Km In Summer Research Articles

Simulation of a Pulsed Metastable Helium Lidar

Measurements of atmosphere density in the upper thermosphere and exosphere are of great significance for studying space–atmosphere interactions. However, the region from 200 km to 1000 km has been a blind area for traditional ground-based active remote sensing techniques due to the limitation of facilities and the paucity of neutral atmosphere. To fulfill this gap, the University of Science and Technology of China is developing a powerful metastable helium resonance fluorescent lidar incorporating a 2 m aperture telescope, a high-energy 1083 nm pulsed laser, as well as a superconducting nanowire single-photon detector (SNSPD) with high quantum efficiency and low dark noise. The system is described in detail in this work. To evaluate the performance of the lidar system, numerical simulation is implemented. The results show that metastable helium density measurements can be achieved with a relative error of less than 20% above 370 km in winter and less than 200% in 270–460 km in summer, demonstrating the feasibility of metastable helium lidar.

Spatial density patterns of herbivore response to seasonal dynamics in the tropical deciduous forest of central India

AbstractResource dynamism in seasonal forests leads ungulates to differential habitat preference; hence, their distribution aligns with environmental covariates across spatial and temporal scales. Seasonal patterns of four species of ungulates, namely sambar, chital, nilgai, and wild pig, were investigated and identified as the environmental variables driving the density gradient across two seasons, summer and winter, in the tropical dry deciduous forest of Panna Tiger Reserve, central India. Distance sampling data were analyzed using density surface modeling for ungulates with a survey effort of 518 km in winter and 356 km in summer in a generalized additive modeling framework. We found that season significantly affected the spatial densities of all ungulates, with sambar, chital, and nilgai congregating in summer and wild pig in winter. All ungulates showed a clear seasonal shift to the valley in summer and preferred plateaus in winter. The spatially explicit map outputs draw attention to the seasonal hot spots for ungulates abundance and the species and season‐specific roles of environment variables in defining their distribution. These results provide a scientific basis for direct conservation efforts to the spatially prioritized habitats for cost‐effective management interventions.

Fine vertical structures of cloud from a ground‐based cloud radar over the western Tianshan mountains

AbstractIn this study, the grounded‐based Ka‐band millimetre‐wave cloud radar (MMCR) is used to analyse the cloud characteristics over the western Tianshan mountains. The cloud top height (CTH) obtained by MMCR is verified by comparing it with the Fengyun‐4A (FY‐4A) observations. Overall, the MMCR‐obtained CTHs are attenuated and lower than the FY‐4A‐obtained CTHs under precipitation conditions. Thus, the FY‐4A data is used to complement MMCR data when there is rainfall. The diurnal, seasonal variation and vertical structures of the cloud are further examined using the combination of MMCR and FY‐4A. The result indicates that the CTH and cloud base height (CBH) are highest in summer and lowest in winter. Also, clouds tend to form frequently at night in spring, summer, and winter. Although in autumn, clouds tend to form most frequently in the morning. This may be related to the diurnal variation of temperature, humidity, wind speed, and wind direction. Moreover, the CTHs occur most frequently at heights of 8–9 km in spring and autumn, 9–10 km in summer, and 7–8 km in winter. The high CTHs caused by the strong convective activities in summer may be related to sufficient water vapour transport. These corrected CTH data are also used to classify cloud types, the results indicate that the proportion of high clouds is highest in summer, while the proportion of medium clouds is lower than in the other three seasons. Also, the average CTH of the low, medium, and high clouds is highest in summer and lowest in winter.

