Potential Temperature Difference Research Articles

Atmospheric conditions conducive to thunderstorms with downbursts in Canada and a downburst precursor parameter

Thunderstorm winds are the costliest wind hazard in Canada, but they have not been well studied. This paper investigates atmospheric conditions prone to downbursts in Canada using a combination of reported downbursts from damage survey reports, ERA5 reanalysis data, lightning flash observations, radar imagery, and upper air measurements for the period of 2019–2023. We propose a novel Downburst Precursor Parameter (DPP) by integrating two low-correlated thermodynamic and kinematic parameters. DPP outperforms other existing instability indices in downburst diagnosis in Canada. Our spatial analysis reveals the absence of damaging downbursts west of the Rockies, while approximately half of all recorded damaging downbursts occurred in southeastern Canada. Squall lines and supercells are the most common types of downburst-generating storms in Canada. Furthermore, nearly two-thirds of storms are squall lines when a downburst is accompanied by lightning flashes, which occurred in ∼71% of all cases. The remaining ∼29% of downbursts without lightning are mostly dry downbursts, typically displaying a disorganized pattern of convective storms on a radar screen. The equivalent potential temperature difference between the surface and mid-level in the troposphere (∆θe) exceeds 13 K prior to 61% of damaging downbursts. This result indicates that ∆θe might be a useful downburst indicator in Canada despite lower values compared to many regions in the United States.

Influence of air-conditioning intermittent operation on the cooling load from opaque envelopes in residences

Air-conditioning (AC) intermittent operation is extensively employed owing to occupancy regulations and residents' habits. This conditioning operation type significantly affects the indoor air temperature as a result of the on-off cycles of AC, leading to potential high temperature differences between adjacent rooms due to varying operation rules. Based on this, numerical simulations were done to deal with the overall thermal behavior of typical bedrooms under intermittent operation, and the individual contributions from different envelope components were discussed considering four models of AC operation in adjacent rooms. The results demonstrate that optimizing these four envelope components can lower average transient heat flows by 16.6 %–65.03 % with the consideration of the operation model. Among these components, the ceiling exhibited the highest energy efficiency potential with a reduction ratio of up to 42.8 %. Furthermore, the AC operation model in adjacent rooms had a significant impact on cooling load, and optimizing these four envelope components greatly reduced thermal disturbances from neighboring rooms. The fluctuation percentage of daily heat transfer capacities decreased from 33.89%-37.16 % to 20.64%–23.76 %. Specifically, optimizing opaque envelopes resulted in reductions in heat transfer capacities by 32.06 % and 30.48 % respectively, with ceilings contributing most significantly at 15.1 % and 14.2 %.

A CloudSat–CALIPSO view of cloud and precipitation in the occluded quadrants of extratropical cyclones

AbstractUsing 10 years of satellite‐borne radar and lidar observations coupled with a novel method for automated occlusion identification, composite transects of cloud and precipitation across occluded thermal ridges of extratropical cyclones are, for the first time, constructed. These composites confirm that occluded sectors are characterized by the most extensive cloud cover and heaviest precipitation in any of the frontal regions of the cyclone. Hydrometeor frequency in occluded sectors is sensitive to the cyclone's ascent strength but not to the mean precipitable water in the cyclone's environment. This result is in contrast to the strong relationships between hydrometeor frequency and both precipitable water and ascent strength as previously reported in warm frontal regions. In both hemispheres, cloud and precipitation increase with the maximum value of the equivalent potential temperature at 700 hPa within the occluded thermal ridge, until a threshold is reached. For very large values of maximum equivalent potential temperature, hydrometeors become less frequent while precipitation rates increase. It is suggested that this conjunction is a by‐product of an increase in the frequency of convection in those instances. While in the Northern Hemisphere occluded sectors exhibit deeper and wider cloud structures than their Southern Hemisphere counterparts, their hydrometeor occurrence frequencies are less. The differences in maximum equivalent potential temperature of the thermal ridges in both hemispheres does not appear to explain the more frequent hydrometeors in the Southern Hemisphere. These relationships offer new perspectives on the interplay between cloud processes and cyclone evolution, as well as new observational constraints for process evaluation of Earth system models.

