWC) scheme is newly introduced to a three-dimensionally monotonic scheme using existing techniques of an upwind-biased scheme and the intermediate density method. The CWC guarantees mass conservation and Lagrangian conservation, both of which are important properties for tracer transport. In addition, the scheme is positive definite. This transport scheme is implemented with the nonhydrostatic icosahedral atmospheric model and transport simulations are performed. A comparison with radon observations reveals that the proposed method reproduces high-concentration events more realistically than the original method does. Furthermore, results show that the proposed method has less numerical diffusion and is superior to the original method in terms of a sharper gradient.

Impact of different definitions of clear-sky flux on the determination of longwave cloud radiative forcing: NICAM simulation results

Abstract. Using one month of the cloud-resolving Nonhydrostatic Icosahedral Atmospheric Model (NICAM) simulations, we examined the impact of different definitions of clear-sky flux on the determination of longwave cloud radiative forcing (CRF). Because the satellite-like cloud-free composite preferentially samples drier conditions relative to the all-sky mean state, the conventional clear-sky flux calculation using the all-sky mean state in the model may represent a more humid atmospheric state in comparison to the cloud-free state. The drier bias is evident for the cloud-free composite in the NICAM simulations, causing an overestimation of the longwave CRF by about 10% compared to the NICAM simulated longwave CRF. Overall, water vapor contributions of up to 10% of the total longwave CRF should be taken account for making model-generated cloud forcing comparable to the satellite measurements.

Multiscale Interactions in the Life Cycle of a Tropical Cyclone Simulated in a Global Cloud-System-Resolving Model. Part II: System-Scale and Mesoscale Processes*

Abstract The life cycle of Tropical Storm Isobel was simulated reasonably well in the Nonhydrostatic Icosahedral Atmospheric Model (NICAM), a global cloud-system-resolving model. The evolution of the large-scale circulation and the storm-scale structure change was discussed in Part I. Both the mesoscale and system-scale processes in the life cycle of the simulated Isobel are documented in this paper. In the preconditioned favorable environment over the Java Sea, mesoscale convective vortices (model MCVs) developed in the mesoscale convective systems (MCSs) and convective towers with cyclonic potential vorticity (PV) anomalies throughout the troposphere [model vortical hot towers (VHTs)] appeared in the model MCVs. Multiple model VHTs strengthened cyclonic PV in the interior of the model MCV and led to the formation of an upright monolithic PV core at the center of the concentric MCV (primary vortex enhancement). As the monolithic PV core with a warm core developed near the circulation center, the intensification and the increase in horizontal size of the cyclonic PV were enhanced through the system-scale intensification (SSI) process (the secondary vortex enhancement), leading to the genesis of Isobel over the Timor Sea. The SSI process can be well explained by the balanced dynamics. After its genesis, the subsequent evolution of the simulated Isobel was controlled by both the external influence and the internal dynamics. Under the unfavorable environmental conditions, the development of asymmetric structure reduced the axisymmetric diabatic heating in the inner core and the SSI process became ineffective and the storm weakened. Later on, as the eyewall reformed as a result of the axisymmetrization of an inward-propagating outer spiral rainband, the SSI process became effective again, leading to the reintensification of Isobel. Therefore, the large-scale environmental flow provided the precondition for the genesis of Isobel and the triggering mechanism for subsequent storm-scale structure change as discussed in Part I. The system-scale and mesoscale processes, such as the evolution of MCVs and merging VHTs, were responsible for the genesis, while the eyewall processes were critical to the storm intensity change through the SSI process.

