Investigating the Summertime Low-Level Jet Over the East Coast of the U.S.A.: A Case Study

The vertical structure of the marine atmospheric boundary layer (MABL) and the summertime low-level jet (LLJ) along the east coast of the U.S.A. was studied in the framework of a joint expedition (CBLAST-Low), carried out during the summer of 2003 (31 July–27 August) at Nantucket island, Massachusetts, U.S.A. Analyzing measurements from radiosondes, in-situ and remote sensing systems, it was found that within the lower part of the stable MABL, intermittent and persistent LLJ events were frequently observed. Evidence is presented implying that the analyzed jet case was induced by the interaction of a slow-moving cyclone over north-eastern U.S.A. and the stationary high pressure system over the Atlantic Ocean, as well as by the sloping inversion of the MABL. Focused on a 5-day period of persistent south-westerly (marine) flow, the analysis of sodar and radiosonde data revealed the presence of a stable layer associated with increased static stability just before the emergence of low-level flow acceleration. As indicated by the Richardson number profiles, the increased stability of the lower MABL suppressed turbulence, allowed the decoupling of LLJ from friction, providing a favourable environment for the development of inertial oscillations. Significant amplitudes of inertial motions, which were confirmed by the application of a Hilbert–Huang transform, are associated with the acceleration at the LLJ’s core, due to the frontal events and the subsequent frictional decoupling, both leading to a modification of the large-scale flow structure.     

An analytical study of diurnal wind-structure variations in the boundary layer and the low-level nocturnal jet

An analytical framework is proposed for studying variations in the diurnal wind structure in the planetary boundary layer (PBL) and the evolution of the low-level nocturnal jet. A time-dependent eddy-diffusivity coefficient corresponding to solar input is proposed, and an appropriate coordinate transformation ensures that mixing height varies continuously with ground heat-flux changes. The solution exhibits the receding character of the daytime PBL as evening approaches, thereby dividing the PBL into two regimes — the one just above the ground, representing the nocturnal boundary layer, and the region above it. It is assumed that inertial oscillations (IO) are triggered in the upper layer at about the time of sunset when the reversal in the direction of ground heat flux is felt in the upper layer. Two approaches are adopted to determine the characteristic features of IO and the evolution of the nocturnal low-level jet. The first one is based on the physical principle that release of horizontal momentum due to deviation from the geostrophic wind gives rise to the IO. The solution captures all the characteristic features of the IO, such as phase shift and decreasing amplitude of the IO with increasing height. According to this analysis the IO is triggered at a level as soon as the top of the receding boundary layer leaves that level. The solution is discontinuous with respect to the vertical coordinate. In the second approach we solve an initial-value problem to determine the solution in the upper layer, assuming that at about the time of sunset there is a rapid collapse of the daytime PBL to the steady, nocturnal boundary layer. The assumption is based on the mixing-height profiles prepared from climatological data collected at Delhi. The solution for the nocturnal boundary-layer regime is then obtained as a boundary-value problem. The solutions so obtained are continuous throughout the domain of interest and exhibit the characteristic features of an IO. The analysis leads to the conditions under which a low-level nocturnal jet is produced and provides quantitative estimates of the parameters, such as length of night, latitude, mixing height at sunset and nocturnal mixing height, that are conducive to the generation of a jet. The nocturnal wind profile produced by this approach compares well both with typical atmospheric data observed at Delhi and with output from a mesoscale numerical model. There is still some uncertainty related to the time of initiation of the IO as a function of latitude.     

The Koorin nocturnal low-level jet

It has been long known by Australian meteorologists that a nocturnal jet is a frequent feature of the low-level wind profile at Daly Waters in the Northern Territory of Australia. The Koorin Expedition during the southern winter of 1974 obtained data on wind and temperature profiles in the boundary layer which form an ideal base for the study of this phenomenon. In this paper, a simple model of the jet is described and the results of the model are compared with observations. The model is essentially a development of the pioneering work of Blackadar, who proposed that a low-level jet could develop as an inertial oscillation. The observations do show some of the features of the model.     

