Four recent reanalyses—the 55-yr Japanese Reanalysis Project (JRA-55), Interim ECWMF Re-Analysis (ERA-I), NCEP Climate Forecast System Reanalysis (CFSR), and NASA Modern-Era Retrospective Analysis ...for Research and Applications (MERRA)—are assessed to clarify their quality in representing the diurnal cycle over East Asia. They are found to present similar patterns/structure and summer progress of the mean wind diurnal cycle, whereas they exhibit some differences in diurnal amplitude, particularly for the low-level meridional wind. An evaluation with intense soundings suggests that the amplitude difference mainly results from the diurnal variation of mean bias that differs among reanalyses. The root-mean-square (RMS) error is found to have a diurnal variation more evident in CFSR and MERRA than that in JRA-55 and ERA-I, which strongly affects the representation of the varying diurnal amplitude at the peak hours of RMS error.
Compared with satellite-derived rainfall, the four reanalyses are shown to reproduce well the rainfall diurnal cycle over East Asia in terms of large-scale terrain contrast, summer progress, and interannual variability. JRA-55 even presents a long-term increase of morning rainfall percentage over the east China plain over the past four decades, consistent with rain gauge observations. The four reanalyses exhibit some considerable discrepancies at regional scale; JRA-55 gives the best capture of the rainfall diurnal cycle over the Tibetan Plateau and the eastward propagation to the eastern lees. These results suggest that new reanalyses are potentially applicable for studying the large-scale diurnal variability over East Asia, whereas their different preferences, especially at regional scale, should be of concern in data application.
Brewer‐Dobson circulation diagnosed from JRA‐55 Kobayashi, Chiaki; Iwasaki, Toshiki
Journal of geophysical research. Atmospheres,
27 February 2016, Letnik:
121, Številka:
4
Journal Article
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We examine the Brewer‐Dobson circulation (BDC) in the lower stratosphere diagnosed from the Japanese 55 year Reanalysis (JRA‐55) data set and compare it with the BDCs diagnosed from the other ...reanalyses (i.e., ERA‐Interim and JRA‐25), and JRA‐55‐related data sets (i.e., JRA‐55C, created by assimilating only in situ observations, and JRA‐55AMIP, a simulation by a prediction model). The climatological mean seasonal change of the BDC in JRA‐55 is similar to that in ERA‐Interim but considerably different from that in JRA‐25. Dynamical and thermodynamical consistencies among the variables are greatly improved in the JRA‐55 data set. The interannual variations of the annual mean tropical upwellings in JRA‐55 are almost coincident with those in JRA‐55C. It suggests that the weakly increasing trend of the BDC found in JRA‐55 does not have anything to do with changes in the satellite observing system. The climatological mean tropical upwelling diagnosed from JRA‐55 is stronger than that from JRA‐55AMIP. This difference is presumed to partly link to the model's inability to simulate the quasi‐biennial oscillation (QBO). The JRA‐55AMIP data set, a simple simulation performed without data assimilation, does not simulate the QBO, whereas the JRA‐55 data set represents the QBO with the aid of data assimilation. The climatological mean zonal mean states of JRA‐55AMIP considerably deviate from those of JRA‐55 in the lower stratosphere. The deviation of JRA‐55AMIP is similar to the modulation pattern by the QBO in specific phase regions in which the BDC is rather weak. The simulated QBO might enhance the BDC and reduce the model biases in the tropical lower stratosphere.
Key Points
Climatological seasonal change of the BDC in JRA‐55 is similar to that in ERA‐Interim
Strengthening trend of the BDC in JRA‐55 is not due to changes in satellite observations
Model biases in the lower stratosphere depend on the simulation of the QBO
Abstract Moist convection occurred repeatedly in the midnight-to-morning hours of 11–16 June 1998 and yielded excessive rainfall in a narrow latitudinal corridor over East Asia, causing severe flood. ...Numerical experiments and composite analyses of a 5-day period are performed to examine the mechanisms governing nocturnal convection. Both simulations and observations show that a train of MCSs concurrently developed along a quasi-stationary mei-yu front and coincided with the impact of a monsoon surge on a frontogenetic zone at night. This process was regulated primarily by a nocturnal low-level jet (NLLJ) in the southwesterly monsoon that formed over southern China and extended to central China. In particular, the NLLJ acted as a mechanism of moisture transport over the plains. At its northern terminus, the NLLJ led to a zonal band of elevated conditionally unstable air where strong low-level ascent overcame small convective inhibition, triggering new convection in three preferred plains. An analysis of convective instability shows that the low-tropospheric intrusion of moist monsoon air generated CAPE of ~1000 J kg −1 prior to convection initiation, whereas free-atmospheric forcing was much weaker. The NLLJ-related horizontal advection accounted for most of the instability precondition at 100–175 J kg −1 h −1 . At the convective stage, instability generation by the upward transport of moisture increased to ~100 J kg −1 h −1 , suggesting that ascending inflow caused feedback in convection growth. The convection dissipated in late morning with decaying NLLJ and moisture at elevated layers. It is concluded that the diurnally varying summer monsoon acted as an effective discharge of available moist energy from southern to central China, generating the morning-peak heavy rainfall corridor.
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.
Plain Language Summary
In winter, the polar regions are reservoirs of extremely cold air masses. Sometimes, the polar cold air outflows to the midlatitudes and results in very cold weather. This study quantitatively evaluated the future changes in the polar cold air mass and its outflow to the midlatitudes in the winter hemispheres. We found that both the polar cold air mass and its outflow are projected to decrease significantly by the end of the 21st century in the Northern and Southern Hemispheres due to the increase in the global mean surface temperature. These findings provide a possible scenario of future polar climate change.
