Ammonia (NH3) direct combustion is attracting attention for energy utilization without CO2 emissions, but fundamental knowledge related to ammonia combustion is still insufficient. This study was ...designed to examine effects of radiation heat loss on laminar ammonia/air premixed flames because of their very low flame speeds. After numerical simulations for 1-D planar flames with and without radiation heat loss modeled by the optically thin model were conducted, effects of radiation heat loss on flame speeds, flame structure and emissions were investigated. Simulations were also conducted for methane/air mixtures as a reference. Effects of radiation heat loss on flame speeds were strong only near the flammability limits for methane, but were strong over widely diverse equivalence ratios for ammonia. The lower radiative flame temperature suppressed the thermal decomposition of unburned ammonia to hydrogen (H2) at rich conditions. The equivalence ratio for a low emission window of ammonia and nitric oxide (NO) in the radiative condition shifted to a lower value than that in the adiabatic condition.
Interannual variability of the East Asian winter monsoon is investigated through composite analysis applied to observational data for 50 recent years. Although the monsoon activity itself is confined ...into the lower troposphere, its midwinter variability tends to accompany upper-tropospheric geopotential height anomalies similar to the Eurasian (EU) and western Pacific (WP) teleconnection patterns. The “EU-like” pattern is characterized by a wavy signature over the Eurasian continent and the North Atlantic, with surface temperature anomalies over the Far East and North America. In the “WP-like” pattern, a meridional dipole of upper-level height anomalies is evident over the Far East.
These anomaly patterns related to the anomalous winter monsoon activity are found to accompany marked modulations of the climatological development of the upper-tropospheric planetary waves from late autumn to midwinter. Enhanced monsoon activity in January associated with the WP-like pattern involves anomalous seasonal development of a planetary wave ridge with enhanced positive height tendencies from November to January over eastern Siberia and Alaska, while the corresponding tendencies are anomalously negative under the weakened monsoon activity. The stronger monsoon also accompanies an enhanced seasonal decline of geopotential height over the midlatitude North Pacific, corresponding to the enhanced southeastward development of a planetary wave trough. Similar modulations of the planetary wave evolution are observed with the anomalous monsoon activity associated with the EU-like pattern. In addition, the anomalous midwinter activity of the monsoon is also accompanied by noticeable variability of the seasonal development of the planetary waves over the Euro-Atlantic sector.
The western Pacific (WP) pattern, characterized by north–south dipolar anomalies in pressure over the Far East and western North Pacific, is known as one of the dominant teleconnection patterns in ...the wintertime Northern Hemisphere. Composite analysis reveals that monthly height anomalies exhibit baroclinic structure with their phase lines tilting southwestward with height in the lower troposphere. The anomalies can thus yield not only a poleward heat flux across the climatological thermal gradient across the strong Pacific jet but also a westward heat flux across the climatological thermal gradient between the North Pacific and the cooler Asian continent. The resultant baroclinic conversion of available potential energy (APE) from the climatological-mean flow contributes most efficiently to the APE maintenance of the monthly WP pattern, acting against strong thermal damping effects by anomalous heat exchanges with the underlying ocean and anomalous precipitation in the subtropics and by the effect of anomalous eddy heat flux under modulated storm-track activity. Kinetic energy (KE) of the pattern is maintained through barotropic feedback forcing associated with modulated activity of transient eddies and the conversion from the climatological-mean westerlies, both of which act against frictional damping. The net feedback forcing by transient eddies is therefore not particularly efficient. The present study suggests that the WP pattern has a characteristic of a dynamical mode that can maintain itself through efficient energy conversion from the climatological-mean fields even without external forcing, including remote influence from the tropics.
The relative importance between the sensible heat supply from the ocean and latent heating is assessed for the maintenance of near-surface mean baroclinicity in the major storm-track regions, by ...analyzing steady linear responses of a planetary wave model to individual components of zonally asymmetric thermal forcing taken from a global reanalysis dataset. The model experiments carried out separately for the North Atlantic, North Pacific, and south Indian Oceans indicate that distinct local maxima of near-surface baroclinicity observed along the storm tracks can be reinforced most efficiently as a response to the near-surface sensible heating. The result suggests the particular importance of the differential sensible heat supply from the ocean across an oceanic frontal zone for the efficient restoration of surface baroclinicity, which acts against the relaxing effect by poleward eddy heat transport, setting up conditions favorable for the recurrent development of transient eddies to anchor a storm track. Unlike what has been suggested, the corresponding reinforcement of the near-surface baroclinicity along a storm track as the response to the latent heating due either to cumulus convection or large-scale condensation is found less efficient. As is well known, poleward eddy heat flux convergence acts as the primary contributor to the reinforcement of the surface westerlies, especially in the core of a storm track. In its exit region, a substantial contribution to the reinforcement arises also from a planetary wave response to the sensible heat supply from the ocean. In contrast, the surface wind acceleration as a planetary wave response to the latent heating is found to contribute negatively to the maintenance of the surface westerlies along any of the major storm tracks.
