Yuki Kanno

Marine cold air outbreaks (CAOs), cold continental air masses outflow over extratropical oceans, are key drivers of wintertime air–sea interactions. This study presents a Lagrangian climatology of marine CAOs over the northwestern Pacific and marginal seas during the past 39 Januaries (1982–2020), focusing on their pathways, thermodynamic evolution, and downstream hydrological impacts. By tracking cold air masses that undergo ocean-induced modifications, we identify distinct regions of initial air–sea interaction and their associated transformation characteristics. On average, 46.1% of CAO trajectories first interact with the Sea of Japan, followed by the Sea of Okhotsk (18.5%) and the Yellow Sea/East China Sea (YS/ECS; 12.0%). They play a significant role in climatological surface turbulent heat fluxes, accounting for more than 80% of the total flux over the midlatitude basins. The most pronounced airmass transformation occurs within 2–3 days of the first interaction with the ocean with rapid heat and moisture uptake, as the air masses traverse increasingly warmer ocean surfaces. The characteristics of airmass transformations and downstream impacts vary by regions: CAOs over the Sea of Japan, Sea of Okhotsk, and Bering Sea tend to exhibit rapid moisture turnover. In contrast, CAOs over the YS/ECS more frequently exhibit prolonged moisture retention during subsequent air–sea interactions, contributing to a remote water cycle. Our findings thus underscore the importance of treating northwestern Pacific CAOs as a collection of distinct regional processes, each shaped by basin-specific air–sea interactions. Significance Statement: Arrival of cold air outbreaks over extratropical oceans is a major driver of wintertime air–sea interactions. This study tracks the movement and evolution of properties of these cold air masses interacting with the northwestern Pacific Ocean during the past 39 Januaries. We found that the arrival of the cold air masses over the ocean basins determines over 80% of the mean heat fluxes between the air and the ocean, and nearly half of these events occur over the Sea of Japan, followed by the Sea of Okhotsk, Yellow Sea/East China Sea, and Bering Sea. The most pronounced changes in temperature and moisture occur within the first few days of these interactions. Each ocean basin influences the cold air differently, affecting the hydrological cycle in downstream regions like Japan, Siberia, Korea, and Taiwan in unique ways.

Wet snow poses substantial threats to infrastructure during winter. However, bulk cloud microphysics schemes used in current operational weather prediction models cannot explicitly represent wet snow. This study modifies the Milbrandt–Yau (MY) microphysics scheme of the Weather Research and Forecasting Model to explicitly simulate melting snow. The modification introduces an additional prognostic variable for the snow liquid mixing ratio, representing the liquid component of melting snow, along with its source and sink terms. Furthermore, the formulation of fall speed for melting snow and the assumptions regarding meltwater distribution are designed to be consistent with observations. The modified scheme was evaluated during a heavy wet snow accretion event that occurred in Hokkaido, Japan, on 22–23 December 2022. Two numerical simulations with a horizontal resolution of 1.667 km, using the modified and original MY schemes, respectively, were compared with observations from the Kushiro test power line in eastern Hokkaido. The results showed slightly colder surface temperatures in the modified scheme due to meltwater evaporation. Disdrometer observations indicated that the precipitation particles included melting snow with low snow liquid fractions, as well as rain and snow, which were reasonably well reproduced by the modified scheme. The analysis of source–sink terms revealed that the partial melting of snowflakes along with refreezing and evaporation of meltwater are dominant processes governing the tendency equation. Sensitivity experiments further highlighted the influence of fall velocity formulation on precipitation characteristics. Overall, the modified scheme effectively represents melting snow, demonstrating its potential for improving the prediction of wet snow events. Significance Statement: Partially melted snow is one of the greatest threats to infrastructure during winter. This study enhances cloud modeling in numerical weather prediction models to better simulate melting snow, a key factor in forecasting such damage. A case study of heavy wet snow in northern Japan showed that the improved model successfully predicted wet snow occurrence and reproduced the observed relationship between precipitation particle size and fall speed. This advancement has the potential to improve winter weather forecasts and help reduce infrastructure risks.