Key Points
Under the RCP8.5 scenario, the polar cold air mass is projected to decrease by 37% in the boreal winter by the end of 21st century
The equatorward polar cold air mass fluxes in the midlatitudes are projected to decrease significantly by around 2050
Thermodynamical (dynamical) change is the main (secondary) contributor to the projected reduction in the polar cold air mass fluxes
An extreme cold surge event caused record-breaking low temperatures in 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 a threshold potential temperature of 280 K. Because cold air mass is an adiabatically conservative quantity, it is suitable for tracing and examining the extreme cold surges. We further introduced 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 the 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.
The equatorward cold airmass flux below potential temperatureθT
= 280 K across 45°N integrated from 90°E to 180° is used as an index to quantitatively measure cold air outbreaks (CAOs) in the East ...Asian winter monsoon. Intermittent CAOs over East Asia significantly contribute to the global equatorward cold airmass flux. An autocorrelation analysis indicates that CAO events persist for approximately 5 days. The geographical distributions of lagged correlations/regressions with the CAO index (CAOI) clarify the temporal evolution of synoptic conditions associated with CAOs. The developing Siberian high located northwest of Lake Baikal (65°N, 100°E) on day −4 slowly moves southeastward, reaches maximum intensity over Siberia (50°N, 110°E) on day 0, and then decays while moving rapidly southward. By contrast, the Aleutian low is almost stagnant and maintains a strong intensity. The eastward pressure gradient geostrophically induces the equatorward cold airmass flux. After day −2, the cold air mass significantly decreases over Siberia, but increases over East Asia and the western North Pacific Ocean. The cold air mass continues to migrate southward while spreading eastward, and disappears mainly over the ocean. The leading edge of the high pressure anomaly moves southward at 13 m s−1and reaches the equator simultaneously with the equatorward wind anomaly on about day +4. An additional analysis of separating the equatorward flux into 90°–135°E and 135°E–180° suggests that CAOs are, to some extent, caused by the Siberian high and the Aleutian low acting separately.
•Morphodynamics, chute cutoff and bar/channel coevolution were reproduced.•Sand bar growth and discharge unsteadiness were key factors in meandering increase.•Vegetation reduced channel width, ...resulting in a single-thread meandering channel.•No vegetation changed the pattern from a single-thread to a multi-thread channel.•Braided channel could be a moderately meandered channel by presence of vegetation.
Recent studies suggest that braided river could be single-thread channel by colonization of riparian vegetation; however, this kind of mutual interactions between physical and ecological processes in rivers are still poorly understood. Here we investigate the development of meandering channel in a river, which was originally braided and currently vegetated, the Otofuke River in Japan. The significant morphological processes of this river during a destructive flood event was studied using a two-dimensional morphodynamic model. Using well-calibrated parameters, this model qualitatively reproduced observed morphological changes such as the co-development of sand bars, bar-induced meandering and a chute cutoff. We find that for vegetated channels, meandering could maintain moderate sinuosity; in contrast, in the absence of riparian vegetation, bar-induced meandering channels could become braided. This suggests that distinct meandering channels could be a fundamental channel morphology in the originally braided, but currently vegetated river; however, the simultaneous occurrence of the chute cutoff and meandering indicates that this channel could not be a fully-developed high amplitude meandering channel.
Abstract
The present study develops a diagnostic framework for investigating the three-dimensional (3D) structure of mass-weighted isentropic time-mean (T-MIM) meridional circulations and conducts a ...preliminary analysis of the winter hemispheres. The T-MIM meridional velocity can unfold, in the zonal direction, time-averaged two-dimensional (2D) mass-weighted isentropic zonal means. Furthermore, the T-MIM velocity can be decomposed into the unweighted isentropic time-mean (uTM) velocity and the temporal eddy-correlated transport velocity, the so-called bolus velocity. The bolus velocity greatly contributes to the 2D extratropical direct circulation in the troposphere and to the Brewer–Dobson circulation in the stratosphere. The 3D bolus velocity seems to reflect the geographical distributions of baroclinic instability wave activity. In the boreal winter, both low-level equatorward flows and upper-level poleward flows are located around the North Pacific and North Atlantic storm tracks. In the austral winter, low-level equatorward flows extend zonally across the midlatitudes. In the subtropics, the 3D bolus velocity is found to be significant in the upper branch of the Hadley circulation. A zonal momentum equation is formulated to examine the 3D momentum balance of the meridional circulation in the T-MIM framework. In the extratropics, the uTM and bolus meridional velocities are in geostrophic balance with the stationary and transient components of the 3D Eliassen–Palm (EP) flux divergence, respectively. The pressure gradient force of transient baroclinic instability waves balances with the low-level equatorward flows of the bolus velocity in the storm tracks.
An analysis method is proposed for polar cold airmass streams from generation to disappearance. It designates a threshold potential temperature T at around the turning point of the extratropical ...direct (ETD) meridional circulation from downward to equatorward in the mass-weighted isentropic zonal mean (MIM) and clarifies the geographical distributions of the cold air mass, the negative heat content (NHC), their horizontal fluxes, and their diabatic change rates on the basis of conservation relations of the air mass and thermodynamic energy. In the Northern Hemispheric winter, the polar cold air mass below T = 280 K has two main streams: the East Asian stream and the North American stream. The former grows over the northern part of the Eurasian continent, flows eastward, turns down southeastward toward East Asia via Siberia, and disappears over the western North Pacific Ocean. The latter grows over the Arctic Ocean, flows toward the eastern coast of North America via Hudson Bay, and disappears over the western North Atlantic Ocean. In their exit regions, wavemean flow interactions are considered to transfer the angular momentum from the cold airstreams to the upward EliassenPalm flux and convert the available potential energy to wave energy.
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