Toshiki Iwasaki

The Japan Meteorological Agency defines extreme climate events as phenomena occurring once every 30 years or less. To prepare for disasters, we investigated future projections of 30-year return values of heavy precipitation in the Tohoku District of northern Japan based on a large number of ensemble warming projections that are dynamically downscaled to 5-km grids under the Social Implementation Program on Climate Change Adaptation Technology. The 30-year return values of daily and hourly precipitation are projected to significantly increase under global warming, indicating the strengthening of extremely heavy rainfall. Their averaged ratios across the Tohoku District in the 2-K and 4-K warmer climates to those in the present climate are 1.12 and 1.30 times for daily precipitation and 1.18 and 1.45 times for hourly precipitation, respectively. In particular, the 30-year return values are enormous on the eastern slopes of the Kitakami and Abukuma Mountains and the Ou Mountain Range. The rates of increase in the 30-year return values are pronounced in the northern part of Tohoku, where the surface air temperature increase is more significant than in other regions. These results suggest the need to upgrade disaster prevention measures for heavy rainfall as a climate change adaptation in the Tohoku District. We also examined the regional dependency of seasonal variation in the occurrence rates of daily and hourly extreme events. The results indicated that the occurrence rates tend to be relatively high on the Sea of Japan side of the Tohoku District in July due to the Baiu front, over the entire Tohoku District in August due to strong convective instability, and on the Pacific Ocean side due to September typhoons and autumn stationary fronts. This seasonality is projected to remain almost unchanged under warmer conditions.

This study evaluates the climatology and trends of the Brewer–Dobson circulation (BDC) in the Japanese Reanalysis for Three Quarters of a Century (JRA‐3Q) reanalysis compared with those in the Japanese 55‐year reanalysis (JRA‐55) and ERA5 reanalyses. The spatial structure of the BDC shows good agreement among the reanalyses. However, zonal mean tropical upwelling near the lower stratosphere in JRA‐3Q is approximately 80% of that in JRA‐55 throughout the reanalysis period. The long‐term trend of BDC intensity for the period 1948–2022 showed a slight increase in JRA‐3Q and in JRA‐55, whereas that in ERA5 decreased. To investigate the cause of the difference in tropical upwelling between JRA‐3Q and JRA‐55, data assimilation sensitivity experiments were conducted by modifying the forecast model used in the data assimilation system. These results indicate that the difference in BDC intensity between JRA‐3Q and JRA‐55 can be primarily attributed to diabatic heating in the tropical troposphere produced in the forecast model. However, the difference between JRA‐3Q and ERA5 cannot be accounted for by diabatic heating in the forecast model. It is also found that the implementation of the non‐orographic gravity wave drag parameterisation scheme in the JRA forecast model is not the reason for the weaker BDC in JRA‐3Q.

The northern polar region possesses the most extensive cold airmass on Northern Hemisphere. The generation of this polar cold airmass and its outflow to lower latitudes play an important role in the climate system in terms of mass and heat exchanges. However, long-term changes in the polar cold airmass, especially in summer, and the climatic effects on mid-high latitudes are still unclear. Using an isentropic approach, we quantitatively show that the polar cold airmass amount has decreased rapidly since the 1980s, with a decade lagging behind the global warming. The equatorward flux of the cold airmass has also weakened, trapping the cold airmass in its source region. These profound changes in the cold airmass coincide with a period characterized by rapid surface warming and increasingly frequent heat waves in recent three decades over the mid-high latitude continents. Owing to regional differences in the cold airmass reduction, Europe and North America have experienced a surface warming faster than the Northern Hemisphere mean. Furthermore, such a long-term trend in the polar cold airmass can be attributed to Arctic sea ice loss and internal decadal variability of sea surface temperature in high-latitude oceans. Our results highlight that the isentropic analysis of cold airmass may serve a good detection of the climate change at polar region and mid-high latitudes.

This study quantifies future changes in the polar cold air mass (PCAM) below a threshold potential temperature of 280 K and its horizontal fluxes, which are an indicator for the intensity of cold air outbreaks, by analyzing the outputs from five climate models from the Fifth Coupled Model Intercomparison Project. Under the business‐as‐usual scenario, the multimodel means of the wintertime total hemispheric PCAM decrease by 37% in the Northern Hemisphere and by 34% in the Southern Hemisphere by the end of the 21st century. The midlatitude equatorward PCAM fluxes are projected to decrease significantly by around 2050. This is primarily due to the reduction of the PCAM and to a lesser extent due to the weakening of the extratropical meridional circulation. Finally, intermodel differences in the total hemispheric PCAM reduction are shown to be linearly related to the climate sensitivity to the global mean surface air temperature.

