The spectral element (SE) and local Galerkin (LG) methods may be regarded as variants and generalizations of the classic Galerkin approach. In this study, the second-order spectral element (SE2) method is compared with the alternative LG scheme referred to as o2o3, which combines a second-order field representation (o2) with a third-order representation of the flux (o3). The full name of o2o3 is o2o3C⁰C¹, where the continuous basis functions in C⁰-space are used for the field representation and the piecewise third-order differentiable basis functions in C¹-space are used for the flux approximation. The flux in o2o3 is approximated by a piecewise polynomial function that is both continuous and differentiable, contrary to several Galerkin and LG schemes that use either continuous or discontinuous basis functions for flux approximations. We show that o2o3 not only has some advantages of SE schemes but also possesses third-order accuracy similar to o3o3 and SE3, while SE2 possesses second-order accuracy and does not show superconvergence. SE3 has an approximation order greater than or equal to three and uses the irregular Gauss-Lobatto collocation grid, whereas SE2 and o2o3 have a regular collocation grid; this constitutes an advantage for physical parameterizations and follow-up models, such as chemistry or solid-earth models. Furthermore, o2o3 has the technical simplicity of SE2. The common features (accuracy, convergence, and numerical dispersion relations) and differences between these schemes are described in detail for one-dimensional homogeneous advection tests. A two-dimensional test for cut cells indicates the suitability of o2o3 for realistic applications.

Decadal variations of the quasi-biennial oscillation (QBO) in the equatorial stratosphere are investigated, using the Singapore data and reanalysis data from the 1950s to 2019/2020. It is found that the QBO is decadally modulated in the amplitude as well as in the period. These two decadal variations are negatively correlated with each other after the 1980s, while they show approximately positive correlations before the 1980s. In the time series of the QBO amplitude from the 1950s to 2014, there are four maxima (QBO_max) around 1967, 1983, 1995, and 2005, and three minima (QBO_min) around 1973, 1988, and 2000. Composite analyses of QBO_max and QBO_min based on these extrema reveal that the decadal amplitude variations have a maximum amplitude of about 3 m s⁻¹ at 20 hPa in the vertical structure. In the horizontal structure, off-equator extrema of about 3.5 m s⁻¹ appear around 5°N at 20 hPa, while extrema of about 1.8 m s⁻¹ are situated around 5°S at 50 hPa. The decadal amplitude variations of the QBO are closely and positively correlated with the decadal components of Niño 3.4 sea surface temperature anomalies (SSTa) and the Pacific Decadal Oscillation index, suggesting that the tropical SSTa in the Central Pacific substantially influences the QBO in the decadal timescales.

The relationships between the radar reflectivity factor for horizontal polarization (Z_h) at the X-band and the liquid-equivalent snowfall rate (R) are presented for six hydrometeor classes of solid precipitation. These relationships were derived by comparing the values of Z_h, R, and the hydrometeor class obtained by performing simultaneous observations in Niigata Prefecture, Japan. The relationships were assumed to be of the form Z_h = B R^1.67, where B denotes an experimentally determined coefficient. The values of R and the hydrometeor class were determined using a high-resolution precipitation gauge and an optical disdrometer, respectively, set up inside a windshield net. The average Z_h value was determined for an analysis area of approximately 100 km², located upwind of the ground-observation site. The prevailing hydrometeor class, its representative size, and the fall speed, along with the average Z_h, R, and B values, were determined for 48 cases considered over three winters. The average B value for the “heavily rimed snow aggregate” was smaller than that for the “rimed snow aggregate”. The largest B value was derived for a case of aggregates of unrimed dendritic particles (unrimed-D class). The case involving aggregates of unrimed low-temperature-type crystals (unrimed-C class) showed the smallest B value. For graupel cases, the average B value was roughly twice that of the rimed and heavily rimed snow aggregate classes and much smaller than that of the unrimed-D class. Snow aggregates demonstrated a stronger or weaker backscattering than graupel depending on the riming degree and types of constituent ice crystals in the X-band.

