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The evolution of the atmospheric boundary layer (ABL) varies greatly with terrain, so that the spatial and temporal variabilities of the ABL height remain poorly understood over complex terrain. Using radar wind-profiler measurements obtained from rural mountainous (Yanqing) and adjoining urban-plain (Haidian) landscapes of Beijing, China in 2019, ABL heights are calculated based on a normalized signal-to-noise-ratio threshold. The seasonally contrasting features of ABL height variation and growth rate over the two sites are revealed for clear-sky conditions. Interestingly, the ABL in spring remains suppressed during the morning and evolves rapidly in the afternoon over Haidian; however, a usual diurnal ABL evolution is observed over Yanqing. During the winter, more rapid evolution of the ABL is observed over Haidian, although on average the daytime ABL height remains less than 800 m above ground level. The growth rate of ABL height is found to undergo a more pronounced seasonal variation over Haidian while being relatively less variable over Yanqing. As expected, the lowest (highest) growth rate of 90 m h−1 (188 m h−1) occurs in winter (summer) over Haidian. The analysis of the seasonal variations in wind profiles reveals deeper insights into the development of the local plain-to-mountain flow circulation over the region and possible implications on the contrasting seasonal ABL variations, particularly during the spring and summer. Additionally, the slower ABL evolution over Haidian in autumn and winter could be associated with an aerosol-induced stable ABL as well as stronger urban heat accumulation. The findings have implications for the better understanding of air pollution meteorology in regions with mountainous terrain. [ABSTRACT FROM AUTHOR]

Weather radars measure changes in the refractive index of air in the atmospheric boundary layer. The technique uses the phase of signals from ground targets located around the radar to provide information on atmospheric refractivity related to meteorological quantities such as temperature, pressure and humidity. The approach has been successfully implemented during several field campaigns using operational S-band radars in Canada, UK, USA and France. In order to better characterize the origins of errors, a recent study has simulated temporal variations of refractivity based on Automatic Weather Station (AWS) measurements. This reveals a stronger variability of the refractivity during the summer and in the afternoon when the refractivity is the most sensitive to humidity, probably because of turbulence close to the ground. This raises the possibility of retrieving information on the turbulent state of the atmosphere from the variability in radar refractivity. An analysis based on a 1-year dataset from the operational C-band radar at Trappes (near Paris, France) and AWS refractivity variability measurements was used to measure those temporal and spatial variabilities. Particularly during summer, a negative bias increasing with range is observed between radar and AWS estimations, and is well explained by a model based on Taylor's hypotheses. The results demonstrate the possibility of establishing, depending on season, a quantitative and qualitative link between radar and AWS refractivity variability that reflects low-level coherent turbulent structures. [ABSTRACT FROM AUTHOR]

A dataset obtained using a wind-profile radar located at the Yangtze River Delta in China ( $$31.14^{\circ }$$ 31 . 14 ∘ N, $$121.81^{\circ }$$ 121 . 81 ∘ E) in 2009 was used to investigate the characteristics and evolution of low-level jets (LLJs) along the east China coast. The study investigated the daily and seasonal structures of LLJs as well as several possible causes. A total of 1,407 1-h LLJ periods were detected based on an adaptive definition that enabled determination of four LLJ categories. The majority (77 %) of LLJs were found to have speeds $$<$$ < 14.0 m s $$^{-1}$$ - 1 (maximum of 34.6 m s $$^{-1})$$ - 1 ) and occur at an average altitude below 600 m (76 % of the observed LLJs). The dominant direction of the LLJs was from the south-south-west, which accounted for nearly 32 %, with the second most common wind direction ranging from $$040^{\circ }$$ 040 ∘ to $$100^{\circ }$$ 100 ∘ , albeit with a number of stronger LLJs from the west-south-west. A comparison of LLJs and South-west Jets revealed that the frequencies of occurrence in summer are totally different. Results also revealed that in spring and summer, most LLJs originate from the south-south-west, whereas in autumn and winter, north-east is the dominant direction of origin. The peak heights of LLJs tended to be higher in winter than in other seasons. The horizontal wind speed and peak height of the LLJs displayed pronounced diurnal cycles. The Hilbert–Huang transform technique was applied to demonstrate that the intrinsic mode functions with a cycle of nearly 23 h at levels below 800 m, and the instantaneous amplitudes of inertial events (0.0417–0.0476 h $$^{-1}$$ - 1 frequencies) have large values at 300–600 m. The variations in the occurrences of LLJs suggested connections between the formation mechanisms of LLJs and the South-west Jet stream, steady occupation of synoptic-scale pressure system, and land–sea temperature contrasts. [ABSTRACT FROM AUTHOR]

Weather radar refractivity depends on low-level moisture, temperature, and pressure and is available at high space-time resolutions over large areas. It is of definite meteorological interest for assimilation, verification, and process-study purposes. In this study, the path-averaged refractivity change is simulated from the Arome cloud-resolving atmospheric system analyses and compared with corresponding radar observations over a 35-day period with various meteorological conditions. For that, a novel post-processing procedure is applied to radar data to improve its quality. Also, an observation operator is developed that ingests Arome analyses and simulates a 3-h path-averaged refractivity change. A sensitivity study shows that simulated path-averaged refractivity change is immune to the modelling of the beam height as long as it remains below approximately 60 m above the ground. Comparisons show overall consistency between observed and simulated path-averaged refractivity change, with discrepancies at times that suggest an improvement in analyses once radar refractivity change observations are assimilated. Finally, errors introduced when retrieving local refractivity from path-averaged refractivity are estimated and it is found for our dataset that such retrievals halve the range of usable observations. [ABSTRACT FROM AUTHOR]