Rapid 2-Dimensional prediction of supercritical CO2 heat transfer behaviors in inclined tubes based on deep learning

At present, there is a dearth of empirical correlations for the prediction of the inclined supercritical heat transfer. Considering the shortage of the rapid prediction method in the initial design stages for the supercritical fluid heat exchanger with inclined tubes or under inclined conditions, a 2-Dimensional rapid prediction method based on the numerical method combining deep learning method was proposed to reconcile the accuracy and the efficiency of the inclined supercritical heat transfer behaviors prediction. In order to train the deep learning model, 200 numerical cases with 520,000 data were conducted. After that, 7 new cases were chosen to analyze the inclined heat transfer behaviors and validate the current model. The results were indicative of a significant circumferential inner wall temperature difference in the inclined supercritical heat transfer. The temperature difference was larger when the inclined angle was closer to 90°, even though the circumferentially averaged temperature is smooth and acceptable. This potential temperature difference would not only cause large thermal stress which may accelerate fatigue failure, but also increase the risk of over-temperature. Fortunately, the current deep learning model could predict the 2D temperature distribution of these new cases with the max absolute relative error of 16.60%. Therefore, the circumferential inner wall temperature distribution of the inclined supercritical CO2 heat transfer could be evaluated during the initial heat exchanger design. Moreover, the trained model and the relative files are available at Supplementary materials.

Characteristics and Establishment of Objective Identification Criteria and Predictors for Foehn Winds in Urumqi, China

The special terrain of Urumqi (in the valley area) often triggers strong foehn winds, causing huge losses to local people’s lives and social economies. By using the surface observation data of the hourly temperature, pressure, humidity, and wind from the downwind Urumqi Meteorological Station and the upwind Dabancheng Meteorological Station in the Middle Tianshan Canyon and the NCEP/NCAR reanalysis data during 2008–2022, this paper establishes the dataset of foehn processes at Urumqi Station in the past 15 years and analyzes the variation rules of the associated meteorological variables during the foehn processes. In addition, based on the physical mechanism of the occurrence of foehn, a three-element identification criterion (i.e., 94° ≤ 2 min average wind direction ≤ 168°, 2 min average wind speed ≥ 2.0 m/s, and Δθ between Urumqi station and Dabancheng station ≥ 0.29 K) for foehn in Urumqi is established by comparing and analyzing the variations of wind direction (WD), wind speed (WS), and the potential temperature difference (Δθ) between the two weather stations during the periods of foehn and non-foehn winds from 2013 to 2022. In addition, the performance of the three-element identification criterion is verified, and the results suggest that this criterion has an accuracy of 82.96% and a hit rate of 89.50% for the 2008–2012 foehn events in Urumqi. Moreover, the hit rate of this criterion for foehn wind of gale or above level (i.e., a 2 min wind ≥ 10.8 m/s on average) is 100%. In addition, combined with two predictors of sea-level pressure difference (ΔP) and Δθ between downwind stations and upwind stations, the foehn forecast can be more accurate than that provided by a single predictor. With ΔP ≤ −12 hPa and Δθ ≥ 5 K, the chances for foehn to occur are over 90%. This finding would have some reference and application values for the foehn forecasting.

Physico-chemical and structural characterization of cellulose nanocrystals obtained by two drying methods: Freeze-drying and spray-drying

This study evaluated the physical-chemical properties and structure of cellulose nanocrystals (CNC) produced from pine cellulose pulp, and dried by two different methods: Spray-Drying (SD) or Freeze-Drying (FD). After drying, CNC were characterized in terms of size, z-potential, elemental analysis, conductometric titration, crystallinity and thermogravimetry. Complementary, analysis by force microscopy, dynamic sorption and infrared spectroscopy were carried out.Results showed that different drying methods did not produce significant differences in zeta potential, crystallinity and degradation temperature. However, CNC dried by FD showed smaller size but higher aspect ratio, and higher sulfur content, which would explain the significant higher sorption observed in CNC-FD at relative humidity higher than 80%. Drying method does not influence the formation or modification of cellulose bonds as FTIR suggested.Our study suggests that how CNC is dried will influence the CNC's structure and will modify some of their physico-chemical properties. This information is relevant since it can be used as an input for the scale-up processing of CNC and would define the performance in some of their technological applications.

