Using the satellite data, spatial patterns of precipitation diurnal cycles and their seasonality were examined with emphasis on southeastern China (SEC). Results show that spatial distributions of ...diurnal cycles over SEC have a robust large‐scale seasonality in which the regional differences are evidently embedded. Rainfall diurnal variability is weak in spring but it becomes more pronounced from presummer. Both the mean rain rates and amplitudes of diurnal cycles experience remarkable amplification during presummer. The widespread and strong morning rainfall dominates the SEC area, especially inland valleys and plains, and offshore areas. The morning peak rainfall over western SEC is largely contributed by the increasing rain frequency and diurnally varying intense rain rates. Even over eastern SEC, morning rainfall still has a comparable magnitude to afternoon rainfall. In contrast, spatial distributions of diurnal cycles in midsummer are dependent primarily on topography. The morning (afternoon) rainfall is mainly located over valleys, basins, and oceans (plateaus and mountains). The afternoon peak rainfall becomes a notable feature over southern China. The signature of widespread morning rainfall decays during midsummer and remains apparent only in central eastern China, which is likely related to the north shift of summer rainband.
The flow vectors of radioactive cesium-137 (
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Cs) plume emitted from the Fukushima Daiichi nuclear power plant in March 2011 were quantitatively depicted by a mass flux analysis in this study.
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...Cs plumes were calculated by an Eulerian dispersion model with a 3-km horizontal resolution. The vertically column-integrated mass flux was consistent with the flow approximation based on ground surface
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Cs observations, even though there were some discrepancies that were caused by differences in the wind direction between the ground surface and the dominant plume layer. These discrepancies were explained by combining the use of the ground surface horizontal mass flux with the column-integrated mass flux. The mass flux analysis clearly provided an illustration of
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Cs dominant stream locations, directions, and depositions by reducing high-dimensional model outputs into a lower-dimensional plot. Mass flux (i.e. the product of the mass density and wind velocity) has often been used in dynamic meteorology but has not been used as frequently in atmospheric chemistry or pollutant dispersion studies. However, the concept of mass flux is a robust alternative for conventional validation approaches that only utilize a time series of pollutant concentrations. Mass flux analyses can be used further in atmospheric chemistry as a quantitative visualization tool to track the emission, advection, dispersion, and deposition of atmospheric constituents.
In boreal winter, the cold air mass (CAM) flux of air with a potential temperature below 280K forms climatological mean CAM streams in East Asia and North America (NA). This study diagnoses the ...interannual variability of the NA stream by an analysis of the CAM flux across 60°N between Greenland and the Rocky Mountains. The first empirical orthogonal function (EOF) represents the variations in intensity of the NA stream. When the first principal component (PC1) is highly positive, the central part of the NA stream is intensified, with cold anomalies east of the Rocky Mountains. At the same time, a stratospheric polar vortex tends to split or displace toward NA. PC1 is highly correlated with the tropical Northern Hemisphere pattern, implying that this pattern is associated with the intensity of the NA stream. The second EOF shows a longitudinal shift of the NA stream toward Greenland or the Rocky Mountains. A highly negative PC2 results in a cold anomaly from western Canada to the Midwestern United States and anomalous heavy snowfall in the northeastern United States. PC2 is positively correlated with the Arctic Oscillation, which suggests that the longitudinal position of the NA stream varies with the Arctic Oscillation. These results illustrate how the intensity and location of cold air outbreaks vary with large-scale modes of atmospheric variability, with corresponding implications for the predictability of winter severity in NA.
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 (AZ) is converted to the zonal‐mean kinetic energy (KZ) through mean‐meridional direct circulations, and KZ 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 AZ and KZ 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(AZ, KZ) and C(KZ, W). On the other hand, the diabatic wave energy generation rate (QE) is projected to increase, particularly in the SH. The future W change is consistent with the change in the sum of C(KZ, W) and QE. 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.
Plain Language Summary
Atmospheric energy can be divided into three components: the zonal (averaged along a latitudinal circle) kinetic energy, zonal potential energy, and wave energy, which is the sum of eddy kinetic and potential energy. We have investigated future changes in energy flow between these components using a novel coordinate system with a better representation of wave dynamics. A comparison between the late 20th century and late 21st century in a high emission scenario shows that the two zonal components increase, whereas the wave energy significantly decreases in the Northern Hemisphere winter and slightly increases in the Southern Hemisphere winter. Global warming suppresses energy conversions between the components by decreasing a pole‐to‐equator temperature difference. In contrast, wave energy generation by radiation and latent heat increases particularly in the Southern Hemisphere winter. We have found a positive correlation between changes in the wave energy and changes in the sum of the energy conversion and generation, which will provide further insights into the mechanism of future changes in high‐ and low‐pressure patterns and winter monsoon.