At Frankfurt airport (EDDF) vertical soundings of the lower atmosphere from two independent sources are available. One of them is a wind and temperature profiler (wind temperature radar and radio acoustic sounding system, WTR/RASS) located at the western end of the main pair of runways. The second source is aircraft meteorological data relay (AMDAR), i.e. measurements operationally collected by approaching and departing aircraft. Together, both offer a rare opportunity to compare the performance of these widely used systems. We use 1 year of continuous data to calculate systematic temperature and wind vector differences between both measurement systems. The differences show a clear season-dependent structure in conjunction with typical inversion heights. Possible causes for this behaviour are discussed. Second, we compare the ability of both systems to identify inversion and wind-shear layers above the airport. AMDAR-detected layers are typically higher than wind profiler detections. The layer base is usually detected with more agreement than the top. The mutual probability of detection of inversions is found to be mostly between 40 and 60%. [ABSTRACT FROM AUTHOR]

Direct measurements of the air–sea turbulent fluxes of momentum and heat, along with surface currents, waves and supporting meteorological variables, were acquired during a recent field campaign. Surface currents, measured from a very high frequency radar, were found to steer the stress away from the mean wind direction. Although this effect has been reported in a recent scatterometer study, this is the first time it has been observed in an in situ study with co-located flux, wind and surface current measurements. Data collected during a week of stationary conditions are used to investigate and quantify the sampling variability of the air–sea fluxes of momentum and sensible heat. [ABSTRACT FROM AUTHOR]

A case study of a particularly intense cold air outbreak over the north Atlantic Ocean extending from the northeast coast of the United States to the Gulf Stream is described. A RADARSAT satellite synthetic aperture radar (SAR) image of this outbreak dramatically illustrates the spatial evolution of convection. Nearly coincident images from the National Oceanic and Atmospheric Administration's Advanced Very High Resolution Radiometer are used to compare many interesting features. In addition, National Weather Service rawinsonde data, National Data Buoy Center buoy data, and Woods Hole Oceanographic Institute Coastal Mixing and Optics mooring data are presented. We use these data to help describe the spatial evolution of the atmospheric boundary-layer processes involved in this outbreak. Rows of cellular convective clouds begin to appear some distance offshore and then slowly increase in horizontal diameter and wavelength in the downwind direction, with a subsequent jump in cloud diameter downwind of the Gulf Stream North Wall (GSNW). The SAR image shows a similar evolution of sea-surface footprints of these boundary-layer features. This change in boundary-layer structure is attributed to corresponding changes in static stability. About 300 km south of the GSNW in the SAR image, an even larger jump in cell diameter appears and the cells become non-uniform with bright crescents and filled semi-circles on the downwind sides of the cells. These are believed to be surface effects of gust fronts induced by the mesoscale cellular convection and enhanced by the overall northwesterly flow. [ABSTRACT FROM AUTHOR]

On 11 August 1999, a near-total solar eclipse (80%) was observed in Campistrous, France. The influence of this particular event on the atmospheric boundary layer was observed with a UHF-RASS radar, a sodar and an instrumented mast. The changes in turbulence intensity, radar reflectivity, and temperature on the radiative budget are described in relation to collocated ground meteorological data. The impact of the eclipse induces a clear response of the atmosphere, with a time lag of 15 to 30 min, perceptible in several mean and turbulent meteorological variables up to the top of the atmospheric boundary layer. [ABSTRACT FROM AUTHOR]

Mountain waves are observed in the free atmosphere, even when the mountain boundary layer (the source region of these waves) is neutral or convectively unstable, and filled with convective rolls, revealed by cloud streets. This paper investigates if mountain waves are caused not simply by air flow over mountain ridges, but also by flow over boundary-layer convective activity, similar to convection waves above plains and oceans. Disturbance of stable air flow above mountains by convective activity, mechanical and convective turbulence and rotors, near the ground, could move the effective mountain-wave launching height to higher than the mountain peaks. [ABSTRACT FROM AUTHOR]

In two preceding papers, coherent structures of the atmospheric boundary layer (ABL), such as roll vortices or cells, were investigated through radar and aircraft observations collected during the TRAC-93 (Turbulence Radar Aircraft Cells) experiment held in France in June 1993. The analysis of this experimental data set provided information on the spatial characteristics of these organisations (length scale, orientation, type ... ), their temporal and vertical evolution, and their relation with the dynamic and thermodynamic conditions of the ABL. For the third paper in this series, a large eddy simulation model is used to examine the impact of the coherent structures on the ABL vertical fluxes. The analysis of the simulated horizontal fields is made with two-dimensional auto and cross-correlations applied on different pertinent ABL variables. The results emphasise a directional anisotropy of these organised fields throughout the ABL, much more pronounced in the heat flux fields, not only at the length scale of organisations but also at the turbulence scales. This finding has an important consequence for traditional ABL flux measurements based on the hypothesis of isotropic and homogeneous turbulence. It can explain part of the underestimation of the surface fluxes often mentioned in the literature. This approach makes it possible to modify the concept of diffusion time (in chemical modelling) and could also lead to revised ABL parameterisations in Range Scale models. [ABSTRACT FROM AUTHOR]