A simulation-based study to evaluate the cooling potential of nocturnal radiative cooling systems for residential buildings in Egypt

During the first years of the last decade, Egypt used to face recurrent electricity cut-offs in summer. In the past few years, the electricity tariff dramatically increased. Radiative cooling to the clear night sky is a renewable energy source that represents a relative solution. The dry desert climate promotes nocturnal radiative cooling applications. This study investigates the potential of nocturnal radiative cooling systems (RCSs) to reduce the energy consumption of the residential building sector in Egypt. The system technology proposed in this work is based on uncovered solar thermal collectors integrated into the building hydronic system. By implementing different control strategies, the same system could be used for both cooling and heating applications. The goal of this paper is to analyze the performance of RCSs in residential buildings in Egypt. The dynamic simulation program TRNSYS was used to simulate the thermal behavior of the system. The relevant issues of Egypt as a case-study are firstly overviewed. Then the paper introduces the work done to develop a building model that represents a typical residential apartment in Egypt. Typical occupancy profiles were developed to define the internal thermal gains. The adopted control strategy to optimize the system operation is presented as well. To fully understand and hence evaluate the operation of the proposed RCS, four simulation cases were considered: 1. a reference case (fully passive), 2. the stand-alone operation of the RCS, 3. ideal heating & cooling operation (fully-active), and 4. the hybrid-operation (when the active cooling system is supported by the proposed RCS). The analysis considered the main three distinct climates in Egypt, represented by the cities of Alexandria, Cairo and Asyut. The hotter and drier weather conditions resulted in a higher cooling potential and larger temperature differences. The simulated cooling power in Asyut was 28.4 W/m² for a 70 m² absorber field. For a smaller field area of 10 m², the cooling power reached 109 W/m² but with humble temperature differences. To meet the rigorous thermal comfort conditions, the proposed sensible RCS cannot fully replace conventional air-conditioning units, especially in humid areas like Alexandria. When working in a hybrid system, a 10% reduction in the active cooling energy demand could be achieved in Asyut to keep the cooling set-point at 24 °C. This percentage reduction was nearly doubled when the thermal comfort set-point was increased by two degrees (26 °C). In a sensitivity analysis, external shading devices as a passive measure as well as the implementation of the Egyptian code for buildings (ECP306/1–2005) were also investigated. The analysis of this study raised other relevant aspects to discuss, e.g. system-sizing, environmental effects, limitations and recommendations.

ПРИМЕНЕНИЕ ТЕРМОЭЛЕКТРИЧЕСКОГО МОДУЛЯ ПЕЛЬТЬЕ В ТЕХНОЛОГИЯХ ОХЛАЖДЕНИЯ И ГЕНЕРАЦИИ ЭЛЕКТРОЭНЕРГИИ

Relevance Currently, the phenomenon of thermoelectricity is used quite widely in various industries. This phenomenon involves producing a potential difference or temperature difference, and is used for power generation, refrigeration and air conditioning, which helps minimize environmental pollution caused by conventional systems. This is due to the fact that the temperature difference is directly converted into a voltage gradient without the use of any mechanical systems. Aim of research The purpose of the study is to analyze various applications of thermoelectric phenomena using Peltier module technology. Research methods Analysis of literary sources, physical foundations of thermoelectricity. Results The study analyzed the use of thermoelectric technology in various industries, and calculated the parameters of the device based on thermoelectric modules.