Multiscale Interactions in the Life Cycle of a Tropical Cyclone Simulated in a Global Cloud-System-Resolving Model. Part I: Large-Scale and Storm-Scale Evolutions*

Abstract The Nonhydrostatic Icosahedral Atmospheric Model (NICAM), a global cloud-system-resolving model, successfully simulated the life cycle of Tropical Storm Isobel that formed over the Timor Sea in the austral summer of 2006. The multiscale interactions in the life cycle of the simulated storm were analyzed in this study. The large-scale aspects that affected Isobel’s life cycle are documented in this paper and the corresponding mesoscale processes are documented in a companion paper. The life cycle of Isobel was largely controlled by a Madden–Julian oscillation (MJO) event and the associated westerly wind burst (WWB). The MJO was found to have both positive and negative effects on the tropical cyclone intensity depending on the location of the storm relative to the WWB center associated with the MJO. The large-scale low-level convergence and high convective available potential energy (CAPE) downwind of the WWB center provided a favorable region to the cyclogenesis and intensification, whereas the strong large-scale stretching deformation field upwind of the WWB center may weaken the storm by exciting wavenumber-2 asymmetries in the eyewall and leading to the eyewall breakdown. Five stages are identified for the life cycle of the simulated Isobel: the initial eddy, intensifying, temporary weakening, reintensifying, and decaying stages. The initial eddy stage was featured by small-scale/mesoscale convective cyclonic vortices developed in the zonally elongated rainband organized in the preconditioned environment characterized by the WWB over the Java Sea associated with the onset of an MJO event over the East Indian Ocean. As the MJO propagated eastward and the cyclonic eddies moved southward into an environment with weak vertical shear and strong low-level cyclonic vorticity, a typical tropical cyclone structure developed over the Java Sea, namely the genesis of Isobel. Isobel experienced an eyewall breakdown and a temporary weakening when it was located upwind of the WWB center as the MJO propagated southeastward and reintensified as its eyewall reformed as a result of the axisymmetrization of an inward spiraling outer rainband that originally formed downwind of the WWB center. Finally Isobel decayed as it approached the northwest coast of Australia.

Analysis of the tropical tropopause layer using the Nonhydrostatic Icosahedral Atmospheric Model (NICAM): Aqua planet experiments

The dynamical characteristics of the tropical tropopause layer (TTL) are investigated using the Nonhydrostatic Icosahedral Atmospheric Model (NICAM) run on the Earth Simulator under an aqua planet condition. Two experiments are performed: one with a 3.5 km horizontal spacing and a three‐dimensional snapshot output and another with a 7 km horizontal spacing and 3‐hourly averages for 1 month. The number of vertical levels is 54 and the model top is at 40 km; the vertical spacing in and around the TTL is ∼700 m. Large‐scale organized convection associated with convectively coupled equatorial Kelvin waves prevails around the equator. The zonal mean vertical distribution of cloud top height near the equator shows a realistic trimodal structure. The simulation results reveal that cumulus clouds penetrate the lapse‐rate tropopause and the bottom boundary of the TTL (defined as the lapse rate minimum) for ∼0.1% and ∼25%, respectively, in the equatorial area. The frequency distribution of vertical wind may provide a good indicator of the TTL bottom boundary. A significant reduction in the speed of strong vertical winds is observed at ∼16 km. High variability in temperature and the water vapor mixing ratio observed around the tropopause is mainly caused by equatorial Kelvin waves generated by the organized convection in these experiments. Horizontal variability in tropopause height over a large‐scale convective system is much smaller than that in the area of Kelvin waves. The gravity waves generated by this large‐scale convective system locally control the temperature around the tropopause. In addition, large‐amplitude gravity waves with a scale of 600 km are superimposed on the cold phase of Kelvin waves, producing one of the coldest regions around the tropopause. It is suggested that the combination of Kelvin waves and gravity waves may be one of the most effective dehydration processes in the TTL.

