Presence Of Low-level Jet Research Articles

Effects of Low‐Level Jets on Near‐Surface Turbulence and Wind Direction Changes in the Nocturnal Boundary Layer

AbstractIn this study we examined a data set of nearly two‐year collection and investigated the effects of low‐level jets (LLJ) on near‐surface turbulence, especially wind direction changes, in the nocturnal boundary layer. Typically, nocturnal boundary layer is thermally stratified and stable. When wind profiles exhibit low gradient (in the absence of LLJ), it is characterized by very weak turbulence and very large, abrupt, but intermittent wind direction changes (∆WD) in the layers near the surface. In contrast, presence of LLJs can cause dramatic changes through inducing wind velocity shears, enhancing vertical mixing, and weakening the thermal stratification underneath. Ultimately, bulk Richardson number (Rb) is reduced and weakly stable conditions prevail, leading to active turbulence, close coupling across the layers between the LLJ height and ground surface, relatively large vertical momentum and sensible heat fluxes, and suppressed ∆WD values. Rb can be a useful parameter in assessing turbulence strength and ∆WD as well. The dependence of ∆WD on Rb appears to be well defined under weakly stable conditions (0.0 < Rb ≤ 0.25) and ∆WD is generally confined to small values. However, the relationship between ΔWD and Rb breaks when Rb increases, especially Rb > 1.0 (very stable conditions), under which ΔWD varies across a very wide range and the potential for large ΔWD increases greatly. Our findings have provided important implications to the plume dispersion in the nocturnal boundary layers.

A Three-Year Climatology of the Wind Field Structure at Cape Baranova (Severnaya Zemlya, Siberia) from SODAR Observations and High-Resolution Regional Climate Model Simulations during YOPP

Measurements of the atmospheric boundary layer (ABL) structure were performed for three years (October 2017–August 2020) at the Russian observatory “Ice Base Cape Baranova” (79.280° N, 101.620° E) using SODAR (Sound Detection And Ranging). These measurements were part of the YOPP (Year of Polar Prediction) project “Boundary layer measurements in the high Arctic” (CATS_BL) within the scope of a joint German–Russian project. In addition to SODAR-derived vertical profiles of wind speed and direction, a suite of complementary measurements at the observatory was available. ABL measurements were used for verification of the regional climate model COSMO-CLM (CCLM) with a 5 km resolution for 2017–2020. The CCLM was run with nesting in ERA5 data in a forecast mode for the measurement period. SODAR measurements were mostly limited to wind speeds <12 m/s since the signal was often lost for higher winds. The SODAR data showed a topographical channeling effect for the wind field in the lowest 100 m and some low-level jets (LLJs). The verification of the CCLM with near-surface data of the observatory showed good agreement for the wind and a negative bias for the 2 m temperature. The comparison with SODAR data showed a positive bias for the wind speed of about 1 m/s below 100 m, which increased to 1.5 m/s for higher levels. In contrast to the SODAR data, the CCLM data showed the frequent presence of LLJs associated with the topographic channeling in Shokalsky Strait. Although SODAR wind profiles are limited in range and have a lot of gaps, they represent a valuable data set for model verification. However, a full picture of the ABL structure and the climatology of channeling events could be obtained only with the model data. The climatological evaluation showed that the wind field at Cape Baranova was not only influenced by direct topographic channeling under conditions of southerly winds through the Shokalsky Strait but also by channeling through a mountain gap for westerly winds. LLJs were detected in 37% of all profiles and most LLJs were associated with channeling, particularly LLJs with a jet speed ≥ 15 m/s (which were 29% of all LLJs). The analysis of the simulated 10 m wind field showed that the 99%-tile of the wind speed reached 18 m/s and clearly showed a dipole structure of channeled wind at both exits of Shokalsky Strait. The climatology of channeling events showed that this dipole structure was caused by the frequent occurrence of channeling at both exits. Channeling events lasting at least 12 h occurred on about 62 days per year at both exits of Shokalsky Strait.

