Background Wind Conditions Research Articles

Flow patterns and heat transfer of an idealized square city in non-uniform heat flux and different background wind conditions

The city scale buoyancy-driven flow is crucial for the wind and thermal environment in urban areas, exerting a significant impact on the city ventilation, pollutants dispersion and heat removal. The intra-urban variations of hot spots and heat flux are drawing more attention on beating the urban heat. This study aims at analyzing the influences of non-uniform heat flux and background wind on the city-scale heat removal ability and underlying mechanisms. The large eddy simulation model was used and verified with water tank experiments. Results show that positive heat flux and negative heat flux in the rural areas will enhance and depress the strength of the urban heat dome flow respectively compared to the adiabatic condition. The high-speed regions are wider and stronger when the rural area has positive heat flux condition. Under positive rural heat flux condition, the time-average mixed layer height in urban areas across the city center plane is nearly twice that under negative rural heat flux condition. Besides, the city-scale heat transfer coefficients in positive rural heat flux condition are higher than those under negative rural heat flux and adiabatic conditions. For cases of non-uniform heat flux in urban areas, the differences of time-average mixed layer height and heat transfer coefficients are not obvious. The heat transfer coefficients decrease initially and then increase with increasing background wind speed indicating the non-linear interaction between approaching wind and urban heat dome flow. Moreover, the growth rate of the heat transfer coefficient decreases when the non-dimensional wind speed exceeds 1.19.

Observing Gravity Waves Generated by Moving Sources With Ground‐Based Rayleigh Lidars

AbstractTemperature measurements by zenith‐pointing ground‐based Rayleigh lidars are often used to detect middle atmospheric gravity waves. In time‐height diagrams of temperature perturbations, stationary mountain waves are identifiable by horizontal phase lines. Vertically tilted phase lines, on the other hand, indicate that the wave source or the propagation conditions are transient. Idealized numerical simulations illustrate that and how a wave source moving in the direction of the mean wind entails upward‐tilted phase lines. The inclination angle depends on the horizontal wavelength and the wave source’s propagation speed. On this basis, the goal is to identify and characterize non‐orographic gravity waves (NOGWs) from propagating sources, for example, upper‐level jet/front systems, in simulated lidar observations and actual Rayleigh lidar measurements. Compositions of selected atmospheric variables from a meteorological forecast or reanalysis are thoughtfully combined to associate NOGWs with processes in the troposphere and stratosphere. For a virtual observation over the Southern Ocean, upward‐tilted phase lines indeed dominate the time‐height diagram during the passage of an upper‐level trough. The example also emphasizes that temporal filtering of temperature measurements is appropriate for NOGWs, especially in the presence of a strong polar night jet that implies large vertical wavelengths. During two selected observational periods of the COmpact Rayleigh Autonomous Lidar (CORAL) in the lee of the southern Andes, upward‐tilted phase lines are mainly associated with mountain waves and transient background wind conditions. One nighttime measurement by CORAL coincides with the passage of an upper‐level trough, but large‐amplitude mountain waves superpose the small‐amplitude NOGWs in the middle atmosphere.

Climatology of turbulent kinetic energy dissipation rate in the middle troposphere using the 205 MHz S–T radar at Kochi, India

Turbulent kinetic energy dissipation rate (ɛ) in the mid-troposphere (5–12 km) using data obtained from the S–T wind profiler radar located at Kochi (10.04°N, 76.33°E) is analyzed for a period of 1595 days, ranging from March 2017 to December 2022, at a vertical resolution of 180 m. The spectral width method was employed for the estimation of ɛ after initial data filtering, and corrections for beam and shear broadening. Monthly variations in ɛ exhibited a marginal decrease with height. The background wind conditions, such as horizontal wind speed, vertical shear of horizontal wind, and wind direction, have minimal impact vertically. It is found that turbulence is more significantly affected by wind speed and wind direction. Westerly winds, despite being infrequent in the mid-troposphere, emerged as the primary contributors to elevated turbulence values. This observation was attributed to the unique topographical characteristics, creating an interplay of land–sea contrast and orography, leading to enhanced turbulence during westerly winds. During the monsoon season, enhanced turbulence is observed throughout the altitude. The presence of the tropical easterly jet was identified as a significant contributor to the increased wind speed and shear observed during the monsoon season and this alignment correlates with an increase in turbulence.