Investigating the Semi-Analytical Models of Momentum Relaxation Mean Free Time and Path and Ionization Coefficient of Trilayer Graphene Nanoribbon- Based FETs

Trilayer graphene nanoribbon as a superlative semiconductor promises potential applications in the diodes, FETs, gas and biosensors. It is a suitable candidate for nanoelectronic applications due to attractive electronic, mechanical, thermoelectric and optoelectronics properties. The aim of the present paper is to analytically investigate the momentum relaxation mean free time and path and also ionization coefficient of trilayer graphene nanoribbon in nanoscale FETs. The models are derived based on energy band structure and calculating the effective mass, carrier velocity and scattering rate. Furthermore, the effects of interlayer potential energies, temperature and potential difference of layers are also taken into account on the modeled parameters. As a result, the obtained results and findings of this research are in rational agreement with published data, in terms of value and trend. The results and figures of merit for the proposed device showed a promising performance for transistor applications.

Cascading effects of the Changbai Mountains on an extreme weather disaster in northern Japan in January 2021

A sequence of a snowstorm, a quasi-stationary convergence zone (Japan Sea polar airmass convergence zone, JPCZ) and a mesoscale coastal front (MCF) caused an extreme weather disaster along the Sea of Japan (SOJ) coast in northern Japan from January 7 to 11, 2021. To examine how the presence of the Changbai Mountains (CMs) affects these multi-scale successive phenomena, we conducted numerical simulations with and without modified topography of the CMs and their surrounding regions using a regional atmospheric model with a horizontal resolution of 4 km. Backward-trajectory analyses and an additional experiment that removed the sublimation process of snow were also performed to clarify the formation and maintenance processes of the MCF. The CMs lead to the structural change of the snowstorm developing over the SOJ, enhancement of the JPCZ, and formation of the MCF, affecting heavy snowfall in the SOJ coastal areas. The MCF, which is a mesoscale phenomenon smaller than the JPCZ, is formed and maintained by the monsoonal flows that detour from the north and south of the CMs. Several of the parcels that detour from north of the CMs intrude into the MCF through the boundary layer, which is thermodynamically modified by the heat and moisture supply from the sea, whereas the inland flows that penetrate the MCF from its south are composed of two major routes: one is the flow detouring from south of the CMs that crosses the high mountains on the mainland of Japan, and the other is the parcels detouring from north of the CMs that penetrate from the inland after they land on the northern coast. The difference in equivalent potential temperature between the inland flows and the monsoonal flow from the sea is responsible for the MCF formation. A cold pool near the surface also contributes to the prolonged MCF. Understanding of the CMs’ cascading effects on multi-scale phenomena from synoptic scale to mesoscale is necessary for the prediction of extreme weather disasters over Japan in winter.

Quantification of Seasonal and Interannual Variations of the Tibetan Plateau Surface Thermodynamic Forcing Based on the Potential Vorticity

AbstractIn this study, a new index based on the potential vorticity (PV) framework is proposed for the quantification of the Tibetan Plateau (TP) surface thermodynamic and dynamic forcing. The results show that the derived TP surface PV (SPV) includes the topographical effect, near‐surface absolute vorticity, and land–air potential temperature differences. The climatological annual cycle of the SPV suggests that the TP transitions from a cooling to a heating source in April. The SPV reaches a maximum from June to August, which is consistent with the evolution of the Asian summer monsoon precipitation. Further analysis suggests that the intensified SPV in the boreal summer results in a low‐level cyclonic circulation anomaly associated with increased precipitation over the southeastern slope of the TP and South China and decreased precipitation over the Indian Ocean. In winter, the intensified SPV is associated with local cold air and divergence at the TP surface.

