Variations in latent heat flux (LHF), calculated from daily variables in the OAFlux dataset, were investigated over the Japan Sea for December of 1990-2010 because the December is the month with the largest LHF release. The largest temporal variations in LHF occurred over the eastern Japan Sea (EJS; 137°E-140°E, 38°N-43°N), showing an interannual-scale variation. By performing simple experiments with combinations of raw daily data and daily climatological data, we quantitatively assessed the relative contributions of saturated specific humidity at sea surface temperature (SST), surface air humidity, and surface wind speed in determining the LHF over the EJS. Results showed that SST (saturated specific humidity), which is associated with the Tsushima Warm Current, predominantly determines the LHF; a huge evaporation in the state of the positive SST anomaly. The LHF in the EJS has a strong influence on precipitation over the northern Japanese Islands.

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A regional atmospheric model was employed to demonstrate that a strong westerly wind, which occurred around December 2001 prior to the 2002/03 El Niño event, was enhanced by sea surface temperature gradient along the equator in the western Pacific warm pool region. Furthermore, a regional oceanic model was employed to demonstrate that the enhanced westerly wind event substantially increased oceanic Kelvin wave response along the equatorial Pacific Ocean.

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We investigated the temporal evolution of two types of diagram produced by a three-dimensional large-eddy simulation: the Contoured Frequency Optical Depth Diagram (CFODD) and the correlation pattern between the cloud droplet effective radius and the cloud optical thickness (RE-COT pattern). The CFODD obtained by statistically method, in the past study, suggested that the transition of cloud stages were included in the diagram, but this paper is the first paper that confirms similar structure in the CFODD obtained by time-series analysis. Our results suggest that a part of the cloud lifecycle; transition from the non-drizzle to drizzle stage can be well expressed by the CFODD of individual clouds. While, the cloud lifecycle on the RE-COT pattern was obscure. This advantage of CFODD over the RE-COT pattern arises because the CFODD uses vertical information of the inner cloud, whereas the effective radius of the RE-COT diagram presents property near the cloud top. Our results emphasize that future satellite observations should retrieve vertical information such as the effective radius at each height with high frequency to capture the lifecycle of individual clouds. This paper presents useful suggestion that will be useful to the satellite user community for retrieving the shallow-cloud lifecycle.

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The impact of intraseasonal oscillation on tropical cyclone genesis (TCG) over the western North Pacific during boreal summer was examined in association with categorized synoptic-scale low-level flow patterns. Five synoptic-scale flow patterns were considered as synoptic-scale environment connecting the large-scale intraseasonal oscillation and meso-scale TCG: a shear line, a confluence region, an easterly wave, a monsoon gyre, and a pre-existing tropical cyclone. The phase and amplitude of the boreal summer intraseasonal oscillation (BSISO) were used. It is found that the phase 7 was favorable for TCG associated with the shear line, and the phases 7 and 8 were also favorable for TCG associated with the confluence region. During the favorable phases for each flow pattern associated with TCG, the BSISO affected the large-scale zonal wind distribution, and the large-scale zonal wind forms synoptic-scale flow patterns over the suitable environment for TCG. The link between the intraseasonal oscillation and synoptic-scale flow patterns is an important factor in TCG.

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This study focuses on the main factor of regional difference in Altitudinal Dependency of Snow Depth (ADSD) and discusses the applicable range of snow cover estimation method with ADSD. We use the high-density surface observational data and a regional climate model in Niigata Prefecture. The estimation method with ADSD produces significant estimation error if the method is applied to broad areas. The high-density observational data show the regional difference of ADSD between windward and leeward areas with a coastal mountain. Numerical simulation reproduces these observational results. We evaluate mountain effects on the regional difference of ADSD using sensitivity experiments. In the sensitivity experiment, the altitude is changed from a mountain to a flat plain. The sensitivity experiment shows that the regional differences of ADSD are not simulated. The results indicate that the applicable range of the estimation method with ADSD is limited to a single slope and mountain. It should be noted that this method has a high probability of increasing the estimation error in complex mountainous areas, particularly in the coastal area of the Japan Sea.

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Air samples collected over the western North Pacific using a cargo aircraft C-130H have been analyzed for O₂/N₂ ratio (δ(O₂/N₂)), Ar/N₂ ratio (δ(Ar/N₂)), δ¹⁵N of N₂, δ¹⁸O of O₂ and δ⁴⁰Ar since May 2012. The relationships of δ(Ar/N₂), δ¹⁸O and δ⁴⁰Ar with δ¹⁵N indicate a significant artificial fractionation due to thermal diffusion during the air sample collection. The observed δ(O₂/N₂) and the atmospheric potential oxygen (APO = O₂ + 1.1 × CO₂), corrected for the artificial fractionation, show clear seasonal cycles at all altitudes over Minamitorishima, with amplitudes decreasing with height. Both the seasonal amplitude of the mid-tropospheric APO and its ratio to the surface value are found to decrease significantly from the mid-latitude to the subtropical region. The amplitude of the mid-tropospheric CO₂ seasonal cycle does not change significantly meridionally in a latitude band.