Comparison of high‐level clouds represented in a global cloud system–resolving model with CALIPSO/CloudSat and geostationary satellite observations

Vertical and horizontal distributions of high‐level clouds (ice and snow) simulated in high‐resolution global cloud system–resolving simulations by the Nonhydrostatic Icosahedral Atmospheric Model (NICAM) are compared with satellite observations. Ice and snow data in a 1 week experiment by the NICAM 3.5 km grid mesh global simulation initiated at 0000 UTC 25 December 2006 are used in this study. The vertical structure of ice and snow represented by NICAM was compared with Cloud‐Aerosol Lidar and Infrared Pathfinder Satellite Observation (CALIPSO) and CloudSat observations. High‐level clouds (cumulonimbus and cirrus type clouds) classified by the split window (11 and 12 μm) data on board geostationary meteorological satellites (GMSs) were used for comparison of the horizontal distributions of ice and snow in NICAM. The vertical distributions of ice and snow simulated by NICAM qualitatively agree well with those of cloud signals observed by CALIPSO and CloudSat. We computed corresponding cloud lidar backscatter coefficients and cloud radar reflectivity signals from ice and snow data of NICAM using Cloud Feedback Model Intercomparison Project (CFMIP) observational simulator packages. The contoured frequency by altitude diagram for the cloud lidar backscatter coefficients shows lower frequency at higher altitude of 8–14 km by NICAM than CALIOP observations. This suggests that the amount of ice is not well represented in NICAM. The simulated cloud radar reflectivity signals by NICAM indicated higher frequency at 8–10 km altitude than CloudSat observations, although there were some differences between over oceans and continents. This implies that the amount of snow is larger in NICAM simulations. The horizontal pattern of ice clouds (column‐integrated ice and snow of greater than 0.01 kg/m2) in NICAM shows good agreement with that of high‐level clouds identified by the split window analysis. During this 1 week simulation, 48–59% of ice clouds in NICAM matches with observed high‐level clouds. The cross correlation between the spatial distributions of simulated ice clouds and satellite‐observed high‐level clouds is 0.40–0.51, and the equitable threat score is 0.31–0.45. Furthermore, temporal variations of column‐integrated ice clouds in NICAM are compared with high‐level clouds classified by the split window at the decaying stage of deep convection over the tropics. The results indicate that the mean decaying speed of ice clouds of NICAM and high‐level clouds by satellite observations agrees well for this analysis area and period, although the variances are larger in NICAM. This implies that the fall speed of snow in this NICAM experiment is appropriate to depict the decay of anvil clouds by compensating for the excess of snow in NICAM simulations, when we assume that the decay of anvil clouds is largely controlled by the evaporation of ice and snow.

Ensemble Simulation of Cyclone Nargis by a Global Cloud-System-Resolving Model-Modulation of Cyclogenesis by the Madden-Julian Oscillation

An ensemble simulation of cyclone Nargis was performed using the Non-hydrostatic ICosahedral Atmospheric Model (NICAM) at 14-km mesh size in order to examine the effect on cyclogenesis of disturbances associated with intraseasonal oscillations. An analysis of observational data reveals that cyclone Nargis formed during the northward propagation of low-level zonal wind, associated with active cloud areas and precipitation from the equator to 20°N in the Bay of Bengal, when the active convective region associated with the Madden-Julian Oscillation (MJO) passed through the bay and then resided over the Maritime continent. The northward migration of low-level zonal wind, outgoing longwave radiation (OLR), and precipitation are successfully simulated in the ensemble results. Each simulated tropical cyclone (TC) genesis also occurs with northward migration and with a timing such that the active convective region associated with the MJO resides over the east side of the Maritime continent. The incipient disturbances that contributed to the initiation of cyclone Nargis formed during the period when the westerly wind burst passed through the Bay of Bengal after the monsoon onset and developed to TCs in the ensemble simulation. However, for an ensemble member for which northward migration as a monsoon onset is not simulated, no TC is formed in the Bay of Bengal. It is also found that the effect of the easterlies across the northern part of the Malay Peninsula is important for TC formation in our simulation.