Long-Term Variation in Wintertime Atmospheric Diffusion Conditions Over the Sichuan Basin

Atmospheric diffusion is one of the factors affecting local air quality, dominating the evolution of air pollution episodes. Previous work has emphasized the unfavorable diffusion conditions in the Sichuan Basin resulting from its complex terrain. However, the recent spatiotemporal variation in atmospheric diffusion conditions in the basin and their effects on local air quality remain unclear. Based on the wind speed, boundary layer height, vertical potential temperature difference of ERA5 analysis, two independent metrics containing information on horizontal and vertical diffusion ability, i.e., ventilation coefficient (VE) and the air stagnant conditions (ASI), are involved to indicate wintertime atmospheric diffusion conditions in the basin. Both VE and ASI reveal a decrease tendency of atmospheric diffusion condition from the northwestern portion of the basin to the southeast. In terms of the long-term variation in diffusion conditions, VE showed a broader increasing trend from 1979 to 2019, with a distinct increase in the western region. In contrast, the occurrence of air stagnation events has declined −3∼−6%/decade in the basin, more significantly over the western basin. Both the increase in VE and the decrease in air stagnation frequency indicate the improvement of atmospheric diffusion conditions in the Sichuan Basin from 1979 to 2019, which mitigates the effects of air pollutant emissions to some extent. The enhancement of diffusion conditions is due to the improvement of vertical diffusion conditions. The lowest seasonal PM2.5 concentrations occur in the northwestern basin, where VE is highest and ASI is lowest. Atmospheric diffusion conditions can explain approximately 25–50% of the interannual variation in PM2.5 concentrations in Chengdu.

Numerical simulations of upslope fog observed at Beppu Bay in Oita Prefecture, Japan

AbstractThe present study focussed on the formation of upslope fog. This fog causes chronic expressway closures near Beppu Bay (BB) in Oita Prefecture, Japan. A dense fog event, occurring on 3 and 4 May 2020, resulting from the passage of an extratropical cyclone through the southern Japanese islands, was simulated using the Weather Research and Forecasting (WRF) numerical model. The simulation results indicated that warm and humid air with a lifting condensation level less than 20 m flowed into a plain from BB. Simulated fog and poor visibility were more severe on mountain slope adjacent to BB than on the surroundings. Here, the westward horizontal flux of water vapour by the wind flow was prolonged at lower heights. Analyses of the simulated elements verified that the upslope fog was induced by moist adiabatic cooling of the air lifted by the upslope winds. At the mature stage of upslope fog, a simulated local vertical circulation generated in conjunction with the upper synoptic flow extended the dense fog into the upper atmosphere over BB. During the decaying stage of upslope fog on mountain, a fog covered on the entire BB. This was associated with weakening winds at lower heights. Sensitivity experiments for model boundary conditions supported the importance of mountain steepness and the rectangular shape of BB for the generation of upslope fog. Finally, an application possibility of dense fog forecasts was discussed by using a simulated difference in the equivalent potential temperature between the base of the mountain and Beppuwan expressway interchange located midslope.

日本初のアイウォール貫通型航空機観測により捉えられた2017年台風第21号（ラン）の二重暖気核構造

Upper-tropospheric aircraft reconnaissance was carried out for Typhoon Lan (2017) using a civil jet with a newly developed dropsonde system. This was the first time a Japanese research group observed the inner core of an intense typhoon using dropsondes. This paper describes the warm-core structure in the eye and the associated thermodynamic and kinematic features of the eyewall. During two days of reconnaissance, the typhoon preserved its peak intensity in an environment with strengthening vertical shear. The dropsondes captured a double warm-core structure with a higher perturbation temperature in the middle and upper troposphere, which persisted between the two missions. The two warm cores showed a difference in equivalent potential temperature (θe) of more than 10 K, suggesting different air origins. Saturation-point analysis suggests that the air observed in the upper warm core was entrained from the eyewall. The eyewall updraft in the left-of-shear semicircle had a two-layer structure with a higher θe and lower absolute angular momentum on the inner side of the updraft core. Analyses of the saturation point and parcel method suggest that the warmer air with a θe exceeding 370 K on the inner side of the updrafts originated from the eye boundary layer and was eventually transported into the upper warm core. These results led us to hypothesize that the vertical transport of high-θe air from the eye boundary layer contributed to the continuous eye warming in the upper troposphere against the negative effect of strengthening environmental wind shear on storm intensity. This study demonstrates the significance of eyewallpenetrating upper-tropospheric reconnaissance for monitoring the warm-core structure in the present situation, where accurate measurements of both humidity and temperature for calculating θe can only be made with dropsonde-type expendables.

