scholarly journals Comments on “An Evaluation of Hurricane Superintensity in Axisymmetric Numerical Models”

2020 ◽  
Vol 77 (11) ◽  
pp. 3971-3975 ◽  
Author(s):  
A. M. Makarieva ◽  
A. V. Nefiodov ◽  
D. Sheil ◽  
A. D. Nobre ◽  
A. V. Chikunov ◽  
...  
Keyword(s):  
Kinetic Energy ◽  
Wind Velocity ◽  
Liquid Water ◽  
Numerical Models ◽  
Surface Fluxes ◽  
Carnot Cycle ◽  
Sensible Heat ◽  
Velocity Estimate ◽  
Upper Limit ◽  
Significant Term

AbstractIn a recent paper Rousseau-Rizzi and Emanuel (2019) presented a derivation of an upper limit on maximum hurricane velocity at the ocean surface. This derivation was based on a consideration of an infinitely narrow (differential) Carnot cycle with the warmer isotherm at the point of the maximum wind velocity. Here we show that this derivation neglected a significant term describing the kinetic energy change in the outflow. Additionally, we highlight the importance of a proper accounting for the power needed to lift liquid water. Finally, we provide a revision to the formula for surface fluxes of heat and momentum showing that, if we accept the assumptions adopted by Rousseau-Rizzi and Emanuel (2019), the resulting velocity estimate does not depend on the flux of sensible heat.

scholarly journals Reply to “Comments on ‘An Evaluation of Hurricane Superintensity in Axisymmetric Numerical Models’”

2020 ◽  
Vol 77 (11) ◽  
pp. 3977-3980
Author(s):  
Kerry Emanuel ◽  
Raphaël Rousseau-Rizzi
Keyword(s):  
Boundary Layer ◽  
Kinetic Energy ◽  
Tropical Cyclones ◽  
Numerical Models ◽  
Dissipative Heating ◽  
Surface Fluxes ◽  
Heat Fluxes ◽  
Carnot Cycle ◽  
Closed Theory ◽  
Work Done

AbstractWe concur with Makarieva et al. that in our earlier work on the hurricane differential Carnot cycle, we neglected the work done in lifting water and the dissipation of kinetic energy in the outflow (we explicitly acknowledged neglecting these terms). Here, we relax those assumptions, affirm the conclusion of Makarieva et al. that the water lifting term is small, and show that the effect of outflow dissipation is negligible. We remind readers that the differential Carnot theory is not a closed theory for potential intensity as it does not specify the outflow temperature or the boundary layer moist enthalpy at the radius of maximum winds. The addition of enthalpy to the inflow can raise the boundary layer enthalpy, reducing subsequent surface fluxes, regardless of whether that addition comes from surface fluxes themselves or from dissipative heating. We show that while this may indeed reduce the effect of dissipative heating, it does not eliminate it. We disagree with Makarieva et al.’s assertions that dissipative heating does not increase potential intensity and that only latent heat fluxes can drive tropical cyclones when dissipative heating is included.

scholarly journals Stochastic modeling of transient surface scalar and momentum fluxes in turbulent boundary layers

2021 ◽  
Author(s):  
Marten Klein ◽  
David O. Lignell ◽  
Heiko Schmidt
Keyword(s):  
Boundary Layers ◽  
Control Parameter ◽  
Layer Structure ◽  
Numerical Models ◽  
Turbulent Boundary Layers ◽  
Transport Processes ◽  
Surface Fluxes ◽  
One Dimensional ◽  
Weather And Climate ◽  
Normal Transport

<p>Turbulence is ubiquitous in atmospheric boundary layers and manifests itself by transient transport processes on a range of scales. This range easily reaches down to less than a meter, which is smaller than the typical height of the first grid cell layer adjacent to the surface in numerical models for weather and climate prediction. In these models, the bulk-surface coupling plays an important role for the evolution of the atmosphere but it is not feasible to fully resolve it in applications. Hence, the overall quality of numerical weather and climate predictions crucially depends on the modeling of subfilter-scale transport processes near the surface. A standing challenge in this regard is the robust but efficient representation of transient and non-Fickian transport such as counter-gradient fluxes that arise from stratification and rotation effects.</p><p>We address the issues mentioned above by utilizing a stochastic one-dimensional turbulence (ODT) model. For turbulent boundary layers, ODT aims to resolve the wall-normal transport processes on all relevant scales but only along a single one-dimensional domain (column) that is aligned with the vertical. Molecular diffusion and unbalanced Coriolis forces are directly resolved, whereas effects of turbulent advection and stratification are modeled by stochastically sampled sequence of mapping (eddy) events. Each of these events instantaneously modifies the flow profiles by a permutation of fluid parcels across a selected size interval. The model is of lower order but obeys fundamental conservation principles and Richardson's 1/4 law by construction.</p><p>In this study, ODT is applied as stand-alone tool in order to investigate nondimensional control parameter dependencies of the scalar and momentum transport in turbulent channel, neutral, and stably-stratified Ekman flows up to (friction) Reynolds number <em>Re</em> = <em>O</em>(10<sup>4</sup>). We demonstrate that ODT is able to capture the state-space statistics of transient surface fluxes as well as the boundary-layer structure and nondimensional control parameter dependencies of low-order flow statistics.<br>Very good to reasonable agreement with available reference data is obtained for various observables using fixed model set-ups. We conclude that ODT is an economical turbulence model that is able to not only capture but also predict the wall-normal transport and surface fluxes in multiphysics turbulent boundary layers.</p>

