Showing papers on "Wind shear published in 2023"

Turbulent structure of the Arctic boundary layer in early summer driven by stability, wind shear and cloud-top radiative cooling: ACLOUD airborne observations

Abstract: Abstract. Clouds are assumed to play an important role in the Arctic amplification process. This motivated a detailed investigation of cloud processes, including radiative and turbulent fluxes. Data from the aircraft campaign ACLOUD were analyzed with a focus on the mean and turbulent structure of the cloudy boundary layer over the Fram Strait marginal sea ice zone in late spring and early summer 2017. Vertical profiles of turbulence moments are presented from contrasting atmospheric boundary layers (ABLs) from 4 d. They differ by the magnitude of wind speed, boundary-layer height, stability, the strength of the cloud-top radiative cooling and the number of cloud layers. Turbulence statistics up to third-order moments are presented, which were obtained from horizontal-level flights and from slanted profiles. It is shown that both of these flight patterns complement each other and form a data set that resolves the vertical structure of the ABL turbulence well. The comparison of the 4 d shows that especially during weak wind, even in shallow Arctic ABLs with mixing ratios below 3 g kg−1, cloud-top cooling can serve as a main source of turbulent kinetic energy (TKE). Well-mixed ABLs are generated where TKE is increased and vertical velocity variance shows pronounced maxima in the cloud layer. Negative vertical velocity skewness points then to upside-down convection. Turbulent heat fluxes are directed upward in the cloud layer as a result of cold downdrafts. In two cases with single-layer stratocumulus, turbulent transport of heat flux and of temperature variance are both negative in the cloud layer, suggesting an important role of large eddies. In contrast, in a case with weak cloud-top cooling, these quantities are positive in the ABL due to the heating from the surface. Based on observations and results of a mixed-layer model it is shown that the maxima of turbulent fluxes are, however, smaller than the jump of the net terrestrial radiation flux across the upper part of a cloud due to the (i) shallowness of the mixed layer and (ii) the presence of a downward entrainment heat flux. The mixed-layer model also shows that the buoyancy production of TKE is substantially smaller in stratocumulus over the Arctic sea ice compared to subtropics due to a smaller surface moisture flux and smaller decrease in specific humidity (or even humidity inversions) right above the cloud top. In a case of strong wind, wind shear shapes the ABL turbulent structure, especially over rough sea ice, despite the presence of a strong cloud-top cooling. In the presence of mid-level clouds, cloud-top radiative cooling and thus also TKE in the lowermost cloud layer are strongly reduced, and the ABL turbulent structure becomes governed by stability, i.e., by the surface–air temperature difference and wind speed. A comparison of slightly unstable and weakly stable cases shows a strong reduction of TKE due to increased stability even though the absolute value of wind speed was similar. In summary, the presented study documents vertical profiles of the ABL turbulence with a high resolution in a wide range of conditions. It can serve as a basis for turbulence closure evaluation and process studies in Arctic clouds.

Daily and Subdaily Wind and Divergence Variations Observed by a Shipborne Doppler Radar off the Southwestern Coast of Sumatra

Abstract: This study investigated the daily cycle of the wind and divergence fields observed off the southwestern coast of Sumatra during a field campaign of the Years of the Maritime Continent pilot study. An algorithm was developed to retrieve kinematic variables from the single-Doppler data collected aboard the research vessel Mirai from November 24 to December 13, 2015. The observed daily cycles of the wind and divergence fields consisted of diurnal, semidiurnal, and short-term variations. Diurnal wind variation was characterized by deep and three-dimensional circulation. There was an approximate phase locking of the semidiurnal variation to the diurnal variation, both in the wind and divergence fields. The short-term wind variation occurred at a time scale of ∼1–3 h, and this pattern was associated with density currents or mesoscale gravity waves. Up to 73% of the daily vertical motion variance can be attributed to the diurnal and semidiurnal vertical motion variations with comparable strengths. Concurrently, precipitation propagated offshore in phase with density currents and mesoscale gravity waves. Our results suggest that diurnal and semidiurnal wind variations dominate the daily evolution of precipitation, whereas density currents and mesoscale gravity waves control offshore propagation. Additionally, it appears that the daily precipitation cycle is modulated by multiple timescale wind variabilities of less than a day, which is also responsible for the development of strong nocturnal convection off the southwestern coast of Sumatra.

