scholarly journals Convective self–aggregation in a mean flow

2021 ◽  
Vol 21 (13) ◽  
pp. 10337-10345
Author(s):  
Hyunju Jung ◽  
Ann Kristin Naumann ◽  
Bjorn Stevens
Keyword(s):  
Momentum Flux ◽  
Surface Flux ◽  
Surface Heat ◽  
Horizontal Wind ◽  
Convective System ◽  
Mean Flow ◽  
Near Surface ◽  
Surface Heat Exchange ◽  
The Mean ◽  
Self Aggregation

Abstract. Convective self-aggregation is an atmospheric phenomenon seen in numerical simulations in a radiative convective equilibrium framework thought to be informative of some aspects of the behavior of real-world convection in the deep tropics. We impose a background mean wind flow on convection-permitting simulations through the surface flux calculation in an effort to understand how the asymmetry imposed by a mean wind influences the propagation of aggregated structures in convection. The simulations show that, with imposing mean flow, the organized convective system propagates in the direction of the flow but slows down compared to what pure advection would suggest, and it eventually becomes stationary relative to the surface after 15 simulation days. The termination of the propagation arises from momentum flux, which acts as a drag on the near-surface horizontal wind. In contrast, the thermodynamic response through the wind-induced surface heat exchange feedback is a relatively small effect, which slightly retards the propagation of the convection relative to the mean wind.

scholarly journals Convective self–aggregation in a mean flow

2020 ◽  
Author(s):  
Hyunju Jung ◽  
Ann Kristin Naumann ◽  
Bjorn Stevens
Keyword(s):  
Large Scale ◽  
Deep Convection ◽  
Surface Flux ◽  
Horizontal Wind ◽  
Convective System ◽  
Mean Flow ◽  
Near Surface ◽  
Tropical Deep Convection ◽  
Large Scale Flow ◽  
Self Aggregation

Abstract. Convective self-aggregation is an atmospheric phenomenon found in numerical simulations in a radiative convective equilibrium framework of which configuration captures the main characteristics of the real-world convection in the deep tropics. As tropical deep convection is typically embedded in a large-scale flow, we impose a background mean wind flow on convection-permitting simulations through the surface flux calculation. The simulations show that with imposing mean flow, the organized convective system propagates in the direction of the flow but slows down compared to what pure advection would suggest, and eventually becomes stationary relative to the surface after 15 simulation days. The termination of the propagation arises from momentum flux, which acts as a drag on the near-surface horizontal wind. In contrast, the thermodynamic response through the wind-induced surface heat exchange feedback is a relatively small effect, which slightly retards (by about 15 %) the convection relative to the mean wind.

scholarly journals On strongly nonlinear gravity waves in a vertically sheared atmosphere

2020 ◽  
Vol 6 (1) ◽  
pp. 63-74
Author(s):  
Mark Schlutow ◽  
Georg S. Voelker
Keyword(s):  
Gravity Waves ◽  
Lower Layer ◽  
Vertical Shear ◽  
Horizontal Wind ◽  
Necessary Condition ◽  
Mean Flow ◽  
Individual Layer ◽  
Strongly Nonlinear ◽  
The Mean ◽  
The Stability

Abstract We investigate strongly nonlinear stationary gravity waves which experience refraction due to a thin vertical shear layer of horizontal background wind. The velocity amplitude of the waves is of the same order of magnitude as the background flow and hence the self-induced mean flow alters the modulation properties to leading order. In this theoretical study, we show that the stability of such a refracted wave depends on the classical modulation stability criterion for each individual layer, above and below the shearing. Additionally, the stability is conditioned by novel instability criteria providing bounds on the mean-flow horizontal wind and the amplitude of the wave. A necessary condition for instability is that the mean-flow horizontal wind in the upper layer is stronger than the wind in the lower layer.

Field measurement of near-surface typhoon characteristics using a smart monitoring system on a long-span arch bridge site

2019 ◽  
Vol 22 (8) ◽  
pp. 1977-1987
Author(s):  
Xu Wang ◽  
Huaqiang Li ◽  
Zengshun Chen ◽  
Yuanhao Qian ◽  
Yanru Wang ◽  
...  
Keyword(s):  
Monitoring System ◽  
Wind Velocity ◽  
Turbulence Intensity ◽  
Eastern China ◽  
Mean Flow ◽  
Time Duration ◽  
Lateral Direction ◽  
Near Surface ◽  
Gust Factor ◽  
The Mean