An MJO Simulated by the NICAM at 14- and 7-km Resolutions

Abstract This study discloses detailed Madden–Julian oscillation (MJO) characteristics in the two 30-day integrations of the global cloud-system-resolving Nonhydrostatic Icosahedral Atmospheric Model (NICAM) using the all-season real-time multivariate MJO index of Wheeler and Hendon. The model anomaly is derived by excluding the observed climatology because the simulation is sufficiently realistic. Results show that the MJO has a realistic evolution in amplitude pattern, geographical locations, eastward propagation, and baroclinic- and westward-tilted structures. In the central Indian Ocean, convection develops with the low-level easterly wind anomaly then matures where the low-level easterly and westerly anomalies meet. Anomalous moisture tilts slightly with height. In contrast, over the western Pacific, the convection grows with a low-level westerly anomaly. Moisture fluctuations, leading convection in eastward propagation, tilt clearly westward with height. The frictional moisture convergence mechanism operates to maintain the MJO. Such success can be attributed to the explicit representation of the interactions between convection and large-scale circulations. The simulated event, however, grows faster in phases 2 and 3, and peaks with 30% higher amplitude than that observed, although the 7-km version shows slight improvement. The fast-growth phases are induced by the fast-growing low-level convergence in the Indian Ocean and the strongly biased ITCZ in the west Pacific when the model undergoes a spinup. The simulated OLR has a substantial bias in the tropics. Possible solutions to the deficiencies are discussed.

Diurnal Cycle of Precipitation in the Tropics Simulated in a Global Cloud-Resolving Model

Abstract This study analyzes the diurnal cycle of precipitation simulated in a global cloud-resolving model (GCRM) named the Nonhydrostatic Icosahedral Atmospheric Model (NICAM). A 30-day integration of NICAM successfully simulates the precipitation diurnal cycle associated with the land–sea breeze and the thermally induced topographic circulations as well as the horizontal propagation of diurnal cycle signals. The first harmonic of the diurnal cycle of precipitation in the 7-km run agrees well with that from satellite observations in its geographical distributions although its amplitude is slightly overestimated. The NICAM simulation revealed that the precipitation diurnal cycle over the Maritime Continent is strongly coupled with the land–sea breeze that controls the convergence/divergence pattern in the lower troposphere around the islands. The analysis also suggests that the cold pool often forms over the open ocean where the precipitation intensity is high, and the propagation of the cold pool events is related to the precipitation diurnal cycle as well as the land–sea breeze. Sensitivity experiments suggest a prominent horizontal resolution dependence of the simulated precipitation diurnal cycle. Over continental areas the 14-km run induces the diurnal peak about three hours later than the 7-km run. The 3.5-km run produces the peak time and amplitude that are very similar to those in Tropical Rainfall Measuring Mission (TRMM) precipitation radar (PR) observations. Meanwhile, the resolution dependence in phase and amplitude is negligibly small over the open oceans. This contrast sensitivity to the horizontal resolution is attributed to the differences in structure and life cycle of convective systems over land and ocean. Diurnal peaks of precipitable water vapor, precipitation, and outgoing longwave radiation (OLR) are compared over land areas using the NICAM 7-km run. The daily precipitable water vapor maximum appears around 1500 local time (LT), which is followed by the precipitation peak around 1630 LT. The diurnal cycle of high clouds tends to peak around 1930 LT, three hours later than the precipitation peak. These results from NICAM simulations can explain the cause of the phase differences among precipitation products based on several satellite observations. The authors demonstrate that the GCRM is a promising tool for realistically simulating the precipitation diurnal cycle and could be quite useful for studying the role of the diurnal cycle in the climate systems in a global context.

Importance of the subgrid-scale turbulent moist process: Cloud distribution in global cloud-resolving simulations