Time-Series Prediction of Intense Wind Shear Using Machine Learning Algorithms: A Case Study of Hong Kong International Airport

Abstract: Machine learning algorithms are applied to predict intense wind shear from the Doppler LiDAR data located at the Hong Kong International Airport. Forecasting intense wind shear in the vicinity of airport runways is vital in order to make intelligent management and timely flight operation decisions. To predict the time series of intense wind shear, Bayesian optimized machine learning models such as adaptive boosting, light gradient boosting machine, categorical boosting, extreme gradient boosting, random forest, and natural gradient boosting are developed in this study. The time-series prediction describes a model that predicts future values based on past values. Based on the testing set, the Bayesian optimized-Extreme Gradient Boosting (XGBoost) model outperformed the other models in terms of mean absolute error (1.764), mean squared error (5.611), root mean squared error (2.368), and R-Square (0.859). Afterwards, the XGBoost model is interpreted using the SHapley Additive exPlanations (SHAP) method. The XGBoost-based importance and SHAP method reveal that the month of the year and the encounter location of the most intense wind shear were the most influential features. August is more likely to have a high number of intense wind-shear events. The majority of the intense wind-shear events occurred on the runway and within one nautical mile of the departure end of the runway.

Observed Quasi 16-Day Wave by Meteor Radar over 9 Years at Mengcheng (33.4°N, 116.5°E) and Comparison with the Whole Atmosphere Community Climate Model Simulation

Abstract: In this study, we present nearly 9 years of the quasi16-day wave (Q16DW) in the mesosphere and lower thermosphere (MLT) wind at middle latitudes based on long-term wind observations between April 2014 and December 2022 by the Mengcheng (33.4°N, 116.5°E) meteor radar. There are two maxima in the Q16DW amplitude in the winter and early spring (near the equinox) and a minimum during the summer. The Q16DWs are relatively weak in meridional winds with no obvious seasonal variations. On average, the phase of the zonal Q16DW is larger than the meridional components with a mean difference that is slightly less than 90°, which suggests that there are orthogonal relationships between them. During the bursts of Q16DW, the periods in winter range between 15 and 18 d, whereas in summer, the periods of the planetary waves have a wider range. The wintertime Q16DW anomalies are, on average, amplified when the zonal wind shear anomalies increase, suggesting that barotropic instability may be a source of the Q16DW. Although the interannual variability of Q16DW amplitudes has been suggested observationally, there is no significant relationship between the interannual wind shear variability and Q16DW at most altitudes.

Investigating the physical mechanisms that modify wind plant blockage in stable boundary layers

Abstract: Abstract. Wind plants slow down the approaching wind, a phenomenon known as blockage. Wind plant blockage undermines turbine performance for front-row turbines and potentially for turbines deeper into the array. We use large-eddy simulations to characterize blockage upstream of a finite-size wind plant in flat terrain for different atmospheric stability conditions, and investigate the physical mechanisms modifying the flow upstream of the turbines. To examine the influence of atmospheric stability, we compare simulations of two stably stratified boundary layers using the Weather Research and Forecasting model in large-eddy simulation mode, representing wind turbines using the generalized actuator disk approach. For a wind plant, a faster cooling rate at the surface, which produces stronger stably stratified flow in the boundary layer, amplifies blockage. As a novelty, we investigate the physical mechanisms amplifying blockage by evaluating the different terms in the momentum conservation equation within the turbine rotor layer. The velocity deceleration upstream of a wind plant is caused by an adverse pressure gradient and momentum advection out of the turbine rotor layer. The cumulative deceleration of the flow upstream of the front-row turbines sets in motion a secondary circulation. The horizontal flow is diverted vertically, reducing momentum availability in the turbine rotor layer. Although the adverse pressure gradient upstream of the wind plant remains unchanged with atmospheric stability, vertical momentum advection is amplified in the more strongly stable boundary layer, mainly by larger shear of the horizontal velocity, thus increasing the blockage effect.