During landfall of Typhoon Haikui in Eastern China in 2012, ground level wind data were recorded using a smart monitoring system installed on JiuBao Bridge in Hangzhou, China. This article documents the mean flow and turbulence characteristics from data recorded during the storm. The results show that both turbulence intensity and gust factor decrease with the increase in the mean wind velocity. However, as the mean wind velocity increases, this trend gradually attenuates. The peak factor distribution with gust averaging time duration derived with the Typhoon Haikui data agrees well with the Durst curve. However, the longitudinal gust factor derived from the typhoon wind-speed record in this study is higher compared with the curves proposed by Durst and Krayer-Marshall. Analyses of the gust factor distribution with the turbulence intensity during the passage of the storm reveal a similarity to the empirical curves of Ishizaki and Choi. Results show that the relationship between lateral turbulence and gust factors can be well represented by a quadratic polynomial. Turbulence scale increased with mean wind velocity. The values of autocorrelation coefficients in longitudinal direction are larger than those in lateral direction. There has no obvious dependency of cross-correlation coefficients with mean wind velocity. In general, the wind characteristics in this study are shown to be very similar to those of winds under normal circumstance.

scholarly journals Near-Surface Transport Properties and Lagrangian Statistics during Two Contrasting Years in the Adriatic Sea

2020 ◽  
Vol 8 (9) ◽  
pp. 681
Author(s):  
Saeed Hariri
Keyword(s):  
Transport Properties ◽  
General Circulation Model ◽  
General Circulation ◽  
Circulation Model ◽  
Mean Flow ◽  
Near Surface ◽  
Surface Transport ◽  
The Real ◽  
Lagrangian Statistics ◽  
The Mean

This paper describes the near-surface transport properties and Lagrangian statistics in the Adriatic semi-enclosed basin using synthetic drifters. Lagrangian transport models were used to simulate synthetic trajectories from the mean flow fields obtained by the Massachusetts Institute of Technology general circulation model (MITgcm), implemented in the Adriatic from October 2006 until December 2008. In particular, the surface circulation properties in two contrasting years (2007 had a mild winter and cold fall, while 2008 had a normal winter and hot summer) are compared here. In addition, the Lagrangian statistics for the entire Adriatic Basin after removing the Eulerian mean circulation for numerical particles were calculated. The results indicate that the numerical particles were slower in this simulation when compared with the real drifters. This is because of the reduced energetic flow field generated by the MIT general circulation model during the selected years. The numerical results showed that the balanced effects of the wind-driven recirculation in the northernmost area(which would be a sea response to the Bora wind field) and the Po River discharge cause the residence times to be similar during the two selected years (182 and 185 days in 2007 and 2008, respectively). Furthermore, the mean angular momentum, diffusivity, and Lagrangian velocity covariance values are smaller than in the real drifter observations, while the maximum Lagrangian integral time scale is the same.

Application of the IDEMIX Concept for Internal Gravity Waves in the Atmosphere

2020 ◽  
Vol 77 (10) ◽  
pp. 3601-3618
Author(s):  
B. Quinn ◽  
C. Eden ◽  
D. Olbers
Keyword(s):  
Energy Balance ◽  
Wave Field ◽  
Gravity Wave ◽  
Gravity Waves ◽  
Momentum Flux ◽  
Middle Atmosphere ◽  
Internal Gravity Waves ◽  
Wave Drag ◽  
Mean Flow ◽  
The Mean

AbstractThe model Internal Wave Dissipation, Energy and Mixing (IDEMIX) presents a novel way of parameterizing internal gravity waves in the atmosphere. IDEMIX is based on the spectral energy balance of the wave field and has previously been successfully developed as a model for diapycnal diffusivity, induced by internal gravity wave breaking in oceans. Applied here for the first time to atmospheric gravity waves, integration of the energy balance equation for a continuous wave field of a given spectrum, results in prognostic equations for the energy density of eastward and westward gravity waves. It includes their interaction with the mean flow, allowing for an evolving and local description of momentum flux and gravity wave drag. A saturation mechanism maintains the wave field within convective stability limits, and a closure for critical-layer effects controls how much wave flux propagates from the troposphere into the middle atmosphere. Offline comparisons to a traditional parameterization reveal increases in the wave momentum flux in the middle atmosphere due to the mean-flow interaction, resulting in a greater gravity wave drag at lower altitudes. Preliminary validation against observational data show good agreement with momentum fluxes.

scholarly journals Seasonal Variations of High-Frequency Gravity Wave Momentum Fluxes and Their Forcing toward Zonal Winds in the Mesosphere and Lower Thermosphere over Langfang, China (39.4° N, 116.7° E)