A case study of wind farm effects using two wake parameterizations in the Weather Research and Forecasting (WRF) model (V3.7.1) in the presence of low-level jets

Abstract. While the wind farm parameterization by Fitch et al. (2012) in the Weather Research and Forecasting (WRF) model has been used and evaluated frequently, the explicit wake parameterization (EWP) by Volker et al. (2015) is less well explored. The openly available high-frequency flight measurements from Bärfuss et al. (2019a) provide an opportunity to directly compare the simulation results from the EWP and Fitch scheme with in situ measurements. In doing so, this study aims to complement the recent study by Siedersleben et al. (2020) by (1) comparing the EWP and Fitch schemes in terms of turbulent kinetic energy (TKE) and velocity deficit, together with FINO 1 measurements and synthetic aperture radar (SAR) data, and (2) exploring the interactions of the wind farm with low-level jets (LLJs). This is done using a bug-fixed WRF version that includes the correct TKE advection, following Archer et al. (2020). Both the Fitch and the EWP schemes can capture the mean wind field in the presence of the wind farm consistently and well. TKE in the EWP scheme is significantly underestimated, suggesting that an explicit turbine-induced TKE source should be included in addition to the implicit source from shear. The value of the correction factor for turbine-induced TKE generation in the Fitch scheme has a significant impact on the simulation results. The position of the LLJ nose and the shear beneath the jet nose are modified by the presence of wind farms.

Effect of low-level jet on turbine aerodynamic blade loading using large-eddy simulations

Low-level jets (LLJs) are winds with high-shear and large wind energy potential. We perform large-eddy simulations (LES) with actuator line modeling of a turbine operating in a moderately stable boundary layer in the presence of LLJs. We find that the turbine tip and root vortices break down quickly when the LLJ is above the turbine rotor swept area. In contrast, the wake recovery is slow, and the vortices are stable when the LLJ is in the middle or even below the rotor swept area. The LLJ shear causes significant azimuthal variation in the external aerodynamic blade loading, increasing fatigue loading on the turbines. We observe that both tangential and axial forces on the blades are highest when the blade directly interacts with the LLJ. Azimuthal variation in the tangential forces on the blades is the highest when the LLJ is above the rotor swept area, i.e. when the turbine operates in the positive shear region of LLJ, with the blade tip interacting with the LLJ.

A case study of wind farm effects using two wake parameterizations in WRF (V3.7) in the presence of low level jets

Abstract. While the wind farm parameterization by Fitch et al. (2012) in Weather Research and Forecasting (WRF) model has been used and evaluated frequently, the Explicit Wake Parameterization (EWP) by Volker et al. (2015) is less well explored. The openly available high frequency flight measurements from Barfuss et al. (2019) provide an opportunity to directly compare the simulation results from the EWP and Fitch scheme with in situ measurements. In doing so, this study aims to compliment the recent study by Siedersleben et al. (2020) by (1) comparing the EWP and Fitch schemes in terms of turbulent kinetic energy (TKE) and velocity deficit, together with FINO 1 measurements and Synthetic Aperture Radar (SAR) data and (2) exploring the interactions of the wind farm with Low Level Jets. Both the Fitch and the EWP schemes can capture the mean wind field in the presence of the wind farm consistently and well. However, their skill is limited in capturing the flow acceleration along the farm edge. TKE in the EWP scheme is significantly underestimated, suggesting that an explicit turbine-induced TKE source should be included in addition to the implicit source from shear. The position of the LLJ nose and the shear beneath the jet nose are modified by the presence of wind farms.

Análise do perfil vertical do vento no Centro de Lançamento de Alcântara com uso de Análise de Componentes Principais

O objetivo deste estudo é fazer uma análise do comportamento do perfil vertical do vento, utilizando dados de radiossondagem no Centro de Lançamento de Alcântara (CLA), por meio do uso da técnica estatística multivariada da Análise das Componentes Principais (ACP), verificar se há presença de fluxos no perfil vertical do vento. Estes fluxos podem caracterizar circulações locais e estar associados com circulações do tipo brisa marítima, e também pode estar associado a circulações ao nível de mesoescala sinótica, tais como os Jatos de Baixos Níveis (JBN). Os resultados mostraram que nas três primeiras componentes no período noturno há presença de JBN, com 85% da variância explicada. A ocorrência dos JBN apresentou alturas típicas nas componentes, sendo a primeira com altura de 800 metros (direção média 60 a 70 graus), a segunda com altura de 400 metros (direção média 40 a 50 graus) e a terceira com 200 metros (direção média 50 a 60 graus). Tais JBN encontrados podem estar associados a processos relacionados a Camada Limite Planetária (CLP) no período noturno, como por exemplo o vento máximo de inversão e brisas.

