scholarly journals Observation of Turbulent Mixing Characteristics in the Typical Daytime Cloud-Topped Boundary Layer over Hong Kong in 2019

2020 ◽  
Vol 12 (9) ◽  
pp. 1533 ◽  
Author(s):  
Tao Huang ◽  
Steve Hung-lam Yim ◽  
Yuanjian Yang ◽  
Olivia Shuk-ming Lee ◽  
David Hok-yin Lam ◽  
...  
Keyword(s):  
Boundary Layer ◽  
Hong Kong ◽  
Vertical Velocity ◽  
Turbulent Mixing ◽  
Wind Shear ◽  
Vertical Wind Shear ◽  
Vertical Wind ◽  
Surface Heating ◽  
Cloud Base ◽  
Radiative Cooling

Turbulent mixing is critical in affecting urban climate and air pollution. Nevertheless, our understanding of it, especially in a cloud-topped boundary layer (CTBL), remains limited. High-temporal resolution observations provide sufficient information of vertical velocity profiles, which is essential for turbulence studies in the atmospheric boundary layer (ABL). We conducted Doppler Light Detection and Ranging (LiDAR) measurements in 2019 using the 3-Dimensional Real-time Atmospheric Monitoring System (3DREAMS) to reveal the characteristics of typical daytime turbulent mixing processes in CTBL over Hong Kong. We assessed the contribution of cloud radiative cooling on turbulent mixing and determined the altitudinal dependence of the contribution of surface heating and vertical wind shear to turbulent mixing. Our results show that more downdrafts and updrafts in spring and autumn were observed and positively associated with seasonal cloud fraction. These results reveal that cloud radiative cooling was the main source of downdraft, which was also confirmed by our detailed case study of vertical velocity. Compared to winter and autumn, cloud base heights were lower in spring and summer. Cloud radiative cooling contributed ~32% to turbulent mixing even near the surface, although the contribution was relatively weaker compared to surface heating and vertical wind shear. Surface heating and vertical wind shear together contributed to ~45% of turbulent mixing near the surface, but wind shear can affect up to ~1100 m while surface heating can only reach ~450 m. Despite the fact that more research is still needed to further understand the processes, our findings provide useful references for local weather forecast and air quality studies.

Assessing the influence of complex terrain on severe convective environments in northeastern Alabama

2021 ◽  
Author(s):  
Branden Katona ◽  
Paul Markowski
Keyword(s):  
Boundary Layer ◽  
Complex Terrain ◽  
Wind Shear ◽  
Vertical Wind Shear ◽  
Vertical Wind ◽  
Spatial Distributions ◽  
Convective Storm ◽  
Self Organizing Maps ◽  
Low Level ◽  
Wind Regimes

AbstractStorms crossing complex terrain can potentially encounter rapidly changing convective environments. However, our understanding of terrain-induced variability in convective stormenvironments remains limited. HRRR data are used to create climatologies of popular convective storm forecasting parameters for different wind regimes. Self-organizing maps (SOMs) are used to generate six different low-level wind regimes, characterized by different wind directions, for which popular instability and vertical wind shear parameters are averaged. The climatologies show that both instability and vertical wind shear are highly variable in regions of complex terrain, and that the spatial distributions of perturbations relative to the terrain are dependent on the low-level wind direction. Idealized simulations are used to investigate the origins of some of the perturbations seen in the SOM climatologies. The idealized simulations replicate many of the features in the SOM climatologies, which facilitates analysis of their dynamical origins. Terrain influences are greatest when winds are approximately perpendicular to the terrain. In such cases, a standing wave can develop in the lee, leading to an increase in low-level wind speed and a reduction in vertical wind shear with the valley lee of the plateau. Additionally, CAPE tends to be decreased and LCL heights are increased in the lee of the terrain where relative humidity within the boundary layer is locally decreased.

scholarly journals PRELIMINARY STUDY OF HORIZONTAL AND VERTICAL WIND PROFILE OF QUASI-LINEAR CONVECTIVE UTILIZING WEATHER RADAR OVER WESTERN JAVA REGION, INDONESIA

2019 ◽  
Vol 15 (2) ◽  
pp. 177
Author(s):  
Abdullah Ali ◽  
Riris Adrianto ◽  
Miming Saepudin
Keyword(s):  
Velocity Profile ◽  
Vertical Velocity ◽  
Wind Shear ◽  
Vertical Wind Shear ◽  
Convective Cloud ◽  
Vertical Wind ◽  
Squall Line ◽  
Vertical Velocity Profile ◽  
Preliminary Study ◽  
Wind Profiles