Meiyu front precipitation makes the region prone to frequent floods, mudslides, landslides, and other disasters and has been the focus of ongoing and challenging meteorological research. Investigation of the Raindrop size distribution (RSD) is essential for exploring the characteristics and underlying physical precipitation processes. In this study, the precipitation characteristics in Lushan mountainous areas during the Meiyu season were investigated using laser disdrometer observed RSD data from 2016 to 2020. For the average spectra of five rain rate classes, the concentrations of large raindrops (> 0.5 mm) increased with rain rate (R), whereas the concentrations of small raindrops (< 0.5 mm) increased only under rain rates higher than 10 mm h⁻¹. The gamma distribution parameters of N₀ (intercept parameter) and Λ (slope parameter) increased/decreased with rain rate, and the shape parameter µ exhibited negative values in different rain rate classes. The distribution pattern features were N(D) =721D^−1.79e^−1.20D. Distributions of the frequency for mass-weighted mean diameter (D_m) and the logarithm of the generalized intercept parameter (log₁₀ N_w) both showed a unique bimodal type, and an exceptionally high N_w (log₁₀N_w > 4.5) subset with small D_m was determined. The stratiform and convective rains of RSD were also investigated. D_m–R and N_w–R showed similar variations in two types of precipitation. The lower µ values resulted in higher primary and constant coefficients in the quadratic polynomial fitting for the µ–Λ relationship (Λ = 0.0347µ² + 1.180µ + 2.495). The Z–R relationship (Z for radar reflectivity factor) in stratiform precipitation characteristics was Z = 203R^1.59. Further investigations showed that high N_w values usually occurred in persistent precipitation. The RSD can be characterized as high concentrations of the first two diameter classes with a narrow spectrum width (< 1 mm), which were captured during in-cloud rain with a low but continuous rain rate (< 5 mm h⁻¹). The mountainous topography plays an important role in reshaping the characteristics of RSD and physical processes of precipitation.

The interannual and interdecadal variabilities of Indonesian rainfall in dry seasons (June–November) are investigated by using rainfall data from the Climate Research Unit (CRU) from 1939 to 2016 and from the Global Precipitation Climatology Project (GPCP) from 1979 to 2016. The first principal component (PC1) of both the CRU and GPCP data shows that the canonical El Niño–Southern Oscillation (ENSO), ENSO Modoki, and Indian Ocean Dipole (IOD) are major climate modes influencing the interannual variability of rainfall in Indonesia, and the Interdecadal Pacific Oscillation (IPO) is the major decadal phenomenon affecting the decadal variability of the rainfall. Furthermore, the IPO modulates the influence of IOD on Indonesian rainfall, with a weaker influence during the positive IPO phase during 1979–1997 and a stronger influence during the negative IPO phases during 1939–1978 and 1998–2016. The dependency of Indonesian rainfall response to the canonical ENSO and ENSO Modoki on IPO phases is not significant, although the response to the ENSO Modoki (canonical ENSO) becomes significant (less significant) in the negative IPO phase during 1998–2016 when compared with earlier periods.

Early morning precipitation (EMP) events occur most frequently during January and February over the northern coast of West Java and are characterized by propagating systems originating from both inland and offshore. The timing of EMP is determined by the initial location, direction, and speed of the propagating precipitating system. This study explores processes that characterize such propagating precipitation systems by performing composite analysis and real-case numerical simulations of selected events using the Weather and Research Forecasting (WRF) model with a cloud-permitting horizontal resolution of 3 km. In the composite analysis, EMP events are classified according to the strength of the northerly background wind (V_BG), defined as the 925 hPa meridional wind averaged over an area covering western Java and the adjacent sea. We find that under both strong northerly (SN) and weak northerly (WN) wind conditions, EMP is mainly induced by a precipitation system that propagates from sea to land. For WN cases, however, precipitating systems that propagate from inland areas to the sea also play a role. The WRF simulations suggest that mechanisms akin to cold pool propagation and advection by prevailing winds are responsible for the propagating convection that induces EMP, which also explains the dependence of EMP frequency on the strength of V_BG. On the basis of the WRF simulations, we also discuss the roles of sea breeze and gravity waves in the initiation of convection.