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A procedure for estimating the precipitable water vapor (PWV) distribution around ground-based stations of the global navigation satellite system (GNSS) on a scale of several kilometers is presented. This procedure utilizes the difference between the zenith total delay above a GNSS station and the zenith mapped slant path delay (SPD). This difference can be used to estimate the PWV gradient in each SPD direction by assuming an exponential distribution for the horizontal water vapor gradient.
The procedure was tested using an estimation of the PWV variation associated with the parent storm of an F3 Fujita scale tornado that occurred in Ibaraki prefecture on 6 May, 2012. Differential reflectivity observed by a dual-polarimetric radar indicated the existence of a developed parent cloud approximately 1h before the tornado occurred. A high-resolution numerical weather model simulation suggested the existence of a strong PWV gradient around the parent cloud, made evident by the co-existence of a strong updraft and downdraft within an approximately 5-km radius. The PWV gradient, calculated using the GNSS observation network with an average spacing of approximately 17 km, could not detect such a small-scale, strong PWV gradient. The PWV gradient estimated using the proposed procedure revealed a strong PWV gradient and its enhancement. In this case, a higher-order inhomogeneity component of each SPD played a critical role.

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Most global climate models (GCMs) suffer from prediction biases in their dynamic and thermodynamic structures in and around the boundary layer (BL). It remains unclear which of these biases within the large-scale conditions are crucial to the accurate reproduction of BL clouds. To develop a better understanding of the effects of variations in the simulated large-scale conditions, this paper uses large-eddy simulations to evaluate the effects of the fluctuation based on the latest GCM ensemble data on the prediction of a Californian stratocumulus under perturbed environments. The result indicates the relative importance of each component, and the most important factors controlling cloud behavior are the amplitudes of jumps in vapor and temperature across a BL top. The given variations in wind velocity and its vertical shear, large-scale subsidence, and surface heat fluxes have a lesser effect. This suggests that to reduce model biases predicted in GCMs, greater attention should be paid to the stratification structure across the BL top.

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We perform decadal hindcast experiments with initialization every year and assess sensitivity of the hindcasted errors to the errors in the initial climate states. The hindcasted sea surface temperature (SST) over the extratropical North Pacific shows significant impacts of initialization, yet the hindcasted indices of the Pacific decadal variability usually suffer from limited predictability. Our sensitivity analyses reveal that, in the decadal SST hindcasts over the extratropical North Pacific, the annual-to-decadal errors of the Aleutian Low fluctuation before the time of starting hindcast experiments work as a major source of uncertainty through delayed responses of the ocean. As we directly assimilate only the ocean states to the atmosphere-ocean coupled model in initialization, the Aleutian Low fluctuation in the assimilation exhibits large errors even though the SST is well correlated with the observation. These assimilation errors in the Aleutian Low fluctuation are primarily due to the distorted responses of the extratropical atmosphere to the tropical SST changes in the model. A close examination indicates that the observed and assimilated Aleutian Lows are sensitive to the eastern and central equatorial SSTs, respectively. Toward further reducing uncertainty in the Pacific decadal hindcasts, therefore, it should be an effective way to raise quality of initial conditions for the extratropical atmosphere and the tropical atmosphere-ocean coupling.

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The seasonal (monthly) variation in temperature difference between a meteorological station in an urban-area and nearby farmland in Kumagaya City was examined over 2010-2012. Kumagaya, one of the hottest cities in Japan, has an urban area of about 5 × 5 km and is surrounded by farmland. The daily mean, maximum, and minimum temperatures (T_mean, T_max, and T_min) routinely observed at the meteorological station (urban site) were higher than at the nearby farmland site across all seasons. Differences in the monthly temperatures between the two sites were 0.17-0.90°C (T_mean), 0.55-1.63°C (T_max), and 0.20-0.62°C (T_min), and the maximum differences were in August (T_mean and T_max) and April (T_min). Large temperature differences (> 1°C) for T_max were observed during the paddy rice-growing season in farmland from July to September. Differences in daily T_max and T_mean between the two sites were found to increase with daily solar radiation Sd. The sensitivities of these temperature differences to Sd were larger from July to September. The number of “extremely hot days” (T_max ≥ 35°C) at the farmland site was only 36% of the number at the urban site, while the relative proportion of “sultry nights” (T_min ≥ 25°C) was 62%.