Downward Momentum Flux: An Important Mechanism of Typhoon Maintaining Ground Destructive Force

Abstract: Under the background of global climate change, typhoons have been attracting increasing attention due to their extraordinary destructive potential and great impact on the coastal areas of the South China Sea. Although the risk of strong winds related to typhoons has long been of interest, less is known about the underlying mechanism responsible for the severe near-surface winds. Using eddy covariance data collected at four different heights on a 365-m meteorological tower located in a coastal region, the characteristics of the convective typhoon boundary layer and the associated turbulence structures are compared with their counterparts in the “textbook” dynamically unstable boundary layer. In the convective typhoon boundary layer, bulk wind shear predominates the generation of mechanical turbulence, enhancing the vertical correlation between vertical layers. The spectral analysis highlights the salient features of turbulent structures under the convective typhoon boundary layer, confirming that the gust disturbance with the nondimensional frequency ranging from 0.003 to 0.3 modulates not only turbulent transports but also the horizontal flow. Such gusts with reduced phase difference enhance the downward momentum transport, mainly responsible for the maintenance of the strong near-surface winds during typhoon landfalls.

Investigating the physical mechanisms that modify wind plant blockage in stable boundary layers

Abstract: Abstract. Wind plants slow down the approaching wind, a phenomenon known as blockage. Wind plant blockage undermines turbine performance for front-row turbines and potentially for turbines deeper into the array. We use large-eddy simulations to characterize blockage upstream of a finite-size wind plant in flat terrain for different atmospheric stability conditions and investigate the physical mechanisms modifying the flow upstream of the turbines. To examine the influence of atmospheric stability, we compare simulations of two stably stratified boundary layers using the Weather Research and Forecasting model in large-eddy simulation mode, representing wind turbines using the generalized actuator disk approach. For a wind plant, a faster cooling rate at the surface, which produces stronger stably stratified flow in the boundary layer, amplifies blockage. As a novelty, we investigate the physical mechanisms amplifying blockage by evaluating the different terms in the momentum conservation equation within the turbine rotor layer. The velocity deceleration upstream of a wind plant is caused by an adverse pressure gradient and momentum advection out of the turbine rotor layer. The cumulative deceleration of the flow upstream of the front-row turbines instigates vertical motions. The horizontal flow is diverted vertically, reducing momentum availability in the turbine rotor layer. Although the adverse pressure gradient upstream of the wind plant remains unchanged with atmospheric stability, vertical advection of horizontal momentum is amplified in the more strongly stable boundary layer, mainly by larger shear of the horizontal velocity, thus increasing the blockage effect.

Effects of wind field characteristics on pitch bearing reliability: a case study of 5 MW reference wind turbine at onshore and offshore sites

Abstract: Abstract This paper presents a study on pitch bearing basic rating life affected by wind field characteristics at both onshore and offshore wind sites. The National Renewable Energy Laboratory 5 MW reference wind turbine is selected for the study. Wind field characteristics including reference hub height mean wind speed, wind speed distribution, wind shear, and vertical inflow are studied. A decoupled approach is employed where global analysis is performed first. Second, the load effects from the global analysis are applied on a reference pitch bearing designed based on best industrial practices. For the case study onshore site, it is found that the Kernel density estimation best fits the wind distribution, while the International Electrotechnical Commission proposed distribution appears to be not suitable. Moreover, it is shown that the seed number has high effect on the bearing life in turbulence wind and the wind speeds around rated have the highest contribution in both bearing fatigue damage and extreme load failure. The results contribute to better understanding of the wind field characteristics on the pitch bearing life.

Importance of self‐induced vertical wind shear and diabatic heating on the Fujiwhara effect

Abstract: This study hypothesises that the motion of binary tropical cyclones (TCs) can be modified by diabatic heating (DH) asymmetry associated with self‐induced vertical wind shear. To demonstrate this hypothesis in the quiescent environment, a set of the idealised f‐plane numerical experiments such as identical (varied in the initial separation distance) and non‐identical (varied in TC intensity and size) experiments are conducted. Results show that the binary TC tracks are affected by the initial separation distance, and TC intensity and size, that is, TC influence radius. Identical experiments show that the critical distance for separating, merging or repelling motion is 8° (where 1° ≃ 111 km). The binary TC interaction shows that single large‐scale cyclonic and anticyclonic circulations can develop as a result of the superposition of the circulation of the binary TCs in the lower and upper troposphere. These two different large‐scale circulations result in a vertically sheared environment in each TC. Potential vorticity (PV) tendency diagnosis shows that the TC motion is consistent with a positive region of the local tendency of PV. To quantify the contribution of each PV diagnostic term to the TC motion, we calculate each vector of the PV tendency diagnostic equation. In the merger case, the horizontal advection vector (HADV) appears to be greater than the DH vector (DHV) due to attraction. In the repulsion case, the DHV is largely compensated by the HADV. Specifically, the strong shear‐induced DH generated in the lower troposphere and downshear or downshear‐left quadrant can modify the TC motion in collaboration with the transport of PV to the middle and upper troposphere by offsetting the PV imbalance via the HADV. Overall, these results validate our hypothesis, which we refer to as the three‐dimensional Fujiwhara effect.

