SPC Mesoscale Analysis Compared to Field-Project Soundings: Implications for Supercell Environment Studies

Two-hundred-fifty-seven supercell proximity soundings obtained for field programs over the central U.S. are compared to profiles extracted from the SPC mesoscale analysis system (the SFCOA) to understand how errors in the SFCOA and in its baseline model analysis system – the RUC/RAP – might impact climatological assessments of supercell environments. A primary result is that the SFCOA underestimates the low-level storm-relative winds and wind shear, a clear consequence of the lack of vertical resolution near the ground. The near-ground (≤ 500 m) wind shear is underestimated similarly in near-field, far-field, tornadic, and nontornadic supercell environments. The near-ground storm-relative winds, however, are underestimated the most in the near field and in tornadic supercell environments. Under-prediction of storm-relative winds is therefore a likely contributor to the lack of differences in storm-relative winds between nontornadic and tornadic supercell environments in past studies that use RUC/RAP-based analyses. Furthermore, these storm-relative wind errors could lead to an under emphasis of deep-layer SRH variables relative to shallower SRH in discriminating nontornadic from tornadic supercells. The mean critical angles are 5–15° larger and farther from 90° in the observed soundings than in the SFCOA, particularly in the near field, likely indicating that the ratio of streamwise to crosswise horizontal vorticity is often smaller than that suggested by the SFCOA profiles. Errors in thermodynamic variables are less prevalent, but show low-level CAPE to be too low closer to the storms, a dry bias above the boundary layer, and the absence of shallow near-ground stable layers that are much more prevalent in tornadic supercell environments.

Does the wind stress always damp an oceanic eddy?

Up to now, the literature has shown that the relative wind stress does negative work on ocean mesoscale eddies. In other words, the relative wind stress inhibits the development of the eddies. However, based on a newly derived simplified theoretical model, the present study finds that under the action of a steady and uniform wind field, eddies can rapidly obtain kinetic energy from the wind field following several hours of adaption and adjustment, in which the wind stress transitions from doing negative to positive work. The finding is supported by the fact that the relative wind stress work on oceanic eddies over the northeastern tropical Pacific ocean is positive with the nearly constant gap wind. This implies that energy input from the wind is sensitive to eddy velocity structure, and hence, wind stress is not always a killer of eddies.

Numerical Study of the Action of Convection on the Volume and Length of the Flammable Zone Formed by Hydrogen Emissions from the Vent Masts Installed on an International Ship

International ships carrying liquefied fuel are strongly recommended to install vent masts to control the pressure of cargo tanks in the event of an emergency. However, the gas emitted from a vent mast may be hazardous for the crew of the ship. In the present study, the volume and length of the flammable zone (FZ) created by the emitted gas above the ship was examined. Various scenarios comprising four parameters, namely, relative wind speed, arrangement of vent masts, combination of emissions among four vent masts, and direction of emission from the vent-mast outlet were considered. The results showed that the convection acts on the volume and length of an FZ. The volume of an FZ increases when there is a reduction in convection reaching the FZ and when strong convection brings hydrogen from a nearby FZ. The length of the FZ is also related to convection. An FZ is elongated if the center of a vortex is located inside the FZ, because this vortex traps hydrogen inside the FZ. The length of an FZ decreases if the center of the vortex is located outside the FZ, as such a vortex brings more fresh air into the FZ.

Impacts of ocean currents on the South Indian Ocean extratropical storm track through the relative wind effect

This study examines the role of the relative wind (RW) effect (wind relative to ocean current) in the regional ocean circulation and extratropical storm track in the South Indian Ocean. Comparison of two high-resolution regional coupled model simulations with/without the RW effect reveals that the most conspicuous ocean circulation response is the significant weakening of the overly energetic anticyclonic standing eddy off Port Elizabeth, South Africa, a biased feature ascribed to upstream retroflection of the Agulhas Current (AC). This opens a pathway through which the AC transports the warm and salty water mass from the subtropics, yielding marked increases in sea surface temperature (SST), upward turbulent heat flux (THF), and meridional SST gradient in the Agulhas retroflection region. These thermodynamic and dynamic changes are accompanied by the robust strengthening of the local low-tropospheric baroclinicity and the baroclinic wave activity in the atmosphere. Examination of the composite lifecycle of synoptic-scale storms subjected to the high THF events indicates a robust strengthening of the extratropical storms far downstream. Energetics calculations for the atmosphere suggest that the baroclinic energy conversion from the basic flow is the chief source of increased eddy available potential energy, which is subsequently converted to eddy kinetic energy, providing for the growth of transient baroclinic waves. Overall, the results suggest that the mechanical and thermal air-sea interactions are inherently and inextricably linked together to substantially influence the extratropical storm tracks in the South Indian Ocean.

