Fallacies of the Enthalpy Transfer Coefficient over the Ocean in High Winds

2011 ◽  
Vol 68 (7) ◽  
pp. 1435-1445 ◽  
Author(s):  
Edgar L Andreas
Keyword(s):  
Wind Speed ◽  
Latent Heat ◽  
Large Scale ◽  
Scaling Laws ◽  
Surface Fluxes ◽  
Neutral Stability ◽  
Transfer Coefficients ◽  
Wide Range ◽  
Interfacial Transfer ◽  
High Winds

Abstract Mesoscale and large-scale atmospheric models use a bulk surface flux algorithm to compute the turbulent flux boundary conditions at the bottom of the atmosphere from modeled mean meteorological quantities such as wind speed, temperature, and humidity. This study, on the other hand, uses a state-of-the-art bulk air–sea flux algorithm in stand-alone mode to compute the surface fluxes of momentum, sensible and latent heat, and enthalpy for a wide range of typical (though randomly generated) meteorological conditions over the open ocean. The flux algorithm treats both interfacial transfer (controlled by molecular processes right at the air–sea interface) and transfer mediated by sea spray. Because these two transfer routes obey different scaling laws, neutral-stability, 10-m transfer coefficients for enthalpy CKN10, latent heat CEN10, and sensible heat CHN10 are quite varied when calculated from the artificial flux data under the assumption of only interfacial transfer—the assumption in almost all analyses of measured air–sea fluxes. That variability increases with wind speed because of increasing spray-mediated transfer and also depends on surface temperature and atmospheric stratification. The analysis thereby reveals as fallacious several assumptions that are common in air–sea interaction research—especially in high winds. For instance, CKN10, CEN10, and CHN10 are not constants; they are not even single-valued functions of wind speed, nor must they increase monotonically with wind speed if spray-mediated transfer is important. Moreover, the ratio CKN10/CDN10, where CDN10 is the neutral-stability, 10-m drag coefficient, does not need to be greater than 0.75 at all wind speeds, as many have inferred from Emanuel’s seminal paper in this journal. Data from the literature and from the Coupled Boundary Layers and Air–Sea Transfer (CBLAST) hurricane experiment tend to corroborate these results.

Effect of Surface Fluxes versus Radiative Heating on Tropical Deep Convection

2015 ◽  
Vol 72 (9) ◽  
pp. 3378-3388 ◽  
Author(s):  
Usama Anber ◽  
Shuguang Wang ◽  
Adam Sobel
Keyword(s):  
Large Scale ◽  
Multiple Equilibria ◽  
Initial Conditions ◽  
Deep Convection ◽  
Surface Fluxes ◽  
Radiative Heating ◽  
Net Energy ◽  
Wide Range ◽  
Tropical Deep Convection ◽  
Gross Moist Stability

Abstract The effects of turbulent surface fluxes and radiative heating on tropical deep convection are compared in a series of idealized cloud-system-resolving simulations with parameterized large-scale dynamics. Two methods of parameterizing the large-scale dynamics are used: the weak temperature gradient (WTG) approximation and the damped gravity wave (DGW) method. Both surface fluxes and radiative heating are specified, with radiative heating taken as constant in the vertical in the troposphere. All simulations are run to statistical equilibrium. In the precipitating equilibria, which result from sufficiently moist initial conditions, an increment in surface fluxes produces more precipitation than an equal increment of column-integrated radiative heating. This is straightforwardly understood in terms of the column-integrated moist static energy budget with constant normalized gross moist stability. Under both large-scale parameterizations, the gross moist stability does in fact remain close to constant over a wide range of forcings, and the small variations that occur are similar for equal increments of surface flux and radiative heating. With completely dry initial conditions, the WTG simulations exhibit hysteresis, maintaining a dry state with no precipitation for a wide range of net energy inputs to the atmospheric column. The same boundary conditions and forcings admit a rainy state also (for moist initial conditions), and thus multiple equilibria exist under WTG. When the net forcing (surface fluxes minus radiative heating) is increased enough that simulations that begin dry eventually develop precipitation, the dry state persists longer after initialization when the surface fluxes are increased than when radiative heating is increased. The DGW method, however, shows no multiple equilibria in any of the simulations.