This study quantitatively examined the relative importance of Rossby wave breaking (RWB) east of Japan to a formation of the Pacific–Japan (PJ) pattern compared with that of tropical atmospheric and oceanographic variabilities. First, cases of the positive and negative PJ patterns are classified into those with and without the RWB occurrence. The result of the classification indicates that the cases of the positive PJ pattern triggered by the RWB account for approximately 20 % of the whole cases of the positive PJ pattern. The number of positive PJ cases with the RWB further accounts for approximately 80 % of those in the cases associated with the RWB. Results of a lag composite analysis and the related Q-vector diagnosis for the cases of the positive PJ pattern with the presence of RWB show that the RWB east of Japan promotes the formation of the PJ pattern through the southwestward intrusion of high potential vorticity air mass toward the subtropical western North Pacific (WNP) and the consequent dynamically induced enhanced convection over the region, consistent with results of previous studies. By contrast, the composite for the cases the negative PJ pattern accompanied by the RWB indicates that the RWB-related upper-tropospheric zonally elongated anomalous circulation and basin-wide sea surface temperature (SST) warming over the Indian Ocean can contribute suppressed convective activities over the subtropical WNP and the consequent formation of the negative PJ pattern although the RWB occurs. The composite for the cases of positive and negative PJ patterns with the absence of RWB further indicates that the convective activities over the subtropical WNP move northwestward with time, causing the formation of the PJ pattern. The formation of the PJ pattern with the absence of RWB is also closely associated with tropical SST and phase of the boreal summer intraseasonal oscillation, illuminating “pure” tropical impacts on the formation of a PJ pattern.

Ensemble forecasts with 101 members, including one ensemble mean, using ensemble Kalman filter analysis were performed to understand the atmospheric conditions favorable for the development of a meso-β-scale vortex (MBV) that caused shipwrecks as a result of sudden gusty winds in the southwestern part of the Sea of Japan on 1 September 2015. A composite analysis was conducted to reveal the differences in the structure of the MBV and atmospheric conditions around the MBV between the strongest 8 (STRG) and weakest 10 (WEAK) ensemble members. Two of the strongest ten members that developed the MBV much earlier than the other members were excluded from the analysis. The analysis revealed that the near-surface cyclonic horizontal shear to the northeast and the south of the MBV was stronger for STRG than for WEAK. In addition, larger low-level water vapor and its horizontal flux for STRG contribute to greater convective available potential energy to the southeast of the MBV, which results in stronger convection around the MBV. The results of the composite analysis are statistically supported by an ensemble-based sensitivity analysis. Differences in the near-surface horizontal shear were closely related to the structure of the extratropical cyclone in which the MBV was embedded. Although the strength of the extratropical cyclone for STRG was comparable with that for WEAK, the cyclonic horizontal shear of winds in the northeastern quadrant of the extratropical cyclone was greater for STRG than for WEAK.

The role of extratropical forcing on the summertime tropical synoptic-scale disturbances (TSDs) in the western North Pacific has been investigated by conducting parallel integrations of the Regional Climate Model. The suite of experiments consists of a control run (CTRL) with European Centre for Medium-Range Forecasts Reanalysis data as boundary conditions, and an experimental run (EXPT) with the same setting, except that signals with zonal wavenumber > 6 were suppressed at the northern boundary (located at 42°N) of the model domain. Comparison between CTRL and EXPT showed that without extratropical forcing, there is weaker TSD activity in the June-to-August season, with reduced precipitation over the TSD pathway. Associated with suppressed TSD, southeastward-directed wave activity is also reduced, leading to less active mixed Rossby gravity waves in the equatorial western Pacific area. Further analysis revealed that extratropical forcing and associated circulation changes can modulate the TSD wavetrain and its coherence structure, in relation to low-level vorticity in the far western North Pacific. In CTRL, west of about 140°E, TSD-related circulation tends to be stronger; in EXPT, vorticity signals and vertical motions are found to be slightly more coherent in the more eastern portion of the TSD wavetrain. The latter enhanced coherency of TSD east of 140°E, from the EXPT simulations, might be due to changes in wave activity transport channeled by modulated upper-level mid-latitude westerlies in EXPT. Our results serve to quantify how extratropical forcing and related general circulation features influence western North Pacific summertime TSD activities. Implications on the understanding of the initiation of TSD are discussed.