Comparison of horizontal wind observed by wind profiler radars with ERA5 reanalysis data in Anhui, China

The wind profiler radar (WPR) network in Anhui Province in China completed the construction in 2019 to improve the warning capacity for meteorological disasters. To verify its proper performance, vertical profiles of horizontal winds were compared with ERA5 reanalysis data from January to December 2021. The results show that the WPRs demonstrate good capability in measuring atmospheric wind due to their high data collection rate (> 80%). Overall, the data availability of the WPRs decreases with increasing altitude, exceeding 90% below 3 km during the daytime without precipitation. In terms of seasonal variability, the annual mean effective detection height reaches a maximum value at 4–6 km in summer and a minimum value at approximately 2 km in winter. Comparison analyses show that the wind measurements by the WPRs show excellent agreement with the ERA5 zonal (correlation coefficient > 0.89 and standard deviation < 2.57 m/s) and meridional winds (correlation coefficient > 0.76 and standard deviation < 3.26 m/s) over most WPR stations in Anhui, China. Additionally, the biases of winds between WPR and ERA5 generally follow a Gaussian distribution pattern, most of which are all close to zero. In particular, the differences in horizontal wind between WPR and ERA5 increase with altitude, with a much lower bias present in the lower layer. In addition, due to the difference in atmospheric turbulence between day and night, statistical parameters of comparison during daytime behave better than those at night. These findings indicate that the WPR network in Anhui is capable of providing good quality wind data, laying a solid data foundation for the improvement and development of the boundary layer scheme in the future.

Investigation on cloud vertical structures based on Ka-band cloud radar observations at Wuhan in Central China

Based on vertically pointing observations of Ka-band frequency-modulated continuous millimeter wave cloud radar (MMCR) in 2020, we investigate cloud vertical structures at Wuhan in central China. Case comparison indicates that the cloud top locations in the MMCR observation are largely consistent with those in satellite data. The precipitation has the occurrence frequency of 9.5% over the year, with the maximum value of 16.3% in July and the submaximal value of 14.7% in January, showing the significant influences of monsoons on precipitation at Wuhan. As the humid, cloudy and rainy climate, the probability of precipitation and non-precipitation clouds increases gradually from 53.9% in January to a large value of 71.5% in July. The non-precipitation clouds have the probability of 44.5% in the year, with two prominent peaks at the reflectivities of −35 to −30 dBZ around 12 km in summer and − 37– −32 dBZ around 10 km in spring. The cloud top can reach the heights of 14.49, 16.62, 13.38 and 11.73 km from spring to winter, respectively. The monthly mean cloud layer thickness is the largest (2.18 km) in May and September, and the smallest (1.36 km) in November. The seasonally averaged base and top heights and thickness of clouds have small diurnal variations. The high cloud is predominant in the warm months from May to September, and in the other cool months, the middle cloud has the most fraction. The quantitative investigation of clouds is helpful in improving the cloud parameterization in climate models.

Future seasonal changes in habitat for Arctic whales during predicted ocean warming.

Ocean warming is causing shifts in the distributions of marine species, but the location of suitable habitats in the future is unknown, especially in remote regions such as the Arctic. Using satellite tracking data from a 28-year-long period, covering all three endemic Arctic cetaceans (227 individuals) in the Atlantic sector of the Arctic, together with climate models under two emission scenarios, species distributions were projected to assess responses of these whales to climate change by the end of the century. While contrasting responses were observed across species and seasons, long-term predictions suggest northward shifts (243 km in summer versus 121 km in winter) in distribution to cope with climate change. Current summer habitats will decline (mean loss: −25%), while some expansion into new winter areas (mean gain: +3%) is likely. However, comparing gains versus losses raises serious concerns about the ability of these polar species to deal with the disappearance of traditional colder habitats.

Impact of deep basin terrain on PM2.5 distribution and its seasonality over the Sichuan Basin, Southwest China

