This review paper aims to provide readers with a broad range of meteorological backgrounds with basic information on marine low clouds and the concept of their parameterizations used in global climate models. The first part of the paper presents basic information on marine low clouds and their importance in climate simulations in a comprehensible way. It covers the global distribution and important physical processes related to the clouds, typical examples of observational and modeling studies of such clouds, and the considerable importance of changes in low clouds for climate simulations. In the latter half of the paper, the concept of cloud parameterizations that determine cloud fraction and cloud water content in global climate models, which is sometimes called cloud “macrophysics”, is introduced. In the parameterizations, the key element is how to assume or determine the inhomogeneity of water vapor and cloud water content in model grid boxes whose size is several tens to several hundreds of kilometers. Challenges related to cloud representation in such models that must be tackled in the next couple of decades are discussed.

Dual-frequency Ku/Ka-band radar retrievals of snow parameters such as liquid-equivalent snowfall rate (R) and mass-weighted diameter (D_m) have two principal errors, namely, the differences between the assumed particle size distribution (PSD) model from the actual PSD and inadequacies in characterizing the single-scattering properties of snowflakes. Regarding the first issue, this study, based on radar simulations from a large amount of observed PSD data, shows that there exist relatively high correlations between the estimated snow parameters and their true values derived directly from the measured PSD. For PSD data with R greater than 0.1 mm h⁻¹, a gamma PSD model with a fixed shape factor (µ) equal to 0 (or exponential distribution) provides the best estimates of R and D_m. This is despite negative biases of up to −15 % in R and underestimates and overestimates in D_m for small and large D_m, respectively. The µ = 0 assumption, however, produces relatively poor estimates of normalized intercepts of a gamma PSD (N_w), whereas the best estimates are obtained when µ is considered either 3 or 6. However, the use of an inappropriate scattering table increases the errors in snow retrieval. Simple evaluations are made for cases where the scattering databases used for the algorithm input differ from that used for retrieval. The mismatched scattering databases alone could cause at least 30–50 % changes in the estimates of snow water content (SWC) and R and could affect the retrievals of D_m and N_w and their dependence on µ.

The usefulness of Clausius–Clapeyron (CC) scaling in explaining extremely heavy precipitations is explored in the present-day climate and in pseudo-global-warming (PGW) conditions. This is analyzed by conducting regional-scale numerical simulations at 1-km grid resolution for two recent extreme rainfall events that occurred in Japan: the case in northern Kyushu during 5–6 July 2017 and the case in Shikoku Island during 5–8 July 2018. The Weather Research and Forecasting (WRF) model was used for the simulation, and the data samples were collected at each grid point individually for each hour over the two regions. We found that the frequency and intensity of extremely heavy precipitation associated with the two events are increased under PGW conditions. The extremely heavy precipitations (> 50 mm h⁻¹) followed CC scaling for the temperatures up to 22°C in the present-day climate, while those under the PGW conditions followed CC-scaling up to 24°C. The peak intensity of the extremely heavy precipitations in the precipitation-temperature relationship is found as ∼ 140 mm h⁻¹ at 25°C in the present-day climate, while the same with PGW conditions is projected as ∼ 160 mm h⁻¹ at 27°C. The increasing rate of the extremely heavy precipitations in the present-climate condition is noticed as ∼ 3 % °C⁻¹ and that under the PGW conditions is anticipated as ∼ 3.5 % °C⁻¹. The increase in peak precipitation intensity and the rate of precipitation increase against temperature in future warming climate are attributed to the decrease in temperature lapse rate and increase in atmospheric water vapor and convective available potential energy. To our knowledge, this is one of the first quantitative investigations of CC scaling of extremely heavy precipitations based on case studies.

Ultra-high-resolution numerical weather prediction (NWP) experiments over a large domain have been conducted to investigate the impacts of different factors of an NWP model in simulating the Hiroshima heavy rain event in August 2014. This is a continuation of the study in Part 1 in which similar experiments were carried out for the Izu Oshima heavy rain event in October 2013. We have demonstrated the benefit of using a high-resolution model (500-m grid spacing or less) with a large domain in simulating torrential rain events.