Observations of Typhoon Generated Gravity Waves From the CIPS and AIRS Instruments and Comparison to the High‐Resolution ECMWF Model

AbstractThe satellite‐based Cloud Imaging and Particle Size (CIPS) instrument and Atmospheric Infrared Sounder (AIRS) observed concentric gravity waves (GWs) generated by Typhoon Yutu in late October 2018. This work compares CIPS and AIRS nadir viewing observations of GWs at altitudes of 50–55 and 30–40 km, respectively, to simulations from the high‐resolution European Centre for Medium‐Range Weather Forecasting Integrated Forecasting System (ECMWF‐IFS) and ECMWF reanalysis v5 (ERA5). Both ECMWF‐IFS with 9 km and ERA5 with 31 km horizontal resolution show concentric GWs at similar locations and timing as the AIRS and CIPS observations. The GW wavelengths are ∼225–236 km in ECMWF‐IFS simulations, which compares well with the wavelength inferred from the observations. After validation of ECMWF GWs, five category five typhoon events during 2018 are analyzed using ECMWF to obtain characteristics of concentric GWs in the Western Pacific regions. The amplitudes of GWs in the stratosphere are not strongly correlated with the strength of typhoons, but are controlled by background wind conditions. Our results confirm that amplitudes and shapes of concentric GWs observed in the stratosphere and lowermost mesosphere are heavily influenced by the background wind conditions.

Performance of summertime temperature and wind fields under different background winds in Kowloon, Hong Kong

The increasingly frequent urban extreme heat events are adversely affecting human health and the economy worldwide. In the last three decades, Kowloon, Hong Kong, one of the most densely populated urban areas in the world, has witnessed a rapid warming rate of 0.31 °C/decade. An in-depth study of the wind and temperature environment is needed to explore the urban warming mechanisms in Kowloon. Using semi-idealised Weather Research and Forecasting (WRF) simulations, we investigated the wind flow fields over the Kowloon area under various background wind conditions. The evolution and interaction of urban heat island circulation, sea–land breeze circulation and mountain circulation under calm weather conditions were also studied. We examined weather photos of Kowloon to identify cloud locations and evaluate our predicted urban plumes. We also explored the association between background wind and spatial air temperature distribution in Kowloon. Northeasterly wind conditions can lead to high temperatures at the foothills of mountains situated northeast of Kowloon; a streamline analysis showed that heat advection accounts for this phenomenon during the day, while a possible foehn-like wind plays an important role at night. These insights into the wind and thermal environment in Kowloon can contribute to the interpretation and prediction of extreme heat events.

Investigation on the Anomalous Weakening of Diurnal Tides in the Mesosphere‐Lower Thermosphere

AbstractVariations in tidal characteristics assume importance as far as the atmosphere‐ionosphere coupling is concerned. The present study examines short‐term variabilities in diurnal tidal characteristics, using one decade (2006–2015) of wind measurements in the mesosphere‐lower thermosphere (MLT) region over a near equatorial location, Thumba (8.5°N, 77°E). The study presents two distinct events of anomalous tidal weakening, with decrease in monthly mean tidal amplitude by −38 ms−1 (−26 ms−1) in February 2010 and −17 ms−1 (−8 ms−1) in January 2012 at 98 km (91 km). These events are observed to be coinciding with the occurrence of strong quasi‐two‐day wave (QTDW). Compared to pre‐event conditions, decrease (increase) in tidal (QTDW) amplitudes at 91 km is −13.03 ms−1 (34.97 ms−1) and −15.65 ms−1 (52.81 ms−1) during the first and second event, respectively. However, causative mechanisms are found to be different for both the events. While the former event is primarily due to parametric excitation of phase locked 2‐day waves by diurnal tide, the latter one is mainly due to non‐linear interaction between tide and QTDW and subsequent generation of secondary waves. While QTDW phase is relatively stable during the former event, it shows steep temporal variations during the latter one. Changes in background wind conditions are also found to be more conducive for weakening of diurnal tide during the former event. These anomalous changes in tidal characteristics would have significant impacts on ionospheric processes. The significance of this study lies in providing observational evidence for the existence of different pathways through which QTDW affects tidal amplitudes over the low‐latitude MLT region.

