scholarly journals An Observational Case for the Prevalence of Roll Vortices in the Hurricane Boundary Layer*

2005 ◽  
Vol 62 (8) ◽  
pp. 2662-2673 ◽  
Author(s):  
Ian Morrison ◽  
Steven Businger ◽  
Frank Marks ◽  
Peter Dodge ◽  
Joost A. Businger
Keyword(s):  
Boundary Layer ◽  
Prediction Models ◽  
Weather Prediction ◽  
Doppler Velocity ◽  
Velocity Data ◽  
Roll Vortices ◽  
Secondary Circulations ◽  
Velocity Anomalies ◽  
The Mean ◽  
Hurricane Boundary Layer

Abstract Doppler velocity data from Weather Surveillance Radar-1988 Doppler (WSR-88D) radars during four hurricane landfalls are analyzed to investigate the presence of organized vortices in the hurricane boundary layer (HBL). The wavelength, depth, magnitude, and track of velocity anomalies were compiled through analysis of Doppler velocity data. The analysis reveals alternating bands of enhanced and reduced azimuthal winds closely aligned with the mean wind direction. Resulting statistics provide compelling evidence for the presence of organized secondary circulations or boundary layer rolls across significant areas during four hurricane landfalls. The results confirm previous observations of the presence of rolls in the HBL. A potential limitation of the study presented here is the resolution of the WSR-88D data. In particular, analysis of higher-resolution data (e.g., from the Doppler on Wheels) is needed to confirm that data aliasing has not unduly impacted the statistics reported here. Momentum fluxes associated with the secondary circulations are estimated using the covariance between the horizontal and vertical components of the wind fluctuations in rolls, with resulting fluxes 2–3 times greater than estimated by parameterizations in numerical weather prediction models. The observational analysis presented here, showing a prevalence of roll vortices in the HBL, has significant implications for the vertical transport of energy in hurricanes, for the character of wind damage, and for improvements in numerical simulations of hurricanes.

scholarly journals On the Generation of Roll Vortices due to the Inflection Point Instability of the Hurricane Boundary Layer Flow

2014 ◽  
Vol 71 (11) ◽  
pp. 4292-4307 ◽  
Author(s):  
Kun Gao ◽  
Isaac Ginis
Keyword(s):  
Boundary Layer ◽  
Growth Rate ◽  
Internal Waves ◽  
Mixed Layer ◽  
Inflection Point ◽  
Numerical Approach ◽  
Stable Stratification ◽  
Roll Vortices ◽  
The Mean ◽  
Hurricane Boundary Layer

Abstract Horizontal roll vortices, or rolls, are frequently observed in the hurricane boundary layer (HBL). Previous studies suggest that these rolls can be generated by the inflection point instability of the HBL flow. In this study we investigate the formation of rolls due to this mechanism in the axisymmetric HBL using a numerical approach that explicitly resolves rolls. The effects of mean HBL wind and stratification distributions on rolls are evaluated. We identify two important factors of the mean HBL wind that affect the characteristics of rolls. The dynamical HBL height affects the wavelength of rolls, and the magnitude of the mean wind shear affects the growth rate of rolls. As a result, under neutrally stratified HBL, the wavelength of rolls increases with the radius (out of the radius of maximum wind), while the growth rate of rolls decreases. The stratification also plays an important role in the generation of rolls. The stable stratification suppresses the growth of rolls because of the negative work done by the buoyancy force. Nonuniform stratification with a mixed layer has less suppressing effect on rolls. Rolls can trigger internal waves in the stably stratified layer, which have both vertically propagating and decaying properties. We derive analytical solutions for the internal waves, which relate the properties of the internal waves to the boundary layer rolls. We find the properties of the internal waves are affected by the mixed-layer height.

scholarly journals Why Rolls are Prevalent in the Hurricane Boundary Layer

2005 ◽  
Vol 62 (8) ◽  
pp. 2647-2661 ◽  
Author(s):  
Ralph C. Foster
Keyword(s):  
Boundary Layer ◽  
Boundary Layer Flow ◽  
Large Fraction ◽  
Main Body ◽  
Roll Vortices ◽  
Starting Point ◽  
The Mean ◽  
Linear And Nonlinear ◽  
Layer Flow ◽  
Hurricane Boundary Layer

Abstract Recent remote sensing observations show that the hurricane boundary layer flow, although energetic, is not a region of homogeneous turbulence. In fact, the observations convincingly demonstrate that a large fraction of the turbulent flow in the regions away from the deep convective rainbands is highly organized into intense horizontal roll vortices that are approximately aligned with the mean wind and span the depth of the boundary layer. These observations show that rolls strongly increase the flux of momentum between the underlying surface and the main body of the storm compared to an equivalent hurricane boundary layer flow without rolls. The linear and nonlinear dynamics of hurricane boundary layer roll formation are outlined and it is shown why rolls are, in fact, the expected basic hurricane boundary layer state. The model presented here explains the hurricane roll features currently documented in field programs and makes predictions that can be tested in future experiments. The primary effects of rolls on the boundary layer fluxes are inherently nonlocal and nongradient and hence cannot be captured by standard downgradient turbulence parameterizations used in hurricane simulations. However, the nonlinear theory is the proper starting point for developing boundary layer parameterizations that include roll modification of the turbulent fluxes.