Northeastward propagation of nocturnal precipitation over the Sichuan Basin

AbstractThe Sichuan Basin (SCB) is located to the east of the Tibetan Plateau (TP) and features nocturnal precipitation. This study explores spatial variation of nocturnal precipitation during the warm season over the SCB using high spatiotemporal resolution merged precipitation products from 2008 to 2017. It is found that there is a prominent northeastward timing delay of precipitation peak over the SCB, that is, the diurnal peak of precipitation occurs in the midnight over the southwestern SCB whereas in the early morning over the northeastern SCB. This time delay is partly ascribed to a northeastward propagation of precipitation over the SCB, account for about 40% and 25% of the diurnal variation of precipitation over the central and northeastern SCB, respectively. Precipitation first occurs over the southwestern edge of the SCB at about 1900–2000 LST, and then propagates northeastward steered by the large‐scale mid‐tropospheric southwesterly airflow, which is consistent with the northeastward propagation of the large moisture convergence. The precipitation reaches the northeastern SCB in the early morning. The diurnal variation of 850‐hPa wind has strong easterly wind deviations in the early evening over the southwestern SCB, which favours for an initiation of precipitation by orographic lift of air. The diurnal variation of difference in equivalent potential temperature between 500 hPa and 850 hPa has a maximum negative deviation in the early evening over the southwestern SCB, which enhances the instability and therefore favours an initiation of precipitation. The Yunnan‐Guizhou Plateau (YGP) may play an important role in the initiation of precipitation over the southwestern SCB in the early evening.

On the Relationship Between the Marine Cold Air Outbreak M Parameter and Low‐Level Cloud Heights in the Midlatitudes

AbstractFocusing on conditions of subsidence when low clouds are present, ground‐based observations in both the North Atlantic and the Southern Ocean reveal strong relationships between cloud boundary (base and top heights) and different measures of lower tropospheric instability. The difference in potential temperature between the surface and 800 hPa (a metric called M) provides a stronger relationship than measures of inversion strength such as the lower tropospheric stability and estimated inversion strength. This is because (1) inversion strength itself does not correlate well with cloud boundaries, and (2) M contains information that appears important for cloud boundaries. These include the surface forcing through the use of sea surface rather than near‐surface air temperature and an upper level close to the real cloud top. These results expand upon previous work on the importance of M as a predictor of cloud morphology. However, important differences are found in low‐cloud conditions for the North Atlantic as compared to the Southern Ocean (for a given value of M): stronger inversions, deeper boundary layers, and much larger sea level pressures. Therefore, the relationship between cloud boundaries and M differs between the two regions. A general circulation model provides similar relationships as observed between M and both cloud top height and temperature but tends to place clouds higher and at colder temperatures than observed for a given M. This might cause issues with the representation of precipitation, cloud cover and radiation in the Southern Ocean.

Particle simulation of an anode-layer Hall thruster plume using an anisotropic scattering model

The large-angle scattered ions are generated in the plasma plume mainly due to the ion-atom collisions. These ions can induce thrust loss, backflow, or sputtering on the spacecraft. It is necessary, therefore, to give an accurate prediction of ion movement for the study on electric propulsions. In the present work, a particle simulation procedure, which includes (1) a Direct Simulation Monte Carlo method and (2) a particle-in-cell method with a Monte Carlo collisions technique coupled with an anisotropy scattering model, is applied to simulate the plume of a D55 anode-layer Hall thruster. The effective Xe+-Xe and Xe2+-Xe interaction potentials are employed to simulate the ion-atom scattering. The electron properties are obtained via a Boltzmann relation and a model that utilizes conservation-law-type equations, respectively. Comparisons are made between the experimental data together with the simulation results from the anisotropy model and those from an isotropic scattering model to analyze their disparities. Results show that, compared with the isotropic model, the electron number density from the anisotropy model matches better with the experimental data. However, the differences in ion current density, plasma potential, and electron temperature between the two scattering models are insignificant. The anisotropy model can give a more accurate prediction of the ion energy distribution, resulting from its more reasonable hypothesis than the isotropic model. For different electron models, the computed results from one using conservation-law-type equations are in better agreement with the measurements than a simple Boltzmann relation.

