scholarly journals Multiple-Doppler Radar Observations of the Evolution of Vorticity Extrema in a Convective Boundary Layer

2006 ◽  
Vol 134 (1) ◽  
pp. 355-374 ◽  
Author(s):  
Paul Markowski ◽  
Christina Hannon
Keyword(s):  
Boundary Layer ◽  
Vertical Velocity ◽  
Convective Boundary Layer ◽  
Doppler Radar ◽  
Vertical Wind Shear ◽  
Evolutionary Stage ◽  
Convective Boundary ◽  
Vertical Vorticity ◽  
Complex Interactions ◽  
Convection Initiation

Abstract Overdetermined, dual-Doppler wind syntheses are used to document the evolution, structure, and dynamics of vertical vorticity extrema observed in a convective boundary layer during the 12 June 2002 International H2O Project (IHOP) mission. Discrete vertical vorticity extrema having horizontal scales of 1–2 km can be observed continuously for periods exceeding an hour. The evolution of the vorticity field is characterized by complex interactions among vorticity extrema and between the vertical vorticity and vertical velocity fields. The most prominent vorticity maxima have amplitudes of approximately 0.01 s−1 and are associated with retrieved pressure deficits of order 0.1 mb. The vorticity extrema weaken with height and tilt in the presence of vertical wind shear. Advection and propagation both contribute substantially to the motion of the vorticity extrema. Amplifications of vertical vorticity are closely linked to the intensification of updrafts. Both stretching and tilting can contribute significantly to the vorticity budgets of the air parcels comprising the vorticity extrema, and their relative importance varies with elevation, evolutionary stage, and from one vorticity extremum to another. It is therefore difficult to generalize about the dynamics of the vorticity extrema. It also is difficult to generalize about the helicity of the vorticity maxima and suppression of mixing for similar reasons. The weakening of vertical vorticity extrema is closely tied to the weakening of updrafts. In some cases, downward-directed vertical pressure gradient forces due to vertical gradients of rotation bring about updraft weakening and vorticity demise. An improved understanding of the nature of boundary layer vortices could have large relevance to convection initiation owing to feedbacks between vertical velocity and vorticity.

scholarly journals Properties of a Simulated Convective Boundary Layer in an Idealized Supercell Thunderstorm Environment

2014 ◽  
Vol 142 (11) ◽  
pp. 3955-3976 ◽  
Author(s):  
Christopher J. Nowotarski ◽  
Paul M. Markowski ◽  
Yvette P. Richardson ◽  
George H. Bryan
Keyword(s):  
Boundary Layer ◽  
Convective Boundary Layer ◽  
Convective Available Potential Energy ◽  
Vertical Wind Shear ◽  
Surface Fluxes ◽  
Convective Boundary ◽  
Eddy Simulation ◽  
Sensitivity Tests ◽  
Supercell Thunderstorms ◽  
Background Shear

Abstract Nearly all previous numerical simulations of supercell thunderstorms have neglected surface fluxes of heat, moisture, and momentum. This choice precludes horizontal inhomogeneities associated with dry boundary layer convection in the near-storm environment. As part of a broader study on how mature supercell thunderstorms are affected by a convective boundary layer (CBL) with quasi-two-dimensional features (i.e., boundary layer rolls), this paper documents the methods used to develop a realistic CBL in an idealized environment supportive of supercells. The evolution and characteristics of the modeled CBL, including the horizontal variability of thermodynamic and kinematic quantities known to affect supercell evolution, are presented. The simulated rolls result in periodic bands of perturbations in temperature, moisture, convective available potential energy (CAPE), vertical wind shear, and storm-relative helicity (SRH). Vertical vorticity is shown to arise within the boundary layer through the tilting of ambient horizontal vorticity associated with the background shear by vertical velocity perturbations in the turbulent CBL. Sensitivity tests suggest that 200-m horizontal grid spacing is adequate to represent rolls using a large-eddy simulation (LES) approach.

scholarly journals Vertical velocity and turbulence aspects during Mistral events as observed by UHF wind profilers

2004 ◽  
Vol 22 (11) ◽  
pp. 3927-3936 ◽  
Author(s):  
J.-L. Caccia ◽  
V. Guénard ◽  
B. Benech ◽  
B. Campistron ◽  
P. Drobinski
Keyword(s):  
Boundary Layer ◽  
Planetary Boundary Layer ◽  
Vertical Velocity ◽  
Convective Boundary Layer ◽  
Dissipation Rate ◽  
Convective Boundary ◽  
Turbulent Dissipation ◽  
The Alps ◽  
Rhone Valley ◽  
Wind Profilers