The terrain effect on atmospheric environment is poorly understood in particular for the polluted region with underlying complex topography. Therefore, this study targeted the Sichuan Basin (SCB), a deep basin with severe PM2.5 pollution enclosed by the eastern Tibetan Plateau (TP), Yunnan-Guizhou Plateaus (YGP) and mountains over Southwest China, and we investigated the terrain effect on seasonal PM2.5 distribution and the meteorological mechanism based on the WRF-Chem simulation with stuffing the basin topography. It is characterized that the three-dimensional distribution of topography-induced PM2.5 concentrations over the SCB with the seasonal shift of regional PM2.5 averages from approximately 30 μg m−3 in summer to 90 μg m−3 in winter at surface layer and from summertime 10 μg m−3 to wintertime 30 μg m−3 in the lower free troposphere. Such basin-forced PM2.5 changes presented the vertically monotonical declines concentrated within the lower troposphere below 3.6 km in spring, 2.3 km in summer, 2.6 km in autumn and 4.8 km in winter. Impacts of deep basin aggravated PM2.5 accumulation within the SCB and transport toward the surrounding plateaus contributing approximately 50–90% to PM2.5 levels over the regions of eastern TP and northern YGP. In the SCB, atmospheric thermal structure in the lower troposphere could build a vertical convergence layer between the boundary layer and free troposphere, acting as a lid inhibiting air diffusion, which was regulated by the terrain effects on interactions of westerlies and Asian monsoons, especially the wintertime strong warm lid deteriorating air pollution in the SCB. Furthermore, warm and humid air conditions within the basin prompted sulfur oxidation ratio by +0.02 and nitrogen oxidation ratio by +0.22 effectively producing the secondary PM2.5 in atmospheric environment.

Climatology Analysis of the Daytime Topside Ionospheric Diffusive O+ Flux Based on Incoherent Scatter Radar Observations at Millstone Hill

AbstractThis paper reports the characteristics of the topside ionospheric O+ diffusive flux () during both geomagnetically quiet (0 ≤ Kp ≤ 2) and moderate (2 &lt; Kp ≤ 4) times using incoherent scatter radar observations at Millstone Hill (42.6°N, 288.5°E) for solar minimum from 1970 to 2018. partially characterizes plasma mass exchange between the upper and lower part of the topside ionosphere through diffusion and sometimes serves as upper boundary conditions for ionosphere‐thermosphere models. The altitude where the flux sign changes (mainly during daytime) is termed the transition height and the time when the flux sign changes (mainly at dawn and dusk) is termed the transition time. At quiet times, the daytime transition height is ∼100 km above the peak height (hmF2) in summer, and it is about 50 km above hmF2 in other seasons; the transition time is before 18 solar local time (SLT) in spring and winter, but after 18 SLT in summer and autumn. The daytime average upward above the transition height shows a significant seasonal variation with a minimum of in summer and a maximum of in autumn. Under geomagnetically moderate conditions, the transition height increases by ∼20 km in spring, winter, and autumn, but moves up by about 20–50 km in summer. The transition time occurs later by ∼1 hr in summer but ∼1 hr earlier in other seasons. The mean upward peaks in summer and minimizes in spring.

The role of shallow convection in the momentum budget of the trades from large‐eddy‐simulation hindcasts

AbstractMotivated by the abundance of low clouds in the subtropics, where the easterly trade winds prevail, we study the role of shallow convection in the momentum budget of the trades. To this end, we use ICON‐LEM hindcasts run over the North Atlantic for 12 days corresponding to the NARVAL1 (winter) and NARVAL2 (summer) flight campaigns. The simulation protocol consists of several nested domains, and we focus on the inner domains (≈100 × 100 km2) which have been run at resolutions of 150–600 m and are forced by analysis data, thus exhibiting realistic conditions. Combined, the resolved advection and the subgrid stresses decelerate the easterly flow over a frictional layer that balances the prevailing geostrophic wind forcing. Irrespective of the horizontal resolution, this layer is about 2 km deep in the strong winter trades and 1 km in summer, as winds and geostrophic forcing weaken and cloudiness reduces. The unresolved processes are strongest near the surface and are well captured by traditional K‐diffusion theory, but convective‐scale motions which are not considered in K‐diffusion theory contribute the most in the upper part of the mixed layer and are strongest just below cloud base. The results point out that convection in the mixed layer – the roots of trade‐wind cumuli and subcloud‐layer circulations – play an important role in slowing down easterly flow below cloud base (but little in the cloud layer itself), which helps make the zonal wind jet more distinct. Most of the friction within the clouds and near the wind jet stems from smaller‐scale turbulence stresses.