Active flap control with the trailing edge flap hinge moment as a sensor: using it to estimate local blade inflow conditions and to reduce extreme blade loads and deflections

Active trailing edge flaps are a promising technology that can potentially enable further increases in wind turbine sizes without the disproportionate increase in loads, thus reducing the cost of wind energy even further. Extreme loads and critical deflections of the blade are design-driving issues that can effectively be reduced by flaps. In this paper, we consider the flap hinge moment as a local input sensor for a simple flap controller that reduces extreme loads and critical deflections of the DTU 10 MW Reference Wind Turbine blade. We present a model to calculate the unsteady flap hinge moment that can be used in aeroelastic simulations in the time domain. This model is used to develop an observer that estimates the local angle of attack and relative wind velocity of a blade section based on local sensor information including the flap hinge moment of the blade section. For steady wind conditions that include yawed inflow and wind shear, the observer is able to estimate the local inflow conditions with errors in the mean angle of attack below 0.2∘ and mean relative wind speed errors below 0.4 %. For fully turbulent wind conditions, the observer is able to estimate the low-frequency content of the local angle of attack and relative velocity even when it is lacking information on the incoming turbulent wind. We include this observer as part of a simple flap controller to reduce extreme loads and critical deflections of the blade. The flap controller's performance is tested in load simulations of the reference turbine with active flaps according to the IEC 61400-1 power production with extreme turbulence group. We used the lifting line free vortex wake method to calculate the aerodynamic loads. Results show a reduction of the maximum out-of-plane and resulting blade root bending moments of 8 % and 7.6 %, respectively, when compared to a baseline case without flaps. The critical blade tip deflection is reduced by 7.1 %. Furthermore, a sector load analysis considering extreme loading in all load directions shows a reduction of the extreme resulting bending moment in an angular region covering 30∘ around the positive out-of-plane blade root bending moment. Further analysis reveals that a fast reaction time of the flap system proves to be critical for its performance. This is achieved with the use of local sensors as input for the flap controller. A larger reduction potential of the system is identified but not reached mainly because of a combination of challenging controller objectives and the simple controller architecture.

The influence of ground-relative flow and friction on near-surface storm-relative helicity

Recent studies have highlighted the importance of near-ground storm-relative helicity (SRH) in supercell and tornado processes and how surface friction can play a role. In this study, we use an analytical approach to examine how uniform changes to the ground-relative wind profile above the near-ground layer influence SRH within the near-ground layer. We show how the ground-relative influence of surface friction alters the near-ground shear profile. For idealized semicircular and straight shear profiles, increasing preexisting ground-relative flow above the near-ground layer yields increasing SRH. The magnitude of the SRH increase is sensitive to storm motion, with more deviant motion yielding greater SRH increases given the same increase in ground-relative flow. Supercells may be more susceptible to storm-induced SRH enhancements given their deviant motion and ability to increase ground-relative flow in the background environment.

Local ventilation and its impacts on urban heat islands and outdoor thermal comfort