scholarly journals Turbulence in a coastal Mediterranean area: surface fluxes and related parameters at Castel Porziano, Italy

2009 ◽  
Vol 6 (2) ◽  
pp. 3355-3372 ◽  
Author(s):  
S. A. Cieslik ◽  
G. Gerosa ◽  
A. Finco ◽  
G. Matteucci ◽  
N. Cape ◽  
...  
Keyword(s):  
Wind Speed ◽  
Latent Heat ◽  
Mediterranean Area ◽  
Surface Fluxes ◽  
Heat Fluxes ◽  
Meteorological Variables ◽  
Measuring Campaign ◽  
Ozone Fluxes

Abstract. During the ACCENT/VOCBAS measuring campaign conducted at Castel Porziano, Italy over a Mediterranean macchia ecosystem located near the coastline, a series of micrometeorological observations were made. Sensible and latent heat fluxes, as well as ozone fluxes, are presented. The behaviour of the main meteorological variables such as temperature, humidity, wind speed and direction, is analysed.

Wind Speed, Surface Flux, and Convection Coupling from CYGNSS Data

2021 ◽  
Author(s):  
Eric Maloney ◽  
Hien Bui ◽  
Emily Riley Dellaripa ◽  
Bohar Singh
Keyword(s):  
Heat Flux ◽  
Wind Speed ◽  
Latent Heat ◽  
Latent Heat Flux ◽  
Surface Wind ◽  
Surface Flux ◽  
Surface Wind Speed ◽  
Surface Fluxes ◽  
Heat Fluxes ◽  
Boreal Summer

<p>This study analyzes wind speed and surface latent heat flux anomalies from the Cyclone Global Navigation Satellite System (CYGNSS), aiming to understand the physical mechanisms regulating intraseasonal convection, particularly associated with the Madden-Julian oscillation (MJO). The importance of wind-driven surface flux variability for supporting east Pacific diurnal convective disturbances during boreal summer is also examined. An advantage of CYGNSS compared to other space-based datasets is that its surface wind speed retrievals have reduced attenuation by precipitation, thus providing improved information about the importance of wind-induced surface fluxes for the maintenance of convection. Consistent with previous studies from buoys, CYGNSS shows that enhanced MJO precipitation is associated with enhanced wind speeds, and that associated surface heat fluxes anomalies have a magnitude about 7%-12% of precipitation anomalies. Thus, latent heat flux anomalies are an important maintenance mechanism for MJO convection through the column moist static energy budget. A composite analysis during boreal summer over the eastern north Pacific also supports the idea that wind-induced surface flux is important for MJO maintenance there. We also show the surface fluxes help moisten the atmosphere in advance of diurnal convective disturbances that propagate offshore from the Colombian Coast during boreal summer, helping to sustain such convection.  </p>

scholarly journals A Bulk Turbulent Air–Sea Flux Algorithm for High-Wind, Spray Conditions

2008 ◽  
Vol 38 (7) ◽  
pp. 1581-1596 ◽  
Author(s):  
Edgar L. Andreas ◽  
P. Ola G. Persson ◽  
Jeffrey E. Hare
Keyword(s):  
Latent Heat ◽  
Large Scale ◽  
Surface Flux ◽  
Heat Fluxes ◽  
High Wind ◽  
Molecular Processes ◽  
Tropical Ocean ◽  
Wind Speeds ◽  
The Mean ◽  
High Winds