Visibility Variation in Zhengzhou from 2008–2017

Visibility is closely related to air quality and is therefore a source of concern. Using data from the National Basic Weather Station (NBWS), this study measured visibility variation in Zhengzhou from 2008 to 2017. The effects of PM2.5 and meteorological parameters on visibility were also assessed. Results indicated that the mean annual visibility fluctuated between 8.07 km and 14.02 km, decreasing during the period from 2008 to 2013 and then increasing slightly after 2013. The seasonal mean visibility was 11.69 km in spring (the highest value), 11.37 km in summer, 9.42 km in autumn, and 8.98 km in winter (the lowest value). The mean annual concentrations of PM2.5 from 2015 to 2017 were 90.83 μg m-3, 79.89 μg m-3, and 66.69 μg m-3, respectively. These were 159.51%, 128.26%, and 90.54% higher than 35 μg m-3 of the second grade of China National Ambient Air Quality Standards (CNAAQS). Additionally, direct radiation, total radiation, temperature, and relative humidity were positively correlated with visibility, whereas PM2.5 mass concentration, scattered radiation, and wind velocity were negatively correlated with visibility. Further research is needed to analyse the effect of PM2.5 on visibility and the mechanism through which this occurs.

Measurement Report: Determination of aerosol vertical features on different timescales over East Asia based on CATS aerosol products

Abstract. The Cloud-Aerosol Transport System (CATS) lidar, on board the International Space Station (ISS), provides a new opportunity for studying aerosol vertical distributions, especially the diurnal variations, from space observations. In this study, we investigate the seasonal variations and diurnal cycles in the vertical aerosol extinction coefficients (AECs) over East Asia by taking advantage of 32 months of continuous and uniform aerosol measurements from the CATS lidar. Over the Tibetan Plateau, a belt of AECs at approximately 6 km between 30 and 38∘ N persistently exists in all seasons with an obvious seasonal variation. In summer, the aerosols at 6 km are identified as a mixture of both anthropogenic aerosols transported from India and coarse dust particles from Asian dust sources. In addition, the high AECs up to 8 km in summer over the Tibetan Plateau are caused by smoke aerosols from thermal dynamic processes. In fall and winter, the northern slope of the plateau is continuously influenced by both dust aerosols and polluted aerosols transported upslope from cities located at lower elevations in northwestern Asia. The diurnal variation in AECs in North China is mainly related to the diurnal variations in the transported dust and local polluted aerosols. Below 2 km, the AEC profiles in North China at 06:00 and 12:00 CST (China standard time) are significantly higher than those at 00:00 and 18:00 CST, reaching a maximum at midday. The aerosol vertical profiles over the Tarim Desert region in summer have obvious diurnal variations, and the AECs at 12:00 and 18:00 CST are significantly higher than those at 00:00 and 06:00 CST, which are induced by the strong diurnal variations in near-surface wind speeds. In addition, the peak in the AEC profiles has a significant seasonal variation, which is mainly determined by the boundary layer height.

Temperature Correction of the Vertical Ozone Distribution Retrieval at the Siberian Lidar Station Using the MetOp and Aura Data