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The snow darkening module evaluating dust, black carbon (BC), and organic carbon (OC) depositions on the mass of snow impurities and albedo has been developed for the NASA Goddard Earth Observing System, Version 5 (GEOS-5) Earth System Model, as the GOddard SnoW Impurity Module (GOSWIM). GOSWIM consists of the updated snow albedo scheme from a previous study (Yasunari et al. 2011) and a newly developed mass concentration calculation scheme, directly using aerosol depositions from the chemical transport model (GOCART) in GEOS-5. Compared to observations at Sapporo, the off-line simulations, forced by observation-based meteorology and aerosol depositions from GOES-5, reasonably simulated the seasonal migration of snow depth, albedos, and impurities of dust, BC, and OC in the snow surface. However, the simulated dust and BC mass concentrations in snow were especially underestimated except for the BC in the early winter, compared to the observations. Increasing the deposition rates of dust and BC could explain the observations. Removing BC deposition could possibly lead to an extension of snow cover duration in Sapporo of four days. Comparing the off-line GOSWIM and the GEOS-5 global simulations, we found that determining better local precipitation and deposition rates of the aerosols are key factors in generating better GOSWIM snow darkening simulation in NASA GEOS-5.

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We examined proximity soundings at intervals of a few minutes and at distances of less than 20 km from a significant tornadic (SIGTOR) supercell that occurred on 6 May 2012 in Japan. We used a 1-dimensional variational (1DVAR) technique that combined the observations of a ground-based microwave radiometer with outputs from a numerical model. Based on the results of the 1DVAR, several supercell and tornado forecast parameters were calculated and compared with values typical of SIGTOR supercell environments in the United States. One and a half hours before the occurrence of the tornado, the value of convective available potential energy increased significantly to about 1000 J kg⁻¹, a value that is smaller than the typical value in the United States. Low-level vertical wind shear and some composite parameters attained maximum values at the time when the distance to the supercell was the smallest. The vertical wind shear parameters and some composite parameters indicated that the environment fell into the SIGTOR supercell category. This result shows that the thermodynamic environments became unstable before the approach of the supercell, and the low-level vertical wind shear changed locally near the supercell.

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A series of videosondes that were continuously launched revealed the microphysical structures of snow clouds during the entire period of an intense cold air outbreak that occurred during December 23-25, 2010. The evolution of the microphysical features of graupel and snowflakes associated with the cold air outbreak were examined. It was found that the precipitation particle distributions in the snow clouds varied as the development of the cold air outbreak progressed. Graupel was the predominant precipitation particle throughout the cold air outbreak, while the number of snowflakes was increasing in the latter half of the period. When the lightning activity was relatively weak, high concentrations of graupel were located at lower level warmer than −10°C. It is suggested that lightning activity is associated with the vertical distribution of graupel.

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We examined the orographic effect of Mindanao Island in the Philippines on the distribution of precipitation from Typhoon Washi. The National Centers for Environmental Protection (NCEP) and US Department of Energy (DOE) Reanalysis data were downscaled using a regional spectral model to reproduce the typhoon in detail. A heat budget analysis revealed the reason for the absence of precipitation over the mountain in the center of Mindanao Island. Sensitivity experiments with various artificial terrains were conducted to estimate the orographic effect on the precipitation distribution. The peak rainfall, which occurred on the west side of Mt. Ragang, was caused by the interaction between the typhoon and both Mt. Ragang and Mt. Malindang. A rich source of heat and moisture is indispensable for typhoon rainfall. However, without a mountain, a typhoon would not cause heavy precipitation even after trespassing. Divergence of the typhoon wind due to one or more mountains is shown to be the key factor behind the heavy rainfall over land from Typhoon Washi in 2011.

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Diurnal variations of precipitation around western Japan during the warm season were studied by using the Japan Meteorological Agency (JMA) Radar-Raingauge analyzed precipitation (R/A) dataset from 1995 to 2012. Prominent diurnal variations in monthly mean precipitation were detected in June, with morning maxima (0600-1100 JST) around Kyushu over a large area of both land and sea (30°N-34°N, 128°E-138°E), and afternoon maxima (1200-1700 JST) around the Nansei Islands (26°N-30°N, 126°E-132°E).
Case studies of the precipitation maxima around Kyushu and the Nansei Islands in June revealed that both regions were covered by the Baiu cloud band during the days with significant maxima. During this period, cloud systems repeatedly appeared over the East China Sea in the early morning and developed rapidly while traveling eastward. Cloud systems associated with morning precipitation maxima around Kyushu tended to weaken around noon. Afternoon precipitation maxima around the Nansei Islands, however, were accompanied by organized deep cloud systems that continuously developed from the early morning to the afternoon. The JMA 55-year Reanalysis dataset suggested that the June precipitation maxima around Kyushu and the Nansei Islands were both closely related to diurnal variations in low-level southerly winds.

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As a subset of the Japanese 55-year Reanalysis (JRA-55) project, the Meteorological Research Institute of the Japan Meteorological Agency is conducting a global atmospheric reanalysis that assimilates only conventional surface and upper air observations, with no use of satellite observations, using the same data assimilation system as the JRA-55. The project, named the JRA-55 Conventional (JRA-55C), aims to produce a more homogeneous dataset over a long period, unaffected by changes in historical satellite observing systems. The dataset is intended to be suitable for studies of climate change or multi-decadal variability. The climatological properties deduced from the early results of the JRA-55C are similar to those of the JRA-55 in the troposphere and lower stratosphere, except for high southern latitudes. On the basis of forecast skill, the quality of the JRA-55C is inferior to that of the JRA-55, but the JRA-55C has better temporal homogeneity than the JRA-55. The skill of the latter changes during the JRA-55 period. We have completed 85% of the entire JRA-55C calculation as of February 2014. We expect that the JRA-55C will contribute to a much better understanding of the impact of changes in observing systems on climate trends and variability estimated from the JRA-55.