The simulated location and intensity of the rain band in the Hiroshima case has been shown to be sensitive to the model resolution. The simulation at the 2-km grid spacing could reproduce the rain band, but with a shift to the northeast. This displacement error was reduced when the grid spacing was further reduced to 500 m and 250 m. The best simulation both in the location and intensity was obtained at the 250-m grid spacing. The planetary boundary layer schemes had a smaller impact in this case, which is different from the Izu Oshima case.

This study also investigated the dependency of simulated convective cores (CCs) on model resolutions. The local rate of change of the number of CCs with respect to the model resolution was found to start decreasing at very high resolutions that are around the 500-m grid spacing. This implies the number of CCs tends to converge when the resolution goes beyond 500 m.

To reveal a maintenance mechanism for Rossby wave breaking (RWB) east of Japan and Pacific-Japan (PJ) pattern, which are triggered due to quasi-stationary Rossby wave propagation along the Asian jet, the past 44 RWB cases east of Japan are analyzed using a reanalysis dataset. A comparison between the composites of seven persistent and seven non-persistent cases, which are classified based on the duration of the RWB and PJ pattern, indicates that the persistent case demonstrates stronger and longer-lived quasi-stationary Rossby wave propagation along the Asian jet. The subsequent stronger RWB in the persistent case causes the consequential formation of the more enhanced PJ pattern through the stronger high potential vorticity intrusion toward the subtropical western North Pacific. The persistent case further demonstrates a persistent northward-tilting vertical structure of the anomalous anticyclone east of Japan, accompanied by the enhanced anomalous warm air advection in the lower to middle troposphere north of the anomalously extended North Pacific Subtropical High associated with the PJ pattern. The diagnosis of the Q-vector and partial correlation analysis indicate that the anomalous warm air advection in the middle troposphere is closely associated with dynamically induced anomalous ascent from Japan to the east by an adiabatic process. Enhanced anomalous moisture flux convergence from Japan to the east, which is due to the moisture inflow along the fringe of North Pacific Subtropical High from the subtropical western North Pacific, also causes the anomalous ascent over the region by a diabatic process. A simple correlation analysis reveals nearly equivalent associations of the adiabatic and diabatic factors with the anomalous ascent. The anomalous ascent contributes to enhanced and persistent RWB through a negative vorticity tendency due to vortex squashing in the upper troposphere, which further contributes to the enhanced and persistent PJ pattern in the persistent case.

In order to explore large-scale atmospheric factors causing heavy rainfall events that occurred widely in western Japan, a composite analysis of atmospheric fields during the past heavy rainfall events in the region is conducted using the Japanese 55-year Reanalysis. During heavy rainfall events, atmospheric fields are characterized by an upper-tropospheric trough over the Korean Peninsula (KP), an upper-tropospheric ridge to the east of Japan, a surface high-pressure system to the southeast of Japan, and southwesterly moisture flux. The composite analysis indicates that a clear wave train due to quasi-stationary Rossby wave packet propagation (RWPP) along the polar front jet over Siberia tends to occur just before extreme events. Further analysis considering various time scale variabilities in the atmosphere reveals that surface high-pressure anomalies to the southeast of Japan are dominated by variability with a 25–90 day period, whereas variability with an 8–25 day period dominates lower-pressure anomalies over the East China Sea (ECS) in relation to the development of the upper-tropospheric trough around the KP.

We also investigate atmospheric fields during an extreme heavy rainfall event that occurred in early July 2018 (HR18). Atmospheric features during HR18 are generally similar to those of the other heavy rainfall events. However, a remarkable RWPP occurred along the subtropical jet (STJ) in late June 2018 and intensified a surface high-pressure system to the southeast of Japan. Additionally, a low-pressure system with an 8–25 day period to the south of Japan developed in association with wave breaking induced by the remarkable RWPP along the STJ and propagated northwestward toward the ECS and then to Japan. The simultaneous development of high- and low-pressure systems contributed to the extreme southerly moisture flux into western Japan. HR18 is also characterized by a sharp upper-tropospheric trough over the KP that is dominated by high frequency variability with a period < 8 days.