The purpose of the work is to study the influence of temperature correction on ozone vertical distribution (OVD) in the upper troposphere–stratosphere in the altitude range~(5–45) km, using differential absorption lidar (DIAL), operating at the sensing wavelengths 299/341 nm and 308/353 nm. We analyze the results of lidar measurements, obtained using meteorological data from MLS/Aura and IASI/MetOp satellites and temperature model, at the wavelengths of 299/341 nm and 308/353 nm in 2018 at Siberian Lidar Station (SLS) of Institute of Atmospheric Optics, Siberian Branch, Russian Academy of Sciences. To estimate how the temperature correction of absorption cross-sections influences the OVD retrieval from lidar measurements, we calculated the deviations of the difference between two profiles, retrieved using satellite- and model-based temperatures. Two temperature seasons were singled out to analyze how real temperature influences the retrieved OVD profiles. In the stratosphere, when satellite-derived temperature and model are used for retrieval, the deviations may reach absolute values of ozone concentration in the range from −0.97 × 1012 molecules × cm−3 at 19.7 km to 1.05 × 1012 molecules × cm−3 at 25.3 km during winter–spring season, and from −0.17 × 1012 molecules × cm−3 at height of 17.4 km to 0.27 × 1012 molecules × cm−3 at 40 km in summer–fall period. In the troposphere, when satellite-derived temperature is used in the retrieval, the deviations may reach absolute values of ozone concentration in the range from −1.95 × 1012 molecules × cm−3 at 18.6 km to 1.23 × 1012 molecules × cm−3 at 18.2 km during winter–spring season, and from −0.15 × 1012 molecules × cm−3 at height of 11.4 km to 0.3 × 1012 molecules × cm−3 at 8 km during summer–fall season.

Spatio-Temporal Variability of Aerosol Optical Depth, Total Ozone and NO2 Over East Asia: Strategy for the Validation to the GEMS Scientific Products

In this study, the spatio-temporal variability of aerosol optical depth (AOD), total column ozone (TCO), and total column NO2 (TCN) was identified over East Asia using long-term datasets from ground-based and satellite observations. Based on the statistical results, optimized spatio-temporal ranges for the validation study were determined with respect to the target materials. To determine both spatial and temporal ranges for the validation study, we confirmed that the observed datasets can be statistically considered as the same quantity within the ranges. Based on the thresholds of R2&gt;0.95 (temporal) and R&gt;0.95 (spatial), the basic ranges for spatial and temporal scales for AOD validation was within 30 km and 30 min, respectively. Furthermore, the spatial scales for AOD validation showed seasonal variation, which expanded the range to 40 km in summer and autumn. Because of the seasonal change of latitudinal gradient of the TCO, the seasonal variation of the north-south range is a considerable point. For the TCO validation, the north-south range is varied from 0.87° in spring to 1.05° in summer. The spatio-temporal range for TCN validation was 20 min (temporal) and 20–50 km (spatial). However, the nearest value of satellite data was used in the validation because the spatio-temporal variation of TCN is large in summer and autumn. Estimation of the spatio-temporal variability for respective pollutants may contribute to improving the validation of satellite products.

차량 배출량 산정을 위한 터널유동계수 연구

PM2.5 is well known to be greatly affected by the vehicle emissions, which can affect climate change and the human body. Recently, the number of vehicles has been increasing continuously in Korea. However, there are still very few studies on the relationships between chemical components actually emitted from vehicles. Therefore, this study aims to analyze the actual tunnel flow coefficient for car emission rate based on the measurements of individual physical parameters (e.g., tunnel wind speed and traffic volume). As a result, the average tunnel flow coefficient were shown as 187, 335, 294, and 219 ㎥/vehicle·km in Summer, Fall, Winter, and Spring, respectively. The overall average of tunnel flow coefficient was determined as 259 ㎥/vehicle·km for vehicle emission rate. The emission factors can apply to investigate the both seasonal and chemical unit of vehicle emissions in real atmosphere, which can contribute to calculate the total road emissions related to their environmental effects.

Aerosol layers in the free troposphere and their seasonal variations as observed in Wuhan, China