One of the weather phenomena that potentially cause extreme weather conditions is the linear-shaped mesoscale convective systems, including squall lines. The phenomenon that can be categorized as a squall line is a convective cloud pair with the linear pattern of more than 100 km length and 6 hours lifetime. The new theory explained that the cloud system with the same morphology as squall line without longevity threshold. Such a cloud system is so-called Quasi-Linear Convective System (QLCS), which strongly influenced by the ambient dynamic processes, include horizontal and vertical wind profiles. This research is intended as a preliminary study for horizontal and vertical wind profiles of QLCS developed over the Western Java region utilizing Doppler weather radar. The following parameters were analyzed in this research, include direction pattern and spatial-temporal significance of wind speed, divergence profile, vertical wind shear (VWS) direction, and intensity profiles, and vertical velocity profile. The subjective and objective analysis was applied to explain the characteristics and effects of those parameters to the orientation of propagation, relative direction, and speed of the cloud system’s movement, and the lifetime of the system. Analysis results showed that the movement of the system was affected by wind direction and velocity patterns. The divergence profile combined with the vertical velocity profile represents the inflow which can supply water vapor for QLCS convective cloud cluster. Vertical wind shear that effect QLCS system is only its direction relative to the QLCS propagation, while the intensity didn’t have a significant effect.

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.

scholarly journals A new paradigm for intensity modification of tropical cyclones: thermodynamic impact of vertical wind shear on the inflow layer

2009 ◽  
Vol 9 (3) ◽  
pp. 10711-10775 ◽  
Author(s):  
M. Riemer ◽  
M. T. Montgomery ◽  
M. E. Nicholls
Keyword(s):  
Boundary Layer ◽  
Tropical Cyclones ◽  
Wind Shear ◽  
Vertical Wind Shear ◽  
Vertical Wind ◽  
Vertical Shear ◽  
Carnot Cycle ◽  
Close Link ◽  
Storm Intensity ◽  
Intensity Evolution

Abstract. An important roadblock to improved intensity forecasts for tropical cyclones (TCs) is our incomplete understanding of the interaction of a TC with the environmental flow. In this paper we re-visit the classical idealised numerical experiment of tropical cyclones (TCs) in vertical wind shear on an f-plane. We employ a set of simplified model physics – a simple bulk aerodynamic boundary layer scheme and "warm rain" microphysics – to foster better understanding of the dynamics and thermodynamics that govern the modification of TC intensity. A suite of experiments is performed with intense TCs in moderate to strong vertical shear. In all experiments the TC is resilient to shear but significant differences in the intensity evolution occur. The ventilation of the TC core with dry environmental air at mid-levels and the dilution of the upper-level warm core are two prevailing hypotheses for the adverse effect of vertical shear on storm intensity. Here we propose an alternative and arguably more effective mechanism how cooler and drier (lower θe) air – "anti-fuel" for the TC power machine – can enter the core region of the TC. Strong and persistent downdrafts flux low θe air from the lower and middle troposphere into the boundary layer, significantly depressing the θe values in the storm's inflow layer. Air with lower θe values enters the eyewall updrafts, considerably reducing eyewall θe values in the azimuthal mean. When viewed from the perspective of an idealised Carnot-cycle heat engine a decrease of storm intensity can thus be expected. Although the Carnot cycle model is – if at all – only valid for stationary and axisymmetric TCs, a strong correlation between the downward transport of low θe into the boundary layer and the intensity evolution offers further evidence in support of our hypothesis. The downdrafts that flush the inflow layer with low θe air are associated with a quasi-stationary region of convective activity outside the TC's eyewall. We show evidence that, to zero order, the formation of the convective asymmetry is driven by the balanced dynamical response of the TC vortex to the vertical shear forcing. Thus a close link is provided between the thermodynamic impact in the near-core boundary layer and the balanced dynamics governing the TC vortex evolution.