scholarly journals On the Equilibrium-State Roll Vortices and Their Effects in the Hurricane Boundary Layer

2016 ◽  
Vol 73 (3) ◽  
pp. 1205-1222 ◽  
Author(s):  
Kun Gao ◽  
Isaac Ginis
Keyword(s):  
Boundary Layer ◽  
Wind Shear ◽  
Momentum Flux ◽  
Wind Profile ◽  
Layer Height ◽  
Roll Vortices ◽  
The Mean ◽  
The Cross ◽  
Hurricane Boundary Layer ◽  
The Impact

Abstract In this study, the authors numerically simulate roll vortices (rolls) generated by the inflection-point instability in the hurricane boundary layer (HBL). The approach is based on embedding a two-dimensional high-resolution single-grid roll-resolving model (SRM) at selected horizontal grid points of an axisymmetric HBL model. The results from a set of idealized experiments indicate that the mixed-layer height is an important factor affecting the magnitude of the roll velocities and the structure of the internal waves triggered in the stably stratified layer above. This study reveals the important difference between the roll-induced cross-roll (nearly radial) and along-roll (nearly azimuthal) momentum fluxes: while the cross-roll momentum flux is well correlated to the cross-roll mean wind shear, the along-roll momentum flux is typically not correlated with the along-roll mean wind shear. Therefore, the commonly used K theory in the boundary layer parameterizations cannot reasonably capture the vertical distribution of the roll-induced along-roll momentum flux. Moreover, the authors find that the rolls induce more significant changes in the mean radial wind profile than in the mean azimuthal wind profile. Specifically, rolls reduce the inflow near surface, enhance the inflow at upper levels, and increase the inflow-layer height. Based on a linear dynamical HBL model, the authors find that the impact of rolls on the mean radial wind profile is essentially due to their redistribution effect on the mean azimuthal momentum in the HBL.

Estimation of the Minimum Canopy Resistance for Croplands and Grasslands Using Data from the 2002 International H2O Project

2008 ◽  
Vol 136 (11) ◽  
pp. 4452-4469 ◽  
Author(s):  
Joseph G. Alfieri ◽  
Dev Niyogi ◽  
Peter D. Blanken ◽  
Fei Chen ◽  
Margaret A. LeMone ◽  
...  
Keyword(s):  
Land Surface ◽  
Prediction Models ◽  
Weather Prediction ◽  
Moisture Flux ◽  
Mean Value ◽  
Surface Model ◽  
Canopy Resistance ◽  
Land Surface Models ◽  
Surface Models ◽  
The Mean

Abstract Vegetated surfaces, such as grasslands and croplands, constitute a significant portion of the earth’s surface and play an important role in land–atmosphere exchange processes. This study focuses on one important parameter used in describing the exchange of moisture from vegetated surfaces: the minimum canopy resistance (rcmin). This parameter is used in the Jarvis canopy resistance scheme that is incorporated into the Noah and many other land surface models. By using an inverted form of the Jarvis scheme, rcmin is determined from observational data collected during the 2002 International H2O Project (IHOP_2002). The results indicate that rcmin is highly variable both site to site and over diurnal and longer time scales. The mean value at the grassland sites in this study is 96 s m−1 while the mean value for the cropland (winter wheat) sites is one-fourth that value at 24 s m−1. The mean rcmin for all the sites is 72 s m−1 with a standard deviation of 39 s m−1. This variability is due to both the empirical nature of the Jarvis scheme and a combination of changing environmental conditions, such as plant physiology and plant species composition, that are not explicitly considered by the scheme. This variability in rcmin has important implications for land surface modeling where rcmin is often parameterized as a constant. For example, the Noah land surface model parameterizes rcmin for the grasslands and croplands types in this study as 40 s m−1. Tests with the coupled Weather Research and Forecasting (WRF)–Noah model indicate that the using the modified values of rcmin from this study improves the estimates of latent heat flux; the difference between the observed and modeled moisture flux decreased by 50% or more. While land surface models that estimate transpiration using Jarvis-type relationships may be improved by revising the rcmin values for grasslands and croplands, updating the rcmin will not fully account for the variability in rcmin observed in this study. As such, it may be necessary to replace the Jarvis scheme currently used in many land surface and numerical weather prediction models with a physiologically based estimate of the canopy resistance.