As suggested by previous studies, the entrainment of the low-level high-entropy eye air can provide additional energy for tropical cyclone (TC) intensification, but the previous trajectory analysis only indicated that considerable air parcels below the eye inversion could be entrained into the eyewall. In the present study, the 1 min output data from a semi-idealized experiment are utilized to quantitatively evaluate the relative importance of the entrainment of the high-entropy eye air by enhancing the eyewall convection.

It is confirmed that a significant amount of high-entropy eye air below the eye inversion can be entrained into the eyewall. The entrainment occurs favorably on the quadrants of enhanced eyewall convection and is enhanced in the presence of small-scale disturbances in the inner edge of the eyewall. However, the eyewall air parcels below 3 km experience a fast cycling. There are 84.4 % and 7.7 % eyewall air from the low-level inflow and the middle-level dry environment, respectively. The low-level high-entropy eye air only accounts for 1.7 % of the eyewall air, whereas 6.2 % eyewall air remains in the eyewall below 3 km during the 90 min period. The eye air from the low-level high-entropy reservoir accounts for 5.8 % of the equivalent potential temperature change below 3 km and 4.5 % of the total mass transport at 3 km in the TC eyewall. The present study suggests that the low-level high-entropy air from the eye has little direct influence on TC intensity through enhancing the eyewall convection by providing relatively small mass and thermodynamic contributions.

La Niña is the negative phase of the El Niño-Southern Oscillation (ENSO) cycle. It occurs in the equatorial Pacific, and events known as multiyear La Niña often persists for more than two years. During a conventional La Niña event, the seasonal cycle of surface temperature over Japan is amplified (i.e., hotter summer and colder winter than normal years), but the influence of multiyear events on temperatures over Japan is unclear. In this study, we evaluate the teleconnection associated with multiyear La Niña events. Our research uses composite analyses of observations, reanalysis data, and a large ensemble of atmospheric general circulation model (AGCM) simulations for 1951–2010, driven by observed boundary conditions. We propose two distinct mechanisms involved in multiyear La Niña events that cause hot summers over Japan.

Observational data composites show significant positive temperature anomalies over Japan in the boreal summer season preceding the two consecutive La Niña events reaching their mature phases. This robust summer signal can be reproduced by AGCM large-ensemble simulations, indicating that it is forced by multiyear La Niña. The time evolution of the anomalous summer temperature over Japan differs between the first and second years of multiyear La Niña. In the first summer, warm conditions occur in August–October in the southwestern part of Japan, due to anomalous southwesterly winds in the lower troposphere. This atmospheric circulation anomaly is attributable to a La Niña-induced decrease in precipitation over the equatorial Pacific. In the second summer, warm anomalies occur in June–August over northeastern Japan, and these are accompanied by an anomalous barotropic high-pressure induced by negative precipitation anomalies over the equatorial Pacific. The seasonal march in atmospheric background states and the delayed effect of a preceding El Niño may explain the distinct teleconnection during multiyear La Niña.

Polar mesocyclones (PMCs) occur frequently over the northern Sea of Japan. In the present study, topographic effects on PMC genesis in this region were investigated using long-term numerical simulations extending over 36 winter seasons. Sensitivity experiments showed that PMC genesis decreases in the part of the northern Sea of Japan when the mountain region at the eastern end of the Eurasian continent is removed. For instance, the generation of PMCs over offshore west of Hokkaido decreases significantly when the mountain range is removed, whereas the generation of PMCs over the Strait of Tartary remains unchanged. According to a composite analysis, this result can be attributed to the different responses of subregional oceanic surface wind to the removal of the mountains. In the experiment without mountains, cold air outbreaks from the continent blow directly over the Sea of Japan causing strong westerly winds over the offshore west of Hokkaido. Consequently, PMCs tend to make landfall earlier and before reaching maturity. The uniformly distributed westerly wind also has a negative impact on PMC genesis because of weakened horizontal wind shear and meridional temperature gradient. By contrast, the low-level wind over the Strait of Tartary before PMC genesis is unaffected by the mountains, and thus, topographic effects are not required for PMC genesis in this region. These results indicate that the responses of PMCs to topographic forcing have a regional variability.

