scholarly journals A Direct Measure of Entrainment

2010 ◽  
Vol 67 (6) ◽  
pp. 1908-1927 ◽  
Author(s):  
David M. Romps
Keyword(s):  
Spatial Distribution ◽  
Direct Measurement ◽  
Deep Convection ◽  
Large Eddy Simulations ◽  
The Other ◽  
Vertical Profiles ◽  
Direct Measure ◽  
Total Water ◽  
Convective Flux ◽  
Large Eddy

Abstract A method is introduced for directly measuring convective entrainment and detrainment in a cloud-resolving simulation. This technique is used to quantify the errors in the entrainment and detrainment estimates obtained using the standard bulk-plume method. The bulk-plume method diagnoses these rates from the convective flux of some conserved tracer, such as total water in nonprecipitating convection. By not accounting for the variability of this tracer in clouds and in the environment, it is argued that the bulk-plume equations systematically underestimate entrainment. Using tracers with different vertical profiles, it is also shown that the bulk-plume estimates are tracer dependent and, in some cases, unphysical. The new direct-measurement technique diagnoses entrainment and detrainment at the gridcell level without any recourse to conserved tracers. Using this method in large-eddy simulations of shallow and deep convection, it is found that the bulk-plume method underestimates entrainment by roughly a factor of 2. The directly measured entrainment rates are then compared to cloud height and cloud buoyancy. Contrary to existing theories, fractional entrainment is not found to scale like the inverse of height, the cloud buoyancy, or the gradient of cloud buoyancy. On the other hand, fractional detrainment is found to scale linearly with cloud buoyancy. Finally, direct measurement is used to diagnose the spatial distribution of entrainment and detrainment during the evolution of an individual deep cumulonimbus.

LES Simulations of Rotating Two-Pass Ribbed Duct With Coriolis and Centrifugal Buoyancy Forces at Re=100,000

2016 ◽  
Author(s):  
Cody Dowd ◽  
Danesh Tafti
Keyword(s):  
Heat Transfer ◽  
Reynolds Number ◽  
Large Eddy Simulations ◽  
The Other ◽  
Coriolis Forces ◽  
Flow And Heat Transfer ◽  
Large Eddy ◽  
Buoyancy Forces ◽  
First Pass ◽  
Duct Geometry

The focus of this research is to predict the flow and heat transfer in a rotating two-pass duct geometry with staggered ribs using Large-Eddy Simulations (LES). The geometry consists of a U-Bend with 17 ribs in each pass. The ribs are staggered with an e/Dh = 0.1 and P/e = 10. LES is performed at a Reynolds number of 100,000, a rotation number of 0.2 and buoyancy parameters (Bo) of 0.5 and 1.0. The effects of Coriolis forces and centrifugal buoyancy are isolated and studied individually. In all cases it is found that increasing Bo from 0.5 to 1.0 at Ro = 0.2 has little impact on heat transfer. It is found that in the first pass, the heat transfer is quite receptive to Coriolis forces which augment and attenuate heat transfer at the trailing and leading walls, respectively. Centrifugal buoyancy, on the other hand has a bigger effect in augmenting heat transfer at the trailing wall than in attenuating heat transfer at the leading wall. On contrary, it aids heat transfer in the second half of the first pass at the leading wall by energizing the flow near the wall. The heat transfer in the second pass is dominated by the highly turbulent flow exiting the bend. Coriolis forces have no impact on the augmentation of heat transfer on the leading wall till the second half of the passage whereas it attenuates heat transfer at the trailing wall as soon as the flow exits the bend. Contrary to phenomenological arguments, inclusion of centrifugal buoyancy augments heat transfer over Coriolis forces alone on both the leading and trailing walls of the second pass.