Abstract Sensible and latent heat can cross the air–sea interface by two routes: as interfacial fluxes controlled by molecular processes right at the interface, and as spray fluxes from the surface of sea spray droplets. Once the 10-m wind speed over the ocean reaches approximately 11–13 m s−1, the spray sensible and latent heat fluxes become significant fractions (i.e., 10% or greater) of the corresponding interfacial fluxes. The analysis here establishes that result by combining the Tropical Ocean-Global Atmosphere Coupled Ocean–Atmosphere Response Experiment (COARE) version 2.6 bulk interfacial flux algorithm with a microphysical spray model to partition measured heat fluxes from two good high-wind datasets into spray and interfacial flux contributions. The measurements come from the Humidity Exchange over the Sea (HEXOS) experiment and the Fronts and Atlantic Storm-Tracks Experiment (FASTEX); wind speeds in these two datasets span 5 to 20 m s−1. After the measured heat fluxes are separated into spray and interfacial contributions, the spray fluxes are used to develop a fast spray flux algorithm to combine with the COARE version 2.6 interfacial flux algorithm in a unified turbulent surface flux algorithm for use in large-scale and ocean storm models. A sensitivity analysis of the spray and interfacial components of this unified flux algorithm demonstrates how the two component fluxes scale differently with the mean meteorological variables and why they must therefore be parameterized separately in models intended to treat air–sea fluxes in high winds.

scholarly journals Generation of free convection due to changes of the local circulation system

2009 ◽  
Vol 9 (21) ◽  
pp. 8587-8600 ◽  
Author(s):  
R. Eigenmann ◽  
S. Metzger ◽  
T. Foken
Keyword(s):  
Wind Speed ◽  
Free Convection ◽  
Large Scale ◽  
Sonic Anemometer ◽  
Measurement Data ◽  
Surface Fluxes ◽  
Circulation System ◽  
Local Circulation ◽  
Valley Wind ◽  
Obukhov Length

Abstract. Eddy-covariance and Sodar/RASS experimental measurement data of the COPS (Convective and Orographically-induced Precipitation Study) field campaign 2007 are used to investigate the generation of near-ground free convection conditions (FCCs) in the Kinzig valley, Black Forest, Southwest Germany. The measured high-quality turbulent flux data revealed that FCCs are initiated near the ground in situations where moderate to high buoyancy fluxes and a simultaneously occurring drop of the wind speed were present. The minimum in wind speed – observable by the Sodar measurements through the whole vertical extension of the valley atmosphere – is the consequence of a thermally-induced valley wind system, which changes its wind direction from down to up-valley winds in the morning hours. Buoyancy then dominates over shear within the production of turbulence kinetic energy near the ground. These situations are detected by the stability parameter (ratio of the measurement height to the Obukhov length) when the level of free convection, which starts above the Obukhov length, drops below that of the sonic anemometer. An analysis of the scales of turbulent motions during FCCs using wavelet transform shows the occurrence of large-scale turbulence structures. Regarding the entire COPS measurement period, FCCs in the morning hours occur on about 50% of all days. Enhanced surface fluxes of latent and sensible heat are found on these days.

Impacts of Air–Sea Flux Parameterizations on the Intensity and Structure of Tropical Cyclones

2013 ◽  
Vol 141 (7) ◽  
pp. 2308-2324 ◽  
Author(s):  
Benjamin W. Green ◽  
Fuqing Zhang
Keyword(s):  
Wind Speed ◽  
Tropical Cyclone ◽  
Tropical Cyclones ◽  
Latent Heat ◽  
Weather Prediction ◽  
Surface Flux ◽  
Surface Fluxes ◽  
Sensible Heat ◽  
Near Surface ◽  
Flux Parameterization