Heavy snowfall event over the Swiss Alps: did wind shear impact secondary ice production?

Abstract: Abstract. The change in wind direction and speed with height, referred to as vertical wind shear, causes enhanced turbulence in the atmosphere. As a result, there are enhanced interactions between ice particles that break up during collisions in clouds which could cause heavy snowfall. For example, intense dual-polarization Doppler signatures in conjunction with strong vertical wind shear were observed by an X-band weather radar during a wintertime high-intensity precipitation event over the Swiss Alps. An enhancement of differential phase shift (Kdp>1∘ km−1) around −15 ∘C suggested that a large population of oblate ice particles was present in the atmosphere. Here, we show that ice–graupel collisions are a likely origin of this population, probably enhanced by turbulence. We perform sensitivity simulations that include ice–graupel collisions of a cold frontal passage to investigate whether these simulations can capture the event better and whether the vertical wind shear had an impact on the secondary ice production (SIP) rate. The simulations are conducted with the Consortium for Small-scale Modeling (COSMO), at a 1 km horizontal grid spacing in the Davos region in Switzerland. The rime-splintering simulations could not reproduce the high ice crystal number concentrations, produced too large ice particles and therefore overestimated the radar reflectivity. The collisional-breakup simulations reproduced both the measured horizontal reflectivity and the ground-based observations of hydrometeor number concentration more accurately (∼20 L−1). During 14:30–15:45 UTC the vertical wind shear strengthened by 60 % within the region favorable for SIP. Calculation of the mutual information between the SIP rate and vertical wind shear and updraft velocity suggests that the SIP rate is best predicted by the vertical wind shear rather than the updraft velocity. The ice–graupel simulations were insensitive to the parameters in the model that control the size threshold for the conversion from ice to graupel and snow to graupel.

Does Low-level Vertical Wind Shear Matter for Hail Production?

Abstract: <p>Vertical wind shear (or more precisely, bulk wind vector magnitude differences between specified altitudes) has long been used for severe convective storm science and forecasting, in part owing to its relative success in correlating to various storm behaviors and hazards. However, theoretical, modeling, and observational work has suggested that this success may arise because vertical wind shear is associated with and/or a proxy for other, more dynamically relevant environmental characteristics.</p> <p>Recent research has explored various environmental controls on hail sizes in supercell storms, and found that vertical wind shear (and, by extension, hodograph shape) is an important determinant on a storm’s proclivity for hail production. Idealized numerical simulations of supercells showed that, as deep-layer shear increases (specifically, zonal shear in the 2-6 km AGL layer; this also means increased 0-6-km shear), the resulting broader updrafts increased hailstone residence time and thus size. Paradoxically, increasing the 0-2-km shear (predominantly, but not entirely, in the meridional direction) broadened the updraft but decreased hail size: increased southerly winds within the hail growth zone increased residence times along the main growth pathway. These studies left a lingering question: what is the underlying driver to changes in supercellular hail production, the low-level shear magnitude, its orientation relative to the deep-layer shear, or neither?    </p> <p>To answer this question, we present the results of simple idealized numerical modeling experiments in which the low-level (0-2 km) vertical wind shear magnitudes and directions are systematically and independently varied, keeping all other environmental factors the same. The resulting storms are used to drive the Kumjian & Lombardo (2020) hailstone growth trajectory model. The simulations lead to differences in hail sizes produced, despite having identical 0-2-km shear values. Rather, the differences in storm motion and hodograph shape lead to markedly different low-level storm-relative wind profiles. As a consequence of varied low-level storm-relative wind speeds and directions, the mesocyclonic flow speeds and directions within the hail growth zone differ amongst the experiments, which directly affects residence times and thus hail growth.</p>

Changes to Sea Surface Temperatures and Vertical Wind Shear and Their Influence on Tropical Cyclone Activity in the Caribbean and the Main Developing Region

