Structure Analysis of the Sea Breeze Based on Doppler Lidar and Its Impact on Pollutants

The Doppler lidar system can accurately obtain wind profiles with high spatiotemporal resolution, which plays an increasingly important role in the research of atmospheric boundary layers and sea–land breeze. In September 2019, Doppler lidars were used to carry out observation experiments of the atmospheric wind field and pollutants in Shenzhen. Weather Research and Forecasting showed that the topography of Hongkong affected the sea breeze to produce the circumfluence flow at low altitudes. Two sea breezes from the Pearl River Estuary and the northeast of Hong Kong arrived at the observation site in succession, changing the wind direction from northeast to southeast. Based on the wind profiles, the structural and turbulent characteristics of the sea breeze were analyzed. The sea breeze front was accurately captured by the algorithm based on fuzzy logic, and its arrival time was 17:30 on 25 September. The boundary between the sea breeze and the return flow was separated by the edge enhancement algorithm. From this, the height of the sea breeze head (about 1100 m) and the thickness of the sea breeze layer (about 700 m) can be obtained. The fluctuated height of the boundary and the spiral airflow nearby revealed the Kelvin–Helmholtz instability. The influence of the Kelvin–Helmholtz instability could be delivered to the near-surface, which was verified by the spatiotemporal change of the horizontal wind speed and momentum flux. The intensity of the turbulence under the control of the sea breeze was significantly lower than that under the land breeze. The turbulent intensity was almost 0.1, and the dissipation rate was between 10−4 and 10−2 m2·s−3 under the land breeze. The turbulent intensity was below 0.05, and the dissipation rate was between 10−5 and 10−3 m2·s−3 under the sea breeze. The turbulent parameters showed peaks and large gradients at the boundary and the sea breeze front. The peak value of the turbulent intensity was around 0.3, and the dissipation rate was around 0.1 m2·s−3. The round-trip effect of sea–land breeze caused circulate pollutants. The recirculation factor was maintained at 0.5–0.6 at heights where the sea and land breeze alternately controlled (below 600 m), as well as increasing with a decreasing duration of the sea breeze. The factor exceeded 0.9 under the control of the high-altitude breeze (above 750 m). The convergence and rise of the airflow at the front led to collect pollutants, causing a sharp decrease in air quality when the sea breeze front passed.

Relationship between topography, tropospheric wind, and frequency of mountain waves in the upper mesosphere over the Kanto area of Japan

Imaging observations of OH airglow were performed at Meiji University, Japan (35.6° N, 139.5° E), from May 2018 to December 2019. Mountainous areas are located to the west of the imager, and westerly winds are dominant in the lower atmosphere throughout the year. Mountain waves (MWs) are generated and occasionally propagate to the upper atmosphere. However, only four likely MW events were identified, which are considerably fewer than expected. There are two possible reasons for the low incidence: (1) MWs do not propagate easily to the upper mesosphere due to background wind conditions, and/or (2) the frequency of MW excitation was low around the observation site. Former possibility is found not to be a main reason to explain the frequency by assuming typical wind profiles in troposphere and upper mesosphere over Japan. Thus, frequency and spatial distribution of orographic wavy clouds were investigated by analyzing images taken by the Himawari-8 geostationary meteorological satellite in 2018. The number of days when wavy clouds were detected in the troposphere around the observation site (Kanto area) was about a quarter of that around the Tohoku area. This result indicates that frequency of over-mountain flow which is thought to be a source of excitation of MWs is low in Kanto area. We also found that the angle between the horizontal wind direction in troposphere and the orientation of the mountain ridge is a good proxy for the occurrence of orographic wavy clouds, i.e., excitation of MWs. We applied this proxy to the topography around the world to investigate regions where MWs are likely to be excited frequently throughout the year to discuss the likelihood of "MW hotspots" at various spatial scale. Graphical Abstract

Classification and properties of coastal wind profiles with negative gradients – an observational study