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An advanced particle microphysics (APM) model has been incorporated into a regional chemical transport model, NAQPMS, to simulate the seasonal variation of particle number concentrations over China in 2007 for the first time. The NAQPMS+APM can reproduce the particles number concentrations at remote, suburban sites and urban sites reasonably. Most of modeled values were within a factor of two of observations. The simulation indicated that particles number concentration was significantly higher in southeastern China than that in northwestern China. Monthly mean number concentration can be over 20000 cm^-3 in most polluted regions in southeastern China while the value is generally below 7000 cm^-3 in northwestern parts of China. Higher number concentration occurred in January while lower value occurred in April and July. In heavily polluted regions, like Sichuan Basin and central-eastern China, primary particles dominated particles number, while secondary particles formed via the nucleation process account for most of particles number over relative clean areas. The area over which secondary particles dominated showed distinct seasonal variation and its spatial pattern was coupled with primary particles distribution which was strongly influenced by the meteorological conditions, e.g., East Asia Monsoon.

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The evolutions of Asian summer monsoon (ASM) are detected and evaluated based on the models in Couple Model Intercomparison Projects Phase-3 and Phase-5 (CMIP3 and CMIP5) for the 20^th Century climate simulation (20c3m and Historical runs, respectively). Considering that the individual models have various biases in rainfall amount simulation, instead of applying a fixed rainfall criterion as used in observation, we use model-dependent rainfall criteria to identify the simulated ASM onset, retreat, and duration. This model-dependent criterion is defined as the height in cumulative distribution function (CDF) of simulated precipitation that the observed criterion occurs. Based on this method, the multi-model ensembles (MMEs) of CMIP3 and CMIP5 both show a delayed monsoon onset but an earlier retreat relative to the observations, indicating that models tend to underestimate the monsoon period. The MME results show a skill in capturing the ASM domain which features monsoon rainfall characteristics, whereas a large spread is found among individual models. Overall, the state-of-the-art CMIP5 models show slightly improvements from the CMIP3 models in the simulations of ASM domain and evolutions. Models with a hybrid method based on bulk mass flux and CAPE closure schemes perform better than models with other types of convection parameterization.

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The scheme developed by Nguyen and Chen (2011) is used to produce 18 TCs (2004-2013) over the Northwestern Pacific that are well adjusted to the environment. The environment, including SST, in which the storm is embedded has a significant effect on the intensity and rainband patterns of these TCs. During the early season, TCs have a tendency to exhibit a “9” type asymmetric structure with an upper-level outflow channel extending southwestward from the southeastern quadrate of the storm. At low levels, the convergence area between the TC circulation and the southwesterly monsoon flow is a favorable location for the development of spiral rainbands. Late season TCs have a tendency to produce a “6” type storm structure with an outflow channel extending northeastward from the northwestern part of the eyewall, especially when an upper-level cold low or trough is present to the northwest of the storm. At low levels, the convergence of the northeasterly monsoon flow and the TC circulation is favorable for the occurrences of spiral rainbands. For intense TCs that underwent an eyewall replacement cycle, the scheme also shows considerable skill in reproducing the concentric eyewall structure.

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Features of lightning around Japan over a four-year period are described using World Wide Lightning Location Network data based on global observations of very low frequency radio waves. Lightning density over land is lower than that over the ocean and the oceanic area of high density runs along the Kuroshio Current. The zone of highest density is found to the west of the Nansei Islands; however, very strong lightning rarely occurs there. Conversely, lightning density decreases gradually towards the eastern part of the Kuroshio Current, where the occurrence of very strong lightning increases.
The ratio of very strong lightning in most of the analysis area tends to increase in the cold season, and it becomes large in areas where lightning frequency is low. Furthermore, both very weak and strong lightning have a tendency to occur in areas where rain intensity is not strong.

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In this study, we conducted a linear stability analysis by using a linear baroclinic model (LBM) derived from a three-dimensional spectral primitive equation model with only gravity modes. According to the stability analysis for a non-zonal basic state subject to an inviscid adiabatic condition, some unstable modes appear in a low-frequency range, and one of these unstable modes has an MJO structure. The MJO mode has a life cycle of ∼12.5 days. The dynamical field of the mode has a structure with zonal wavenumber 1 near the equator, and its vertical structure indicates an opposite sign in the upper and lower troposphere. This feature is confirmed by the energy spectra in the zonal, vertical, and meridional directions. It is found from the basic properties of the LBM, that the MJO mode arises from an up-scale energy interaction with the non-zonal basic state.