Abstract: Sea surface temperatures and vertical wind shear are essential to tropical cyclone formation. TCs need warm SSTs and low shear for genesis. Increasing SSTs and decreasing VWS influences storm development. This work analyzes SST and VWS trends for the Caribbean, surrounding region, and the Atlantic hurricane main developing region from 1982–2020. Storm intensity increases significantly during this period. Annual and seasonal trends show that regional SSTs in the MDR are warming annually at 0.0219 °C yr−1 and, per season, 0.0280 °C yr−1. Simultaneously, VWS decreases during the late rainfall season, at 0.056 m/s yr−1 in the MDR and 0.0167 m/s yr−1 in the Caribbean and surrounding area. The Atlantic Warm Pool is expanding at 0.51 km2 per decade, increasing upper atmospheric winds and driving VWS changes. Correlations of large-area averages do not show significant relationships between TC intensity, frequency, and SSTs/VWS during the LRS. The observed changes appear to be associated with regional warming SSTs impacting TC changes. Plain Language Abstract: Tropical cyclone (TC) formation requires warm ocean waters and low wind shear. Changes to sea surface anomalies and wind shear influences are essential to understanding storm development and intensification. The ability to forecast storm changes is vital to human lives and livelihoods. This work analyzes sea surface temperatures (SSTs) and vertical wind shear (VWS) trends in the Caribbean, surrounding areas, and the Atlantic main developing region (MDR). We found increasing SSTs, decreasing wind shears, an expanding Atlantic Warm Pool (AWP), and increased storm intensity during the Atlantic hurricane season.

Contrasting Responses of Atlantic and Pacific Tropical Cyclone Activity to Atlantic Multidecadal Variability

Abstract: This research assesses the influences of Atlantic Multidecadal Variability (AMV) on global tropical cyclones (TCs) using two large ensembles of idealized global climate model simulations with opposite signs of AMV forcings superimposed (i.e., AMV+ and AMV–). We first detect TCs and then compare TC activity by basin in the two AMV experiments. We find contrasting responses of Atlantic and Pacific TC frequency to the AMV anomalies. Compared to AMV–, AMV+ significantly increases TC frequency in the North Atlantic, including those making landfalls. The increase is explained by warmer sea surface temperature, higher relative humidity, increased relative vorticity, and weaker vertical wind shear under AMV+. By contrast, AMV+ decreases TC occurrence over the western North Pacific and South Pacific, which is tied to stronger vertical wind shear and lower relative humidity. The opposite responses of TC activity to AMV+ are attributed to strengthened Walker Circulation between the Atlantic and Pacific.

Triggering effects of large topography and boundary layer turbulence on convection over the Tibetan Plateau

Abstract: Abstract. In this study, we analyze the diurnal variations and formation mechanisms of low clouds at different elevations. We further discuss whether there exists a triggering mechanism for convection over the Tibetan Plateau (TP) and whether there is an association among low air density, strong turbulence, and ubiquitous “popcorn-like” cumulus clouds. The buoyancy term (BT) and shear term (ST) over the TP are significantly greater than those at low elevations, which is favorable for the formation of an increasing planetary boundary layer height (PBLH) and also plays a key role in the convective activities in the lower troposphere. From the viewpoint of global effects, the triggering of convection by boundary layer dynamics is analyzed over the TP, but also in the Northern Hemisphere over the Rocky Mountains. It is found that ST and BT are strong over both high-elevation regions. The strong thermal turbulence and large-scale ascending motion jointly result in obvious positive values of PBLH–LCL (lifting condensation level) under low relative humidity (RH) conditions over the TP. The obvious large-scale subsidence on both sides of the Rocky Mountains, especially the western side, leads to inversion above the PBL and lower RH within the PBL, which further leads to negative values of PBLH–LCL and decreased low cloud cover (LCC) in most parts of the Rocky Mountains. The slightly greater-than-zero PBLH–LCL corresponds spatially to increased LCC in the partial regions of the central Rocky Mountains. Thus, less LCC is generated at the Rocky Mountains compared to the TP.