Atmosphere ◽  
2020 ◽  
Vol 11 (11) ◽  
pp. 1253
Author(s):  
Caixia Tian ◽  
Xiong Hu ◽  
Yurong Liu ◽  
Xuan Cheng ◽  
Zhaoai Yan ◽  
...  
Keyword(s):  
Seasonal Variations ◽  
High Frequency ◽  
Momentum Flux ◽  
Lower Thermosphere ◽  
Mean Flow ◽  
Zonal Winds ◽  
Short Period ◽  
Momentum Fluxes ◽  
The Mean ◽  
Mesosphere And Lower Thermosphere

Meteor radar data collected over Langfang, China (39.4° N, 116.7° E) were used to estimate the momentum flux of short-period (less than 2 h) gravity waves (GWs) in the mesosphere and lower thermosphere (MLT), using the Hocking (2005) analysis technique. Seasonal variations in GW momentum flux exhibited annual oscillation (AO), semiannual oscillation (SAO), and quasi-4-month oscillation. Quantitative estimations of GW forcing toward the mean zonal flow were provided using the determined GW momentum flux. The mean flow acceleration estimated from the divergence of this flux was compared with the observed acceleration of zonal winds displaying SAO and quasi-4-month oscillations. These comparisons were used to analyze the contribution of zonal momentum fluxes of SAO and quasi-4-month oscillations to zonal winds. The estimated acceleration from high-frequency GWs was in the same direction as the observed acceleration of zonal winds for quasi-4-month oscillation winds, with GWs contributing more than 69%. The estimated acceleration due to Coriolis forces to the zonal wind was studied; the findings were opposite to the estimated acceleration of high-frequency GWs for quasi-4-month oscillation winds. The significance of this study lies in estimating and quantifying the contribution of the GW momentum fluxes to zonal winds with quasi-4-month periods over mid-latitude regions for the first time.

Near-surface particle image velocimetry measurements in a transitionally rough-wall atmospheric boundary layer

2007 ◽  
Vol 580 ◽  
pp. 319-338 ◽  
Author(s):  
SCOTT C. MORRIS ◽  
SCOTT R. STOLPA ◽  
PAUL E. SLABOCH ◽  
JOSEPH C. KLEWICKI
Keyword(s):  
Reynolds Number ◽  
Particle Image Velocimetry ◽  
Environmental Science ◽  
Particle Image ◽  
Shear Velocity ◽  
Mean Flow ◽  
Mean Velocity ◽  
Near Surface ◽  
Image Velocimetry ◽  
The Mean

The Reynolds number dependence of the structure and statistics of wall-layer turbulence remains an open topic of research. This issue is considered in the present work using two-component planar particle image velocimetry (PIV) measurements acquired at the Surface Layer Turbulence and Environmental Science Test (SLTEST) facility in western Utah. The Reynolds number (δuτ/ν) was of the order 106. The surface was flat with an equivalent sand grain roughness k+ = 18. The domain of the measurements was 500 < yuτ/ν < 3000 in viscous units, 0.00081 < y/δ < 0.005 in outer units, with a streamwise extent of 6000ν/uτ. The mean velocity was fitted by a logarithmic equation with a von Kármán constant of 0.41. The profile of u′v′ indicated that the entire measurement domain was within a region of essentially constant stress, from which the wall shear velocity was estimated. The stochastic measurements discussed include mean and RMS profiles as well as two-point velocity correlations. Examination of the instantaneous vector maps indicated that approximately 60% of the realizations could be characterized as having a nearly uniform velocity. The remaining 40% of the images indicated two regions of nearly uniform momentum separated by a thin region of high shear. This shear layer was typically found to be inclined to the mean flow, with an average positive angle of 14.9°.

PIV-POD Investigation of the Wake of a Sharp-Edged Flat Bluff Body Immersed in a Shallow Channel Flow

2009 ◽  
Vol 131 (2) ◽  
Author(s):  
Arindam Singha ◽  
A.-M. Shinneeb ◽  
Ram Balachandar
Keyword(s):  
Velocity Field ◽  
Channel Flow ◽  
Bluff Body ◽  
Maximum Flow ◽  
Velocity Fields ◽  
Central Plane ◽  
Mean Flow ◽  
Mean Velocity ◽  
Near Surface ◽  
The Mean