Lidar investigation of wind turbulence in the presence of low-level jet in the atmosphere

Lidar investigation of wind turbulence in the presence of low-level jet in the atmosphere

Impact of Nocturnal Low-Level Jets on Near-Surface Turbulence Kinetic Energy

We report on the role of low-level jets (LLJs) on the modulation of near-surface turbulence in the stable boundary layer, focusing on the behaviour of the transport terms of the turbulence kinetic energy (TKE) budget. We also examine the applicability of Monin–Obukhov similarity theory (MOST) in light of these terms. Using coincident near-surface turbulence and LLJ data collected over a three-month period in South Carolina, USA, we found that turbulence during LLJ periods was typically stronger and more well-developed in comparison with periods without a LLJ. We found a local imbalance in the near-surface TKE budget, in which the imbalance (residual) term was typically positive (i.e., energy gain) and nearly in equilibrium with buoyant consumption. Based on a comparison with previous studies, we assume that this residual term represents mostly pressure transport. We found the behaviour of the residual term to be better delineated in the presence of LLJs. We found shear production to adhere to MOST remarkably well during LLJs, except under very stable conditions. Gain of non-local TKE via pressure transport, likely consisting of large-scale fluctuations, could be the cause of the observed deviation from the MOST $$z$$ -less prediction. The fact that this deviation was observed for periods with well-developed turbulence with an inertial subrange slope close to $$-5/3$$ indicates that such Kolmogorov turbulence is not a sufficient condition to guarantee the applicability of the MOST $$z$$ -less concept, as recently suggested in the literature. The implications of these results are discussed.

The effects of mean atmospheric forcings of the stable atmospheric boundary layer on wind turbine wake

Interactions between the nocturnal atmospheric boundary layer (ABL) and wind turbines (WTs) can be complicated due to the presence of low level jets (LLJ), a region which creates wind speeds higher than geostrophic wind speed. A study has been performed to isolate the effect of mean forcings of the ABL on turbulence energetics and structures in the wake of WT. Large eddy simulation with an actuator line model has been used as a tool to simulate a full-scale 5-MW WT under two different realistic atmospheric states of the stable ABL corresponding to low- and high-stratification. The study clearly demonstrates that the large-scale forcings of thermally stratified atmospheric boundary characterized by shear- and buoyancy-driven turbulence significantly influence the wake structure of a wind turbine. For the WT in low-stratified ABL, the jets occur above the WT resulting in a strong mixed layer behind the WT. High turbulence results in a faster wake recovery. For the WT in high-stratified ABL, the jets occur near the hub-height resulting in an asymmetric wake structure. The jets confine the mixing to hub-height resulting in a slower wake recovery. Vertical shear causes the interaction of the root- and lower-tip vortices resulting in the instability of the root vortex leading to an enhanced shear stress and turbulent kinetic energy. The tip vortices exhibit mutual inductance between adjacent vortex filaments resulting in vortex merging. LLJs are an important metric associated with mean atmospheric forcings that dictate the turbulence generated in WT wake and the wake recovery of a WT in a stable ABL.

Boundary layer jet on the lee side of Western Ghats during southwest monsoon as revealed by high resolution sodar winds

High resolution winds measured by SOund Detection And Ranging (SODAR) in the atmospheric boundary layer (ABL) over Pune, located on the lee side of the Western Ghats (WG) reveal the presence of Low Level Jet (LLJ) in the atmospheric boundary layer (hereafter called as boundary layer jet, BLJ) during southwest monsoon (June–August) for three consecutive years, viz., 2009, 2010 and 2011. Its prevalence only during the monsoon period, even at monthly scale indicates a possible interconnection to another LLJ, Somali jet, present during this period. An investigation is done to look into the dominant mechanisms those could result in such an occurrence in the lee side of the WG. Baroclinicity, inertial oscillation, upstream topographic blocking and variability of Somali jet are the mechanisms considered. Baroclinicity due to east–west temperature gradient on the leeside to induce thermal wind and inertial oscillation appear to have a little role in LLJ formation. However, Somali jet seems to govern the BLJ on the lee side through its dynamical interaction with the WG causing flow reversal and wave breaking above WG and inducing acceleration of downslope winds. The height and sharpness of Somali jet core on the windward side and its magnitude as well as direction with respect to WG are observed to determine the strength of BLJ on the lee side illustrating the signature of the summer monsoon in the boundary layer over Pune.

Response of Ecosystem Carbon and Water Vapor Exchanges in Evolving Nocturnal Low-Level Jets

The nocturnal low-level jet makes a significant impact on carbon and water exchanges and turbulent mixing processes in the atmospheric boundary layer. This study reports a case study of nocturnal surface fluxes such as <TEX>$CO_2$</TEX> and water vapor in the surface layer observed at a flat and homogeneous site in the presence of low-level jets (LLJs). In particular, it documents the temporal evolution of the overlying jets and the coincident response of surface fluxes. The present study highlights several factors linking the evolution of low-level jets to surface fluxes: 1) wavelet analysis shows that turbulent fluxes have similar time scales with temporal scale of LLJ evolution; 2) turbulent mixing is enhanced during the transition period of low-level jets; and 3) <TEX>$CO_2$</TEX>, water vapor and heat show dissimilarity from momentum during the period. We also found that LLJ activity is related not only to turbulent motions but also to the divergence of mean flow. An examination of scalar profiles and turbulence data reveal that LLJs transport <TEX>$CO_2$</TEX> and water vapor by advection in the stable boundary layer, suggesting that surface fluxes obtained from the micrometeorological method such as nocturnal boundary layer budget technique should carefully interpreted in the presence of LLJs.