Generation of a mountain-valley wind in an atmospheric boundary wind tunnel: a gust-wind generator study

Abstract: Abstract Appropriate modeling of an experimental technology is necessary in order to estimate the aerodynamic characteristic of railway trains and infrastructure (e.g., bridges). Simulation of the earth’s wind characteristics of nature is a well-established practice by using an atmospheric boundary wind tunnel. However, in the mountainous area, the wind characteristics are strikingly different from those of the plain area, the amplitude variation of wind is related to complex terrain. Compared with atmospheric boundary layer winds, which are customarily treated as stationary, winds associated with gust winds originating from mountain areas exhibit rapid changes during a short period. A lack of available field test data and testing techniques has hindered such knowledge of the effect of mountain wind on railway-related applications. To simulate the characteristics of gust winds and prepare for follow-up studies of the impact on the railway-related structures, a gust wind generator was developed in an atmospheric boundary wind tunnel — the CSU wind tunnel. Further, the performance of the gust-wind generator was studied and analyzed under the condition of the combined operation between a gust-wind generator and a wind tunnel.

Boundary-layer aerodynamics over long wind farms over hilly terrains

Abstract: A wind-tunnel campaign with a wind-farm model over two cosinusoidal hills has been performed to assess the changes in the flow above a wind-farm model in presence of topography. The experiments focused on the three-dimensional velocity field above the turbines to characterise the boundary layer evolution along the farm. The flow over the hills was characterised first without the turbines and compared with potential flow theory with good agreement between the two. The presence of the turbines imposed an upward displacement of the velocity field with similar speed-ups on the hill crests as observed in absence of turbines. The vertical velocity was also quite similar in the windward side of the hills and significantly damped in the leeward side, probably due to boundary-layer separation and shear sheltering operated by the turbine top tips. The presence of free-stream turbulence does not change this qualitative picture, although the turbulent activity within the internal boundary layer is slightly increased. The analysis of the streamwise velocity spectra indicated the emergence of the wake meandering as dominant dynamics and its modulation operated by the topography.

Comment on egusphere-2022-1096

Abstract: Abstract. Volcanic eruptions that inject sulphur dioxide into the stratosphere have the potential to alter large-scale circulation patterns, such as the quasi-biennial oscillation (QBO), which can affect weather and transport of chemical species. Here, we conduct simulations of tropical volcanic eruptions using the UM-UKCA aerosol-climate model with an explicit representation of the QBO. Eruptions emitting 60 Tg of SO₂ (i.e., 1815 Mt. Tambora-magnitude) and 15 Tg of SO₂ (i.e., 1991 Mt. Pinatubo-magnitude) were simulated at the equator initiated during two different QBO states. We show that tropical eruptions delay the progression of the QBO phases, with the magnitude of the delay dependent on the initial wind shear in the lower stratosphere and a much longer delay when the shear is easterly than when it is westerly. The QBO response in our model is driven by vertical advection of momentum by the stronger tropical upwelling caused by heating due to the increased volcanic sulfate aerosol loading. Direct aerosol-induced warming with subsequent thermal wind adjustment, as proposed by previous studies, is found to only play a secondary role. This interpretation of the response is supported by comparison with a simple dynamical model. The dependence of the magnitude of the response on the initial QBO state results from differences in the QBO secondary circulation. In the easterly shear zone of the QBO, the vertical component of the secondary circulation is upward and reinforces the anomalous upwelling driven by volcanic aerosol heating, whereas in the westerly shear zone the vertical component is downward and opposes the aerosol-induced upwelling. We also find a change to the latitudinal structure of the QBO, with the westerly phase of the QBO strengthening in the hemisphere with the lowest sulfate aerosol burden. Overall, our study suggests that tropical eruptions of Pinatubo-magnitude or larger could force changes to the progression of the QBO, with particularly disruptive outcomes for the QBO if the eruption occurs during the easterly QBO shear.

Future Projections of Multiple Tropical Cyclone Events in the Northern Hemisphere in the CMIP6‐HighResMIP Models

Abstract: How future multiple tropical cyclone events (MTCEs) could change is crucial for effective risk management and ensuring human safety, however, it remains unclear. This study projects changes in MTCEs by 2050 in the major basins of the Northern Hemisphere using high-resolution climate models. Results show a significant increase in the frequency and duration of MTCEs over the North Atlantic (NA), a notable decrease over the western North Pacific (WNP), and little change over the eastern North Pacific (ENP). The increase in MTCEs over the NA is concentrated in August–September, while the decrease over the WNP occurs in most months. In contrast, the ENP exhibits large yet insignificant seasonal variation, suggesting considerable uncertainty in this basin. Further analysis shows that mid-level vertical motion dominates the MTCE changes over the WNP, while vertical wind shear contributes the most to the NA, which may be linked to future changes in tropical convection.