Wind profiles with a negative gradient are frequently occurring over the Baltic Sea and are important to take into consideration for offshore wind power as they affect not only the power production, but also the loads on the structure and the behavior of the wake behind the turbine. In this study, we classified non-normal profiles as wind profiles having negative shear in at least one part of the profile between 28 and 300 m: low-level jets (with a local wind maximum in the profile), profiles with a local minimum, and negative profiles. Using observations spanning over 3 years, we show that the non-normal wind profiles are common over the Baltic Sea in late spring and summer, with a peak of 40 % relative occurrence in May. Negative profiles (in the 28–300 m layer) were mostly occurring during unstable conditions, in contrast to low-level jets that primarily occurred in stable stratification. There were indications that the the zone with strong shear during low-level jets could cause a relative suppression of the variance for large turbulent eddies compared to the peak of the velocity spectra, in the layer below the jet core. Swell conditions were found to be favourable for the occurrence of negative profiles and profiles with a local minimum, as the waves fed energy into the surface layer, resulting in an increase of the wind speed from below.

Influence of Aerosols on Atmospheric Gravity Waves at an Urban Tropical Location

The elevated layer of heat-absorbing pollutant aerosols causes temperature perturbations in the pre-monsoon period above the boundary layer height (1.6-4 km) as observed over a polluted tropical urban location Kolkata (22°34' N, 88°22' E) during 2007-2016. Satellite observations of different types of aerosols show an increase in aerosol extinction coefficient around 1.6-4 km altitude, enhancing the perturbations in both temperature and wind profiles at that height. The opposing air mass movement within and above the boundary layer, which is strengthened by elevated heat-absorbing aerosols, is illustrated by height profiles of atmospheric vorticity and divergence. This results in higher Brunt-Vaisala frequencies indicating increased atmospheric oscillations. Consequently, atmospheric gravity waves, which manifest the temperature and wind profile perturbations, have enhanced energy in the upper troposphere (6-10 km). Based on multi- technique observations consisting of radiosonde, space-borne lidar and model data, this study reveals the interactions between aerosol and other atmospheric processes such as temperature variations and wind perturbations, which affect the atmospheric instability and increase gravity wave activities during the pre-monsoon period over a tropical metropolis.

Comparative Study of the Near-Surface Typhoon Wind Profile Fitting between Offshore and Onshore Areas

The study of typhoon wind profiles, especially offshore typhoon wind profiles, has been constrained by the scarcity of observational data. In this study, the Doppler wind lidar was used to observe the offshore wind profiles during Super Typhoon Mangkhut and onshore wind profiles during Super Typhoon Lekima. Four wind profile models, including the power law, logarithmic law, Deaves–Harris (D-H), and Gryning, were selected in the height range of 0–300 m to fit the wind profile. The variations in the power exponent with the mean wind speed and roughness length were also analyzed. The results showed that the wind profiles fitted by the four models were generally in good agreement with the observed wind profiles with correlation coefficients greater than 0.98 and root mean square deviations less than 0.5 m s−1. For the offshore case, the fitting degree of all wind profile models improved with increasing mean wind speed. Specifically, the D-H model had the highest fitting degree when the horizontal mean wind speed at 40 m was in the range of 8–25 m s−1, while the log-law model had the highest fitting degree when the wind speed exceeded 30 m s−1. For the onshore case, the fitting degree of the four wind profile models deteriorated with increasing mean wind speed, and the log-law model had the highest fitting degree in all wind speed intervals from 8 to 30 m s−1. For both offshore and onshore cases, the power exponent was less affected by mean wind speed and increased with increasing roughness length, and the logarithmic empirical model proposed in this study could well characterize the relationship between the power exponent and roughness length.

Modelling Wind Damage to Southeastern U.S. Trees: Effects of Wind Profile, Gaps, Neighborhood Interactions, and Wind Direction