Large-eddy simulations of convection initiation over heterogeneous, low terrain

2022 ◽  
Keyword(s):  
Kinetic Energy ◽  
Free Convection ◽  
Deep Convection ◽  
Physical Space ◽  
Peak Height ◽  
Spectral Shape ◽  
Large Eddy Simulations ◽  
Power Spectral ◽  
Large Eddy ◽  
Convection Initiation

Abstract Large-eddy simulations are conducted to investigate and physically interpret the impacts of heterogeneous, low terrain on deep-convection initiation (CI). The simulations are based on a case of shallow-to-deep convective transition over the Amazon River basin, and use idealized terrains with varying levels of ruggedness. The terrain is designed by specifying its power-spectral shape in wavenumber space, inverting to physical space assuming random phases for all wave modes, and scaling the terrain to have a peak height of 200 m. For the case in question, these modest terrain fields expedite CI by up to 2-3 h, largely due to the impacts of the terrain on the size of, and subcloud support for, incipient cumuli. Terrain-induced circulations enhance subcloud kinetic energy on the mesoscale, which is realized as wider and longer-lived subcloud circulations. When the updraft branches of these circulations breach the level of free convection, they initiate wider and more persistent cumuli that subsequently undergo less entrainment-induced cloud dilution and detrainment-induced mass loss. As a result, the clouds become more vigorous and penetrate deeper into the troposphere. Larger-scale terrains are more effective than smaller-scale terrains in promoting CI because they induce larger enhancements in both the width and the persistence of subcloud updrafts.

On the lifecycle of a shallow cumulus cloud: Is it a bubble or plume, active or forced?

2021 ◽  
Author(s):  
David M. Romps ◽  
Rusen Öktem ◽  
Satoshi Endo ◽  
Andrew M. Vogelmann
Keyword(s):  
Cloud Cover ◽  
Large Eddy Simulations ◽  
The Other ◽  
Doppler Lidar ◽  
Cumulus Cloud ◽  
Cumulus Clouds ◽  
Shallow Cumulus ◽  
Large Eddy ◽  
Stereo Cameras ◽  
Using Data

AbstractA cloud’s lifecycle determines how its mass flux translates into cloud cover, thereby setting Earth’s albedo. Here, an attempt is made to quantify the most basic aspects of the lifecycle of a shallow cumulus cloud: the degree to which it is a bubble or plume, and active or forced. Quantitative measures are proposed for these properties, which are then applied to hundreds of shallow cumulus clouds in Oklahoma using data from stereo cameras, a Doppler lidar, and large-eddy simulations. The observed clouds are intermediate between active and forced, but behave more like bubbles than plumes. The simulated clouds, on the other hand, are more active and plume-like, suggesting room for improvement in the modeling of shallow cumulus.

scholarly journals Departure from Flux-Gradient Relation in the Planetary Boundary Layer

Atmosphere ◽  
2021 ◽  
Vol 12 (6) ◽  
pp. 672
Author(s):  
Pedro Santos ◽  
Alfredo Peña ◽  
Jakob Mann
Keyword(s):  
Wind Speed ◽  
Mesoscale Model ◽  
Large Eddy Simulations ◽  
Horizontal Velocity ◽  
Vertical Profiles ◽  
Height Range ◽  
Flux Gradient ◽  
Large Eddy ◽  
Momentum Fluxes ◽  
The Mean

It is well known that when eddies are small, the eddy fluxes can be directly related to the mean vertical gradients, the so-called flux-gradient relation, but such a relation becomes weaker the larger the coherent structures. Here, we show that this relation does not hold at heights relevant for wind energy applications. The flux–gradient relation assumes that the angle (β) between the vector of vertical flux of horizontal momentum and the vector of the mean vertical gradient of horizontal velocity is zero, i.e., these vectors are aligned. Our observations do not support this assumption, either onshore or offshore. Here, we present analyses of a misalignment between these vectors from a Doppler wind lidar observations and large-eddy simulations. We also use a real-time mesoscale model output for inter-comparison with the lidar-observed vertical profiles of wind speed, wind direction, momentum fluxes, and the angle between the horizontal velocity vector and the momentum flux vector up to 500 m, both offshore and onshore. The observations show this within the height range 100–500 m, β=−18∘ offshore and β=−12∘ onshore, on average. However, the large-eddy simulations show β≈0∘ both offshore and onshore. We show that observed and mesoscale-simulated vertical profiles of mean wind speed and momentum fluxes agree well; however, the mesoscale results significantly deviate from the wind-turning observations.