On the Formation of an Elevated Nocturnal Inversion Layer in the Presence of a Low-Level Jet: A Case Study

We report on observed nocturnal profiles, in which an inversion layer is located at the core of a low-level jet, bounded between two well-mixed layers. High-resolution vertical profiles were collected during a field campaign in a small plain in the Israeli desert (Negev), distant 100 km from the eastern shore of the Mediterranean Sea. During the evening hours, the synoptic flow, superposed on the late sea breeze, forms a low-level jet characterized by a maximum wind speed of 12 m s ⁻¹ at an altitude of 150 m above the ground. The strong wind shear at the jet maximum generates downward heat fluxes that act against the nocturnal ground cooling. As a result, the typical ground-based nocturnal inversion is “elevated” towards the jet centre, hence a typical early morning thermal profile is observed a few hours after sunset. Since the jet is advected into the region, its formation does not depend on the presence of a surface nocturnal inversion layer to decouple the jet from surface friction. On the contrary, here the advected low-level jet acts to hinder the formation of such an inversion. These unusual temperature and wind profiles are expected to affect near-ground dispersion processes.     

A Comparative Study and Evaluation of Mixing-Height Estimation Based on Sodar-RASS, Ceilometer Data and Numerical Model Simulations

The unmanned aerial system SUMO (Small Unmanned Meteorological Observer) has been used for the observation of the structure and behaviour of the atmospheric boundary layer above the Advent Valley, Svalbard during a two-week period in early spring 2009. Temperature, humidity and wind profiles measured by the SUMO system have been compared with measurements of a small tethered balloon system that was operated simultaneously. It is shown that both systems complement each other. Above 200?m, the SUMO system outperforms the tethered balloon in terms of flexibility and the ability to penetrate strong inversion layers of the Arctic boundary layer. Below that level, the tethered balloon system provides atmospheric profiles with higher accuracy, mainly due to its ability to operate at very low vertical velocities. For the observational period, a numerical mesoscale model has been run at high resolution and evaluated with SUMO profiles reaching up to a height of 1500?m above the ground. The sensitivity to the choice of atmospheric boundary-layer schemes and horizontal resolution has been investigated. A new scheme especially suited for stable conditions slightly improves the temperature forecast in stable conditions, although all schemes show a warm bias close to the surface and a cold bias above the atmospheric boundary layer. During one cold and cloudless night, the SUMO system could be operated nearly continuously (every 30?C45?minutes). This allowed for a detailed case study addressing the structure and behaviour of the air column within and above Advent Valley and its interaction with the local topography. The SUMO measurements in conjunction with a 10-m meteorological mast enabled the identification of a very stable nocturnal surface layer adjacent to the valley bottom, a stable air column in the valley and a strong inversion layer above the summit height. The results indicate the presence of inertial-gravity waves during the night, a feature not captured by the model.     

青藏高原近地面层微气象学特征

利用1998年5月-7月在改则、当雄和昌都三测站获得的近地面层气象要素变化的观测资料，分析了青藏高原近地面层风速、温度和湿度日变化特征及廓线规律，发现高原近地面层微气象学特征具有自己的特点；同时还讨论了高原近地面层白天出现的逆湿现象。     

A Study of the Relationship between Air Pollutants and Inversion in the ABL over the City of Lanzhou

By analyzing the pollutant concentrations over the urban area and over the rural area of the city of Lanzhou, Gansu Province, China, the relationships between the daytime inversion intensity and the pollutant concentration in the atmospheric boundary layer (ABL) are studied with the consideration of wind speed and direction, potential temperature, specific humidity profiles, pollutant concentration in the ABL, the surface temperature, and global radiation on the ground. It was shown that the daytime inversion is a key factor in controlling air pollution concentration. A clear and positive feedback process between the daytime inversion intensity and the air pollutants over the city was found through the analysis of influences of climatic and environmental factors. The mechanisms by which the terrain and air pollutants affect the formation of the daytime inversion are discussed. The solar radiation as the essential energy source to maintain the inversion is analyzed, as are various out-forcing factors affecting the inversion and air pollutants. At last, a physical frame of relationships of air pollution with daytime inversion and the local and out-forcing factors over Lanzhou is built.     