Scandinavian pattern and its climatic impact Bueh, Cholaw; Nakamura, Hisashi
Quarterly journal of the Royal Meteorological Society,
October 2007 Part B, Letnik:
133, Številka:
629
Journal Article
Dynamical understandings of midlatitude transient eddy activity, especially its midwinter minimum over the North Pacific, are still limited, partly because conventional Eulerian eddy statistics are ...incapable of separating cyclonic and anticyclonic contributions. Here we evaluate the two contributions separately based on local curvature of instantaneous flow fields to compare their seasonality between the North Pacific and North Atlantic storm‐tracks. The anticyclonic contribution is found crucial for the midwinter minimum of the North Pacific transient eddy activity. Eddy energetics reveals that the net efficiency of the anticyclonic contribution in replenishing total transient eddy energy over the North Pacific exhibits a pronounced midwinter minimum leading to net energy loss, while that of its cyclonic counterpart does not, in harmony with a precipitation peak around midwinter. This study suggests that more attention should be paid to anticyclones in studying midlatitude storm‐track dynamics.
Plain Language Summary
Our understanding of the dynamics of midlatitude transient eddy activity, especially its midwinter minimum over the North Pacific, is still limited. This is partly because conventional local statistics based on temporal filtering, which are commonly used as a measure of transient eddy activity, are unable to treat contributions from cyclones and anticyclones separately. Here we evaluate cyclonic and anticyclonic contributions to local eddy statistics separately based on local curvature of instantaneous flow fields, to compare their seasonality between the North Pacific and North Atlantic storm‐tracks. The anticyclonic contribution is found crucial for the midwinter minimum of the North Pacific transient eddy activity. We then apply eddy energetics to assess the relative importance of various processes relevant to the seasonality of eddy activity. The net efficiency of the relevant processes associated with the anticyclonic contribution in replenishing total transient eddy energy over the North Pacific exhibits a pronounced midwinter minimum leading to net energy loss. By contrast, that of the cyclonic counterpart does not, in harmony with a precipitation peak around midwinter. This study suggests that more attention should be paid to anticyclones in studying midlatitude storm‐track dynamics.
Key Points
Anticyclonic contribution is crucial for the midwinter minimum of the North Pacific transient eddy activity
This minimum is consistent with net energy loss for anticyclonic regions in midwinter in energy conversion/generation terms
More attention should be paid to anticyclones in studying midlatitude storm‐track activity and their interaction with a time‐mean flow
Summertime atmospheric circulation over the midlatitude western North Pacific (WNP) is influenced by anomalous convective activity near the Philippines. This meridional teleconnection, observed in ...monthly anomalies and known as the Pacific–Japan (PJ) pattern, is characterized by zonally elongated cyclonic and anticyclonic anomalies around the enhanced convection center and to its northeast, respectively, in the lower troposphere, with an apparent poleward phase tilt with height. The authors’ idealized two-layer linear model, whose basic state consists of a zonal subtropical jet and a pair of a monsoon system and a subtropical anticyclone, can simulate a PJ-like response against diabatic heating located between the pair. Each of the observed and simulated patterns can gain energy through barotropic and baroclinic conversions from the zonally varying baroclinic mean flow, in an efficiency comparable with that of energy generation due to the anomalous diabatic heating, indicating a characteristic of the pattern as a dry dynamical mode. In fact, the conversion efficiency is sensitive to the location of the anomaly pattern relative to the climatological-mean flow. Furthermore, the second-least damped mode identified in the idealized model bears certain resemblance with the observed PJ pattern, indicating its modal characteristics as well as a critical importance of these features in the mean field for the pattern. In addition to the PJ pattern, another meridional teleconnection pattern with high efficiency for its energy conversion is identified observationally in association with anomalous convection near the Bonin Islands.
The anomalous circulation of the PJ pattern, in turn, can intensify the anomalous convective activity near the Philippines through enhancing evaporation and moisture convergence and dynamically inducing anomalous ascent. It is thus hypothesized that the PJ pattern can be regarded as a moist dynamical mode that sustains itself both via dry energy conversion and interaction with moist processes.