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

The effect of background wind on summertime daily maximum air temperature in Kowloon, Hong Kong

The increased intensity of extreme weather events (e.g., heatwaves) that is predicted to occur due to global climate change could significantly impact human health, biodiversity, and infrastructure integrity in cities. As one of the most densely constructed cities in the world, Hong Kong has been experiencing increasingly intense heatwaves in recent years. However, the city presents a challenging site for studying urban climate due to its complex topography. Using observed weather data for clear summer (June to August) days from 2000 to 2020, we examine the relationship between background wind and daily maximum temperature in Kowloon, Hong Kong, and investigate how this relationship is affected by geographical location, land cover, and topography. We reveal the distribution of near-surface wind fields under calm weather conditions, which provides a basis for the analysis of relationship between background wind and maximum temperature in urban areas under windy conditions. We find that the maximum surface air temperature in coastal regions is significantly influenced by the background wind direction. For weather stations located in the Kowloon Peninsula, a larger background wind speed is associated with a faster increase in daily maximum temperature when the background temperature rises. We find that the mountain warming effect is influential in areas at the foot of a mountain, even though the maximum terrain height is only approximately 500 m a.s.l. These findings on the daily maximum temperature behavior under different background wind conditions provide a possible way to predict extreme high-temperature patterns in different regions of Hong Kong.

Potential role of local contributions to record-breaking high-temperature event in Xiamen, China

A record-breaking extreme high-temperature (EHT) event of 39.6 °C occurred in the coastal city of Xiamen, China, on August 9, 2019. The unique weather background and topographical features of Xiamen resulted in this event. In this study, the Weather Research and Forecast model with urban land use and landscape data, obtained using the local climate zone classification method, was used to simulate the aforementioned EHT event. The simulation results based on the refined underlying surface accurately reproduced the temporal and spatial distributions of the meteorological elements to enable investigation of the local contributions to air temperature during the EHT event. Tall buildings weakened the cooling effects of convective motions; thus, the intensity of the urban heat island (UHI) still reached 2.8 °C under strong background wind conditions as geostrophic wind speeds exceeded 10 m s−1. The northwest wind not only generated a foehn wind, which caused the urban area close to the mountain to directly heat up by about 2 °C, but also prevented the southeasterly sea breeze from delivering a cold and humid air mass. Thus, a high temperature was maintained in urban areas. Such local contributions from background wind and the weakening of surface winds by urban buildings are key reasons for this EHT event. Past EHT events also exhibit similar local characteristics. In summary, our results are informative for the prediction of future EHT events in Xiamen and other cities with similar topographical features when similar circulation patterns occur.

Indoor and Outdoor Surface Measurement of 3D Objects under Different Background Illuminations and Wind Conditions Using Laser-Beam-Based Sinusoidal Fringe Projections

In this study, both theoretical analysis and experimental validation are carried out for 3D surface measurement under different indoor/outdoor environmental conditions via combining the projected laser-beam-based sinusoidal optical signal, the optical filtering technique, and the single-shot approach based on Fourier transform profilometry. The designed optical signal generator used in this work is capable of ensuring that the projected fringe pattern is monochromatic, higher-contrast, time-invariant, and truly sinusoidal. The proposed and developed optical setup of 3D surface measurement is portable and is used for in-situ experiments of 3D surface measurements that have been carried out under different sunlight illuminations. The experimental results indicate that accurate reconstructions of measured objects with even or varying surface reflectivity can be obtained under windy conditions and strong environmental illuminations such as the background illuminance of 5600–35,000 Lux. The generated fringe-pattern signal is not sensitive to vibrations from environmental influences including the effects of the wind, which has overcome the outdoor-measurement restrictions of the traditional interferometric system and the profilometry approaches based on phase-shifting methods.

Gravity wave variations and contributions to stratospheric sudden warming using long-term ERA5 model output

This work analyzed ERA5 (ECMWF ReAnalysis) data from 1979 to 2018 to study changes in stratospheric gravity waves (GWs) associated with stratospheric sudden warming (SSWs). GW variations are examined by separating them into polar vortex split and displacement events. In general, GW amplitudes show enhancements prior to the wind reversal at 10 hPa and 60°N and reductions after the wind reversal. GW enhancements are larger during polar vortex split events compared to displacement events, likely due to the location of the polar vortex and background wind conditions. Locations of GW enhancements are different between split events and displacement events and these locations correlate with the locations of polar vortex edge where background wind is strong. GW drag (GWD) and Eliassen-Palm (EP) flux divergences are also analyzed to understand GW contributions to occurrences of SSWs. Our results indicate maximum GWD prior to SSWs are ~18% of the total drag, and timing of eastward zonal wind weakening is coincident with enhancements of westward GWD. These results indicate that although the main driver of SSWs are planetary waves, GWs can contribute to the occurrences of SSWs.