Characteristic seasonal variation of vertical air temperature profile in urban areas of Japan

The seasonality of the vertical air temperature profile in an urban area in the vicinity of the coast was studied. The vertical air temperature profile showed the characteristic seasonality. In the summer, the atmosphere was thermally stratified in the 60–100 m layer, while the 20–60 m layer was unstable or weakly stable throughout the day due to the strong solar radiation. On the other hand, no temperature inversion layer was observed in the winter presumably due to the heat supply from the sea as well as the strong wind speed in the vicinity of the coast. The vertical air temperature profiles at nighttime in the spring and autumn were transitional between the summer and the winter. In contrast, the vertical air temperature profile in the daytime in the spring was similar to that in the summer, while that in the autumn was comparable to that in the winter. Characteristic findings in the study due to the site location, i.e., in the vicinity of the coast, can be summarized as follows: (1) the elevated temperature inversion layer at noon was observed in the summer, and (2) no temperature inversion layer was observed in the winter.     

The Nighttime Accelerations of the Wind in the Boundary Layer

Blackadar’s inertial relaxation of the nighttime boundary layer is modified to account for the finite scale of the pressure gradient normal to the jet. Even though this horizontal dimension may be several thousand times the depth of the daytime convective layer, it severely reduces the period of the free oscillation in the relaxation process and replaces the harmonic acceleration of the jet with a linear growth rate.     

春季晴日蒙古高原半干旱荒漠草原地边界层结构的一次观测研究

在科技部公益专项"沙尘气溶胶辐射模型及气候环境生态效应研究"支持下,执行期间在二连浩特气象站用GFE(L)1型二次测风雷达和GTS1型数字式探空仪,于2006年4月24~25日对内蒙古高原半干旱荒漠化草原地区春季典型晴日边界层特征进行了加密探测。结果表明:二连浩特地区边界层的日变化大,对流边界层最大高度可以达到2000 m,超绝热层平均高度为270 m,夜间稳定边界层高度可达到900 m;白天在4000 m附近存在高空急流,最大风速达到29 m.s-1;夜间近地面层有东风急流,其高度在250 m左右,观测日低空急流最强时间是在23:00,风速达到16 m.s-1;边界层内湿度变化较大,呈现多层逆湿结构。     

梅雨锋强降水与低空急流日变化的观测分析和数值模拟

利用地面加密自动站逐小时观测资料和ERA-Interim再分析资料,分析了2011年6月江淮流域的5次强降水过程和西南低空急流的日变化特征。发现强降水的日变化与西南低空急流的日变化一致:02—08时增强,14时减弱。这主要是由于夜间边界层内的惯性振荡,导致西南低空急流增强从而使得梅雨锋水汽通量辐合增强,降水增强;而白天由于边界层混合摩擦力增大,致使西南低空急流减弱或消失,降水减弱。WRF数值模拟试验不仅重现了观测的日变化特征,而且证实了江淮暴雨和西南低空急流的日变化主要是由非地转风的日变化造成:白天边界层混合强,风为次地转;而夜间边界层混合消失,气压梯度力和科氏力平衡的惯性振荡使得风为超地转      

Frequency of Boundary-Layer-Top Fluctuations in Convective and Stable Conditions Using Laser Remote Sensing

The planetary boundary-layer (PBL) height is determined with high temporal and altitude resolution from lidar backscatter profiles. Then, the frequencies of daytime thermal updrafts and downdrafts and of nighttime gravity waves are obtained applying a fast Fourier transform on the temporal fluctuation of the PBL height. The principal frequency components of each spectrum are related to the dominant processes occurring at the daytime and nighttime PBL top. Two groups of cases are selected for the study: one group combines daytime cases, measured in weak horizontal wind conditions and dominated by convection. The cases show higher updraft and downdraft frequencies for the shallow, convective boundary layer and lower frequencies for a deep PBL. For cases characterized by strong horizontal winds, the frequencies directly depend on the wind speed. The temporal variation of the PBL height is determined also in the likely presence of lee waves. For nighttime cases, the main frequency components in the spectra do not show a real correlation with the nocturnal PBL height. Altitude fluctuations of the top of the nocturnal boundary layer are observed even though the boundary layer is statically stable. These oscillations are associated with the wind shear effect and with buoyancy waves at the PBL top.     