Local-Scale Valley Wind Retrieval Using an Artificial Neural Network Applied to Routine Weather Observations

AbstractWe hereby present a new method with which to nowcast a thermally driven, downvalley wind using an artificial neural network (ANN) based on remote observations. The method allows the retrieval of wind speed and direction. The ANN was trained and evaluated using a 3-month winter-period dataset of routine weather observations made in and above the valley. The targeted valley winds feature two main directions (91% of the total dataset) that are aligned with the valley axis. They result from downward momentum transport, channeling mechanisms, and thermally driven flows. A selection procedure of the most pertinent ANN input variables, among the routine observations, highlighted three key variables: a potential temperature difference between the top and the bottom of the valley and the two wind components above the valley. These variables are directly related to the mechanisms that generate the valley winds. The performance of the ANN method improves on an earlier-proposed nowcasting method, based solely on a vertical temperature difference, as well as a multilinear regression model. The assessment of the wind speed and direction indicates good performance (i.e., wind speed bias of −0.28 m s−1 and 84% of calculated directions stray from observations by less than 45°). Major sources of error are due to the misrepresentation of cross-valley winds and very light winds. The validated method was then successfully applied to a 1-yr period with a similar performance. Potentially, this method could be used to downscale valley wind characteristics for unresolved valleys in mesoscale simulations.

The critical mass flux for positive flow characteristic of heated tubes in the vertical water wall

Positive flow characteristic is favorable in design of vertical water wall of large-scale once-through boilers, since it is beneficial for reducing potential temperature differences and thermal imbalance distribution across the water wall. The identification of the critical mass flux (G0) at which the flow characteristic transits from positive to negative is important for design of supercritical boilers with vertical water wall. In this paper, theoretical analysis was performed to investigate G0 under both supercritical and subcritical pressures. A theoretical solution of G0 was derived through thermal-hydraulic models. The impacts of operation parameters and tube structure parameters on G0 were quantified through numerical analysis. The results show that fluid outlet temperature, fluid pressure, tube inner diameter and tube friction factor have significant impacts on G0. G0 decreases as the increase of the fluid outlet temperature and the decrease of the fluid pressure. Under the condition of identical fluid inlet and outlet temperatures, operating pressure and tube length, G0 is proportional to D/λ. With the proposed model, G0 of a 600 MWe supercritical CFB boiler under different loads were calculated and validated.

Post Cold Frontal Clouds at the ARM Eastern North Atlantic Site: An Examination of the Relationship Between Large‐Scale Environment and Low‐Level Cloud Properties

AbstractUsing cloud and environment observations from the Atmospheric Radiation Measurement (ARM) program Eastern North Atlantic site and an automated cold front detection routine, cloud properties in post cold front (PCF) periods are examined and compared to similar conditions of subsidence (non‐PCF). PCF periods exhibit stronger subsidence and wind speed than non‐PCF periods, with weaker inversions and stronger surface temperature contrasts. Low‐level clouds are predominant and are found to have higher cloud base and top heights, colder cloud top temperature, and greater vertical extent and liquid water path during PCF than non‐PCF periods. The environmental metric that is best correlated with cloud boundaries for both PCF and non‐PCF periods is the difference in potential temperature between the sea surface and 800 hPa, a parameter used to locate cold air outbreak conditions. However, the cloud vertical extent and liquid water path are found to be better correlated with sea‐air temperature contrast, a parameter related to turbulent surface fluxes. The strength of the relationships between the cloud characteristics and these metrics does not differ for PCF and non‐PCF periods. However, the strength of the metrics differs between PCF and non‐PCF periods and can explain cloud property differences. The results suggest both the properties of the boundary layer and the presence of an upper‐level cyclone associated with the cold front determine PCF cloud properties.