Tree damage from a variety of types of wind events is widespread and of great ecological and economic importance. In terms of areas impacted, tropical storms have the most widespread effects on tropical and temperate forests, with southeastern U.S. forests particularly prone to tropical storm damage. This impact motivates attempts to understand the tree and forest characteristics that influence levels of damage. This study presents initial findings from a spatially explicit, individual-based mechanistic wind severity model, ForSTORM, parameterized from winching research on trees in southeastern U.S. This model allows independent control of six wind and neighborhood parameters likely to influence the patterns of wind damage, such as gap formation, the shape of the vertical wind profile, indirect damage, and support from neighbors. We arranged the subject trees in two virtual stands orientations with identical positions relative to each other, but with one virtual stand rotated 90 degrees from the other virtual stand – to explore the effect of wind coming from two alternative directions. The model reproduces several trends observed in field damage surveys, as well as analogous CWS models developed for other forests, and reveals unexpected insights. Wind profiles with higher extinction coefficients, or steeper decrease in wind speed from canopy top to lower levels, resulted in significantly higher critical wind speeds, thus reducing level of damage for a given wind speed. Three alternative formulations of wind profiles also led to significant differences in critical wind speed (CWS), although the effect of profile was less than effect of different extinction coefficients. The CWS differed little between the two alternative stand orientations. Support from neighboring trees resulted in significantly higher critical wind speeds, regardless of type of wind profile or spatial arrangement of trees. The presence or absence of gaps caused marginally significant different in CWS, while inclusion of indirect damage along with direct damage did not significantly change CWS from those caused by direct damage alone. Empirical research that could most benefit this modelling approach includes improving crown area measurement, refining drag coefficients, and development of a biomechanical framework for neighbor support.

Assessment of a Broadband Differential Absorption Lidar (DIAL) for use in a ground-based measurement network

<p>Convective-scale forecasts require more detailed and continuous observational data of thermodynamic profiles and wind profiles in the atmospheric boundary layer (ABL) than currently provided. In order to meet these data requirements in the future, DWD evaluates various surface remote sensing systems targeted on ABL-profiling for routine network operation.</p> <p>One of the candidate systems in operation at the Observatory Lindenberg is a new pre-production broadband DIAL from Vaisala. DIAL instruments are well-established in research activities, but this instrument is developed for operationally providing water vapor profile observations in the ABL during all weather conditions. We present evaluation results of the DIAL’s operational performance regarding the quality of the water vapor profiles and report on its ability to monitor sub-grid scale processes, such as convection and associated weather phenomena. This includes comparisons with radiosounding observations (4 per day) over at least one year of continuous observations and additional comparisons with Raman lidar for a three-month period during summer 2021. Furthermore, we provide observation-minus-background statistics between the DIAL and the ICON limited area model (ICON-LAM) to evaluate the model performance, e.g. under convection, and to identify observational error sources.</p> <p>This contribution provides knowledge regarding the operational viability of the new pre-production broadband DIAL, its value for monitoring water vapour profiles 24/7 and ABL processes for future model applications.</p>

Coupling of Wind and Potential Temperature in an Ekman Model in the Stratified Atmospheric Boundary Layer

A linear Ekman model in the stratified atmospheric boundary layer (ABL) is proposed based on the steady state version of the linearized three-dimensional primitive equations with the inclusion of the vertical diffusivity. Due to the inclusion of the potential temperature equation and hydrostatic equation, pressure and potential temperature couple with wind in the proposed model, and thus are not arbitrarily specified variables as in previous studies on the baroclinicity in the Ekman model. The extended thermal wind balance equation and the Ekman potential vorticity equation are derived to describe the coupling. The two equations, along with the equation describing the constraint on potential temperature, are employed to derive the analytical solutions of the proposed Ekman model. Because potential temperature is not a specified variable but part of the solution, the derived analytical solutions have very different forms from those derived in previous studies. The differences illustrate the impact of the inclusion of the potential temperature equation and hydrostatic equation on wind, pressure and potential temperature in the proposed Ekman model. It is found that the computed wind profiles based on the proposed model can capture some important features of the observed wind profiles.

EXPERIMENTAL COMPARATIVE STUDY BETWEEN MPPT, HYBRID P&O AND FUZZY CONTROL FOR A SMALL WIND TURBINE APPLICATION

ABSTRACT This study focuses on a comparison between different maximum power extraction controls for a small wind turbine application. In this case, a vertical axis wind turbine (VAWT) coupled to a permanent magnet synchronous generator (PMSG) is used. The analysis is performed for variable winds, so that a real small wind turbine application is simulated. An experimental platform and a wind tunnel are used to recreate the different wind profiles. Diverse controls are compared, a "classical" MPPT, a hybrid Perturb & Observe control and a fuzzy control. Artificial intelligence allows the creation of control algorithms that are not bound by the mechanical parameters of the wind turbine, thus avoiding the need to characterize the wind turbine and define the maximum power curve. It is determined that, for almost constant wind profiles, the hybrid Perturb & Observe extracts more energy. However, for the same profile with greater variability, the fuzzy controller extracts more energy.