scholarly journals Seamless Stratocumulus Simulation across the Turbulent Gray Zone

2014 ◽  
Vol 142 (4) ◽  
pp. 1655-1668 ◽  
Author(s):  
I. A. Boutle ◽  
J. E. J. Eyre ◽  
A. P. Lock
Keyword(s):  
Boundary Layer ◽  
Prediction Models ◽  
Weather Prediction ◽  
Eddy Simulation ◽  
Large Eddy ◽  
Aircraft Observations ◽  
Microphysical Parameterization ◽  
Grid Length ◽  
Numerical Weather Prediction Models

Abstract A pragmatic approach for representing partially resolved turbulence in numerical weather prediction models is introduced and tested. The method blends a conventional boundary layer parameterization, suitable for large grid lengths, with a subgrid turbulence scheme suitable for large-eddy simulation. The key parameter for blending the schemes is the ratio of grid length to boundary layer depth. The new parameterization is combined with a scale-aware microphysical parameterization and tested on a case study forecast of stratocumulus evolution. Simulations at a range of model grid lengths between 1 km and 100 m are compared to aircraft observations. The improved microphysical representation removes the correlation between precipitation rate and model grid length, while the new turbulence parameterization improves the transition from unresolved to resolved turbulence as grid length is reduced.

scholarly journals On the Characteristics of Linear-Phase Roll Vortices under a Moving Hurricane Boundary Layer

2018 ◽  
Vol 75 (8) ◽  
pp. 2589-2598 ◽  
Author(s):  
Kun Gao ◽  
Isaac Ginis
Keyword(s):  
Boundary Layer ◽  
Growth Rate ◽  
Shear Layer ◽  
Wind Shear ◽  
Horizontal Distribution ◽  
Linear Phase ◽  
Layer Depth ◽  
Bottom Boundary ◽  
Roll Vortices ◽  
Hurricane Boundary Layer

Abstract Previous theoretical and numerical studies only focused on the formation of roll vortices (rolls) under a stationary and axisymmetric hurricane. The effect of the asymmetric wind structure induced by the storm movement on the roll characteristics remains unknown. In this study, we present the first attempt to investigate the characteristics of linear-phase rolls under a moving hurricane by embedding a linear two-dimensional (2D) roll-resolving model into a 3D hurricane boundary layer model. It is found that the roll horizontal wavelength under the moving hurricane is largely determined by the radial-shear-layer depth, defined as the thickness of the layer with positive radial wind shear. The horizontal distribution of the roll wavelength resembles the asymmetric pattern of the radial-shear-layer depth. Interestingly, the roll growth rate is not only affected by the radial wind shear magnitude alluded to in previous studies but also by the radial-shear-layer depth. A deeper (shallower) radial shear layer tends to decrease (increase) the roll growth rate. Such an effect is due to the presence of the bottom boundary. The bottom boundary constrains the lower-level roll streamlines and reduces the efficiency of rolls in extracting kinetic energy from the radial shear. This effect is more pronounced under a deeper shear layer, which favors the formation of larger-size rolls. This study improves the understanding of the main factors affecting the structure and growth of rolls and will provide guidance for interpreting the spatial distribution of rolls under realistic hurricanes in observations and high-resolution simulations.

Toward Better Understanding of the Contiguous Rain Area (CRA) Method for Spatial Forecast Verification

2009 ◽  
Vol 24 (5) ◽  
pp. 1401-1415 ◽  
Author(s):  
Elizabeth E. Ebert ◽  
William A. Gallus
Keyword(s):  
Correlation Coefficient ◽  
Prediction Models ◽  
Mean Squared Error ◽  
Weather Prediction ◽  
Forecast Errors ◽  
Forecast Verification ◽  
Rain Area ◽  
Squared Error ◽  
Verification Methods ◽  
The Mean

Abstract The contiguous rain area (CRA) method for spatial forecast verification is a features-based approach that evaluates the properties of forecast rain systems, namely, their location, size, intensity, and finescale pattern. It is one of many recently developed spatial verification approaches that are being evaluated as part of a Spatial Forecast Verification Methods Intercomparison Project. To better understand the strengths and weaknesses of the CRA method, it has been tested here on a set of idealized geometric and perturbed forecasts with known errors, as well as nine precipitation forecasts from three high-resolution numerical weather prediction models. The CRA method was able to identify the known errors for the geometric forecasts, but only after a modification was introduced to allow nonoverlapping forecast and observed features to be matched. For the perturbed cases in which a radar rain field was spatially translated and amplified to simulate forecast errors, the CRA method also reproduced the known errors except when a high-intensity threshold was used to define the CRA (≥10 mm h−1) and a large translation error was imposed (>200 km). The decomposition of total error into displacement, volume, and pattern components reflected the source of the error almost all of the time when a mean squared error formulation was used, but not necessarily when a correlation-based formulation was used. When applied to real forecasts, the CRA method gave similar results when either best-fit criteria, minimization of the mean squared error, or maximization of the correlation coefficient, was chosen for matching forecast and observed features. The diagnosed displacement error was somewhat sensitive to the choice of search distance. Of the many diagnostics produced by this method, the errors in the mean and peak rain rate between the forecast and observed features showed the best correspondence with subjective evaluations of the forecasts, while the spatial correlation coefficient (after matching) did not reflect the subjective judgments.