On concentration footprints for a tall tower in the presence of a nocturnal low-level jet

This paper reports on the location of sources contributing to a point concentration measurement in the stable boundary layer. Concentration footprints for sensors located at different heights during the night are examined using a backward-in-time Lagrangian stochastic dispersion model. Simulations of air parcel transport in a non-steady-state atmospheric boundary layer above a bare surface and a forest under different insolation suggest that sources located as far as several hundred kms away contribute to a concentration measurement made at levels as high as 500 m. The origin of the maximal contribution area shifts during the night depending on the wind direction at the sensor location, a feature most prominent in the presence of decoupling between sensor levels and surface sources. Simulations suggest that atmospheric static stability alone is not a sufficient criterion to trigger flow decoupling. The presence of the low-level jet provides vertical mixing of air parcels even for stable boundary layers with Richardson numbers higher than the critical. This finding is in agreement with earlier observations suggesting that the level of the low-level jet nose acts as a strong lid prohibiting vertical gas propagation. These findings have important implications in the interpretation of eddy-flux measurements in nocturnal conditions.

Evaluation of the Weather Research and Forecasting model on forecasting low‐level jets: implications for wind energy

AbstractNocturnal low‐level jet (LLJ) events are commonly observed over the Great Plains region of the USA, thus making this region more favorable for wind energy production. At the same time, the presence of LLJs can significantly modify vertical shear and nocturnal turbulence in the vicinities of wind turbine hub height, and therefore has detrimental effects on turbine rotors. Accurate numerical modeling and forecasting of LLJs are thus needed for precise assessment of wind resources, reliable prediction of power generation and robust design of wind turbines. However, mesoscale numerical weather prediction models face a challenge in precisely forecasting the development, magnitude and location of LLJs. This is due to the fact that LLJs are common in nocturnal stable boundary layers, and there is a general consensus in the literature that our contemporary understanding and modeling capability of this boundary‐layer regime is quite poor. In this paper, we investigate the potential of the Weather Research and Forecasting (WRF) model in forecasting LLJ events over West Texas and southern Kansas. Detailed observational data from both cases were used to assess the performance of the WRF model with different model configurations. Our results indicate that the WRF model can capture some of the essential characteristics of observed LLJs, and thus offers the prospect of improving the accuracy of wind resource estimates and short‐term wind energy forecasts. However, the core of the LLJ tended to be higher as well as slower than what was observed, leaving room for improvement in model performance. Copyright © 2008 John Wiley &amp; Sons, Ltd.

Planetary boundary layer structure and air mass transport during the International Arctic Ocean Expedition 1991

During the International Arctic Ocean Expedition 1991 north of latitudes 70° N from August 1 to October 6 (late summer and early autumn), the dominant air mass origin was from the seas that surround the central Arctic Ocean, while land was a much less important source. This resulted in air temperatures near 0 °C, fogs and extensive stratus layers. In the autumn, long residence time of the air over the pack ice and subsidence became more common, accompanied by decreasing cloudiness, foginess and air temperature. The residence time of air over the pack ice and the time since land contact had medians of 52 and 91 h, respectively. The structure and evolution of the marine boundary layer above the pack ice is described. A stable layer, 300–1700 m high, grew in height proportionally to the square root of the distance from the ice edge, the fetch, up to 3000 km. After 400 km, a mixed layer formed, typically 100–250 m thick, related to the presence of low level jets in 60–90% of the profiles. Layers of subcritical Richardson number were present in 65–80% of the profiles above and below the jet centres. Cloud streets and roll vortices were observed above the pack ice with wavelengths of 4–12 km. The high aspect ratios of 7–9 of the few earlier roll observations above ice were confirmed. Cold air advection and weak convection caused the roll vortices in aged air that had been advected usually 2000 km or more over the ice. Breaking of Kelvin-Helmholtz waves, gravity waves or other reasons for intermittent planetary boundary layer breakdown combined with vertical gradients in concentration are suggested to have caused the observed sudden changes, in an hour or less, in aerosol and tracer concentrations and to have influenced fog formation. Roll vortices are suggested as the cause of periodic changes in aerosol concentrations and fog formation. DOI: 10.1034/j.1600-0889.1996.t01-1-00004.x