1. The introduction of renewable energies is accelerating in numerous countries to achieve the greenhouse gas emission reduction targets of the Paris Agreement for climate change. In particular, wi...

Future changes in the atmospheric energy cycle were estimated by using 12 climate models from the fifth phase of the Coupled Model Intercomparison Project (CMIP5) and mass‐weighted isentropic zonal‐mean framework. In this framework, the zonal‐mean available potential energy (A_Z) is converted to the zonal‐mean kinetic energy (K_Z) through mean‐meridional direct circulations, and K_Z is converted to the wave energy (W), the sum of eddy available potential energy and eddy kinetic energy, through wave‐mean flow interactions. The comparison between the late 21st century in a high emission scenario and the late 20th century in the historical scenario indicates a significant increase in A_Z and K_Z in winter in the Northern Hemisphere (NH) and the Southern Hemisphere (SH). The wave energy significantly decreases in the NH winter but slightly increases in the SH winter. In the NH winter, the stationary wave energy significantly decreases, and the transient wave energy shifts poleward. Global warming reduces the baroclinic instability wave activity, which suppresses the upward Eliassen‐Palm flux and wave‐induced extratropical direct circulation. It decreases the dynamic energy conversion rates C(A_Z, K_Z) and C(K_Z, W). On the other hand, the diabatic wave energy generation rate (Q_E) is projected to increase, particularly in the SH. The future W change is consistent with the change in the sum of C(K_Z, W) and Q_E. Changes in both the dynamical energy conversion associated with wave‐mean flow interactions and the generation of eddy available potential energy associated with diabatic heating processes are necessary to explain the wave energy change.

A model diagnosis for the energy flux of off-equatorial Rossby waves in the atmosphere has previously been done using quasi-geostrophic equations and is singular at the equator. The energy flux of equatorial waves has been separately investigated in previous studies using a space-time spectral analysis or a ray theory. A recent analytical study has derived an exact universal expression for the energy flux which can indicate the direction of the group velocity for linear shallow water waves at all latitudes. This analytical result is extended in the present study to a height-dependent framework for three-dimensional waves in the atmosphere. This is achieved by investigating the classical analytical solution of both equatorial and off-equatorial waves in a Boussinesq fluid. For the horizontal component of the energy flux, the same expression has been obtained between equatorial waves and off-equatorial waves in the height-dependent framework, which is linked to a scalar quantity inverted from the isentropic perturbation of Ertel’s potential vorticity. The expression of the vertical component of the energy flux requires computation of another scalar quantity that may be obtained from the meridional integral of geopotential anomaly in a wavenumber-frequency space. The exact version of the universal expression is explored and illustrated for three-dimensional waves induced by an idealized Madden-Julian Oscillation forcing in a basic model experiment. The zonal and vertical fluxes manifest the energy transfer of both equatorial Kelvin waves and off-equatorial Rossby waves with a smooth transition at around 10°S and around 10°N. The meridional flux of wave energy represents connection between off-equatorial divergence regions and equatorial convergence regions. A model diagnosis for the energy flux of off-equatorial Rossby waves in the atmosphere has previously been done using quasi-geostrophic equations and is singular at the equator. The energy flux of equatorial waves has been separately investigated in previous studies using a space-time spectral analysis or a ray theory. A recent analytical study has derived an exact universal expression for the energy flux which can indicate the direction of the group velocity for linear shallow water waves at all latitudes. This analytical result is extended in the present study to a height-dependent framework for three-dimensional waves in the atmosphere. This is achieved by investigating the classical analytical solution of both equatorial and off-equatorial waves in a Boussinesq fluid. For the horizontal component of the energy flux, the same expression has been obtained between equatorial waves and off-equatorial waves in the height-dependent framework, which is linked to a scalar quantity inverted from the isentropic perturbation of Ertel’s potential vorticity. The expression of the vertical component of the energy flux requires computation of another scalar quantity that may be obtained from the meridional integral of geopotential anomaly in a wavenumber-frequency space. The exact version of the universal expression is explored and illustrated for three-dimensional waves induced by an idealized Madden-Julian Oscillation forcing in a basic model experiment. The zonal and vertical fluxes manifest the energy transfer of both equatorial Kelvin waves and off-equatorial Rossby waves with a smooth transition at around 10°S and around 10°N. The meridional flux of wave energy represents connection between off-equatorial divergence regions and equatorial convergence regions.