scholarly journals Variation of diurnal Turbulence Kinetic Energy using sonic anemometer observation: a case study Ledeng, Bandung

2018 ◽  
Author(s):  
Sandy Hardian Susanto Herho ◽  
Dasapta Erwin Irawan
Keyword(s):  
Kinetic Energy ◽  
Diurnal Variation ◽  
Wind Velocity ◽  
Sonic Anemometer ◽  
Diurnal Variations ◽  
Turbulence Kinetic Energy ◽  
Atmospheric Conditions

Sonic anemometer observation was performed on 29 - 30 September 2014 in Ledeng, Bandung to see diurnal variations of Turbulence Kinetic Energy (TKE) that occurred in this area. The measured sonic anemometer was the wind velocity components u, v, and w. From the observation result, it can be seen that the diurnal variation that happened was quite significant. The maximum TKE occurs during the daytime when atmospheric conditions tend to be unstable. TKE values were small at night when atmospheric conditions are more stable than during the daytime.

Two Experiments on Using a Scintillometer to Infer the Surface Fluxes of Momentum and Sensible Heat

2012 ◽  
Vol 51 (9) ◽  
pp. 1685-1701 ◽  
Author(s):  
Edgar L Andreas
Keyword(s):  
Eddy Covariance ◽  
Correlation Coefficients ◽  
Arctic Sea Ice ◽  
Surface Fluxes ◽  
Index Structure ◽  
Quality Data ◽  
Sensible Heat ◽  
Traditional Use ◽  
Similarity Functions ◽  
Essential Question

AbstractA traditional use of scintillometry is to infer path-averaged values of the turbulent surface fluxes of sensible heat Hs and momentum τ (, where ρ is air density and u* is the friction velocity). Many scintillometer setups, however, measure only the path-averaged refractive-index structure parameter ; the wind information necessary for inferring u* and Hs comes from point measurements or is absent. The Scintec AG SLS20 surface-layer scintillometer system, however, measures both and the inner scale of turbulence ℓ0, where ℓ0 is related to the dissipation rate of turbulent kinetic energy ɛ. The SLS20 is thus presumed to provide path-averaged estimates of both u* and Hs. This paper describes comparisons between SLS20-derived estimates of u* and Hs and simultaneous eddy-covariance measurements of these quantities during two experiments: one, over Arctic sea ice; and a second, over a midlatitude land site during spring. For both experiments, the correlation between scintillometer and eddy-covariance fluxes is reasonable: correlation coefficients are typically above 0.7 for the better-quality data. For both experiments, though, the scintillometer usually underestimates u* and underestimates the magnitude of Hs when compared with the corresponding eddy-covariance values. The data also tend to be more scattered when < 10−14 m−2/3: the signal-to-noise ratio for scintillometer-derived fluxes decreases as decreases. An essential question that arises during these comparisons is what similarity functions to use for inferring fluxes from the scintillometer and ℓ0 measurements. The paper thus closes by evaluating whether any of four candidate sets of similarity functions is consistent with the scintillometer data.

scholarly journals Diffusion in Novae at High Accretion Rates

1990 ◽  
Vol 122 ◽  
pp. 394-396
Author(s):  
Attay Kovetz ◽  
Prialnik Dina
Keyword(s):  
Numerical Models ◽  
Variable Mass ◽  
Apparent Discrepancy ◽  
Full Understanding ◽  
Transfer Rates ◽  
Outstanding Problem ◽  
Upper Limit ◽  
Classical Nova ◽  
Accretion Rates ◽  
Diffusion Convection