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Dust devils are small-scale atmospheric vortices that are often observed on deserts. Using a video of a dust devil that occurred in a close distance, this study attempts to apply Particle Image Velocimetry (PIV) to obtain flows in the dust devil for the first time. Images taken by two cameras enable us to connect a pixel size into actual length scale. The estimated maximum tangential wind and updraft in the dust devil are 16 m s^-1 and 6 m s^-1, respectively. These wind speeds are comparable to these of strong ones reported in previous observational studies.

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We re-examine the internal oscillation dynamically analogous to the equatorial quasi-biennial oscillation (QBO) that was firstly obtained by Held et al. (1993; hereafter HHR93) as a radiative-convective quasi-equilibrium state in a highly-idealized two-dimensional regional model with explicit moist convections under a periodic lateral boundary condition without Coriolis effects. A QBO-like oscillation with a period of 120.6 days is obtained for the control experiment with a similar configuration as HHR93. The QBO-like oscillation is a robust feature, not sensitive to the choice of model configuration such as domain size and horizontal resolution, or boundary conditions such as prescribed zonal wind at the top and sea surface temperature.
The obtained QBO-like oscillations show downward propagation of the zonal mean signals in the stratosphere as revealed by observations and wave-mean flow interaction theories, while unlike the observed equatorial QBO, they have a clear signal in the zonal mean zonal wind and temperature in the troposphere. The zonal mean precipitation also varies in accordance with the oscillation, though its day-to-day fluctuation is very large compared to the long-period oscillation.

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Open-path eddy-covariance CO₂ flux over the ocean have been reported to be much larger in magnitude than the estimated bulk CO₂ flux, and optical window contamination of the open-path gas analyzers has been reported as the cause of the overestimations. During an on-board experiment over the ocean, we manually cleaned the optical window and directly compared CO₂ output signal before and after the window cleaning. It is found that both of the CO₂ fluctuation amplitude and correlation coefficient between vertical wind velocity and CO₂ mixing ratio have apparently increased, then resulting in the downward CO₂ flux overestimation when the optical window contamination accumulate. These apparent increases were reset after the manual window cleaning. Correlation coefficients between CO₂ and H₂O output signals, which represent cross-sensitivity between CO₂ and H₂O, can be useful indicators of optical window contamination of the open-path gas analyzer.

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A severe heat wave occurred in the southwestern United States (US) during June and July 2013. To investigate the effects of natural variability and anthropogenic climate change on this event, we generated large ensemble simulations of possible weather using the MIROC5A climate model forced by “historical external forcing agents, sea surface temperature (SST) observations and sea ice (SIC) observations” both with and without human influence. It was suggested that both the anthropogenic warming and an atmospheric circulation regime related to the natural variability of SST and SIC made the heat wave event more likely. On the other hand, no significant human influence was found in atmospheric circulation patterns. These results were robust for two different estimates of anthropogenic signals on SST and SIC.

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A three-dimensional coherent Doppler lidar (3D-CDL) and an interval camera were used to detect a cyclonic “invisible waterspout” very close to the sea surface near Ikeshima island, Nagasaki Prefecture, on 21 December 2010. High-resolution 3D-CDL images revealed that the waterspout had a subscale vortex. The deduced core diameter of the waterspout (subscale vortex) was 234 m (105 m), and the vorticity was 0.17 s^-1 (0.32 s^-1). The estimated maximum wind speed of the waterspout was about 17 m s^-1, which falls into the category of an F0-scale tornado. Analysis of the 3D-CDL and C-band Doppler radar data shows that the waterspout and two vortices were observed along a gust front ahead of the parent storm.

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The effects of sea-surface temperature (SST) on snowfall in Japan were investigated using a high-resolution 60-km-mesh atmospheric general circulation model (AGCM). We used a high-resolution SST dataset (original resolution, ∼20 km); the high-resolution SST data resolve warm boundary currents, such as those of the Kuroshio and Tsushima warm currents. The AGCM experiment using high-resolution SST data simulates snowfall increase (decrease) over warm (cold) SST regions in January better than the AGCM based on coarse-resolution SST data. A moisture budget analysis shows significant rainfall increase over a warm-SST band along the boundary current, where large evaporative fluxes supply moisture to the atmosphere. On the other hand, the moisture convergence anomaly is generally opposite to that of the evaporative flux anomaly, and hence acts to reduce rainfall. The surface moisture flux change is generally similar to the dynamical term, while the thermodynamic term also plays a non-negligible role. A comparison of satellite observations and AGCM experimental results shows that fine-scale SST variations affect surface wind patterns. Furthermore, we diagnosed surface convergence anomaly using two terms: one that describes the vertical mixing effect and the other that describes the pressure adjustment effect. This analysis shows that both vertical mixing mechanisms and pressure adjustment may contribute to surface convergence anomaly over the Japan Sea.