We have recently experienced several heavy snowfall events, but still do not sufficiently understand how global warming will impact changes in local extreme snowfall events. The analysis relevant to the extreme events requires ensemble experiments with high‐resolution regional climate modeling. In this study, we use a large number of ensemble warming projections downscaled dynamically to 5‐km grids to examine the differences in distribution and mechanism in extreme snowfall events over Japan between present and future climates. Japan has two typical snowfall patterns, the winter monsoon and the south coast cyclone patterns. The domain‐averaged amount of extreme snowfall in the south coast cyclone pattern regions appear to decrease with the rise in temperatures. Meanwhile, the winter monsoon pattern regions showed little change as the increases in extreme snowfall in the mountainous areas are canceled out by the decrease in the coastal areas. Based on the existing mechanism of normal snowfall, the increase in mountain snow is caused by a temporary intensification of cold air outbreaks. The amount of the cold air blowing from Japan Sea clearly increases only during the days preceding the extreme snowfall event, and as the cold dry air obtains more latent heat over the ocean, the precipitable water vapor increases. The winter monsoon then transports this air over land where the intensification of precipitation results from orographic updraft. This sequence of processes beginning with cold air outbreaks may have a stronger control over extreme snowfall phenomena on the warmer field.

Winter air quality can vary markedly due to cold air activities. However, the impacts of the polar cold airmass on aerosol variations has not been clarified in a quantitative manner. Using an isentropic analysis method, we investigated changes in the cold airmass and sulfate aerosol distributions over North China during the winter of 2014/2015. The cold airmass and sulfate concentrations exhibited pronounced out‐of‐phase variations, with comparable amplitudes at subseasonal (30–60 days) and synoptic (4–6 days) scales. Subseasonal sulfate variations were closely associated with distribution of the polar cold airmass, as it shifted between zonal and meridional patterns, regulating cold airmass fluxes over North China. Typically, subseasonal surges in the cold airmass consisted of several synoptic disturbances, including one cold air outbreak event and 3–6 renewal/decay oscillations. These synoptic inflows of clean cold airmass repeatedly pushed the warm polluted airmass away from North China. Thus, spatiotemporal variations in sulfate better reflect cold airmass distributions and fluxes than northerly wind regimes. Using diagnostic equations derived herein, we estimated the contribution of various physical processes to aerosol variations during cold air anomalies. We found that the local changes in sulfate concentrations were mainly attributed to advection of warm airmasses, as well as advection and vertical displacement by horizontal convergence of the polar cold airmass. The latter two processes contributed most to sulfate decreases, leading to a relatively strong aerosol reductions during cold air outbreaks, while first two processes led to weaker aerosol reductions during cold air oscillations.

The East Asian winter exhibits frequent cold air outbreaks (CAOs), or cold surges, which often cause severe cold waves in the extratropics. CAOs are also of importance for forecasters in the Maritime Continent because the associated equatorward outflows can influence tropical weather. Various CAO definitions have been documented, but a definition based on a quantitative approach was only recently proposed. Studies of the CAO impact often linked a surge index in the subtropics instead of an index in the midlatitude where CAOs basically occur. Here, we investigate the impact by using a quantitative CAO index, which is defined by integrating cold air mass flux below a threshold potential temperature over midlatitude East Asia (45°N, 90°-135°E). From a climatological analysis, tropical convections are significantly observed two-to-four days following CAO event (i.e., the peak of CAO index), indicating CAO impacts. However, case studies show that the impacts vary among independent CAO cases due to influences of associated synoptic conditions, which affect the pathways of northerlies propagation and consequently hinder or amplify the impacts. Several impact patterns and their possible causes are discussed. A better understanding of East Asian CAO variability can improve the predictability of weather over the Maritime Continent.

Sea‐breeze front (SBF) can cause dramatic changes in weather and air quality near the coast. However, the observation and forecast of its three‐dimensional (3‐D) fine‐scale structures have been challenging. Using mesoscale‐to‐large eddy simulations (LES) models and high‐resolution lidar measurement over Sendai Airport, here we perform a successful simulation of the observed 3‐D structures of an SBF for the first time. We show that frontal structures are characterized by a series of lobes (spaced ~500 m apart) aligned along the raised sea‐breeze head, where the shear between sea breeze and alongshore ambient flow aloft is evident. Local strong updrafts occur both in the frontal lobes of marine cold air and in the prefrontal warm air ascending the wedge of windward lobes. Downdrafts form behind the lifted marine cold air and trap air pollutants. These fine‐scale structures and vertical motions are repeatedly strengthened by the short‐term disturbances of gravity currents that move onshore and collide with the SBF. They are also affected by buildings and determine the detailed variations of surface winds. We conclude that advanced observation and modeling systems can potentially improve the prediction of coastal weather and environment.