This paper reports particle-image velocimetry measurements of instantaneous velocity fields in the wake of a sharp-edged bluff body immersed vertically in a shallow smooth open channel flow. The maximum flow velocity was 0.19 m/s and the Reynolds number based on the water depth was 18,270. The purpose of the present study is to show the vertical variation of the velocity field in the near region of a shallow wake. Measurements of the flow field in the vertical central plane and in the horizontal near-bed, mid-depth, and near-surface planes were taken. Then, the mean flow quantities such as the mean velocity, turbulence intensity, and Reynolds stress fields were investigated. In addition, the proper orthogonal decomposition technique was used to reconstruct the velocity fields to investigate the energetic vortical structures. The results showed that the largest recirculation zone in the mean velocity fields occurred in the mid-depth velocity field, while the smallest one occurred near the bed. Also, the fluid was entrained from the sides toward the wake central plane in the three horizontal velocity fields but with different rates. This behavior was attributed to the existence of quasi-streamwise vortices near the boundaries. In addition, patterns of ejection and sweep events near the free surface similar to the features commonly observed near the wall-bounded flows were observed.

scholarly journals Sensitivity of Tornado Dynamics to Soil Debris Loading

2016 ◽  
Vol 73 (7) ◽  
pp. 2783-2801 ◽  
Author(s):  
David J. Bodine ◽  
Takashi Maruyama ◽  
Robert D. Palmer ◽  
Caleb J. Fulton ◽  
Howard B. Bluestein ◽  
...  
Keyword(s):  
Surface Wind ◽  
Horizontal Wind ◽  
Initial Surface ◽  
Mean Flow ◽  
Single Vortex ◽  
Specific Flow ◽  
Substantial Impact ◽  
Near Surface ◽  
Wind Speeds ◽  
Loading Effects

Abstract Past numerical simulation studies found that debris loading from sand-sized particles may substantially affect tornado dynamics, causing reductions in near-surface wind speeds up to 50%. To further examine debris loading effects, simulations are performed using a large-eddy simulation model with a two-way drag force coupling between air and sand. Simulations encompass a large range of surface debris fluxes that cause negligible to substantial impact on tornado dynamics for a high-swirl tornado vortex simulation. Simulations are considered for a specific case with a single vortex flow type (swirl ratio, intensity, and translation velocity) and a fixed set of debris and aerodynamic parameters. Thus, it is stressed that these findings apply to the specific flow and debris parameters herein and would likely vary for different flows or debris parameters. For this specific case, initial surface debris fluxes are varied over a factor of 16 384, and debris cloud mass varies by only 42% of this range because a negative feedback reduces near-surface horizontal velocities. Debris loading effects on the axisymmetric mean flow are evident when maximum debris loading exceeds 0.1 kg kg−1, but instantaneous maximum wind speed and TKE exhibit small changes at smaller debris loadings (greater than 0.01 kg kg−1). Initially, wind speeds are reduced in a shallow, near-surface layer, but the magnitude and depth of these changes increases with higher debris loading. At high debris loading, near-surface horizontal wind speeds are reduced by 30%–60% in the lowest 10 m AGL. In moderate and high debris loading scenarios, the number and intensity of subvortices also decrease close to the surface.

scholarly journals Relationship between horizontal wind velocity and normalized surface cross section using data from the HIWRAP dual-frequency airborne radar

2020 ◽  
Author(s):  
R. Meneghini ◽  
L. Liao ◽  
G.M. Heymsfield
Keyword(s):  
Cross Section ◽  
Transfer Functions ◽  
Correlation Coefficients ◽  
Horizontal Wind ◽  
Airborne Radar ◽  
Wind Vector ◽  
Dual Frequency ◽  
Near Surface ◽  
The Mean ◽  
Ku Band

AbstractThe HIWRAP dual-frequency conically-scanning airborne radar provides estimates of the range-profiled mean Doppler and backscattered power from the precipitation and surface. A VAD (velocity azimuth display) analysis yields near-surface estimates of the mean horizontal wind vector, vh, in cases where precipitation is present throughout the scan. From the surface return, the normalized radar cross section (NRCS) is obtained which, by a method previously described, can be corrected for path attenuation.Comparisons between vh and the attenuation-corrected NRCS are used to derive transfer functions that provide estimates of the wind vector from the NRCS data under both rain and rain-free conditions. A reasonably robust transfer function is found by using the mean NRCS, 〈NRCS〉, over the scan along with a filtering of the data based on a Fourier series analysis of vh and the NRCS.The approach gives good correlation coefficients between vh and 〈NRCS〉 at Ku-band at incidence angles of 300 and 400. The correlation degrades if the Ka-band data are used rather that the Ku-band.

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