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Record heavy PM_2.5 air pollution (maximum concentration of ∼1 mg m^-3) observed over China in January 2013 was analyzed. The vertical and horizontal scales of the pollution layer are critically important parameters for the analysis of pollution phenomena, but they are difficult to measure. This is because the PM_2.5 aerosol concentration is so high that ordinary remote-sensing methods such as ground-based and space-borne lidar inversion are difficult to apply. First, we showed the detailed time-height structure of aerosol extinction coefficients based on Beijing lidar observation, by assuming a non-zero boundary extinction coefficient and using 3D chemical transport modeling (CTM). The aerosol structure derived from lidar observations and the CTM results were in close agreement. Using ground-based lidar, we also found that a shallow aerosol layer (height of 200-300 m) remained over Beijing for a long time. We also successfully showed that the horizontal extent of the aerosol layer over the China Plain was several hundred km based on CALIOP observations and CTM.

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A 1-month-long observation of the Ciliwung River, which flows through Jakarta in Indonesia, has revealed evidence of the persistent existence of a diurnal cycle in the water level of a tropical river. This was consistent with the diurnal cycle in rainfall observed by meteorological radar and five rain-gauge stations. The river's diurnal cycle was distinguishable from the effects of oceanic and atmospheric tides and has a locally time-locked 1-day periodicity and an amplitude of 0.05 m. The day-to-day variation in the amplitude of the river's diurnal cycle was smaller than the diurnal cycle of the rainfall.

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A new 4-dimensional variational data assimilation system with 0.5-km grid spacing (NHM-4DVAR.v3) was developed by integrating the nonhydrostatic storm-scale 4D-Var (NHM-4DVAR.v2) and the Japan Meteorological Agency (JMA) nonhydrostatic model (NHM) based Variational Data Assimilation System (JNoVA). Both systems are based on the JMANHM, but horizontal resolutions, their formulations, adjoint models of physical processes, and observation operators are different. NHM-4DVAR.v3 comprises advantages of both systems: a penalty term, optimization of lateral boundary conditions, and observation operators for advanced observations. This development aimed at improving the forecast accuracy of hazardous weather at meso-γ-scales (5∼20 km). In this paper, the characteristics of NHM-4DVAR.v3 and some results, including the integrated formulations, are presented. An assimilation experiment of actual observations using NHM-4DVAR.v3 with 2-km grid spacing was found to show improvement over NHM-4DVAR.v2 at the same resolution. As a final goal, NHM-4DVAR.v3 was applied with a 0.5-km resolution. The comparison between assimilation results by NHM-4DVAR.v3 with 0.5- and 2-km horizontal resolutions indicates that analyses with super high resolutions can reproduce more detailed atmospheric features such as convective clouds.

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The peaks of the appearance frequency of the surface air temperature during precipitation are clearly observed near the melting point of water on the Toyama Plain during the winter monsoon. The peaks could be explained by the hypothesis that the melting of snowfall is the primary cause of the cooling on the Toyama Plain. To verify this hypothesis, we investigated the relation of temperature between the inland and the coast using observed data in January from 1990 to 2009 and applied a simple estimation method of the cooling due to the melting of snowfall. The temperature on the Toyama Plain tends to remain around the melting point when the surface air temperature on the coast is higher than 273.15 K and lower than 277.15 K, which almost corresponds to the changeover from snowfall to rainfall. The relation is unclear when hardly any precipitation is observed. The simply estimated cooling by the melting of snowfall using the observed precipitation can also represents the cooling on the Toyama Plain. Accordingly, the local climatic temperature could be greatly influenced by advection of the air mass cooled by the melting of snowfall until the air mass reaches the Toyama Plain during the winter monsoon.

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Herein, the spatial distribution of stable oxygen isotope ratios (δ¹⁸O) of precipitation in the Atlantic to Indian sectors of the Southern Ocean is examined using the results of in situ observations and numerical modeling. In situ observations of 59 precipitation events reveal poleward decrease of δ¹⁸O, with a larger meridional gradient south of 60°S. Moreover, the estimates from the observations and model (IsoGSM) agree reasonably well, with a mean absolute difference of 4.3‰. Thus, the IsoGSM results generally support the observed poleward increase in the meridional gradient. These results will prove valuable in investigating the atmospheric water cycle and in studying oceanic processes of water mass formation and transport.

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This study investigates the characteristics of local-scale afternoon rainfall and its environmental conditions in and around the Nobi Plain under synoptically undisturbed conditions in summer by using operational meteorological data. The analysis of radar/raingauge analyzed precipitation data revealed that strong rainfall occurs over the mountain slopes in late afternoon and in the northern part of the Nobi Plain in early evening, while light rain frequently occurs around the mountain peaks in early afternoon. Examining mesoscal analysis data indicated that the primary difference in the environmental conditions that distinguish between rain and no-rain days is middle-level moisture field, with moister condition being found on rain days. Because of the significant difference in the middle-level moisture, stability index is useful in distinguishing the environmental conditions between rain and no-rain days.