scholarly journals Origin and Flow History of Air Parcels in Orographic Banner Clouds

2015 ◽  
Vol 72 (9) ◽  
pp. 3389-3403 ◽  
Author(s):  
Sebastian Schappert ◽  
Volkmar Wirth
Keyword(s):  
Wind Field ◽  
Large Eddy Simulations ◽  
The Other ◽  
Current Investigation ◽  
Large Eddy ◽  
Long Time ◽  
History Of ◽  
Low Altitude ◽  
Basic Features ◽  
Different Characteristics

Abstract Banner clouds are clouds in the lee of steep mountains or sharp ridges. Previous work suggests that the main formation mechanism is vertical uplift in the lee of the mountain. On the other hand, little is known about the Lagrangian behavior of air parcels as they pass the mountain, which motivates the current investigation. Three different diagnostics are applied in the framework of large-eddy simulations of airflow past an isolated pyramid-shaped obstacle: Eulerian tracers indicating the initial positions of the parcels, streamlines along the time-averaged wind field, and online trajectories computed from the instantaneous wind field. All three methods diagnose a plume of large vertical uplift in the immediate lee of the mountain. According to the time-mean Eulerian tracers, the cloudy parcels originated within a fairly small coherent area at the inflow boundary. In contrast, the time-mean streamlines indicate a bifurcation into two distinct classes of air parcels with very different characteristics. The parcels in the first class originate at intermediate altitudes, pass the obstacle close to its summit, and proceed directly into the cloud. By contrast, the parcels in the second class start at low altitude and take a fairly long time before they reach the cloud on a spiraling path. A humidity tracer quantifies mixing, revealing partial moistening for the first class of parcels and drying for the second class of parcels. For the online trajectories, the originating location of parcels is more scattered, but the results are still consistent with the basic features revealed by the other two diagnostics.

scholarly journals Why Does Fog Deepen? An Analytical Perspective

Atmosphere ◽  
2020 ◽  
Vol 11 (8) ◽  
pp. 865
Author(s):  
Jonathan G. Izett ◽  
Bas J. H. van de Wiel
Keyword(s):  
Observational Data ◽  
Analytical Description ◽  
Large Eddy Simulations ◽  
Maximum Depth ◽  
Analytical Relation ◽  
Vertical Profiles ◽  
Discrete Observations ◽  
Large Eddy ◽  
Analytical Perspective ◽  
Vertical Gradients

The overall depth of a fog layer is one of the important factors in determining the hazard that a fog event presents. With discrete observations and often coarse numerical grids, however, fog depth cannot always be accurately determined. To address this, we derive a simple analytical relation that describes the change in depth of a fog interface with time, which depends on the tendencies and vertical gradients of moisture. We also present a lengthscale estimate for the maximum depth over which mixing can occur in order for the fog layer to be sustained, assuming a uniform mixing of the vertical profiles of temperature and moisture. Even over several hours, and when coarse observational resolution is used, the analytical description is shown to accurately diagnose the depth of a fog layer when compared against observational data and the results of large-eddy simulations. Such an analytical description not only enables the estimation of sub-grid or inter-observation fog depth, but also provides a simple framework for interpreting the evolution of a fog layer in time.

scholarly journals WRF nested large-eddy simulations of deep convection during SEAC4RS

2017 ◽  
Vol 122 (7) ◽  
pp. 3953-3974 ◽  
Author(s):  
Nicholas K. Heath ◽  
Henry E. Fuelberg ◽  
Simone Tanelli ◽  
F. Joseph Turk ◽  
R. Paul Lawson ◽  
...  
Keyword(s):  
Deep Convection ◽  
Large Eddy Simulations ◽  
Large Eddy
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