The main outstanding problem in our full understanding of the classical nova mechanism is the apparent discrepancy between mass transfer rates (10−9 to 10−8M⊙/yr) inferred from observations (Patterson, 1984, Ap.J.Suppl.231, 789), and those required by numerical models in order to reproduce nova characteristics (10−11 to 10−9M⊙/yr). The low accretion rates are needed in order to obtain powerful runaways and high values of Z in the ejecta by the diffusion-convection mechanism. The discrepancy seems to have sharpened by the realization that accretional heating (Shaviv and Starrfield, 1987, Ap.J.321, L51) and angular momentum transfer (Sparks and Kutter, 1987, Ap.J.321, 394 and Kutter and Sparks, 1987, Ap.J.321, 386) tend to lower the theoretical upper limit for Ṁ to about 10−10M⊙/yr. On the other hand, scenarios invoking variable mass accretion rates — hibernation (Shara et al, 1986, Ap.J.311, 163) or ‘mild’ hibernation (Livio, Shankar and Truran, 1988, Ap.J.330, 264) — have shown that the high observed rates immediately prior and following outbursts can be reconciled with lower average accretion rates over the period between outbursts.

Effect of the Micro-Porous Layer-Gas Diffusion Layer Interface on Water Transport in Polymer Electrolyte Fuel Cells

2009 ◽  
Author(s):  
Joshua Preston ◽  
Richard Fu ◽  
Xiaoyu Zhang ◽  
Ugur Pasaogullari
Keyword(s):  
Polymer Electrolyte ◽  
Cross Section ◽  
Porous Layer ◽  
Diffusion Layer ◽  
Liquid Water ◽  
Water Saturation ◽  
Numerical Models ◽  
Gas Diffusion Layer ◽  
Gas Diffusion ◽  
Diffusion Media

An investigation of the liquid water saturation across the cross-section of an operating polymer electrolyte fuel cell is performed to analyze the saturation discontinuity predicted by numerical models. Numerical models have predicted a discontinuity in the liquid water saturation at the interface of the micro-porous layer and the coarser macroporous region of the gas diffusion layer. High-resolution through plane neutron radiography is used to acquire the water content distribution across the thickness of the gas-diffusion layer and study the effects of the interface. The measured liquid water profiles indicate no obvious discontinuity in the liquid water saturation across the cross-section of the bi-layer gas diffusion layer when large areas are averaged spatially. Evidence of the discontinuity is found when small spatial averaging is used in certain locations. Other locations show no evidence of the discontinuity. Scanning electron microscopy is used to examine the microstructure of two types of the bi-layer diffusion media. The images show that the approximation of the interface as a sudden, distinct feature may not be appropriate. The results suggest that a model that considers the existence of an interfacial region in the diffusion media may be appropriate, in which the properties vary continuously.

Partitioned Heat Sinks for Improved Natural Convection

2020 ◽  
Author(s):  
Todd Salamon ◽  
Roger Kempers ◽  
Brian Lynch ◽  
Kevin Terrell ◽  
Elina Simon
Keyword(s):  
Heat Transfer ◽  
Natural Convection ◽  
Convective Heat Transfer ◽  
Antenna Arrays ◽  
Heat Sink ◽  
Massive Mimo ◽  
Vertical Plate ◽  
Numerical Models ◽  
Heat Sinks ◽  
Sensible Heat

Abstract The main drivers contributing to the continued growth of network traffic include video, mobile broadband and machine-to-machine communication (Internet of Things, cloud computing, etc.). Two primary technologies that next-generation (5G) networks are using to increase capacity to meet these future demands are massive MIMO (Multi-Input Multi-Output) antenna arrays and new frequency spectrum. The massive MIMO antenna arrays have significant thermal challenges due to the presence of large arrays of active antenna elements coupled with a reliance on natural convection cooling using vertical plate-finned heat sinks. The geometry of vertical plate-finned heat sinks can be optimized (for example, by choosing the fin pitch and thickness that minimize the thermal resistance of the heat sink to ambient air) and enhanced (for example, by embedding heat pipes within the base to improve heat spreading) to improve convective heat transfer. However, heat transfer performance often suffers as the sensible heat rise of the air flowing through the heat sink can be significant, particularly near the top of the heat sink; this issue can be especially problematic for the relatively large or high-aspect-ratio heat sinks associated with massive MIMO arrays. In this study a vertical plate-finned natural convection heat sink was modified by partitioning the heat sink along its length into distinct sections, where each partitioned section ejects heated air and entrains cooler air. This approach increases overall heat sink effectiveness as the net sensible heat rise of the air in any partitioned section is less than that observed in the unpartitioned heat sink. Experiments were performed using a standard heat sink and equivalent heat sinks partitioned into two and three sections for the cases of ducted and un-ducted natural convection with a uniform heat load applied to the rear of the heat sink. Numerical models were developed which compare well to the experimental results and observed trends. The numerical models also provide additional insight regarding the airflow and thermal performance of the partitioned heat sinks. The combined experimental and numerical results show that for relatively tall natural convection cooled heat sinks, the partitioning approach significantly improves convective heat transfer and overall heat sink effectiveness.