Free-tropospheric aerosol layers and their seasonal variation over Wuhan (30.5°N, 114.4°E), China, are presented based on a 532-nm polarization lidar measurements on 162 days from January through December 2013. Using the aerosol layer selection criterions, a total of 402 free-tropospheric aerosol layer events were identified. The bottom height of the aerosol layers below 2 km accounts for 68% of the total, while approximately 76% of the layer's top height ranges from 1 km to 4 km. Out of the 402 events, 269 (67%) are optically-thin layers with aerosol optical depth (AOD) less than 0.1. The free tropospheric AOD2-7 contribute ~13–31% to the AOD0-7 and the free-tropospheric aerosol layers show considerable moderate variation. The aerosol layers have the maximum mean geometrical thickness of 1.2 km in spring, while the minimum mean thickness is 0.7 km in autumn, and the mean thickness is 0.93 km and 1 km in summer and winter, respectively. The mean backscatter coefficient of aerosol layers during spring, summer, autumn and winter were 1.8 ± 1.4 Mm−1sr−1, 2.3 ± 2 Mm−1sr−1, 2.8 ± 2.7 Mm−1sr−1 and 2.3 ± 2.2 Mm−1sr−1, respectively. Aerosol layers in different seasonal are classified by particle depolarization ratio, there are a large amount of non-spherical particles and mixed particles present in spring, autumn and winter, and the mean particle polarization ratio of aerosol layers during spring, summer, autumn and winter were 0.22, 0.06, 0.15 and 0.14, respectively.

Variability of the Boundary Layer Over an Urban Continental Site Based on 10 Years of Active Remote Sensing Observations in Warsaw

Atmospheric boundary layer height (ABLH) was observed by the CHM15k ceilometer (January 2008 to October 2013) and the PollyXT lidar (July 2013 to December 2018) over the European Aerosol Research LIdar NETwork to Establish an Aerosol Climatology (EARLINET) site at the Remote Sensing Laboratory (RS-Lab) in Warsaw, Poland. Out of a maximum number of 4017 observational days within this period, a subset of quasi-continuous measurements conducted with these instruments at the same wavelength (1064 nm) was carefully chosen. This provided a data sample of 1841 diurnal cycle ABLH observations. The ABLHs were derived from ceilometer and lidar signals using the wavelet covariance transform method (WCT), gradient method (GDT), and standard deviation method (STD). For comparisons, the rawinsondes of the World Meteorological Organization (WMO 12374 site in Legionowo, 25 km distance to the RS-Lab) were used. The ABLHs derived from rawinsondes by the skew-T-log-p method and the bulk Richardson (bulk-Ri) method had a linear correlation coefficient (R2) of 0.9 and standard deviation (SD) of 0.32 km. A comparison of the ABLHs obtained for different methods and instruments indicated a relatively good agreement. The ABLHs estimated from the rawinsondes with the bulk-Ri method had the highest correlations, R2 of 0.80 and 0.70 with the ABLHs determined using the WCT method on ceilometer and lidar signals, respectively. The three methods applied to the simultaneous, collocated lidar, and ceilometer observations (July to October 2013) showed good agreement, especially for the WCT method (R2 of 0.94, SD of 0.19 km). A scaling threshold-based algorithm was proposed to homogenize ceilometer and lidar datasets, which were applied on the lidar data, and significantly improved the coherence of the results (R2 of 0.98, SD of 0.11 km). The difference of ABLH between clear-sky and cloudy conditions was on average below 230 m for the ceilometer and below 70 m for the lidar retrievals. The statistical analysis of the long-term observations indicated that the monthly mean ABLHs varied throughout the year between 0.6 and 1.8 km. The seasonal mean ABLH was of 1.16 ± 0.16 km in spring, 1.34 ± 0.15 km in summer, 0.99 ± 0.11 km in autumn, and 0.73 ± 0.08 km in winter. In spring and summer, the daytime and nighttime ABLHs appeared mainly in a frequency distribution range of 0.6 to 1.0 km. In winter, the distribution was common between 0.2 and 0.6 km. In autumn, it was relatively balanced between 0.2 and 1.2 km. The annual mean ABLHs maintained between 0.77 and 1.16 km, whereby the mean heights of the well-mixed, residual, and nocturnal layer were 1.14 ± 0.11, 1.27 ± 0.09, and 0.71 ± 0.06 km, respectively (for clear-sky conditions). For the whole observation period, the ABLHs below 1 km constituted more than 60% of the retrievals. A strong seasonal change of the monthly mean ABLH diurnal cycle was evident; a mild weakly defined autumn diurnal cycle, followed by a somewhat flat winter diurnal cycle, then a sharp transition to a spring diurnal cycle, and a high bell-like summer diurnal cycle. A prolonged summertime was manifested by the September cycle being more similar to the summer than autumn cycles.