Assessing the Influence of Convective Downdrafts and Surface Enthalpy Fluxes on Tropical Cyclone Intensity Change in Moderate Vertical Wind Shear

2019 ◽  
Vol 147 (10) ◽  
pp. 3519-3534 ◽  
Author(s):  
Leon T. Nguyen ◽  
Robert Rogers ◽  
Jonathan Zawislak ◽  
Jun A. Zhang
Keyword(s):  
Boundary Layer ◽  
Wind Shear ◽  
Deep Convection ◽  
Vertical Wind Shear ◽  
Lower Troposphere ◽  
Observation Time ◽  
Vertical Wind ◽  
Intensity Change ◽  
Conditional Instability ◽  
Surface Enthalpy

Abstract The thermodynamic impacts of downdraft-induced cooling/drying and downstream recovery via surface enthalpy fluxes within tropical cyclones (TCs) were investigated using dropsonde observations collected from 1996 to 2017. This study focused on relatively weak TCs (tropical depression, tropical storm, category 1 hurricane) that were subjected to moderate (4.5–11.0 m s−1) levels of environmental vertical wind shear. The dropsonde data were analyzed in a shear-relative framework and binned according to TC intensity change in the 24 h following the dropsonde observation time, allowing for comparison between storms that underwent different intensity changes. Moisture and temperature asymmetries in the lower troposphere yielded a relative maximum in lower-tropospheric conditional instability in the downshear quadrants and a relative minimum in instability in the upshear quadrants, regardless of intensity change. However, the instability increased as the intensification rate increased, particularly in the downshear quadrants. This was due to increased boundary layer moist entropy relative to the temperature profile above the boundary layer. Additionally, significantly larger surface enthalpy fluxes were observed as the intensification rate increased, particularly in the upshear quadrants. These results suggest that in intensifying storms, enhanced surface enthalpy fluxes in the upshear quadrants allow downdraft-modified boundary layer air to recover moisture and heat more effectively as it is advected cyclonically around the storm. By the time the air reaches the downshear quadrants, the lower-tropospheric conditional instability is enhanced, which is speculated to be more favorable for updraft growth and deep convection.

scholarly journals Composite VORTEX2 Supercell Environments from Near-Storm Soundings

2014 ◽  
Vol 142 (2) ◽  
pp. 508-529 ◽  
Author(s):  
Matthew D. Parker
Keyword(s):  
Boundary Layer ◽  
Wind Shear ◽  
Convective Available Potential Energy ◽  
Three Dimensional ◽  
Vertical Wind Shear ◽  
Vertical Wind ◽  
Vertical Shear ◽  
Low Level ◽  
Full Analysis ◽  
Favorable Conditions

Abstract Three-dimensional composite analyses using 134 soundings from the second Verification of the Origins of Rotation in Tornadoes Experiment (VORTEX2) reveal the nature of near-storm variability in the environments of supercell thunderstorms. Based upon the full analysis, it appears that vertical wind shear increases as one approaches a supercell within the inflow sector, providing favorable conditions for supercell maintenance (and possibly tornado formation) despite small amounts of low-level cooling near the storm. The seven analyzed tornadic supercells have a composite environment that is clearly more impressive (in terms of widely used metrics) than that of the five analyzed nontornadic supercells, including more convective available potential energy (CAPE), more vertical wind shear, higher boundary layer relative humidity, and lower tropospheric horizontal vorticity that is more streamwise in the near-storm inflow. The widely used supercell composite parameter (SCP) and significant tornado parameter (STP) summarize these differences well. Comparison of composite environments from early versus late in supercells' lifetimes reveals only subtle signs of storm-induced environmental modification, but potentially important changes associated with the evening transition toward a cooler and moister boundary layer with enhanced low-level vertical shear. Finally, although this study focused primarily on the composite inflow environment, it is intriguing that the outflows sampled by VORTEX2 soundings were surprisingly shallow (generally ≤500 m deep) and retained considerable CAPE (generally ≥1000 J kg−1). The numerous VORTEX2 near-storm soundings provide an unprecedented observational view of supercell–environment interactions, and the analyses are ripe for use in a variety of future studies.