Ember risk modelling for improved wildfire risk management in the peri-urban fringes

Although embers are a leading cause of house loss and damage from wildfire, risk reduction activities such as vegetation management and planning requirements do not adequately account for the risk of embers. To help address this, a model of the potential ember risk is presented. It takes into account local vegetation and background wind conditions for both long-range and short-range ember dispersal. This model is developed to provide indications of the importance of embers to the household-level wildfire risk in communities at the wildland-urban interface. The model provides information for householders to improve understanding of the nature of ember risk within the community, and to assist planning in order to respond to that risk. A case study of the 2015 Warringine Park (Coastal Section) bushfire in Australia is presented, which demonstrates how the model could be used to assist with community planning. The utility of this outcome for community and household level wildfire planning and preparation is discussed.

Improving the detailing of atmospheric processes modelling using the Polar WRF model: a case study of a heavy rainfall event at the Akademik Vernadsky station

The Antarctic Peninsula region is of growing interest due to the regional climate change features and related atmospheric circulation patterns. The regional mesoscale atmospheric model Polar Weather Research and Forecasting (WRF) v4.1.1 was used in this research to study a heavy precipitation event over the Ukrainian Antarctic Akademik Vernadsky station region (Antarctic Peninsula). The passage of the cyclone over the Antarctic Peninsula as a typical synoptic process as well as a case of the daily precipitation maximum amount of 2018 were chosen for investigation in this research. The estimation of the modelling quality and downscaling was done by comparing the obtained results with in-situ observation at the Akademik Vernadsky station and cross-domain tracking of average meteorological values and their deviation. The concept of the nested domains allowed to increase the horizontal resolution of the simulated atmosphere up to 1 km and to reproduce the wind regime of this region with high quality. Comparison with measured data showed a significant improvement in wind simulation with increasing of resolution, but worse representation of surface temperature and humidity. The Polar WRF made a general cooling of near surface temperature of 2 °C during the period of simulation and increased precipitation amount by 4.6–8.4 mm (12–21%) on average over the territory relative to the initial data from Global Data Assimilation System. This can be explained by the contribution of noise and imperfection of the model (including static input data of the terrain description). Based on the modelled results, the interaction of wind flow with the mountainous terrain of the Antarctic Peninsula creates a range of complex dynamic effects in the atmosphere. These effects cause local precipitation maxima both over the Peninsula and over the adjacent ocean. These are, respectively, bay-valley areas of increased precipitation and increased precipitation on the crests of shock waves from orographic obstacles. Under certain background wind conditions, the influence of the latter effect can reach the Akademik Vernadsky station and cause the formation of heavy precipitation here.

Experimental study of thermal plumes generated by a cluster of high-rise compact buildings under moderate background wind conditions

When background wind is moderate, urban environment may face great challenges as removal ability of airborne pollutants, anthropogenic heat and moisture reduces. Under this condition, buoyancy-driven flow can break the dominant role of the background wind and affect the urban environment greatly. Knowledge of buoyant flow dynamics is essential for understanding urban wind/thermal environment and related pollutant transport.As one important buoyant flow, thermal plumes generated by a cluster of high-rise compact buildings are modelled using a laboratory water channel in this study. Flow similarity, when the buoyancy is prominent, is well satisfied. Mean flows, turbulent statistics, and rising features under different Froude (Fr) numbers are systematically explored by two-dimensional particle image velocimetry (PIV) technique.Overall, the transition of buoyancy-dominated and wind-dominated flow dynamics is observed when Fr number varies. For horizontal and vertical velocity components, peak magnitude and position are quantitatively determined on four building poles. Fr number obviously changes the relative magnitude of v-peak among four poles. Turbulence statistics, including velocity variance, turbulence kinetic energy, power spectral density, turbulent momentum flux, and turbulence production, are systematically analyzed, which show consistent results. As Fr number decreases, the plume mixing is enhanced. When particularly aiming at the plume region, smaller Fr numbers usually make the maximum velocities occur more in the plume central part rather than the boundaries. A quantitative rational function is given to describe the relation between Fr number and the rising angle, as a = 8.76/(Frq + 0.097).