scholarly journals Progress towards an improved parameterisation of small-scale orographic impacts on the atmospheric boundary layer

2021 ◽  
Author(s):  
Julian Quimbayo-Duarte ◽  
Juerg Schmidli
Keyword(s):  
Boundary Layer ◽  
Atmospheric Boundary Layer ◽  
Prediction Models ◽  
Weather Prediction ◽  
Vertical Flux ◽  
Initial Time ◽  
Small Scale ◽  
Mean Flow ◽  
Horizontal Momentum ◽  
The Impact

<p>An accurate representation of the momentum budget in numerical models is essential in the quest for reliable weather forecasting, from large scales (climate models) to small scales (numerical weather prediction models, NWP). It is well known that orographic waves play an important role in large-scale circulation. The vertical propagation of such waves is associated with a vertical flux of horizontal momentum, which may be transferred to the mean flow by wave-mean flow interaction and wave-breaking (Sandu et al., 2019). The orography scales inducing such phenomena are often smaller than the model resolution, even for NWP models, leading to the need for parameterisation schemes for orographic drag. Yet, such parameterization in current models is fairly limited (Vosper et al., 2020). The present work aims to contribute to an improved understanding and parameterization of the impact of small-scale orography on the lower atmosphere with a focus on the stable atmospheric boundary layer.</p><p>As a first step, an idealized set of experiments has been designed to explore the capabilities of the Icosahedral Nonhydrostatic model in its large eddy simulation mode (ICON-LES, Dipankar et al., 2015) to represent turbulence processes in the stably-stratified atmosphere. Initial experiments testing the model performance over flat terrain (GABLS experiment, Beare et al., 2006), orographic wave generation (shallow bell-shaped topography, Xue et al., 2000) and moderate complex terrain (U-shaped valley, Burns and Chemel 2014) have been conducted. The results demonstrate that ICON-LES adequately represents the boundary layer processes for the investigated cases in comparison to the literature.</p><p>In a second step, an idealized set of experiments of atmospheric flow over idealized sinusoidal and multiscale terrain has been designed to study the impact of the orographically-induced gravity waves on the total surface drag and the vertical flux of horizontal momentum. The influence of different atmospheric conditions is assessed by varying the background wind speed and the temperature stratification at the initial time.</p>

scholarly journals The role of urban boundary layer investigated by high resolution models and ground based observations in Rome area: a step for understanding parameterizations potentialities

2013 ◽  
Vol 6 (3) ◽  
pp. 5297-5344
Author(s):  
E. Pichelli ◽  
R. Ferretti ◽  
M. Cacciani ◽  
A. M. Siani ◽  
V. Ciardini ◽  
...  
Keyword(s):  
Boundary Layer ◽  
High Resolution ◽  
Urban Areas ◽  
Prediction Models ◽  
Weather Prediction ◽  
Sonic Anemometer ◽  
Lower Atmosphere ◽  
Area Of Interest ◽  
The Impact

Abstract. The urban forcing on thermo-dynamical conditions can largely influences local evolution of the atmospheric boundary layer. Urban heat storage can produce noteworthy mesoscale perturbations of the lower atmosphere. The new generations of high-resolution numerical weather prediction models (NWP) is nowadays largely applied also to urban areas. It is therefore critical to reproduce correctly the urban forcing which turns in variations of wind, temperature and water vapor content of the planetary boundary layer (PBL). WRF-ARW, a new model generation, has been used to reproduce the circulation in the urban area of Rome. A sensitivity study is performed using different PBL and surface schemes. The significant role of the surface forcing in the PBL evolution has been verified by comparing model results with observations coming from many instruments (LiDAR, SODAR, sonic anemometer and surface stations). The crucial role of a correct urban representation has been demonstrated by testing the impact of different urban canopy models (UCM) on the forecast. Only one of three meteorological events studied will be presented, chosen as statistically relevant for the area of interest. The WRF-ARW model shows a tendency to overestimate vertical transmission of horizontal momentum from upper levels to low atmosphere, that is partially corrected by local PBL scheme coupled with an advanced UCM. Depending on background meteorological scenario, WRF-ARW shows an opposite behavior in correctly representing canopy layer and upper levels when local and non local PBL are compared. Moreover a tendency of the model in largely underestimating vertical motions has been verified.

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