绿洲周边荒漠戈壁夏末土壤-大气水分传输特征

利用“我国西北干旱区陆-气相互作用试验”2000年8-9月在甘肃敦煌地区戈壁滩上取得的野外观测资料,分析了临近绿洲的戈壁土壤湿度和温度特征以及相对应的大气湿度特征,发现土壤热量活动层约为5cm厚,比一般土壤要薄得多;临近绿洲的荒漠戈壁上,不仅近地层大气多为逆湿,而且浅层土壤有时也出现逆湿。土壤湿度日变化能清楚地被区分为湿维持、水分损失、干维持和水分补充等四个阶段。土壤湿度廓线表明：土壤水分活动层厚度约为10m;湿维持阶段的浅层土壤逆湿是土壤湿度廓线最主要的结构特征,这一土壤湿度结构预示着夜间土壤可能通过凝结吸收大气水分,它与白天的土壤水分蒸发共同构成土壤对大气水分的“呼吸”过程。土壤逆湿的形成与土壤温度状态、大气逆湿强度和大气稳定度都有关。     

The nocturnal boundary layer over Northern Germany: An observational study

The stable boundary layer which evolved over the lowland of Northern Germany during a clear night with moderate geostrophic winds is studied. Because of the lack of turbulence measurements, a vertical flux-profile of heat and momentum is derived from a mean wind and temperature profile using an integral method. The stability parameter h/L _* = 17 indicates that turbulence was sporadic during this particular night. This result is confirmed by the observed inertial oscillations, which occur not only in the residual layer but also in the boundary layer below.The case study shows that turbulent cooling overrules radiational cooling in the lower part of the surface inversion layer. Additionally, warm-air advection occurs. In the upper part, cold-air advection and radiational cooling dominate, while turbulent cooling is reduced. Subsidence warming can be neglected throughout the boundary layer during this particular night.     

The New Canadian Urban Modelling System: Evaluation for Two Cases from the Joint Urban 2003 Oklahoma City Experiment

A new Canadian numerical urban modelling system has been developed at the Meteorological Service of Canada to represent surface and boundary-layer processes in the urban environment. In this system, urban covers are taken into account by including the Town Energy Balance urban-canopy parameterization scheme in the Global Environmental Multiscale meteorological model. The new modelling system is run at 250-m grid size for two intensive observational periods of the Joint Urban 2003 experiment that was held in Oklahoma City, U.S.A. An extensive evaluation against near-surface and upper-air observations has been performed. The Town Energy Balance scheme correctly simulates the urban micro-climate, more particularly the positive nighttime urban heat island, and also reproduces the “cool” island during the morning but does not succeed in maintaining it during all of the daytime period. The vertical structure of the boundary layer above the city is reasonably well simulated, but the simulation of the nocturnal boundary layer is difficult, due to the complex interaction with the nighttime southerly low-level jet that crosses the domain. Sensitivity tests reveal that the daytime convective boundary layer is mainly driven by dry soil conditions in and around Oklahoma City and that the nighttime low-level jet reinforces the urban heat island in the first 300m through large-scale advection, leading to the development of a less stable layer above the city.     

Aircraft and ship observations of the mean structure of the marine boundary layer over the Arabian Sea during MONEX 79

Analysis of the mean wind, equivalent potential temperature and virtual potential temperature profiles observed by the National Center for Atmospheric Research (NCAR) Electra aircraft and obtained from dropwindsondes and ship-launched radiosondes were made in conjunction with synoptic observations to study the structure of the monsoon boundary layer over the Arabian Sea during MONEX 79. Comparison of mean profiles indicates the monsoon boundary layer to be much different from the trade wind boundary layer. Results confirm the existence of a boundary-layer jet known as East African or Somali Jet. Regions of multiple cloud layers at roughly the height of the capping inversion layer were associated with the jet. Regions in which a more well-mixed layer was observed showed a jet structure depressed in height. A free-jet surface-layer model appears to describe the mean wind structure of this jet observed during the present study and by others. An approximate balance of forces was found in the monsoon boundary layer between friction, advective acceleration, Coriolis and pressure gradient forces. Friction and advective acceleration terms were significant in the lower levels of the boundary layer. Forces in a typical trade wind boundary layer were found to be approximately one order of magnitude smaller than those observed in the monsoon boundary layer.     