The Variation With Height of the Non-Geostrophic Component of the Wind

1946 ◽  
Vol 27 (9) ◽  
pp. 532-536
Author(s):  
Joseph Vederman
Keyword(s):  
Vertical Velocity ◽  
General Equation ◽  
Wind Shear ◽  
Vertical Wind Shear ◽  
Geostrophic Wind ◽  
Equation Of Motion ◽  
Vertical Wind ◽  
Wind Component ◽  
Order Of Magnitude ◽  
Local Derivative

The general equation of motion for horizontal, frictionless motion is differentiated with respect to height. The total vertical wind shear is shown to be composed of five parts: the shear of (a) the geostrophic wind, (b) the local derivative, (c) the wind component due to the convergence or divergence of the streamlines, (d) the cyclostrophic component of the wind, and (e) the wind component associated with the vertical velocity. Except for the last term, which is smaller, these terms may be of the same order of magnitude.

scholarly journals Morning boundary layer conditions for shallow to deep convective cloud evolution during the dry season in the central Amazon

2021 ◽  
Author(s):  
Alice Henkes ◽  
Gilberto Fisch ◽  
Luiz Augusto Toledo Machado ◽  
Jean-Pierre Chaboureau
Keyword(s):  
Boundary Layer ◽  
Heat Flux ◽  
Turbulent Mixing ◽  
Rapid Growth ◽  
Sensible Heat Flux ◽  
Vertical Wind Shear ◽  
Early Morning ◽  
Cloud Base ◽  
Sensible Heat ◽  
Morning Transition

Abstract. Observations of the boundary layer (BL) processes are analyzed in four shallow convective days (ShCu) and four shallow-to-deep convective days (ShDeep) using a suite of ground-based measurements from the second Intensive Operating Period as part of the Observation and Modeling of the Green Ocean Amazon (IOP2; GoAmazon 2014/5) Experiment. The BL stages in ShDeep days, from the nighttime to the cloudy mixing layer stage, are then described in comparison with ShCu days. Atmosphere thermodynamics and dynamics, environmental profiles, and surface fluxes were employed to compare these two distinct situations for each stage of the BL evolution. Particular attention is given to the morning transition stage, in which the BL changes from stable to unstable conditions in the early morning hours. Results show that the duration of the morning transition on ShDeep days decreases under high humidity and intense vertical wind shear. Higher humidity since nighttime not only contributes to lowering the cloud base during the rapid growth of the BL but also contributes to the balance between radiative cooling and turbulent mixing during nighttime, resulting in large sensible heat flux in the early morning. A large sensible heat flux promotes rapid growth of the well-mixed layer, thus favoring the deeper BL starting from around 08:00 LST. Under these conditions, turbulent mixing provides a lifting mechanism by which air parcels reach the lifting condensation level, leading to the formation of shallow cumulus clouds and their subsequent evolution into deep convective clouds.

scholarly journals Asymmetric Hurricane Boundary Layer Structure from Dropsonde Composites in Relation to the Environmental Vertical Wind Shear

2013 ◽  
Vol 141 (11) ◽  
pp. 3968-3984 ◽  
Author(s):  
Jun A. Zhang ◽  
Robert F. Rogers ◽  
Paul D. Reasor ◽  
Eric W. Uhlhorn ◽  
Frank D. Marks
Keyword(s):  
Boundary Layer ◽  
Wind Speed ◽  
Mixed Layer ◽  
Wind Shear ◽  
Vertical Wind Shear ◽  
Vertical Wind ◽  
Asymmetric Structure ◽  
Layer Depth ◽  
Tangential Wind ◽  
Hurricane Boundary Layer

Abstract This study investigates the asymmetric structure of the hurricane boundary layer in relation to the environmental vertical wind shear in the inner core region. Data from 1878 GPS dropsondes deployed by research aircraft in 19 hurricanes are analyzed in a composite framework. Kinematic structure analyses based on Doppler radar data from 75 flights are compared with the dropsonde composites. Shear-relative quadrant-mean composite analyses show that both the kinematic and thermodynamic boundary layer height scales tend to decrease with decreasing radius, consistent with previous axisymmetric analyses. There is still a clear separation between the kinematic and thermodynamic boundary layer heights. Both the thermodynamic mixed layer and height of maximum tangential wind speed are within the inflow layer. The inflow layer depth is found to be deeper in quadrants downshear, with the downshear right (DR) quadrant being the deepest. The mixed layer depth and height of maximum tangential wind speed are alike at the eyewall, but are deeper outside in quadrants left of the shear. The results also suggest that air parcels acquire equivalent potential temperature θe from surface fluxes as they rotate through the upshear right (UR) quadrant from the upshear left (UL) quadrant. Convection is triggered in the DR quadrant in the presence of asymmetric mesoscale lifting coincident with a maximum in θe. Energy is then released by latent heating in the downshear left (DL) quadrant. Convective downdrafts bring down cool and dry air to the surface and lower θe again in the DL and UL quadrants. This cycling process may be directly tied to shear-induced asymmetry of convection in hurricanes.

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