A series of 40-day non-hydrostatic global simulations was run with the NASA Goddard Earth Observing System (GEOS) model with horizontal grid spacing ranging from 50 km to 3.5 km. Here we evaluate the diurnal cycle of precipitation and organized convection as a function of resolution. For validation we use the TRMM 3B42 and IMERG precipitation products and 4 km merged infrared brightness temperature, focusing on three regions: the contiguous United States (CONUS), the Maritime Continent, and Amazonia. We find that higher resolution has mixed impacts on the diurnal phase. Regions dominated by non-local propagating convection show the greatest improvement, with better representation of organized convective systems. Precipitation in regions dominated by local thermodynamic forcing tends to peak too early at high resolution. Diurnal amplitudes in all regions develop unrealistic small-scale variability at high resolution, while amplitudes tend to be underestimated at low resolution. The GEOS model uses the Grell-Freitas scale-aware convection scheme, which smoothly reduces parameterized deep convection with increasing resolution. We find that some parameterized convection is beneficial for the diurnal amplitude and phase even with a 3.5 km model grid, but only when throttled with the scale-aware approach. An additional 3.5 km experiment employing the GFDL microphysics scheme and higher vertical resolution shows further improvement in propagating convection, but an earlier rainfall peak in locally forced regions.

This study investigates future changes to extremely cool days (ECDs) during the summer (June–August) season in northeastern Japan by applying self-organizing map (SOM) technique to large ensemble simulations from the “database for Policy Decision making for Future climate change” (d4PDF). Two separate SOMs, one trained on mean sea level pressure using a combination of JRA-55 reanalysis and d4PDF to evaluate model performance, and a “master” SOM, which trained the SOMs using historical, +2K, and +4K simulations, were created to investigate possible climate change impacts to future ECDs. For model evaluation, summer climatology and ECDs were confirmed to occur with similar frequencies between circulation patterns in the JRA-55 and d4PDF. Surface temperature anomalies and horizontal wind composite from several high-frequency ECD nodes exhibit similar spatial patterns for all days and ECD occurring in the node, with ECD composites depicting particularly strong northeasterly winds, commonly referred to as Yamase, blowing from high latitudes toward northeast Japan. Future changes using “master” SOMs suggest a gradual shift (from +2K to +4K) in preferred circulation patterns that result in ECDs, with the greatest increase in frequency associated to those with a strong low-pressure system off eastern Japan and a moderate intensity Okhotsk Sea high, and decreased ECDs to those with either a strong Okhotsk Sea high or westward extension of the North Pacific high. Lastly, changes to the intensity of future ECDs are investigated by examining low-level thermal advection. Results suggest that circulation patterns associated with increased ECD frequency coincide with those with very strong cold air advection for all climates, though the magnitude differs based on circulation patterns. Future changes show a weakening cold air advection and decreasing ECDs, due in large part to the weakening meridional temperature gradient east of Japan.

Global warming already affects weather and climate worldwide; accordingly, various studies have been conducted to understand the effects of climate change on tropical cyclones. The translation speed of a tropical cyclone is a particularly important feature, as a slower translation speed lengthens the duration of a cyclone's impact. Here, on the basis of observational data, we report that tropical cyclone translation speeds in the middle latitudes of the western North Pacific basin have significantly decreased during September over the last 40 years. Historical model simulations with and without observational global warming trends reveal two main factors responsible for the translation speed slowdown: natural decadal climate variabilities (such as the Pacific Decadal Oscillation) and global warming. Both factors produce an anticyclonic anomaly in the westerly jet over western Japan; this anomaly relaxes the latitudinal geopotential height gradient, weakening the environmental synoptic winds by which tropical cyclones are steered. Furthermore, model simulations for a future warmer climate show that global warming further reduces the steering flows, leading to more slowly-moving tropical cyclones in autumn in the future.