Is Karenia brevis really a low-light-adapted species?

Despite nearly annual blooms of the neurotoxic dinoflagellate Karenia brevis (Davis) G. Hansen and Moestrup in the Gulf of Mexico, defining the suite of biological traits that explain its proliferation has remained challenging. Studies have described K. brevis as a low-light-adapted species, incapable of sustaining growth under high light, which is at odds with observed surface aggregations sometimes within centimeters of the sea surface and also with short-term experiments showing photosynthetic machinery accommodating high irradiances. Here, growth and photophysiology of three K. brevis isolates were evaluated under a range of environmentally relevant irradiances (10–1500 μmol photons m−2 s−1) in the laboratory. No differences in growth–irradiance curves were observed among isolates; all sustained maximum growth rates at the highest irradiances examined, even in exposures as long as three weeks. The growth efficiency α of K. brevis under light-limiting conditions appeared mediocre among dinoflagellates, and poorer than that of other phytoplankton (e.g., diatoms, cyanobacteria), implying that K. brevis is not a low-light specialist. This finding substantially alters earlier parameterizations of K. brevis growth–irradiance curves. Therefore, a model was developed to contextualize how these new growth–irradiance curves might affect bottom growth rates. This model was subsequently applied to a case study comparing seasonal light forcing offshore of Pinellas County, FL, USA, with a single empirical value for light attenuation, and seasonal bottom water temperatures. Predictions suggested that light may limit bottom growth as close as 1 km from shore in winter, but would only begin limiting growth 20 km from shore in summer. Population maintenance (no net growth) was possible as far offshore as 90 km in summer and 68 km in winter. These ranges intercept areas thought to be important for bloom initiation.

A numerical simulation of the generation and evolution of nonlinear internal waves across the Kara Strait

Nonlinear internal waves (NIWs) are ubiquitous around the Kara Sea, a part of the Arctic Ocean that is north of Siberia. Three hot spot sources for internal waves, one of which is the Kara Strait, have been identified based on Envisat ASAR. The generation and evolution of the NIWs through the interactions of the tide and topography across the strait is studied based on a nonhydrostatic numerical model. The model captures most wave characteristics shown by satellite data. A typical inter-packets distance on the Barents Sea side is about 25 km in summer, with a phase speed about 0.65 m/s. A northward background current may intensify the accumulation of energy during generation, but it has little influence on the other properties of the generated waves. The single internal solitary wave (ISW) structure is a special phenomenon that follows major wave trains, with a distance about 5–8 km. This wave is generated with the leading wave packets during the same tidal period. When a steady current toward the Kara Sea is included, the basic generation process is similar, but the waves toward the Kara Sea weaken and display an internal bore-like structure with smaller amplitude than in the control experiment. In winter, due to the growth of sea ice, stratification across the Kara Strait is mainly determined by the salinity, with an almost uniform temperature close to freezing. A pycnocline deepens near the middle of the water depth (Barents Sea side), and the NIWs process is not as important as the NIWs process in summer. There is no fission process during the simulation.

