Wind-driven wave heights in the German Bight

Wind speed, friction velocity and significant wave height data from the FINO1 platform in the southern German Bight 45 km off the coast for the years 2004 to 2006 have been evaluated and related to each other. The data show a clear dependence of the hourly mean wave height to the hourly mean friction velocity and wind speed. Wave heights increase with decreasing stratification and increasing fetch. Synoptic weather patterns for the highest wave heights in the southern German Bight are determined. The analysis is made separately for four wind direction sectors. The two strongest storms in the evaluated period, “Britta” and “Erwin”, are analysed in more detail. Finally, the 50-year extreme significant wave height has been estimated to be about 11 m most probably coming from northerly directions.     

中间层顶重力波耗散引起钠原子输送的激光雷达观测研究

利用中国科学院国家空间科学中心廊坊站（40.0°N,116.3°E）钠荧光多普勒激光雷达2011年至2013年共约82 h的钠原子数密度和垂直风观测数据,分析了廊坊地区中间层顶区域大气重力波耗散引起的钠原子输送特征.分析得到,90~100 km处重力波耗散引起的平均钠原子垂直通量整体为负,钠原子向下输送,在93 km处达到最大负值-1.47×10⁸m^-3 m·s^-1,85~90 km处平均钠原子垂直通量为正,钠原子向上输送,但通量值随高度递减.钠原子垂直通量方向在90 km处发生转变,垂直通量随高度的变化造成钠原子汇聚,汇聚效应引起的平均钠原子产生率最大值在91 km处达到了1.40×10⁸ m^-3/h,该值超过了相同高度上模式计算流星烧蚀注入引起的钠原子产生率峰值,说明重力波耗散对钠层结构的形成具有重要影响.与美国SOR和Maui观测结果相比,平均钠原子产生率峰值大小相近,但出现高度不同,说明大气重力波耗散引起的物质输送具有显著的地域变化特征.研究结果可为大气物质输送理论的完善以及大气金属层物理模式的改进提供观测事实参考.

     

Gravity disturbances in regions of negative heights: A reference quasi-ellipsoid approach

Compilation of the bathymetrically and topographically corrected gravity disturbance, the so called BT disturbance, for the purpose of gravity interpretation/inversion, is investigated from the numerical point of view, with special emphasis on regions of negative heights. In regions of negative ellipsoidal (geodetic) heights, such as the Dead Sea region onshore or offshore areas of negative geoidal heights, two issues complicate the compilation and subsequently the inversion of the BT disturbance. The first is associated with the evaluation of normal gravity below the surface of the reference ellipsoid (RE). The latter is tied to the legitimacy of the harmonic continuation of the BT disturbance in these regions. These two issues are proposed to be resolved by the so called reference quasi-ellipsoid (RQE) approach. New bathymetric and topographic corrections are derived based on the RQE and the inverse problem is formulated based on the RQE. The RQE approach enables the computation of normal gravity by means of the international gravity formula, and makes the harmonic continuation in the regions of negative heights of gravity stations legitimate. The gravimetric inversion is then transformed from the RQE approach back to the RE approach, following the now legitimate harmonic upward continuation of the gravity data to stations on or above the RE. Stripping, the removal of an effect of a known density contrast, is considered in the context of the RQE approach. A numerical case study is presented for the RQE approach in a region of NW Canada.     

The criterion of gravity wave instability induced by photochemistry in summer polar mesopause region

This paper studies the effect of photochemistry on the gravity wave instability in summer polar mesopause region. The calculation method of the effects of eddy viscosity, conductivity and eddy diffusion of chemical species on the gravity wave instability induced by photochemistry are studied. The critical wavelength of the instability is given in this paper. The influences of some parameters on it are discussed. The study shows that the gravity wave instability induced by photochemistry is sensitive to the temperature and atomic oxygen profiles.     

Predictability of storm wave heights in the ice-free Beaufort Sea

A predictability study on wave forecast of the Arctic Ocean is necessary to help identify hazardous areas and ensure sustainable shipping along the trans-Arctic routes. To assist with validation of the Arctic Ocean wave model, two drifting wave buoys were deployed off Point Barrow, Alaska for two months in September 2016. Both buoys measured significant wave heights exceeding 4 m during two different storm events on 19 September and 22 October. The NOAA-WAVEWATCH III^? model with 16-km resolution was forced using wind and sea ice reanalysis data and obtained general agreement with the observation. The September storm was reproduced well; however, model accuracy deteriorated in October with a negative wave height bias of around 1 m during the October storm. Utilising reanalysis data, including the most up-to-date ERA5, this study investigated the cause: grid resolution, wind and ice forcing, and in situ sea level pressure observations assimilated for reanalysis. The analysis has found that there is a 20% reduction of in situ SLP observations in the area of interest, presumably due to fewer ships and deployment options during the sea ice advance period. The 63-member atmospheric ensemble reanalysis, ALERA2, has shown that this led to a larger ensemble spread in the October monthly mean wind field compared to September. Since atmospheric physics is complex during sea ice advance, it is speculated that the elevated uncertainty of synoptic-scale wind caused the negative wave model bias. This has implications for wave hindcasts and forecasts in the Arctic Ocean.     