This study investigated the turbulent transport process within the nonhydrostatic icosahedral atmospheric model (NICAM), a global cloud-resolving model (GCRM), with particular focus on the spatial reproducibility of cloud characteristics in NICAM simulations. A turbulent closure model was applied, based on level 2 of the model developed by Nakanishi and Niino ([Nakanishi, M., and Niino, H., 2006: An improved Mellor–Yamada level-3 model: its numerical stability and application to a regional prediction of advection fog. Boundary-Layer Meteorol., 119, 397–407.]; a revised version of the original Mellor–Yamada model), together with a subgrid-scale condensation process. NICAM simulations were conducted for boreal summer of 2004 using mesh sizes of about 14 km and 7 km. Simulated cloud amounts and radiative budget were compared with observed data. The results confirmed an improvement in the spatial distribution of low clouds that develop in offshore regions of subtropical continents compared to past NICAM experiments ([Iga, S., Tomita, H., Tsushima, Y., and Satoh, M., 2007: Climatology of a nonhydrostatic global model with explicit cloud process. Geophys. Res. Lett., 34, L22814, doi:10.1029/2007/GL031048.]). A sensitivity study of subgrid-scale clouds in the turbulent closure scheme revealed that the turbulent transport process modulated by the subgrid-scale cloud strongly controls not only the low-cloud amount but also mid- and high-cloud amounts. This indicates that parameterization of unresolvable subgrid-scale clouds remains an important component of cloud behavior in GCRMs.

Applying the Local Ensemble Transform Kalman Filter to the Nonhydrostatic Icosahedral Atmospheric Model (NICAM)

In this study, we apply the local ensemble transform Kalman filter (LETKF) to the Nonhydrostatic Icosahedral Atmospheric Model (NICAM) to develop the NICAM-LETKF. In addition, an algorithm to adaptively estimate the inflation parameter and the observational errors is introduced to the LETKF. The feasibility and stability of the NICAM-LETKF are investigated under the perfect model scenario. According to the results, we confirm that the converged analysis errors of the NICAM-LETKF are smaller than the observational errors, and the magnitude and distribution of the root mean square errors (RMSEs) are comparable to those of the ensemble spreads. In our experiments, we find that the inflation parameter is optimally tuned and the observational errors are close to the true value. It is concluded that the NICAM-LETKF works appropriately and stably under the perfect model scenario even if the inflation parameter and the observational errors are adaptively estimated within the LETKF.

Global cloud‐system‐resolving model NICAM successfully simulated the lifecycles of two real tropical cyclones

The increasing capability of high‐end computers allows numerical simulations with horizontal resolutions high enough to resolve cloud systems in a global model. In this paper, initial results from the global Nonhydrostatic ICosahedral Atmospheric Model (NICAM) are highlighted to demonstrate the beginning of a potentially new era for weather and climate predictions with global cloud‐system‐resolving models. The NICAM simulation with a horizontal resolution of about 7 km successfully reproduced the lifecycles of two real tropical cyclones that formed in Indian Ocean in the austral summer 2006. Initialized with the atmospheric conditions 1‐2 weeks before the cyclones genesis, the model captured reasonably not only the timing of the observed cyclone geneses but also their motions and mesoscale structures. The model provides a high temporal/spatial resolution dataset for detailed studies of mesoscale aspects of tropical cyclone genesis. These promising results suggest the predictability of tropical cyclones by high‐resolution global cloud‐system‐resolving models.

A joint satellite and global cloud‐resolving model analysis of a Madden‐Julian Oscillation event: Model diagnosis

In this study, a methodology to diagnose a global cloud‐resolving model (GCRM) is explored on the basis of a joint analysis with satellite measurements. The Madden‐Julian Oscillation experiment carried out with the Nonhydrostatic Icosahedral Atmospheric Model (NICAM) is used as the test bed. The NICAM output is compared with Tropical Rainfall Measuring Mission (TRMM) and CloudSat measurements in terms of composite analysis, contoured frequency by altitude diagrams (CFADs), and the joint histogram of cloud top and precipitation top heights. It is found in the composite diagram that the GCRM reproduces a slow, eastward migration of a convective envelope well comparable to the satellite measurement. The GCRM CFAD qualitatively reproduces TRMM and CloudSat CFADs, except that the GCRM tends to overly produce snow in deep convection. The joint histograms reveal that, while the overproduction of snow is evident, NICAM‐simulated snow is incapable of producing 94‐GHz radar echoes higher than 5 dBZ. This deficiency can be mitigated by a modification to microphysical parameterization in the way that a proportion of small particles is enhanced in the snow mass spectrum.