A numerical simulation of Typhoon Faxai (1915), which made landfall with a central pressure of 960 hPa in the Kanto region of Japan, was conducted using a nonhydrostatic numerical model with a 1-km grid spacing. Faxai sustained a symmetric structure until the landfall and caused severe damage due to strong winds. The simulation successfully simulated the realistic track and intensity of Faxai for 48 h around landfall. The simulated intensity was strong until the time of landfall, and the spatial size of the vortex was small. The structure of the simulated Faxai, identified as having an axisymmetric flow field and eyewall, was similar to that of a welldeveloped tropical cyclone (TC) in the tropics. Around the TC center, the surface latent heat flux was over 300 W m⁻² until landfall, and the vertical wind shear was less than 9 m s⁻¹ between the 1.5- and 12.0-km altitudes, which is relatively weak at midlatitudes. The maximum potential intensity (MPI) was calculated using environmental parameters around the simulated TC. The simulated and best track TC intensities exceeded the MPI for approximately 12 h before landfall, that is, the TC was in a superintense state. The superintensity was mainly caused by the presence of supergradient wind, which, in turn, resulted from the strong intensity and axisymmetric structure of the typhoon. The simulated TC satisfies the assumptions for the formulation of the MPI during the quasi-steady state, except for the gradient wind balance, implying that the structure of a TC is similar to that of a developed TC in the tropics. The present analyses suggest that the strong intensity of Faxai results from favorable environmental conditions and vortex structure.

The process of an aerosol rainout in wet deposition induces large uncertainties among atmospheric aerosol simulations, especially for particles in the fine mode. In this study, we performed an intercomparison study of four different rainout schemes on the model (the nonhydrostatic icosahedral atmospheric model or NICAM) to simulate particulate Cs-137 in the emission scenario of the March 2011 accident at the Fukushima Dai-ichi Nuclear Power Plant. The schemes include global climate models (GCMs) approach with a simple tuning parameter to determine the scavenging coefficient, and another optimized for cloud resolving models (CRMs) to account for prognostic precipitation and realistic vertical transport. The third approach was the conventional method under the assumption of a pseudo-first-order approximation based on the surface precipitation flux. The fourth approach was involved in offline chemical transport models (CTMs) with a simplified parametric analysis approach to clouds and precipitation flux. In most experiments, statistical metrics of the Cs-137 concentrations using in-situ measurements were calculated to be within ±30 % (bias), 0.6–0.9 (correlation), 67–112 Bq m⁻³ (uncertainty), and < 40 % (precision within a factor of 10). The CRM-type method yielded the best results but required a lower limit of tuning parameters to compensate for the results. Both the GCM-type and the conventional methods were also useful by setting proper tuning parameters. The CTM-type yielded better correlation and lower uncertainty but larger negative bias. These analyses suggest the overestimation of the conversion rate from cloud droplets into raindrops by the NICAM. However, this cannot be resolved by simply interchanging cloud microphysics schemes. It was found that the sensitivity of the rainout scheme has a stronger influence on the Cs-137 concentration than the different treatments of cloud microphysics. Thus, to replicate the observed Cs-137 distribution, it is essential to have a better meteorological field as well as a proper rainout scheme.

The Japan Meteorological Agency operates gridded temperature guidance to predict two-dimensional snowfall amounts and precipitation types, e.g., rain and snow, because surface temperature is one of the key elements to predict them. Operational temperature guidance is based on the Kalman filter, which uses temperature observation and numerical weather prediction (NWP) outputs only around observation sites. Correcting a temperature field when NWP models incorrectly predict a front's location or when observed temperatures are extremely cold or hot has been challenging.