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.

An extreme cold surge event caused record-breaking low temperatures in the East Asia during 20–25 January 2016. The planetary- and synoptic-scale feature of the event is investigated quantitatively using the isentropic cold air mass analysis with the threshold potential temperature of 280 K. Because cold air mass is adiabatically conservative quantity, it is suitable for tracing and examining the extreme cold surges. We further introduce a metric named mean wind of cold air mass, which divides the factor of cold air mass evolution into convergence and advection parts. The new metric allowed us to trace the evolution of the cold air mass with dynamic consistency for a period of more than a week. A thick cold air mass built up over southern Sakha by a convergent cold air mass flow during 16–18 January. It migrated westward and reached Lake Baikal. On 20 January, an intense Siberian High developed with an eastward-moving mid-upper-level ridge, producing a strong surface pressure gradient over coastal regions of the Asian continent. This ridge and a cutoff low to the adjacent east formed a northerly flow in the mid-upper troposphere. The resultant southward flow through the troposphere blew the cold air mass over 480 hPa in thickness to the subtropical region of East Asia, causing strong cold surges there on 24 and 25 January. The abnormality of the event is further quantified using extreme value theory. The cold air mass gradually became rare along the path of the cold air mass from Lake Baikal to eastern China, which experienced as thick a cold air mass as once in 200 years. The cold air mass itself shows little change in thickness. Therefore, the migration of a cold air mass over 540 hPa in thickness from northern Siberia is the major cause of this cold surge extreme.

Trends and variations of the amount of cold airmass in the Arctic and the Northern Hemisphere are evaluated for the 60 year period, 1959-2018. The two indicators are (1) Polar Cold Air Mass (PCAM), which is the amount of air below a potential temperature threshold, and (2) Negative Heat Content (NHC), which includes a weighting by coldness. Because the metrics of coldness are based on multiple layers in the atmosphere, they provide a more comprehensive framework for assessment of warming than is provided by surface air temperatures alone. The negative trends of PCAM and NHC are stronger (as % per decade) when the threshold is 245 K rather than 280 K, indicating that the loss of extremely cold air is happening at a faster rate than the loss of moderately cold air. The loss of cold air has accelerated, as the most rapid loss of NHC has occurred in recent decades (1989-2018). The spatial patterns of the trends of PCAM and NHC provide another manifestation of Arctic amplification. Of the various teleconnection indices, the Atlantic Multidecadal Oscillation shows the strongest correlations with the spatially integrated metrics of moderate coldness. Several Pacific indices also correlate significantly with these indicators. However, the amount of extremely cold airmass does not correlate significantly with the indices of internal variability used here.

The total hemispheric polar cold air mass (PCAM) amount below a threshold potential temperature is conserved under an adiabatic condition and is a good indicator of polar warming. While the long‐term PCAM trends have been investigated, the cause of its interannual variability remains uncertain. Here we find that the El Niño–Southern Oscillation has a strong impact on the total northern hemispheric PCAM amount below a potential temperature of 280 K. The correlation coefficient between the Niño‐3.4 and the total PCAM reaches −0.66 over 36 recent winters. During El Niño, the PCAM decreases significantly because of the shorter PCAM residence time at high latitudes. The Aleutian Low intensifies and effectively transfers the PCAM from North America to the North Pacific, causing a significant decrease in the PCAM amount over the northern part of North America and a strengthening of cold air outflows over the central North Pacific. The PCAM coming to the warm ocean disappears rapidly and results in the imbalance of the hemispheric PCAM. This study reveals a notable influence of the tropical forcing on the variability of polar climate.