The energy mechanism controlling the continuous development of a super-thick atmospheric convective boundary layer during continuous summer sunny periods in an arid area

In the arid regions of the world, due to the specific climatic and environmental background, a super-thick convective boundary layer (SCBL) often develops on sunny days in summer, whereas such phenomena rarely occur in other areas. This special boundary layer structure has an important synoptic and climatic significance, but there have been few studies of its development mechanism in arid areas, which greatly restricts the parametric improvement of the SCBL and our understanding of the interaction between weather and climate processes. This study was conducted in Dunhuang, which is located in the hinterland of northwest China. Based on data obtained from land-air interaction experiments and long-term operational sounding observations in this region, the energy mechanism controlling the development of the CBL and the developmental process of the SCBL were systematically analyzed. In arid northwest China, it is possible that the thickness of the CBL can extend to over 3 km for most of the year, except winter. Even in the early summer, when there is little rain and strong solar radiation, the thickness of the CBL may reach an extreme state of 5.4 km. The thickness of the CBL at this time is higher than that observed in midsummer, when there is slightly more precipitation in this area. This is basically consistent with the extreme thickness of the CBL recently discovered in the Sahara Desert in Africa. In the general mechanism that controls the development of the CBL, there is a close relationship between the development of the CBL and the sensible heat flux of the surface. However, the correlation between the thickness of the CBL and the surface sensible heat flux at the same time is not strong, whereas the correlation between the thickness of the CBL and the cumulative surface sensible heat flux is very strong. This indicates that the development of the CBL is the result of the continuous accumulation of the sensible heat flux on the surface, which is consistent with the energy mechanism controlling the CBL. Although the development of the CBL is closely related to the cumulative heating effect of the daytime surface sensible heat flux, the CBL would still continue to increase even if the integral value of the daytime surface sensible heat flux remained unchanged or even weakened during the continuous clear sky period. The energy provided through sensible heat does not fully explain the energy required to develop the CBL. This is mainly because the deep near-neutral residual layer (RL) background plays an important role in the development of the SCBL. The entrainment energy from the deep RL to the CBL is the key energy supply for the continuous development of the CBL. The sum of the entrainment energy and surface sensible heat energy coincides with the energy absorbed by the development of the SCBL. The reason for the occurrence of an SCBL in arid areas is not only the strong sensible heating in summer but also the persistent clear skies in such areas. In each continuous clear sky period, the positive feedback mechanism between the CBL and the RL will become operational. Under this mechanism, the daily maximum thickness of the CBL and the thickness of the RL will increase continuously. The thickness of the SCBL is generally over 3 km, although depths of over 5 km can develop through a cyclic growth mechanism during periods of strong surface heating. Otherwise, the thickness of the CBL can only reach 2–3 km in summer, and it is unlikely that an SCBL will develop. Strong sensible heating is the key trigger of the positive feedback cycle growth mechanism between the CBL and the RL, which explains why the SCBL phenomenon can only occur in dry areas, with intense surface heating in summer.

Long-term lidar observations of the gravity wave activity near the mesopause at Arecibo

Abstract. Using 11-year-long K Doppler lidar observations of temperature profiles in the mesosphere and lower thermosphere (MLT) between 85 and 100 km, conducted at the Arecibo Observatory, Puerto Rico (18.35∘ N, 66.75∘ W), seasonal variations of mean temperature, the squared Brunt–Väisälä frequency, N2, and the gravity wave potential energy (GWPE) are estimated in a composite year. The following unique features are obtained. (1) The mean temperature structure shows similar characteristics to an earlier report based on a smaller dataset. (2) Temperature inversion layers (TILs) occur at 94–96 km in spring, at ∼92 km in summer, and at ∼91 km in early autumn. (3) The first complete range-resolved climatology of GWPE derived from temperature data in the tropical MLT exhibits an altitude-dependent combination of annual oscillation (AO) and semiannual oscillation (SAO). The maximum occurs in spring and the minimum in summer, and a second maximum is in autumn and a second minimum in winter. (4) The GWPE per unit volume reduces below ∼97 km altitude in all seasons. The reduction of GWPE is significant at and below the TILs but becomes faint above; this provides strong support for the mechanism that the formation of upper mesospheric TILs is mainly due to the reduction of GWPE. The climatology of GWPE shows an indeed pronounced altitudinal and temporal correlation with the wind field in the tropical mesopause region published in the literature. This suggests the GW activity in the tropical mesopause region should be manifested mainly by the filtering effect of the critical level of the local background wind and the energy conversion due to local dynamical instability.