In this study, an encoder–decoder-based convolutional neural network has been proposed to predict gridded temperatures at the surface around the Kanto region in Japan. Verification results showed that the proposed model greatly improves the operational guidance and can correct NWP model biases, such as a positional error of fronts and extreme temperatures.

Retrograde long waves in the higher latitudes of the Northern Hemisphere can episodically attain large amplitudes and sustain coherent phase propagation for 2–3 weeks. The potential influence of such waves on extendedrange weather forecast has been conjectured but not systematically quantified. Using a set of ensemble reforecast data, this study examined the predictability associated with an extraordinary retrograde-wave episode in the 1979–80 winter. Quantified by the anomaly correlation of the 500 hPa geopotential height in the 40–70°N latitudinal band, increased week-2 predictability was found within the subperiod with the presence of coherent retrograde waves. Some individual forecasts made within the retrograde-wave event exhibited the behavior of “return of skills”. The results suggest a future investigation into the relation between the elevated level of anomaly correlation in week-2 and detailed dynamics of the retrograde waves.

It is desirable that a surface layer scheme in an atmospheric numerical model is consistent with an atmospheric boundary layer scheme incorporated in the same model. In this study, stability functions based on Monin–Obukhov similarity theory for momentum and heat, φ_m and φ_h, in the stable surface layer are derived from the Mellor–Yamada–Nakanishi–Niino (MYNN) scheme. The resulting stability functions are approximated by φ_m = 1 + 4.8z/L and φ_h = 0.74 + 6.0z/L, which can be analytically integrated with respect to height z to obtain momentum and heat fluxes, where L is the Obukhov length. The fluxes thus, obtained are compared with those acquired from stability functions in four previous studies: they are nearly the same as those from two of them, and show better agreement with observational data of the Surface Heat Budget of the Arctic Ocean experiment (SHEBA) over sea ice than those from the other two studies. Detailed comparisons of the results of the MYNN scheme with the SHEBA data suggest that significant variations of the fluxes observed during “winter” when the ice was covered with dry snow may have been caused by those of the surface roughness around the observational site. The stability functions obtained from the MYNN scheme predict that the bulk and flux Richardson numbers approach critical values of 0.26 and 0.21, respectively, in the limit of z/L → ∞. These critical values result from the Kolmogorov hypothesis applied to the turbulent dissipation in the MYNN scheme and are considered to correspond to a transition from Kolmogorov to non-Kolmogorov turbulence.

The present study found that the Madden-Julian Oscillation (MJO) significantly influences the occurrence probability of extreme snowfall and precipitation in Japan during boreal winter (December–February) based on observational data and the Database for Policy Decision Making for Future Climate Change (d4PDF). By analyzing d4PDF containing 90-member and 50-member ensemble historical simulations by global and high-resolution regional models, respectively, we could quantify and elucidate the geographical distribution of the probability of extreme weather in Japan related to the MJO. The d4PDF global simulations well represent the MJO and its teleconnection over the Pacific-North America region.

Our results show that (1) the probability of extreme snowfall on the Sea of Japan side of northwestern Japan (SJA) increases (decreases) by approximately 20 % (30–40 %) associated with enhanced MJO over the Maritime Continent and western Pacific (western Indian Ocean) relative to that for all winter days; (2) the extreme precipitation on the Pacific Ocean side (PAC) of Japan increases (reduces) by 40–50 % (approximately 30 %) when the MJO is active over the Indian Ocean (western Pacific); and (3) the extreme snowfall on the Kanto area in PAC increases by 30–45 % with enhanced MJO over the eastern Indian Ocean and Maritime Continent. Composite analysis reveals that different physical processes associated with the MJO are responsible for extremes in the three regions. The MJO intensifies cold air intrusion from Siberia into Japan associated with a more frequent blocking over East Siberia, causing extreme snowfall in SJA. The MJO stimulates the explosive development of extratropical cyclones due to enhanced moisture flux convergence, leading to extreme precipitation in PAC and extreme snowfall in Kanto. Furthermore, the Kanto snowfall is partly related to a cold air outflow from the blocking induced by the MJO.