scholarly journals VLBI Observations of SN 1979C and SN 1986J

2005 ◽  
Vol 192 ◽  
pp. 105-109
Author(s):  
N. Bartel ◽  
M.F. Bietenholz
Keyword(s):  
Kinetic Energy ◽  
Light Curve ◽  
Mass Loss ◽  
Wind Velocity ◽  
Curve Fitting ◽  
Velocity Ratio ◽  
Radio Spectrum ◽  
Vlbi Observations ◽  
The Galaxy ◽  
Order Of Magnitude

SummaryWe summarize our results on multi-epoch VLBI observations of SN 1979C in the galaxy M100 in Virgo, and of SN 1986 in the galaxy NGC 891. From t = 3.7 to 22 yr after the explosion, SN 1979C expands ∝ tm, almost freely, with m = 0.95 ± 0.03. For a total kinetic energy of 3 × 1051 erg, the expansion result requires a mass-loss to wind-velocity ratio for the progenitor of only 1 × 10−5 M⊙ yr−1per 10 km s−1, an order of magnitude smaller than estimated from radio light-curve fitting. We show a first image with slightly discernible structure of the supernova. For SN 1986J we present five images from 1987 to 2002 and show our result on moderately to strongly decelerated expansion with m = 0.71 ± 0.11. We comment on our result of an inversion of the radio spectrum in terms of the emergence of a possible pulsar nebula.

scholarly journals Searching for a dusty cometary belt around TRAPPIST-1 with ALMA

2020 ◽  
Vol 492 (4) ◽  
pp. 6067-6073 ◽  
Author(s):  
S Marino ◽  
M C Wyatt ◽  
G M Kennedy ◽  
M Kama ◽  
L Matrà ◽  
...  
Keyword(s):  
Liquid Water ◽  
Kuiper Belt ◽  
Dust Emission ◽  
Planetary Systems ◽  
Dynamical Instability ◽  
Solid Mass ◽  
Low Mass Stars ◽  
Upper Limit ◽  
Low Mass ◽  
Collisional Evolution

ABSTRACT Low-mass stars might offer today the best opportunities to detect and characterize planetary systems, especially those harbouring close-in low-mass temperate planets. Among those stars, TRAPPIST-1 is exceptional since it has seven Earth-sized planets, of which three could sustain liquid water on their surfaces. Here we present new and deep ALMA observations of TRAPPIST-1 to look for an exo-Kuiper belt which can provide clues about the formation and architecture of this system. Our observations at 0.88 mm did not detect dust emission, but can place an upper limit of 23 µJy if the belt is smaller than 4 au, and 0.15 mJy if resolved and 100 au in radius. These limits correspond to low dust masses of ∼10−5 to 10−2 M⊕, which are expected after 8 Gyr of collisional evolution unless the system was born with a &gt;20 M⊕ belt of 100 km-sized planetesimals beyond 40 au or suffered a dynamical instability. This 20 M⊕ mass upper limit is comparable to the combined mass in TRAPPIST-1 planets, thus it is possible that most of the available solid mass in this system was used to form the known planets. A similar analysis of the ALMA data on Proxima Cen leads us to conclude that a belt born with a mass ≳1 M⊕ in 100 km-sized planetesimals could explain its putative outer belt at 30 au. We recommend that future characterizations of debris discs around low-mass stars should focus on nearby and young systems if possible.

scholarly journals Modeling the Effect of Desert Urbanization on Local Climate and Natural Dust Generation: A Case Study for Erbil, Iraq

Urban Science ◽  
2020 ◽  
Vol 4 (4) ◽  
pp. 46
Author(s):  
Sherzad T. Tahir ◽  
Huei-Ping Huang
Keyword(s):  
Land Surface ◽  
Numerical Models ◽  
Surface Wind ◽  
Surface Fluxes ◽  
Slight Reduction ◽  
Rapid Urbanization ◽  
Near Surface ◽  
Local Climate ◽  
Surface Dust ◽  
Dust Generation

This study uses a suite of meteorological and land-surface models to quantify the changes in local climate and surface dust fluxes associated with desert urbanization. Formulas connecting friction velocity and soil moisture to dust generation are used to quantify surface fluxes for natural wind-blown dust. The models are used to conduct a series of simulations for the desert city of Erbil across a period of rapid urbanization. The results show significant nighttime warming and weak but robust daytime cooling associated with desert urbanization. A slight reduction in near-surface wind speed is also found in the areas undergoing urbanization. These findings are consistent with previous empirical and modeling studies on other desert cities. Numerical models and empirical formulas are used to produce climatological maps of surface dust fluxes as a function of season, and for the pre- and post-urbanization eras. This framework can potentially be used to bridge the gap in observation on the trends in local dust generation associated with land-use changes and urban expansions.

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