Large-Eddy Simulations of wind turbine wakes in sheared inflows

Abstract: Modern-day wind turbines are growing continuously in size and reach diameters of more than 200m in an effort to meet the fast growing demand for wind energy. As a consequence, the rotors are exposed to larger velocity variations in the approach flow due to the presence of shear, veer and turbulence. The shear of the ambient flow is an important effect that can impact the wake of a turbine twofold: one way is how the wake evolves in the sheared flow; the other way is by impacting the performance and loading of the turbine and, hence, the wake it produces. Both ways can affect the size, shape, spreading and recovery of the turbine wake and, consequently, impact on loads and power output of turbines located downstream. In this study, we analyzed the influence of different inflow wind shear configurations on the evolution of the wake behind the IEA 15MW reference wind turbine by means of high-resolution Large-Eddy Simulations. In order to isolate the shear effects, the mean and hub height wind speed of the inflow was kept constant by prescribing linear shear profiles without turbulence. The influence of Coriolis forces and thermal stratification are neglected. In addition, the effect of the imposed shear on the turbine’s power and thrust, and the effect of including the nacelle in the simulation, were monitored.

identification of robust predictors of tropical cyclones using causal effect network over the north indian ocean

Abstract: Tropical cyclone (TC) activity varies substantially yearly, and tropical cyclone-related damage also changes. Longer-term prediction of tropical cyclones plays an important role in reducing the wear and human loss caused by TCs. In this study, we have used a Causal-network-based algorithm to find the main development regions and precursors responsible for TC genesis and intensification. However, all the extreme events are interconnected through various global links. Therefore, analysis of the teleconnection and correlation of Tropical Cyclones with El Nino Southern Oscillation (ENSO), Indian Ocean Dipole (IOD), and North Atlantic Oscillation (NAO) during the satellite era (1980-2020) over the North Indian Ocean (NIO) basins using this Causal Effect Network (CEN) based algorithms is checked. The most appropriate metric for cyclone energy is Accumulated Cyclone Energy (ACE); its correlation with the various factors are investigated. We examined the variation in TCs activity during all three phases (positive, negative, and neutral phases).The results show an increasing trend in ACE over the NIO region during that specific period. The duration of most intense cyclones is increased, but their frequency decreases in this period. A shift in ACE starts after 1997 and still rises significantly. Analysis of Sea Surface Temperature (SST), Vertical Wind Shear (VWS) between 850 and 250 hPa, mid-tropospheric (800 hPa) Relative Humidity (RH), low level (850 hPa) Relative Vorticity (RV), and Tropical Cyclone Heat Potential (TCHP) is done, and it shows positive changes and variability of ACE. These results may help get better knowledge about the atmospheric or oceanic teleconnections between the events, and improved tropical cyclone prediction can help reduce the loss caused by the TCs.        

Low Altitude Tailing Es (LATTE): Analysis of Sporadic‐E Layer Height at Different Latitudes of Middle and Low Region

Abstract: In this paper, the Earth's sporadic-E (Es) layer vertical motion is investigated by using an image processing technique for automatic scaling ionograms from Mohe (122.37°E, 53.50°N, dip angle 71°), Beijing (116.25°E, 40.25°N, dip angle 59°), Wuhan (114.61°E, 30.53°N, dip angle 46°) and Fuke (109.13°E, 19.52°N, dip angle 27°). Es traces descend with different periodicities, indicating tidal modulation to Es layers. Comparing winds from a combination of the Ionospheric Connection Explorer/Michelson Interferometer for Global High-Resolution Thermospheric Imaging and meteor radar measurements with Es layers, we find that Es traces at high altitudes (above 110 km) rapidly move down in accordance with the descent of the wind shear nulls, which indicates the important role of the tides in the formation and descent of the Es layer at high altitude. The lower-lying Es layers, however, do not descend with the wind shear null, but stay at the bottom of the E region (∼100 km) for a long time, which cannot be explained by tidal wind shear theory. In addition, the time duration of the Es layers staying at low latitudes increases with the decreasing latitude. Simulation results demonstrate that the low altitude tailing Es layer is dominated by the dramatically enhanced collision frequency at the lower height of the mesosphere and the lower thermosphere region.