Key Points
The wave energy will significantly decrease and slightly increase in winter in the Northern and Southern Hemispheres, respectively
Global warming suppresses the wave‐mean flow interactions and reduces the dynamic energy conversion rates
The future change in the wave energy is consistent with the change in the sum of its dynamic conversion and diabatic generation rates
This study shows that the total hemispheric cold air mass (CAM) amount below a designated potential temperature of 280 K serves as a good indicator of long‐term climate change trends in the polar ...region. We conduct quantitative analyses of the warming trends of Northern Hemisphere (NH) and Southern Hemisphere (SH) winters using five reanalysis data sets (Climate Forecast System Reanalysis, ERA‐Interim, Japanese 55 year Reanalysis Japanese 25 year Reanalysis, JRA‐25, and National Centers for Environmental Prediction–National Center for Atmospheric Research) and JRA‐55‐related data sets (JRA‐55 Conventional reanalysis and JRA‐55 Atmospheric Model Intercomparison Project type simulation). Total hemispheric CAM amount for the NH winter shows a significant decreasing trend for most of the reanalyses at a rate of approximately −1.37 to −0.77% decade−1 from 1959 to 2012 and at a rate of approximately −1.57 to −0.84% decade−1 from 1980 to 2012. The CAM amount trends show very different geographic patterns during the two periods, which suggests that geographical patterns of the trends estimated for these periods are largely controlled by internal low‐frequency dynamics. The CAM flux across 45°N, which serves as an indicator of hemispheric scale cold air outbreaks, has so large interannual variability that its trend becomes insignificant. In the SH winter, trends of total hemispheric CAM amount and its equatorward flux across 50°S are not consistent among the new reanalyses for 1980–2012. Although total hemispheric CAM amount decreases for 1959–2012, sparse observations and changes in systematic bias, due to the implementation of assimilation of satellite observation data in 1979, compromise the reliability of the trends. Improvements to the reanalyses are desirable as a means of monitoring historical changes in CAM for the SH winter.
Key Points
Total hemispheric CAM amount below a designated potential temperature of 280 K serves as a good quantitative indicator of polar warming
Total hemispheric CAM amount significantly decreases in the NH winter for most of the reanalyses during the two periods
CAM trends for the SH winter for 1980–2012 are not consistent among new reanalyses
This study shows the variability of polar cold air mass amount below potential temperature of 280 K, and north of 45°N can be understood with a concept of charge and discharge, where anomalously ...large daily discharge indicates an intermittent occurrence of cold air outbreak. The polar cold air mass amount north of 45°N gradually charges up due to diabatic cooling but dramatically discharges due to cold air outbreak with a pulse width of about 5 days. Cold air outbreaks tend to bring colder winter in East Asia and the east coast of North America, while warmer winter prevails on the northern side of these regions. The cold air mass amount south of 45°N increases just after a cold air outbreak but returns to the normal level soon because of its life time of about 3 days. Therefore, monthly mean of total cold air mass amount in the Northern Hemisphere is negatively correlated with the monthly mean discharge.
Key Points
Time‐evolution of polar CAM is explained with conceptual charge‐discharge model
Polar CAM amount dramatically decreases due to cold air outbreaks with period of about 5 days
Interannual variability of NH total CAM amount is also explained with charge‐discharge model
•Morphodynamics of alternate bars under unsteady discharge is investigated.•repeated specific hydrographs lead to an equilibrium of alternate bars.•This equilibrium state causes non-time dependent ...wavelength and migration periods.•Under this state, the migration period becomes identical to the hydrograph cycle.•Relation between hydrodynamics and bar growth controls this state.
Discharge unsteadiness plays a key role in the dynamics of alternate bars, however, how migrating alternate bars determine their shape under unsteady discharge conditions, especially in the long-term, is still unclear. In this study, we numerically investigate long-term behaviors of migrating alternate bars subject to repeated hydrographs. For this, the same discharge variation but different hydrograph cycles were tested. The results show that even under unsteady flow conditions, specific combinations of hydrograph cycles and discharge variations result in an equilibrium state of migrating alternate bars (i.e., non-time dependent wavelength and migration period). This may be a unique feature of the alternate bar morphodynamics, since dynamic equilibrium state arising from steady discharge counterparts is time-dependent feature. Meanwhile, it is observed that when the discharge changes abruptly, the response of alternate bar morphodynamics changes along the time. Within a limited period, the response of alternate bars is not strong enough to destroy the original wavelength, suggesting that the hydrograph cycle that achieves this equilibrium state must be moderately long such that the timescale of the hydrograph is similar but slightly smaller than the bar growth.