First HF radar measurements of summer mesopause echoes at SURA

HF sounding of the mesosphere was first carried out at SURA in summer 1994 at frequencies in the range 8–9 MHz using one of the sub-arrays of the SURA heating facility. The observations had a range resolution of 3 km. Almost all measurements indicated the presence of strong radar returns from altitudes between 83 and 90 km with features very similar to VHP measurements of mesopause summer echoes at mid-latitudes and polar mesopause summer echoes. In contrast to VHP observations, HF mesopause echoes are almost always present.     

Gravity waves in the mesopause region observed by meteor radar: 1. A simple measurement technique

A simple new technique for measuring gravity-wave activity using meteor radars is described. The technique uses the variance of horizontal wind velocities measured by individual meteors as a proxy for the activity of the gravity-wave field. It is sensitive to gravity waves with horizontal wavelengths of up to about 400 km and periods up to about 3 h. The technique can be used to investigate the vertical structure of the gravity-wave field at heights between approximately 80 and 100 km and with a time resolution of approximately 6 h. The technique is demonstrated using data from an all-sky meteor radar based at Rothera, Antarctica (68°S, 68°W). Observations made over Rothera for 2006 and 2007 reveal a seasonal behaviour with a semi-annual cycle in wave activity. Wave activity maximises in summer and winter and minimises at the equinoxes. Monthly mean gravity-wave activity increases with height in all seasons except in summer when gravity-wave variances show little or no increase with height below 90 km. Comparisons between the gravity-wave activity determined by this meteor-variance technique and other measurements at similar latitudes in the Antarctic reveal generally good agreement.     

基于激光雷达手段的海南地区重力波与其波谱的季节分布特性研究

利用子午工程海南激光雷达对我国海南地区上空进行持续观测,通过3年的累积观测数据对我国低纬度地区重力波活动的季节分布特性进行研究,依据重力波线性理论对海南地区上空的大气密度扰动规律、空间功率谱及时间频率谱进行分析,并通过选择波长在1 km至8 km范围内具有特定波长以及具有波动周期为60 min至25 min的特定频率的重力波辅助研究大气密度扰动的季节变化规律,总结得出海南地区重力波活动具有夏季大、春秋季小、而冬季依然频繁的季节性分布规律.结合海南地区特殊的地理位置与当地季节性气候特征分析得出海南地区上空重力波活动季节性变化的可能原因为青藏高原地形及我国南海地区存在的热带强对流与赤道潜流共同作用的结果.     

Statistical properties of nonlinear wave signatures in OH and O2 airglow brightness data observed at lower midlatitudes

Among 2187 nights of airglow observations of the OH(6-2) and O₂b(0-1) bands from Argentina (mainly from El Leoncito, 32°S 69°W), 132 show airglow brightness jumps (ABJs) of short duration (16 min median). ABJs are supposed to be related to mesospheric bores or similar nonlinear waves. Several occurrence patterns were identified, which a successful explanation must take into account. ABJs occur preferably in the OH layer at 87 km, and are less likely in the O₂ layer at 95 km, maybe because ducts prefer lower altitudes. The seasonal distribution of nights when ABJs are observed only in the OH layer clearly shows a winter maximum centered around solstice, and equinox minima. In contrast, the seasonal distribution of ABJ nights in O₂ is flat. Most ABJs simultaneously present in OH and O₂ show anticorrelated variation between both layers. ABJ nights tend to occur in clusters lasting several days, which probably reflects duct lifetime.     

Temperature variations in the mesopause region according to the hydroxyl-emission observations at midlatitudes

The seasonal temperature variations in the mesopause region and the inter-day and nighttime temperature variability, the measure of which is standard deviations, have been studied based on the hydroxyl emission spectral observations at the Zvenigorod station of the Obukhov Institute of Atmospheric Physics in 2000–2011 and Institute of Solar Terrestrial Physics geophysical station (Tory) in 2008–2011. The long-term variations in all temperature variability parameters have been analyzed.     