An analytical model of momentum availability for predicting large wind farm power

Abstract: Turbine-wake and farm-atmosphere interactions influence wind farm power production. For large offshore farms, the farm-atmosphere interaction is usually the more significant effect. This study proposes an analytical model of the `momentum availability factor' to predict farm-atmosphere interactions. It models the effects of net advection, pressure gradient forcing and turbulent entrainment. The model uses steady quasi-1D flow assumptions. Turbulent entrainment is modelled by assuming self-similar vertical shear stress profiles. We used the model with the `two-scale momentum theory' (Nishino & Dunstan 2020, J. Fluid Mech. 894, A2) to predict the power output of large finite-sized farms. The model compared well with existing results of large-eddy simulations (LES) of finite wind farms in conventionally neutral boundary layers. The model captured most of the effects of atmospheric conditions (e.g. atmospheric boundary layer (ABL) height and free-atmosphere stratification) on farm performance by only considering the undisturbed vertical shear stress profile of the ABL. The model predicted the power of staggered wind farms with a typical error of 5% or less. The developed model provides a novel way of quickly predicting the power output of large wind farms, including the farm blockage effects. This study also provides a new general framework for modelling farm-scale flows. Future studies can use the framework to better model large wind farms.

Analysis of the severe thunderstorm in Croatia on September 15th 2022.

Abstract: On September 15th 2022, in the afternoon hours, central part of Croatia was hit by a violent thunderstorm, with gale-  and possibly hurricane-force winds, hail and heavy rain - and caused severe damage to properties and particularly to the forests. Testimonies of local witnesses pointed even to possibility of occurence of a tornado. A comprehensive synoptic and mesoscale analysis was performed, including radiosounding, satelite and new radar observations from the recently established radar network.   Synoptic situation was characterized by the passage of a cold front, bringing cold air in the upper layers of atmosphere which enhanced  atmospheric instabiliy. Sounding analysis pointed out high value of most unstable convective available potential energy, accompanied by pronounced deep layer wind shear, as well as significant low layer shear (0-1 km) which is quite favorable for the formation of a tornado. Severity of the phenomenon was confirmed by lightning and satelite measurements (overshooting tops) and particularly by the radar image, pointing to hail areas and exhibiting so-called 'bow-apex' feature, indicating development of a supercell.  In order to clarify some doubts - for the first time in our practice – a detailed in situ inspection of damaged area was also carried out, and aerial footage was also consulted. Although the atmospheric conditions were prone to development of a tornado, no material proof lead to presence of a tornado vortex, since no evidence of wind spinnig was found, and trees were usually knocked down in the same direction.  The risk for severe thunderstorm activity was well forecasted, and corresponding operational alert was issued by the Met Service. Furthermore – as an early warning for the incoming weather change - a special announcement was issued on our web page. 

The Role of North American Convective Storms on Jet Stream Dynamics: A Negative Potential Vorticity Perspective

Abstract: Synoptic-scale filaments of negative potential vorticity (PV) in the northern hemisphere tropopause can form adjacent to the jet stream in the presence of convection and moderate shear (i.e., severe thunderstorm environments). Case-studies have shown that synoptic-scale negative PV can influence in-situ jet stream dynamics. Negative PV arises due to strong vorticity in convective updrafts, driven by the horizontal gradient of diabatic heating (O < 10 km).&#160; Its origin from scales not resolvable by contemporary global weather models can thus also impinge on jet stream forecast skill.Nevertheless, little is still known about the characteristics of synoptic-scale negative PV. How frequently is it observed? And what are its &#8216;typical&#8217; impacts on the jet stream?Focusing on North America where severe thunderstorms are frequent, we design an algorithm that tracks the temporal evolution of closed contours of upper-level, negative PV air using ERA5 data. We composites instances in which it is in close-proximity to (&#8216;interacts with&#8217;) the jet stream and assess its dynamical response. The role of negative PV on jet evolution and its downstream response over the Atlantic is facilitated through a combination of lagged composite analysis and K-means clustering.Our composite results in combination with preliminary high-resolution model simulations highlight that elongated bands of negative PV frequently interact with the jet stream, intensify jet wind maxima and may serve as an amplification source for Rossby waves.