Abstract Fluxes of momentum and moist enthalpy across the air–sea interface are believed to be one of the most important factors in determining tropical cyclone intensity. Because these surface fluxes cannot be directly resolved by numerical weather prediction models, their impacts on tropical cyclones must be accounted for through subgrid-scale parameterizations. There are several air–sea surface flux parameterization schemes available in the Weather Research and Forecasting (WRF) Model; these schemes differ from one another in their formulations of the wind speed–dependent exchange coefficients of momentum, sensible heat, and moisture (latent heat). The effects of surface fluxes on the intensity and structure of tropical cyclones are examined through convection-permitting WRF simulations of Hurricane Katrina (2005). It is found that the intensity (and, to a lesser extent, structure) of the simulated storms is sensitive to the choice of surface flux parameterization scheme. In agreement with recent studies, the drag coefficient CD is found to affect the pressure–wind relationship (between minimum sea level pressure and maximum 10-m wind speed) and to change the radius of maximum near-surface winds of the tropical cyclone. Fluxes of sensible and latent heat (i.e., moist enthalpy) affect intensity but do not significantly change the pressure–wind relationship. Additionally, when low-level winds are strong, the contribution of dissipative heating to calculations of sensible heat flux is not negligible. Expanding the sensitivity tests to several dozen cases from the 2008 to 2011 Atlantic hurricane seasons demonstrates the robustness of these findings.

scholarly journals Moist Convection and the Thermal Stratification of the Extratropical Troposphere

2008 ◽  
Vol 65 (11) ◽  
pp. 3571-3583 ◽  
Author(s):  
Tapio Schneider ◽  
Paul A. O’Gorman
Keyword(s):  
Latent Heat ◽  
Thermal Stratification ◽  
General Circulation ◽  
Large Scale ◽  
Circulation Model ◽  
Lapse Rate ◽  
Moist Convection ◽  
Wide Range ◽  
Simulation Results ◽  
Warm Climates

Abstract Simulations with an aquaplanet general circulation model show that sensible and latent heat transport by large-scale eddies influences the extratropical thermal stratification over a wide range of climates, even in relatively warm climates with small meridional surface temperature gradients. Variations of the lapse rate toward which the parameterized moist convection in the model relaxes atmospheric temperature profiles demonstrate that the convective lapse rate only marginally affects the extratropical thermal stratification in Earth-like and colder climates. In warmer climates, the convective lapse rate does affect the extratropical thermal stratification, but the effect is still smaller than would be expected if moist convection alone controlled the thermal stratification. A theory for how large-scale eddies modify the thermal stratification of dry atmospheres is consistent with the simulation results for colder climates. For warmer and moister climates, however, theories and heuristics that have been proposed to account for the extratropical thermal stratification are not consistent with the simulation results. Theories for the extratropical thermal stratification will generally have to take transport of sensible and latent heat by large-scale eddies into account, but moist convection may only need to be taken into account regionally and in sufficiently warm climates.

scholarly journals Observational Evidence for Relationships between the Degree of Aggregation of Deep Convection, Water Vapor, Surface Fluxes, and Radiation

2012 ◽  
Vol 25 (20) ◽  
pp. 6885-6904 ◽  
Author(s):  
Isabelle Tobin ◽  
Sandrine Bony ◽  
Remy Roca
Keyword(s):  
Water Vapor ◽  
Large Scale ◽  
Spatial Organization ◽  
Numerical Models ◽  
Deep Convection ◽  
Surface Fluxes ◽  
Radiative Heating ◽  
Shortwave Radiation ◽  
Wide Range ◽  
Atmospheric State

Abstract Tropical deep convection exhibits complex organization over a wide range of scales. This study investigates the relationships between the spatial organization of deep convection and the large-scale atmospheric state. By using several satellite datasets and reanalyses, and by defining a simple diagnostic of convective aggregation, relationships between the degree of convective aggregation and the amount of water vapor, turbulent surface fluxes, and radiation are highlighted above tropical oceans. When deep convection is more aggregated, the middle and upper troposphere are drier in the convection-free environment, turbulent surface fluxes are enhanced, and the low-level and midlevel cloudiness is reduced in the environment. Humidity and cloudiness changes lead to a large increase in outgoing longwave radiation. Cloud changes also result in reduced reflected shortwave radiation. Owing to these opposing effects, the sensitivity of the radiative budget at the top of the atmosphere to convective aggregation turns out to be weak, but the distribution of radiative heating throughout the troposphere is affected. These results suggest that feedbacks between convective aggregation and the large-scale atmospheric state might influence large-scale dynamics and the transports of water and energy and, thus, play a role in the climate variability and change. These observational findings are qualitatively consistent with previous cloud-resolving model results, except for the effects on cloudiness and reflected shortwave radiation. The proposed methodology may be useful for assessing the representation of convective aggregation and its interaction with the large-scale atmospheric state in various numerical models.