Investigating microclimate effects in an oasis-desert interaction zone

To investigate oasis-desert microclimate effects, we performed a series of numerical simulations in an idealized oasis-desert system based on an improved computational fluid dynamics (CFD) model for simulating atmospheric boundary layer flows, air temperature and humidity. Numerical simulations were designed based on the hydrometeorological observations obtained during the HiWATER-MUSOEXE (Heihe Watershed Allied Telemetry Experimental Research, Multi-Scale Observation Experiment on Evapotranspiration over heterogeneous land surfaces) campaign. The results are summarized as follows: (1) Oasis-desert interactions are significantly affected by background wind conditions. We observed the oasis-desert local circulation under calm background wind conditions and the oasis thermal internal boundary layer under low wind speed conditions induced by hydrothermal contracts. These interactions will disappear when the background wind speed is sufficiently high, and there is only an oasis dynamic internal boundary layer caused by the aerodynamic roughness length contrast. (2) Oasis-desert interactions lead to a series of microclimate effects, including the oasis cold-wet island effect, air humidity inversion effect within the surrounding desert and oasis wind shield effect, which are important for the stability and sustainability of the oases-desert ecosystem. (3) The hydrothermal conditions due to the difference between the oasis and desert, the vegetation fraction and distribution patterns impact the oasis-desert microclimate effects. The intensity of oasis-desert interactions increases with the land surface temperature (LST) difference in the oasis-desert. The oasis-desert interactions are gradually strengthened with the increase of the vegetation fraction within the oasis. Integrated ecological and economic benefits of the oasis, the oasis vegetation pattern, which includes the croplands and shelterbelts staggered within the oasis and the shelterbelts surrounding the outside, is beneficial to limiting the loss of water vapor and preventing sandstorms from the oasis. The findings of the current study improve the fundamental understanding of the microclimate and provide implications for maintaining the sustainability of oasis-desert ecosystems.

Secondary Gravity Waves Generated by Breaking Mountain Waves Over Europe

AbstractA strong mountain wave, observed over Central Europe on 12 January 2016, is simulated in 2D under two fixed background wind conditions representing opposite tidal phases. The aim of the simulation is to investigate the breaking of the mountain wave and subsequent generation of nonprimary waves in the upper atmosphere. The model results show that the mountain wave first breaks as it approaches a mesospheric critical level creating turbulence on horizontal scales of 8–30 km. These turbulence scales couple directly to horizontal secondary waves scales, but those scales are prevented from reaching the thermosphere by the tidal winds, which act like a filter. Initial secondary waves that can reach the thermosphere range from 60 to 120 km in horizontal scale and are influenced by the scales of the horizontal and vertical forcing associated with wave breaking at mountain wave zonal phase width, and horizontal wavelength scales. Large‐scale nonprimary waves dominate over the whole duration of the simulation with horizontal scales of 107–300 km and periods of 11–22 minutes. The thermosphere winds heavily influence the time‐averaged spatial distribution of wave forcing in the thermosphere, which peaks at 150 km altitude and occurs both westward and eastward of the source in the 2 UT background simulation and primarily eastward of the source in the 7 UT background simulation. The forcing amplitude is 2 that of the primary mountain wave breaking and dissipation. This suggests that nonprimary waves play a significant role in gravity waves dynamics and improved understanding of the thermospheric winds is crucial to understanding their forcing distribution.

Westward Acceleration of Tropical Stratopause Zonal Winds During Major Sudden Stratospheric Warming Events

AbstractThe tropical Mesosphere‐Lower Thermosphere‐Ionosphere system is found to show significant variabilities during Sudden Stratospheric Warming (SSW) events. Recent studies have highlighted the possible role played by modified background wind conditions in communicating the SSW‐induced stratospheric perturbations to the Mesosphere‐Lower Thermosphere‐Ionosphere region. In the present study, changes in the background zonal winds at the tropical stratopause during major SSW events inferred from MERRA‐2 reanalysis data sets, rocket observations, and model simulations are reported for the first time. The tropical stratopause shows enhanced westward winds during the course of major SSW events. Rocketsonde observations from a low latitude station Thumba (8.5 ° N,76.9 ° E) also showed significant westward acceleration during three major SSW events. Further, Specified Dynamics version of the Whole Atmosphere Community Climate Model simulations for the 2008–2009 SSW replicate the observed features.