Abstract
The North Pacific storm-track activity is suppressed substantially under the excessively strong westerlies to form a distinct minimum in midwinter, which seems inconsistent with linear ...baroclinic instability theory. This “midwinter minimum” of the storm-track activity has been intensively investigated for decades as a test case for storm-track dynamics. However, the mechanisms controlling it are yet to be fully unveiled and are still under debate. Here we investigate the detailed seasonal evolution of the climatological density of surface migratory anticyclones over the North Pacific, in comparison with its counterpart for cyclones, based on a Lagrangian tracking algorithm. We demonstrate that the frequency of surface cyclones over the North Pacific maximizes in midwinter, whereas that of anticyclones exhibits a distinct midwinter minimum under the upstream influence, especially from the Japan Sea region. In midwinter, it is only on such a rare occasion that prominent weakening of the East Asian winter monsoon allows a migratory surface anticyclone to form over the Japan Sea, despite the unfavorable climatological-mean conditions due to persistent monsoonal cold-air outbreaks and the excessively strong upper-tropospheric westerlies. The midwinter minimum of the North Pacific anticyclone density suggests that anticyclones are likely the key to understanding the midwinter minimum of the North Pacific storm-track activity as measured by Eulerian eddy statistics.
Abstract
Storm-track activity over the North Pacific (NP) climatologically exhibits a clear minimum in midwinter, when the westerly jet speed sharply maximizes. This counterintuitive phenomenon, ...referred to as the “midwinter minimum (MWM),” has been investigated from various perspectives, but the mechanisms are still to be unrevealed. Toward better understanding of this phenomenon, the present study delineates the detailed seasonal evolution of climatological-mean Eulerian statistics and energetics of migratory eddies along the NP storm track over 60 years. As a comprehensive investigation of the mechanisms for the MWM, this study has revealed that the
net
eddy conversion/generation rate normalized by the eddy total energy, which is independent of eddy amplitude, is indeed reduced in midwinter. The reduction from early winter occurs mainly due to the decreased effectiveness of the baroclinic energy conversion through seasonally weakened temperature fluctuations and the resultant poleward eddy heat flux. The reduced net normalized conversion/generation rate in midwinter is also found to arise in part from the seasonally enhanced kinetic energy conversion from eddies into the strongly diffluent Pacific jet around its exit. The seasonality of the net energy influx also contributes especially to the spring recovery of the net normalized conversion/generation rate. The midwinter reduction in the normalized rates of both the net energy conversion/generation and baroclinic energy conversion was more pronounced in the period before the late 1980s, during which the MWM of the storm-track activity was climatologically more prominent.
Abstract
In northern Australia (NAUS), mean rainfall during the Australian summer monsoon (AUSM) season exhibits distinct interannual variability despite weak influence from tropical sea surface ...temperature (SST) variability. The present study investigates mechanisms for the strong and persistent rainfall anomalies throughout the AUSM season. When the AUSM is stronger than normal, the low-level monsoonal circulation intensifies in response to the stronger convective activity over NAUS. The intensified surface westerlies over the tropical southeastern Indian Ocean (SEIO) enhance oceanic evaporation locally and downstream moisture transport into NAUS. This wind–evaporation feedback is verified through a moist static energy budget analysis. For this feedback to work effectively, SST cooling due to the stronger AUSM should be weak enough not to suppress the oceanic evaporation in the tropical SEIO. Our mixed layer heat budget analysis based on an ocean model hindcast experiment reveals that anomalous downwelling in the subsurface SEIO, which is induced dynamically by the intensified monsoon westerlies, partially offsets the SST cooling. The land surface evaporation over the continental inland area is also enhanced significantly in the middle and later portions of the monsoon season associated with increased soil moisture, suggesting its memory effect for the persistence of rainfall anomalies. The AUSM variability can therefore be regarded as a self-sustaining internal variability in the atmosphere–ocean–land surface coupled system, rather than just an atmospheric internal variability.
Significance Statement
We aim to understand why summer monsoon rainfall in northern Australia varies markedly from one year to another even under the weak influence of large-scale sea surface temperature fluctuations, such as El Niño/La Niña. Our analyses based mainly on observational datasets reveal that wind-induced changes in oceanic evaporation south of Java significantly modulates the water vapor transport into northern Australia. We also find that ocean dynamics helps this wind–evaporation feedback process and continental soil moisture may act to prolong anomalous rainfall by its memory effect. This study shows the self-sustaining nature of the Australian summer monsoon variability under the atmosphere–ocean–land surface interactions, which deepens our understandings of the monsoon system.
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