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The lack of long-term daily observations limited the study of changes in thermal growing season in China during the 20th century. Changes in the Local Growing Season (LGS) are analyzed based on a set of homogenized monthly temperature series back to the 19th century at 16 stations in eastern China. The analysis contains three steps: (1) to calculate the LGS indices (including the start and end dates) based on the daily temperature records at the 16 stations for the period 1960-2011; (2) to establish a linear relationship between the LGS indices and monthly temperature records; (3) to reconstruct the long-term LGS index series based on this linear relationship and the long-term monthly temperature records. It is found that, in eastern China, start (end) of LGS exhibits an advancing (a delaying) trend of −1.0 (0.5) days per decade, resulting in a lengthening trend of growing season of 1.5 days per decade, for the period 1909-2012. Changes in LGS indices are not monotonic but with mutidecadal variability. In particular, enhanced LGS-lengthening trends occurred during 1910-1940 and 1965-2012. Especially from 1965 onward, the LGS has been significantly extended by 3.5 days per decade, of which about 35% is contributed from mutidecadal variability.

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Climate changes for the end of the 21st century projected by Coupled Model Intercomparison Project phase 5 (CMIP5) models are classified into three clusters by a cluster analysis of annual-mean tropical sea surface temperature (SST) change patterns. The classified SST change patterns are featured by the zonal gradient of the change in the equatorial Pacific and inter-hemispheric contrast of the warming. Precipitation and atmospheric circulation responses are composited for the clusters, and their relationships to the SST changes are examined. Precipitation increase is larger where SST warming is larger than surroundings and vice versa. Common precipitation and atmospheric circulation responses for each cluster are found also over tropical lands and the extratropics as well as in the tropical oceans, suggesting that some remote effects of the tropical SST change patterns could be one reason for less agreement among CMIP5 models in climate changes.

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Birch pollen mainly causes springtime allergy-related diseases, birch pollinoses, widely known in high-latitude countries. By utilizing the observation in Sapporo from 2001 to 2011, we found that the daily pollen amount almost follows the log-normal distribution with its characteristic time-scale of several days. The pollen amount itself was therefore taken as a major predictor for its day-to-day variations. Another predictor was chosen from climatic variables that were possibly related to the pollen amount such as temperature, rainfall, sunshine duration, wind, relative humidity, rainfall, and daily temperature difference to explain daily variations of the pollen amount. A resulting statistical equation with two independent predictors of lagged pollen amount and diurnal temperature range based on the multiple regression analysis provided a reasonable hindcast prediction with the correlation coeffcient with observation being 0.80. Moreover, the equation was better fitted to the observations in abundant years than in poor-yield years.

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Since the late 1990s, surface temperature has been higher than (near or lower than) normal for summer/autumn (winter/spring) over Japan, indicating that the seasonal temperature contrast has become enhanced. In order to relate this to global-scale variability on decadal timescale, atmospheric re-analysis and ocean assimilation datasets were analyzed. It is suggested that the La Niña-like conditions which have been frequently observed in the tropical Pacific oceanic and atmospheric fields in the last decade have contributed to these temperature tendencies observed in Japan. These global characteristics are consistent with the global warming hiatus. The results presented here indicate that not only interannual variability and century-scale long-term trends but also decadal variability in global oceanic and atmospheric fields significantly affect Japan's temperature.

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Future changes in winter stationary waves are investigated using Coupled Model Intercomparison Project Phase 5 (CMIP5) models and a linear baroclinic model (LBM). The CMIP5 models showed a wave-like pattern of stationary waves from East Asia to the North Pacific and a weakening of horizontal divergence over the Maritime Continent. To investigate dynamical relationships among these changes, we performed LBM experiments using the zonal mean basic state and zonally asymmetric thermal forcing. The differences between the future and present experiments were similar to the changes projected by the CMIP5 models, although positions and amplitudes differed slightly. In addition, two of the LBM experiments showed that the change in the basic state explained most of the changes in the stationary wave, whereas the change in thermal forcing accounted for the eastward shift of the stationary wave. The storm track experiments conducted with the LBM to investigate the role of transient eddy feedback on stationary wave changes suggested that the feedback shifts the thermally forced stationary waves northeastward. This shift may explain the difference between the LBM experiments and the CMIP5 future projection.

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We statistically investigated an interannual co-variation among aerosol optical depth (AOD), cloud effective radius (CER), and precipitation, focusing on aerosol-cloud interaction over the tropics. A three-month composite analysis for AOD, CER, and precipitation for 2000-2012 based on El Niño-Southern Oscillation phases during September-October-November (SON) and December-January-February shows that an increase (decrease) in AOD in the El Niño (La Niña) years was associated with a decrease (increase) in precipitation, particularly in SON over the Maritime Continent. Additionally, CER decreased in the El Niño years over the same region, which implies that CER was associated with interannual variation in aerosol burden; these results were statistically significant. Interannual variation in AOD and CER in SON in the Maritime Continent was asymmetrical, which can be explained by stronger aerosol-cloud interactions under drier conditions. Specifically, large amounts of aerosols suppressed cloud and precipitation formation, which leads to decreases in wet deposition and increases in emission under warmer and drier surface conditions. This feedback results in asymmetrical variation. Furthermore, the asymmetrical interannual variation was confirmed statistically.