The Role of Background Wind and Moisture in the Atmospheric Response to Oceanic Eddies During Winter in the Kuroshio Extension Region

The role of background wind and moisture in the atmospheric response to oceanic eddies during winter in the Kuroshio Extension (KE) region is examined by numerical experiments (EXPs) using the Weather Research and Forecasting (WRF) model. We designed two sets of parallel experiments (dry and wet EXPs). The dry EXPs exclude the moisture in the air and the evaporation process. Each experiment differs only in the background wind speed during the initial condition. The wet EXPs include humidity in the initial condition and evaporation during the integration; the other settings are the same as the dry EXPs. The atmosphere in the two sets of EXPs are forced by the same mesoscale sea surface temperature anomaly which resembles the oceanic warm eddy in KE region. The results of these EXPs confirm that under weak background wind conditions, the atmospheric secondary circulation over oceanic eddies is driven by the pressure adjustment process due to weak advection. In the case of the dry run, the increase in background wind enhances the sea surface wind (SSW) by increasing vertical mixing. The convergence of SSW induces vertical motion and heating in the boundary layer, which further decreases the instability. The atmospheric secondary circulation in the dry run remains within the boundary layer. In wet EXPs, the atmospheric response is similar to that in dry runs when the background wind is very weak. When the background wind speed is increased to the climatology value (in KE region) or higher, the vertical motion triggers the precipitation process and diabatic heating above the boundary layer, and the heating in turn reinforces the upward flow.

Computation of the trajectory and attitude of arrows subject to background wind

In this work are performed numerical computations of the trajectory and attitude of archery arrows under the influence of background wind. The aerodynamic response of two commercial arrows with different mass is investigated. The maximum angle of attack increases to $$3.0^{\circ }$$ under a uniform side-wind of $$3\,\hbox {ms}^{-1}$$ , if the arrow was shot with zero angle of attack and zero angular velocity. The maximum angle of attack can be reduced by two orders of magnitude by keeping the boundary layer laminar throughout the arrow’s flight if ideal initial conditions are achieved. The ratio of the drag force to the gravitational force plays a key role in determining the deviation in the trajectory. The radial deviation from the centre of the target for a uniform side-wind of $$3\,\hbox {ms}^{-1}$$ was 0.34 m for the lighter arrow and 0.26 m for the heavier arrow at a distance of 70 m. The deviations decrease to 0.18 m and 0.12 m, for the lighter and the heavier arrow, respectively, if the boundary layers remain laminar. The heavier arrow shows a smaller deviation compared with the lighter arrow under identical background wind conditions. Keeping the arrow’s boundary layer laminar may reduce the wind drift significantly, regardless of the mass.

High-resolution vertical velocities and their power spectrum observed with the MAARSY radar – Part 1: frequency spectrum

Abstract. The Middle Atmosphere Alomar Radar System (MAARSY) installed at the island of Andøya has been run for continuous probing of atmospheric winds in the upper troposphere and lower stratosphere (UTLS) region. In the current study, we present high-resolution wind measurements during the period between 2010 and 2013 with MAARSY. The spectral analysis applying the Lomb–Scargle periodogram method has been carried out to determine the frequency spectra of vertical wind velocity. From a total of 522 days of observations, the statistics of the spectral slope have been derived and show a dependence on the background wind conditions. It is a general feature that the observed spectra of vertical velocity during active periods (with wind velocity > 10 m s−1) are much steeper than during quiet periods (with wind velocity < 10 m s−1). The distribution of spectral slopes is roughly symmetric with a maximum at −5/3 during active periods, whereas a very asymmetric distribution with a maximum at around −1 is observed during quiet periods. The slope profiles along altitudes reveal a significant height dependence for both conditions, i.e., the spectra become shallower with increasing altitudes in the upper troposphere and maintain roughly a constant slope in the lower stratosphere. With both wind conditions considered together the general spectra are obtained and their slopes are compared with the background horizontal winds. The comparisons show that the observed spectra become steeper with increasing wind velocities under quiet conditions, approach a spectral slope of −5/3 at a wind velocity of 10 m s−1 and then roughly maintain this slope (−5/3) for even stronger winds. Our findings show an overall agreement with previous studies; furthermore, they provide a more complete climatology of frequency spectra of vertical wind velocities under different wind conditions. Keywords. Meteorology and atmospheric dynamics (turbulence; waves and tides)