scholarly journals Hybrid Pitch Angle Controller Approaches for Stable Wind Turbine Power under Variable Wind Speed

Energies ◽  
2020 ◽  
Vol 13 (14) ◽  
pp. 3622 ◽  
Author(s):  
Md Rasel Sarkar ◽  
Sabariah Julai ◽  
Chong Wen Tong ◽  
Moslem Uddin ◽  
M.F. Romlie ◽  
...  
Keyword(s):  
Wind Speed ◽  
Wind Turbine ◽  
Output Power ◽  
Pitch Angle ◽  
Pid Controller ◽  
Large Scale ◽  
Wind Speeds ◽  
Wide Range ◽  
Fast Wind ◽  
Simple Implementation

The production of maximum wind energy requires controlling various parts of medium to large-scale wind turbines (WTs). This paper presents a robust pitch angle control system for the rated wind turbine power at a wide range of simulated wind speeds by means of a proportional–integral–derivative (PID) controller. In addition, ant colony optimization (ACO), particle swarm optimization (PSO), and classical Ziegler–Nichols (Z-N) algorithms have been used for tuning the PID controller parameters to obtain within rated stable output power of WTs from fluctuating wind speeds. The proposed system is simulated under fast wind speed variation, and its results are compared with those of the PID-ZN controller and PID-PSO to verify its effeteness. The proposed approach contains several benefits including simple implementation, as well as tolerance of turbine parameters and several nonparametric uncertainties. Robust control of the generator output power with wind-speed variations can also be considered a significant advantage of this strategy. Theoretical analyses, as well as simulation results, indicate that the proposed controller can perform better in a wide range of wind speed compared with the PID-ZN and PID-PSO controllers. The WT model and hybrid controllers (PID-ACO and PID-PSO) have been developed in MATLAB/Simulink with validated controller models. The hybrid PID-ACO controller was found to be the most suitable in comparison to the PID-PSO and conventional PID. The root mean square (RMS) error calculated between the desired power and the WT’s output power with PID-ACO is found to be 0.00036, which is the smallest result among the studied controllers.

Large-Eddy Simulation Study of Log Laws in a Neutral Ekman Boundary Layer

2018 ◽  
Vol 75 (6) ◽  
pp. 1873-1889 ◽  
Author(s):  
Qingfang Jiang ◽  
Shouping Wang ◽  
Peter Sullivan
Keyword(s):  
Boundary Layer ◽  
Wind Speed ◽  
Large Scale ◽  
Geostrophic Wind ◽  
Coriolis Parameter ◽  
Ekman Boundary Layer ◽  
Wide Range ◽  
Large Eddy ◽  
Log Law ◽  
Flux Layer

Abstract The characteristics of wind profiles in a neutral atmospheric boundary layer and their dependence on the geostrophic wind speed Ug, Coriolis parameter f, and surface roughness length z0 are examined utilizing large-eddy simulations. These simulations produce a constant momentum flux layer and a log-law layer above the surface characterized by a logarithmic increase of wind speed with height. The von Kármán constant derived from the mean wind profile is around 0.4 over a wide range of control parameters. The depths of the simulated boundary layer, constant-flux layer, and surface log-law layer tend to increase with the wind speed and decrease with an increasing Coriolis parameter. Immediately above the surface log-law layer, a second log-law layer has been identified from these simulations. The depth of this upper log-law layer is comparable to its counterpart in the surface layer, and the wind speed can be scaled as , as opposed to just in the surface log-law layer, implying that in addition to surface processes, the upper log-law layer is also influenced by Earth’s rotation and large-scale conditions. Here is the friction velocity at the surface, and h is the boundary layer depth. An analytical model is proposed to assist in the interpretation of the log laws in a typical Ekman boundary layer. The physics and implications of the upper log-law layer are discussed.

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