Perturbations to the Spatial and Temporal Characteristics of the Diurnally-Varying Atmospheric Boundary Layer Due to an Extensive Wind Farm

The effect of extensive terrestrial wind farms on the spatio-temporal structure of the diurnally-evolving atmospheric boundary layer is explored. High-resolution large-eddy simulations of a realistic diurnal cycle with an embedded wind farm are performed. Simulations are forced by a constant geostrophic velocity with time-varying surface boundary conditions derived from a selected period of the CASES-99 field campaign. Through analysis of the bulk statistics of the flow as a function of height and time, it is shown that extensive wind farms shift the inertial oscillations and the associated nocturnal low-level jet vertically upwards by approximately 200 m; cause a three times stronger stratification between the surface and the rotor-disk region, and as a consequence, delay the formation and growth of the convective boundary layer (CBL) by approximately 2 h. These perturbations are shown to have a direct impact on the potential power output of an extensive wind farm with the displacement of the low-level jet causing lower power output during the night as compared to the day. The low-power regime at night is shown to persist for almost 2 h beyond the morning transition due to the reduced growth of the CBL. It is shown that the wind farm induces a deeper entrainment region with greater entrainment fluxes. Finally, it is found that the diurnally-averaged effective roughness length for wind farms is much lower than the reference value computed theoretically for neutral conditions.     

A ROUTINE FOR THE CALCULATION OF THE TIME-DEPENDENT HEIGHT OF THE ATMOSPHERIC BOUNDARY LAYER FROM SURFACE-LAYER PARAMETERS

A simple routine has been implemented to deduce the 24-hourevolution of the height of the atmospheric boundary layer. This uses a reduced data set of surface-layer parameters, as obtained for examplefrom surface automatic stations.The routine is based on the combination and fitting of the three alreadyexistent models for the evolution of the convectiveboundary layer, the stable boundary layer, and the surface inversionlayer.Hourly values of temperature, friction velocity and potentialtemperature scale (or sensible heat flux) in the surface layer need onlyto be supplied as input data. The lapse rate at the top of the daytime mixed layer is derived fromthe calculated surface inversion profile at sunrise, so that only a roughevaluation of the lapse rate in the free atmosphere remains to be given.The sensitivity of the mixed-layer height is expected to be not verystrong with respect to this last parameter (final part of the growth). The routine has shown satisfactory performances when compared withsodar measurements, working with only a rough average estimate of thefree atmosphere lapse rate.     

A Study of the Relationship between Air Pollutants and Inversion in the ABL over the City of Lanzhou

By analyzing the pollutant concentrations over the urban area and over the rural area of the city of Lanzhou, Gansu Province, China, the relationships between the daytime inversion intensity and the pollutant concentration in the atmospheric boundary layer (ABL) are studied with the consideration of wind speed and direction, potential temperature, specific humidity profiles, pollutant concentration in the ABL, the surface temperature, and global radiation on the ground. It was shown that the daytime inversion is a key factor in controlling air pollution concentration. A clear and positive feedback process between the daytime inversion intensity and the air pollutants over the city was found through the analysis of influences of climatic and environmental factors. The mechanisms by which the terrain and air pollutants affect the formation of the daytime inversion are discussed. The solar radiation as the essential energy source to maintain the inversion is analyzed, as are various out-forcing factors affecting the inversion and air pollutants. At last, a physical frame of relationships of air pollution with daytime inversion and the local and out-forcing factors over Lanzhou is built.     

Entrainment effects in the well-mixed atmospheric boundary layer

We discuss the structure and evolution of a cloud-free atmospheric boundary layer (ABL) during daytime over land, starting from a shallow ABL at sunrise and developing into a deep ABL with strong convection in the afternoon. The structure of the turbulence in the lower half of a convective ABL capped by an inversion is reasonably well understood. Less is known about the details of the turbulence in higher regions affected by entrainment, because of the difficulty in taking turbulence measurements there. For the evolution in time of the height of the ABL and its mean potential temperature mixed-layer models have been developed that give satisfactory agreement with observations. It has been shown that for many practical applications accurate knowledge of forcing functions and boundary conditions is more important than a refinement of the entrainment hypothesis. Observations show that the assumption of well-mixedness of first-order moments of conservative variables is not valid for all quantities. A simple similarity relation for the inclusion of the effect of entrainment on the shape of the vertical profiles is given.