Abstract. The general purpose of this paper is to experimentally study mesoscale dynamical aspects of the Mistral in the coastal area located at the exit of the Rhône-valley. The Mistral is a northerly low-level flow blowing in southern France along the Rhône-valley axis, located between the French Alps and the Massif Central, towards the Mediterranean Sea. The experimental data are obtained by UHF wind profilers deployed during two major field campaigns, MAP (Mesoscale Alpine Program) in autumn 1999, and ESCOMPTE (Expérience sur Site pour COntraindre les Modèles de Pollution atmosphériques et de Transports d'Emission) in summer 2001. Thanks to the use of the time evolution of the vertical profile of the horizontal wind vector, recent works have shown that the dynamics of the Mistral is highly dependent on the season because of the occurrence of specific synoptic patterns. In addition, during summer, thermal forcing leads to a combination of sea breeze with Mistral and weaker Mistral due to the enhanced friction while, during autumn, absence of convective turbulence leads to substantial acceleration as low-level jets are generated in the stably stratified planetary boundary layer. At the exit of the Rhône valley, the gap flow dynamics dominates, whereas at the lee of the Alps, the dynamics is driven by the relative contribution of "flow around" and "flow over" mechanisms, upstream of the Alps. This paper analyses vertical velocity and turbulence, i.e. turbulent dissipation rate, with data obtained by the same UHF wind profilers during the same Mistral events. In autumn, the motions are found to be globally and significantly subsident, which is coherent for a dry, cold and stable flow approaching the sea, and the turbulence is found to be of pure dynamical origin (wind shears and mountain/lee wave breaking), which is coherent with non-convective situations. In summer, due to the ground heating and to the interactions with thermal circulation, the vertical motions are less pronounced and no longer have systematic subsident charateristics. In addition, those vertical motions are found to be much less developed during the nighttimes because of the stabilization of the nocturnal planetary boundary layer due to a ground cooling. The enhanced turbulent dissipation-rate values found at lower levels during the afternoons of weak Mistral cases are consistent with the installation of the summer convective boundary layer and show that, as expected in weaker Mistral events, the convection is the preponderant factor for the turbulence generation. On the other hand, for stronger cases, such a convective boundary layer installation is perturbed by the Mistral.

Influence of Swell on Marine Surface-Layer Structure

2020 ◽  
Vol 77 (5) ◽  
pp. 1865-1885 ◽  
Author(s):  
Qingfang Jiang
Keyword(s):  
Boundary Layer ◽  
Surface Layer ◽  
Convective Boundary Layer ◽  
Wind Shear ◽  
Vertical Wind Shear ◽  
Vertical Wind ◽  
Convective Boundary ◽  
Stable Conditions ◽  
Marine Surface Layer ◽  
Marine Surface

Abstract The influence of swell on turbulence and scalar profiles in a marine surface layer and underlying physics is examined in this study through diagnosis of large-eddy simulations (LES) that explicitly resolve the surface layer and underlying swell. In general, under stable conditions, the mean wind and scalar profiles can be significantly modified by swell. The influence of swell on wind shear, turbulence structure, scalar profiles, and evaporation duct (ED) characteristics becomes less pronounced in a more convective boundary layer, where the buoyancy production of turbulence is significant. Dynamically, swell has little direct impact on scalar profiles. Instead it modifies the vertical wind shear by exerting pressure drag on the wave boundary layer. The resulting redistribution of vertical wind shear leads to changes in turbulence production and therefore turbulence mixing of scalars. Over swell, the eddy diffusivities from LES systematically deviate from the Monin–Obukhov similarity theory (MOST) prediction, implying that MOST becomes invalid over a swell-dominated sea. The deviations from MOST are more pronounced in a neutral or stable boundary layer under relatively low winds and less so in a convective boundary layer.

Unusually Long Duration, Multiple-Doppler Radar Observations of a Front in a Convective Boundary Layer

2007 ◽  
Vol 135 (1) ◽  
pp. 93-117 ◽  
Author(s):  
John R. Stonitsch ◽  
Paul M. Markowski
Keyword(s):  
Boundary Layer ◽  
Density Gradient ◽  
Doppler Radar ◽  
Deep Convection ◽  
Three Dimensional ◽  
Horizontal Wind ◽  
Normal Density ◽  
Wind Fields ◽  
Convective Boundary ◽  
Convection Initiation

Abstract Dual-Doppler observations acquired by a network of mobile radars deployed in the Oklahoma panhandle on 3 June 2002 are used to document the kinematic structure and evolution of a front. The data were collected during the International H2O Project on a mission to study the initiation of deep convection. Synchronized scanning allowed for the synthesis of three-dimensional wind fields for nearly 5.5 h of the 1557–0000 UTC period. The front initially moved southward as a cold front, stalled, and later retreated northward as a warm front. Deep convection failed to be initiated along the front. In situ thermodynamic measurements obtained by a mobile mesonet were used to document changes in the density gradient at the surface. This paper examines the relationships among the changes in baroclinity, the thermally direct frontal circulation, updraft intensity, alongfront updraft variability, and the intensity of vortices along the front. Increases in the front-normal density gradient tended to be associated with increases in the thermally direct frontal circulation, as expected. Increases in the front-normal density gradient were also associated with an increase in the tilt of the frontal updraft as well as an increase in the contiguity of the updraft along the front, termed the “slabularity.” During periods when the front-normal density gradient and associated thermally direct frontal circulation were weak, the kinematic fields were dominated by boundary layer convection and the slabularity of the front was reduced. Intensification of the front-normal density gradient was accompanied by an increase in the horizontal wind shear and the intensity of vortices that were observed along the front. The vortices modulated the vertical velocity field along the front and therefore the slabularity, too. Thus, although the slabularity was a strong function of the strength of the thermally direct frontal circulation, the slabularity appeared to be modified by vortices in complex ways. Possible implications of the observations for convection initiation are also discussed, particularly with respect to updraft tilt and slabularity.

Sign in / Sign up

Export Citation Format

Share Document