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Typhoon Morakot (2009), a devastating tropical cyclone (TC) that made landfall in Taiwan in August 2009, produced the highest recorded rainfall in southern Taiwan in 50 years. The slow translation speed of Morakot, among many other factors, was found to play an important role in heavy rainfall. Using the WRF model, this study examined the causes of the slow TC translation speed in relation to the interaction of Morakot with Typhoon Goni (2009) and Typhoon Etau (2009). The simulated track of Morakot was relatively consistent with the observation in the control run. However, Morakot deviated more westward and moved slower in a sensitivity simulation without Goni, compared with that in the control run, and had a similar but faster track in another simulation without Etau. Comparisons also show that Goni helped to increase southerly to southwesterly steering flow to Morakot, while Etau's circulation helped to produce slightly weaker northerly to northwesterly steering flow. These two opposite forces counteracted partly the south-southeasterly steering of the Pacific high, resulting in a slowly north-northwestward tracking of Morakot.

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The ECMWF 2.5° gridded analysis was used to re-examine the evolution of the synoptic flow patterns and frontal structure of an early summer monsoon trough that occurred during 10-15 June 1975. A blocking pattern that began with an omega shape developed into a Rex pattern on 12 June. During 10-14 June, the blocking low pressure and associated trough axis were almost stagnant. As a result, the Mei-Yu front was quasi-stationary and affected the Taiwan area for more than four days.
Similar to other frontal systems during the early summer rainy season over southern China, this Mei-Yu front exhibited baroclinic characteristics in the subtropics. In the lower troposphere, appreciable temperature gradients and maximum frontogenesis due to horizontal deformation between the postfrontal northwesterlies and prefrontal southwesterlies were diagnosed. The western section (∼115°E) of this frontal system exhibited a marked northward vertical tilt. An upper-level jet near the tropopause was also present. A moist tongue was located south of the surface cold front within the low-level southwesterlies and extended vertically upward. A thermally direct circulation across the front with ascending motion within the prefrontal warm, moist air and descending motion within the postfrontal cold, dry air underneath the upper-level jet was diagnosed.

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The “Local deepening rate (LDR)”, the local surface pressure tendency, which is normalized by the sine of latitude and similar to the definition of an explosive cyclone, is introduced to extratropical cyclone activity analysis. The LDR has the advantage of being much simpler than conventional methods such as cyclone tracking and time filtering. The time average of positive LDR, which implies cyclone deepening, captures not only individual explosive cyclone's deepening but also the mid-latitude storm track climatology. The probability of explosive deepening, defined as LDR ≥ 1 hPa h^-1 and based on ensemble forecasts, accurately represents the deepening potential and provides information regarding the influence area of storms—analogous to the strong wind area used in typhoon forecasts. The LDR can also be used to assess the quality of storm tracks in reanalyses products. In the 20th century reanalysis, the storm track activity, calculated from ensemble mean surface pressure, is too weak before 1910 in the North Pacific, and in the South Pacific low activity is observed up to the end of the 20th century, because of large ensemble spread due to few surface pressure observations.

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In February 2014, the Global Precipitation Measurement (GPM) satellite was launched successfully and started observing global precipitation using the new DPR (Dual-frequency Precipitation Radar) sensor. This study pioneers to compare the GPM/DPR precipitation products with other satellite-derived products and simulated precipitation from the Nonhydrostatic Icosahedral Atmospheric Model (NICAM) at a 3.5-km horizontal resolution. The NICAM simulation outputs are converted to three-dimensional radar reflectivity using a radar simulator included in the joint simulator for satellite sensors (Joint-Simulator). We focus on the three frontal precipitation cases in the storm track regions, where the NICAM surface precipitation agrees generally well with an existing precipitation product “GSMaP”. The surface precipitation patterns, bright band heights and three-dimensional radar reflectivity of GPM/DPR generally agree with the corresponding variables from GSMaP, NICAM and the Joint-Simulator. However, the radar echo tops of GPM/DPR are systematically lower than that of NICAM-Joint-Simulator, suggesting that NICAM may overestimate mixing ratios of snow and graupel. The general agreement of surface precipitation patterns between GPM/DPR and the NICAM simulation encourages a possible use of GPM-derived precipitation data toward numerical weather prediction through data assimilation.

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This study aims to investigate the impact of observation error correlations and non-orthogonal observation operators on analysis accuracy using a chaotic dynamical model known as the Lorenz-96 40-variable model, extending the previous study by Miyoshi et al. using a simple two-dimensional conceptual model. The results corroborate Miyoshi et al.'s conceptual study and show that the analysis is more accurate when the row vectors of a linear observation operator are correlated positively (negatively) with negatively (positively) correlated observation error. The online estimation of the observation error covariance matrix based on the Desroziers diagnostics is successful when we have reasonable a priori knowledge about the observation error correlations.

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