Gravity wave reflection: Case study based on rocket data

Since gravity waves significantly influence the atmosphere by transporting energy and momentum, it is important to study their wave spectrum and their energy dissipation rates. Besides that, knowledge about gravity wave sources and the propagation of the generated waves is essential. Originating in the lower atmosphere, gravity waves can move upwards; when the background wind field is equal to their phase speed a so-called critical layer is reached. Their breakdown and deposition of energy and momentum is possible. Another mechanism which can take place at critical layers is gravity wave reflection.In this paper, gravity waves which were observed by foil chaff measurements during the DYANA (DYnamics Adapted Network for the Atmosphere) campaign in 1990 in Biscarrosse (44°N, 1°W)—as reported by Wüst and Bittner [2006. Non-linear wave–wave interaction: case studies based on rocket data and first application to satellite data. Journal of Atmospheric and Solar-Terrestrial Physics 68, 959–976]—are investigated to look for gravity wave reflection processes. Following nonlinear theory, energy dissipation rates according to Weinstock [1980. Energy dissipation rates of turbulence in the stable free atmosphere. Journal of the Atmospheric Sciences 38, 880–883] are calculated from foil chaff cloud and falling sphere data and compared with the critical layer heights. Enhanced energy dissipation rates are found at those altitudes where the waves’ phase speed matches the zonal background wind speeds. Indication of gravity wave trapping is found between two altitudes of around 95 and 86 km.     

利用瑞利激光雷达和无线电探空仪观测数据对武汉上空重力波特性的研究

本文利用武汉大学的瑞利激光雷达的瑞利散射回波数据(30～65km)来研究武汉地区上空(30.5°N,114.4°E)重力波的活动规律和统计特性.通过对2003年12月到2005年3月观测的200h数据反演的密度进行处理分析,得到了重力波的一些个例特征,并提取垂直波长为2 km以上的重力波进行统计分析.结果表明,最可几的垂直波长是3～4 km和17～20 km,重力波振幅的月平均值在冬季有较大值,夏季值较小.与武汉2004年无线电探空仪的密度扰动提取到的重力波做比较,发现与瑞利激光雷达得到的重力波振幅的月平均值有很强的相关性,也是冬季值比较大,夏季值比较小.通过无线电探空仪的风场数据,本文还得到了急流的季节变化规律、最大风剪切年变化规律,发现急流和最大风剪切与激光雷达的重力波统计结果有很强的相关性.     

Average density of stratigraphic sequences at Vesuvius and Phlaegraean fields at different heights

In order to measure the density of rocks in the Phlacgraean and Vesuvius edifices, appropriately chosen indirect methods, based on a strict relationship between gravity and density, enabled useful data to be obtained. The data can be related to the particular lithological formations of the above volcanic complexes.     

Gravity waves in the mesopause region observed by meteor radar, 2: Climatologies of gravity waves in the Antarctic and Arctic

A simple technique has been used to investigate gravity waves in the Antarctic and Arctic mesosphere. Data from two meteor radars are used, one at Rothera in the Antarctic (68°S) and one at Esrange in the Arctic (68°N). Observations have been made from 2005 to 2008 in the Antarctic and from 2000 to 2008 in the Arctic. The results reveal a seasonal behaviour with a semi-annual cycle. Over both sites wave activity maximises at the solstices and minimises at the equinoxes. Monthly-mean gravity-wave activity increases with height in all seasons except in summer when gravity-wave variances show little or no increase with height at heights below about 90 km. Above about 90 km in summer there is a rapid increase in wave activity. We suggest that this summer-time behaviour is a consequence of rapid wave growth as gravity waves ascend into the more stable air of the lower thermosphere. Differences between the Arctic and Antarctic gravity-wave field are also evident. In particular, a higher level of gravity-wave activity is observed in the Antarctic spring compared to the Arctic spring. We suggest that these inter-hemispheric differences may be the result of differences in the generation of gravity waves in the polar troposphere and stratosphere.     

Ozone concentration at meteor heights determined from forward-scatter radar echoes

The ozone concentration has been measured in the meteor zone using a method of radar echo duration distribution suggested by Jones and Šimek (1995). Data obtained by the forward-scatter system during the Lyrid meteor shower period in five consecutive years 1992–1996 agree with the Solar Mesosphere Explorer results for the same heights of 95 km. The mean value of the ozone concentration [O₃]=(1.56±0.05)×10¹⁴ m⁻³ has been derived from a total of 6596 radar meteor echoes with duration ≥1 s. The results for individual years indicating a possible decrease in [O₃] are discussed.     

新疆库尔勒地区秋季下平流层重力波特性

利用2011年秋季无线电探空数据,采用矢端曲线法首次分析了新疆库尔勒地区下平流层重力波特征参量,得到36组准单色重力波的结果.结果统计显示：库尔勒秋季下平流层重力波垂直波长、水平波长平均值分别为2.8 km和580 km,固有频率平均值为1.74f.垂直传播方向以上传为主,约占78%,其中下传重力波水平波长较短,固有频率较高.水平传播方向以西北和东南为主,各占1/3,其中上传（下传）重力波水平传播方向以西北（东南）居多,这与热带低纬站点和其他中纬站点观测结果不同.与其他站点比较,库尔勒地区最小,中高纬地区水平波长、垂直波长随纬度增加大致有减小的趋势,库尔勒地区偏离这一趋势,波长偏大.