An extreme rainfall event occurred over western Japan and the adjacent Tokai region mainly in early July, named “the Heavy Rain Event of July 2018”, which caused widespread havoc. It was followed by ...heat wave that persisted in many regions over Japan in setting the highest temperature on record since 1946 over eastern Japan as the July and summertime means. The rain event was attributable to two extremely moist airflows of tropical origins confluent persistently into western Japan and large-scale ascent along the stationary Baiu front. The heat wave was attributable to the enhanced surface North Pacific Subtropical High and upper-tropospheric Tibetan High, with a prominent barotropic anticyclonic anomaly around the Korean Peninsula. The consecutive occurrence of these extreme events was related to persistent meandering of the upper-level subtropical jet, indicating remote influence from the upstream. The heat wave can also be influenced by enhanced summertime convective activity around the Philippines and possibly by extremely anomalous warmth over the Northern Hemisphere midlatitude in July 2018. The global warming can also influence not only the heat wave but also the rain event, consistent with a long-term increasing trend in intensity of extreme precipitation observed over Japan.
The feasibility of regional reanalysis assimilating only conventional observations was investigated as an alternative to dynamical downscaling to estimate the past three-dimensional high-resolution ...atmospheric fields with long-term homogeneity over about 60 years. The two types of widely applied dynamical downscaling approaches have problems. One, with a serial long-term time-integration, often fails to reproduce synoptic-scale systems and precipitation patterns. The other, with frequent reinitializations, underestimates precipitation due to insufficient spin-up. To address these problems maintaining long-term homogeneity, we proposed the regional reanalysis assimilating only the conventional observations. We examined it by paying special attention to summer precipitation, through one-month experiment before conducting a long-term reanalysis. The system was designed to assimilate surface pressure and radiosonde upper-air observations using the Japan Meteorological Agency's nonhydrostatic model (NHM) and the local ensemble transform Kalman filter (LETKF). It covered Japan and its surrounding area with a 5-km grid spacing and East Asia with a 25-km grid spacing, applying one-way double nesting in the Japanese 55-year reanalysis (JRA-55). The regional reanalysis overcame the problems with both types of dynamical downscaling approaches. It reproduced actual synoptic-scale systems and precipitation patterns better. It also realistically described spatial variability and precipitation intensity. The 5-km grid spacing regional reanalysis reproduced frequency of heavy precipitation and described anomalous local fields affected by topography, such as circulations and solar radiation, better than the coarser reanalyses. We optimized the NHM-LETKF for long-term reanalysis by sensitivity experiments. The lateral boundary perturbations that were derived from an empirical orthogonal function analysis of JRA-55 brought stable analysis, saving computational costs. The ensemble size of at least 30 was needed, because further reduction significantly degraded the analysis. The deterministic run from non-perturbed analysis was adopted as a first guess in LETKF instead of the ensemble mean of perturbed runs, enabling reasonable simulation of spatial variability in the atmosphere and precipitation intensity.
Local fronts formed near the coast of the Kanto Plain, mainly in the cold season, called “coastal fronts”, tend to be forecast on the inland side of their actual positions by the operational ...mesoscale Numerical Weather Prediction (NWP; with a horizontal grid spacing of 5 km) model at the Japan Meteorological Agency (JMA). In this study, we confirm a systematic NWP error through statistical validations of coastal fronts that occurred with southerly onshore winds during 2015–2018. Using a nonhydrostatic numerical model, we explore the relevant physical mechanisms through sensitivity experiments involving different horizontal resolutions, envelope orography, and physics parameterization schemes for three cases with typical errors. The operational NWP model was shown to have a systematic error, with local fronts being shifted consistently to the inland side of their actual positions when the forecast period exceeds 5 hours, regardless of precipitation. The sensitivity experiments suggested that the systematic error associated with coastal fronts may be caused primarily by an underestimation of the mountain barrier surrounding the Kanto Plain in the model. The northwestward distance error of coastal fronts, averaged over the three illustrative cases, can be reduced by 27 % and 37 % by increasing the horizontal resolution from 5 km to 2 km and 1 km, respectively, and can be eliminated almost entirely by using the envelope orography. Moreover, the evaporative cooling of precipitation shifts coastal fronts seaward. Most coastal fronts are thought to take the form of cold air trapped on the southeastern slope of the mountains surrounding the Kanto Plain, where the elevation angle of the frontal surface is roughly controlled dynamically. The local front shifts seaward when the ridgelines of the mountains become higher, or by the reduction of the elevation angle when the trapped air becomes colder.
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