In this study, the Advanced Weather Research and Forecasting (WRF-ARW) model is used to investigate possible influences of a predominantly upper-level easterly wave (EW) on Typhoon Megi's (2010) sharp northward turn on 20 October, 2010 after passing over the Philippines. Observational analysis indicates that an upper-level EW with a cold-cored structure was located to the east of Megi. This EW moved westward along with Megi and modified the large-scale environmental flow around the typhoon, thus affecting its movement. In a control experiment, the sharp northward turn that was observed was captured well by a simulation. The retreat of the subtropical high contributed directly to the poleward steering flow for Megi. Sensitivity experiments were conducted by filtering out the synoptic-scale (3–8-day) signals associated with EWs. In the absence of the upper-level EW, the simulation showed that Megi would not have made a sharp northward turn. Two mechanisms are proposed regarding the impact of the easterly wave on Megi. First, an upper-level EW may have impacted the environmental flows, allowing Megi to move at a slower westward speed so that it entered the eastern semicircle of the nearby monsoon gyre where an enhanced southerly steering flow then led to the typhoon making a sharp northward turn. Second, the diabatic heating and associated cyclonic vorticity induced by the middle-level (around 400 hPa) convergence may have eroded the western flank of the subtropical high in the western North Pacific, causing an eastward retreat of the high-pressure system. The present modeling approach provides a reasonable assessment of the contribution of upper-level wave disturbances to sudden changes in tropical cyclones.

It is speculated that floods in many areas of the world have become more severe with global warming. This study describes the 2017 spring floods in Kazakhstan, which, with about six people dead or missing, prompted the government to call for more than 7,000 people to leave their homes. Then, based on the Climatic Research Unit (CRU), the NCEP/NCAR Reanalysis 1, and the Coupled Model Intercomparison Project 5 (CMIP5) simulations, the seasonal trends of temperature were calculated using the linear least-squares regression and the Mann–Kendall trend test. The correlation between the surface air temperature and atmospheric circulation was explored, and the attributable risk of the 2017 spring floods was evaluated using the conventional fraction of the attributable risk (FAR) method. The results indicate that the north plains of Kazakhstan had a higher (March–April) mean temperature anomaly compared to the south plains, up to 3°C, relative to the 1901–2017 average temperature. This was the primary cause of flooding in Kazakhstan. March and April were the months with a higher increasing trend in temperature from 1901 to 2017 compared with other months. In addition, a positive anomaly of the geopotential height and air temperature for the March–April 2017 period (based on the reference period 1961–1990) was the reason for a warmer abnormal temperature in the northwest region of Kazakhstan. Finally, the FAR value was approximately equal to 1, which supported the claim of a strong anthropogenic influence on the risk of the 2017 March–April floods in Kazakhstan. The results presented provide essential information for a comprehensive understanding of the 2017 spring floods in Kazakhstan and will help government officials identify flooding situations and mitigate damage in future.

In July and August (JA) 2018, the monsoon trough in the western North Pacific (WNP) was unusually strong, the anticyclonic ridge was anomalously northward-shifted, and enhanced and northward-shifted tropical cyclone activity was observed. Studies have examined the effects of sea surface temperature (SST) anomalies on the North Atlantic (NA), the Indian Ocean (IO), and the tropical North Pacific. However, a synthetic view of SST forcings has yet to be identified. Based on a series of numerical experiments, this study demonstrated that the SST anomaly in the tropical WNP was the key forcing that formed the structure of the observed anomalous phenomena in the monsoon trough. Moreover, the combined effect of the SST anomaly in both the tropical and extratropical WNP resulted in enhanced circulation anomalies in the WNP. The NA SST anomaly also enhanced the monsoon trough in the presence of WNP SST anomaly. By contrast, the individual SST anomaly in the NA, IO, the extratropical WNP, and the subtropical eastern North Pacific could not force the enhanced monsoon trough. We proposed that the local effect of both the tropical and extratropical WNP SST anomaly as the major driver and the remote effect of NA SST anomaly as a minor contributor jointly induced the anomalous circulation and climate extremes in the WNP during JA 2018.