<p>Many cities are facing urban overheating issues where the reduction of urban ventilation is one of the key drivers. To address the urban overheating problems, this study concentrates on the analysis of local-scale urban ventilation and its impacts of urban heat islands and outdoor thermal comfort, in order to support wind-sensitive urban planning and design. To achieve this, this study develops a framework for analysing local ventilation, urban heat islands and outdoor thermal comfort with the consideration of local morphological characteristics, external meteorological conditions, local ventilation performance, urban heat islands and outdoor thermal comfort. In particular, the consideration of local morphological characteristics is supported by the development of precinct morphology classification scheme based on three-component protocol of building height, street structure and compactness. Based on the three-component protocol, 20 types of the local ventilation zones were identified in the context of Greater Sydney, Australia.</p><p>Field measurement was conducted in three typical local ventilation zones, including open low-rise gridiron, open midrise gridiron and compact high-rise gridiron among the 20, to examine the local ventilation performance, urban heat islands and outdoor thermal comfort in summer 2019. The results indicate that the open midrise gridiron precinct underwent the best precinct ventilation performance, followed by the low-rise gridiron precinct and then the compact high-rise gridiron precinct. The local ventilation created by the sea breeze can help alleviate urban heat islands in the open low-rise gridiron and compact high-rise gridiron precincts with every 0.1 increase in relative wind velocity ratio leading to a 0.09-0.12 °C reduction in UHI intensity. However, in the open midrise gridiron precinct, the local ventilation created by the sea breeze made no difference for urban heat islands. However, the precinct ventilation of the open midrise gridiron precinct still partially exhibited UHI alleviation potential with every 0.1 increase in relative wind velocity ratio leading to a 0.06-0.1 °C reduction in UHI intensity depending on the approaching wind temperature and shading conditions.</p><p>Only the precinct ventilation of the open low-rise gridiron precinct leads to outdoor thermal comfort improvement with every 0.1 increase in relative wind velocity ratio leading to 0.29 °C and 0.50 °C physiological equivalent temperature reductions under sea breeze and varying wind conditions, respectively. The results also indicate that within ‘gridiron’ precincts, street orientation is not critical to precinct ventilation performance and its impact on urban heat islands and outdoor thermal comfort. Under wind conditions, trees do not always alleviate urban heat islands and improve outdoor thermal comfort as trees can block sea breeze penetration and inhibit wind cooling potential. These key findings will serve to inform urban heat island mitigation strategies and future planning and design decisions in the built environment.</p>

Role Of Relative Wind Stress In Generating Eddy Instabilities

<p>Relative wind stress (calculated by including the surface current terms) is known to remove energy from mesoscale eddies, but how they respond to this damping mechanism over their lifetime is poorly understood. A method for predicting eddy energy is made by time stepping forward the energy equation of a linear two-layer model using an analytical relative wind stress damping term. Results of this prediction are then compared with numerical experiments of an idealised two-layer anticyclonic eddy in a high-resolution general circulation model. The energy in both experiments displays a quantitative agreement in relative wind stress damping, though this is not the case when the eddy in the numerical experiment becomes baroclinically unstable. In addition to this well-known relative wind stress damping mechanism, we found that relative wind stress can trigger eddy instabilities sooner, leading to quicker decay. The earlier onset of these instabilities by relative wind stress is observed in a Lorenz energy cycle.</p>

Active Flap Control with the Trailing Edge Flap Hinge Moment as a Sensor: Using it to Estimate Local Blade Inflow Conditions and to Reduce Extreme Blade Loads and Deflections

Active trailing edge flaps are a promising technology that can potentially enable further increases in wind turbine sizes without the disproportionate increase in loads, thus reducing the cost of wind energy even further. Extreme loads and critical deflections of the blade are design driving issues that can effectively be reduced by flaps. In this paper, we consider the flap hinge moment as a local input sensor for a simple flap controller that reduces extreme loads and critical deflections of the DTU 10 MW Reference Wind Turbine blade. We present a model to calculate the unsteady flap hinge moment that can be used in aeroelastic simulations in the time domain. This model is used to develop an observer that estimates the local angle of attack and relative wind velocity of a blade section based on local sensor information including the flap hinge moment of the blade section. For steady wind conditions that include yawed inflow and wind shear, the observer is able to estimate the local inflow conditions with errors in the mean angle of attack below 0.2° and mean relative wind speed errors below 0.4 %. For fully turbulent wind conditions, the observer is able to estimate the low frequency content of the local angle of attack and relative velocity even when it is lacking information on the incoming turbulent wind. We include this observer as part of a simple flap controller to reduce extreme loads and critical deflections of the blade. The flap controller’s performance is tested in load simulations of the reference turbine with active flaps according to the IEC 61400-1 power production with extreme turbulence group. We used the lifting line free vortex wake method to calculate the aerodynamic loads. Results show a reduction of the maximum out-of-plane and resulting blade root bending moment of 8 % and 7.6 % respectively when compared to a baseline case without flaps. The critical blade tip deflection is reduced by 7.1 %. Furthermore, a sector load analysis considering extreme loading in all load directions shows a reduction of the extreme resulting bending moment in an angular region covering 30° around the positive out-of-plane blade root bending moment. Further analysis reveals that a fast reaction time of the flap system proves to be critical for its performance. This is achieved with the use of local sensors as input for the flap controller. A larger reduction potential of the system is identified but not reached mainly because of a combination of challenging controller objectives and the simple controller architecture.