Einmischprozesse am Oberrand der konvektiven atmosphärischen Grenzschicht

Der Prozess des Einmischens von Luft aus der freien Atmosphäre in die konvektive Grenzschicht wird mit Hilfe eines Doppler Lidars und konzeptioneller Ansätze untersucht. Durch detaillierte Betrachtung der Vorgänge an der Oberkante der Grenzschicht einerseits und der quantitativen Erfassung der Wirkung dieser Vorgänge andererseits, konnte eine Erweiterung des Prozessverständnisses erfolgen sowie Möglichkeiten zu einer robusteren numerischen Beschreibung des Prozesses aufgezeigt werden

Einmischprozesse am Oberrand der konvektiven atmosphärischen Grenzschicht

Der Prozess des Einmischens von Luft aus der freien Atmosphäre in die konvektive Grenzschicht wird mit Hilfe eines Doppler Lidars und konzeptioneller Ansätze untersucht. Durch detaillierte Betrachtung der Vorgänge an der Oberkante der Grenzschicht einerseits und der quantitativen Erfassung der Wirkung dieser Vorgänge andererseits, konnte eine Erweiterung des Prozessverständnisses erfolgen sowie Möglichkeiten zu einer robusteren numerischen Beschreibung des Prozesses aufgezeigt werden

Assessment of Surface-Layer Coherent Structure Detection in Dual-Doppler Lidar Data Based on Virtual Measurements

Dual-Doppler lidar has become a useful tool to investigate the wind-field structure in two-dimensional planes. However, lidar pulse width and scan duration entail significant and complex averaging in the resulting retrieved wind-field components. The effects of these processes on the wind-field structure remain difficult to investigate with in situ measurements. Based on high resolution large-eddy simulation (LES) data for the surface layer, we performed virtual dual-Doppler lidar measurements and two-dimensional data retrievals. Applying common techniques (integral length scale computation, wavelet analysis, two-dimensional clustering of low-speed streaks) to detect and quantify the length scales of the occurring coherent structures in both the LES and the virtual lidar wind fields, we found that, (i) dual-Doppler lidar measurements overestimate the correlation length due to inherent averaging processes, (ii) the wavelet analysis of lidar data produces reliable results, provided the length scales exceed a lower threshold as a function of the lidar resolution, and (iii) the low-speed streak clusters are too small to be detected directly by the dual-Doppler lidar. Furthermore, we developed and tested a method to correct the integral scale overestimation that, in addition to the dual-Doppler lidar, only requires high-resolution wind-speed variance measurements, e.g. at a tower or energy balance station.DFG/RA 617/19-

The HD(CP)² Observational Prototype Experiment (HOPE) – an overview

The HD(CP)2 Observational Prototype Experiment (HOPE) was performed as a major 2-month field experiment in Jülich, Germany, in April and May 2013, followed by a smaller campaign in Melpitz, Germany, in September 2013. HOPE has been designed to provide an observational dataset for a critical evaluation of the new German community atmospheric icosahedral non-hydrostatic (ICON) model at the scale of the model simulations and further to provide information on land-surface–atmospheric boundary layer exchange, cloud and precipitation processes, as well as sub-grid variability and microphysical properties that are subject to parameterizations. HOPE focuses on the onset of clouds and precipitation in the convective atmospheric boundary layer. This paper summarizes the instrument set-ups, the intensive observation periods, and example results from both campaigns. HOPE-Jülich instrumentation included a radio sounding station, 4 Doppler lidars, 4 Raman lidars (3 of them provide temperature, 3 of them water vapour, and all of them particle backscatter data), 1 water vapour differential absorption lidar, 3 cloud radars, 5 microwave radiometers, 3 rain radars, 6 sky imagers, 99 pyranometers, and 5 sun photometers operated at different sites, some of them in synergy. The HOPE-Melpitz campaign combined ground-based remote sensing of aerosols and clouds with helicopter- and balloon-based in situ observations in the atmospheric column and at the surface. HOPE provided an unprecedented collection of atmospheric dynamical, thermodynamical, and micro- and macrophysical properties of aerosols, clouds, and precipitation with high spatial and temporal resolution within a cube of approximately 10  ×  10  ×  10 km3. HOPE data will significantly contribute to our understanding of boundary layer dynamics and the formation of clouds and precipitation. The datasets have been made available through a dedicated data portal. First applications of HOPE data for model evaluation have shown a general agreement between observed and modelled boundary layer height, turbulence characteristics, and cloud coverage, but they also point to significant differences that deserve further investigations from both the observational and the modelling perspective

Einmischprozesse am Oberrand der konvektiven atmosphärischen Grenzschicht

Der Prozess des Einmischens von Luft aus der freien Atmosphäre in die konvektive Grenzschicht wird mit Hilfe eines Doppler Lidars und konzeptioneller Ansätze untersucht. Durch detaillierte Betrachtung der Vorgänge an der Oberkante der Grenzschicht einerseits und der quantitativen Erfassung der Wirkung dieser Vorgänge andererseits, konnte eine Erweiterung des Prozessverständnisses erfolgen sowie Möglichkeiten zu einer robusteren numerischen Beschreibung des Prozesses aufgezeigt werden

Meso-scale eddies affect near-surface turbulent exchange: evidence from lidar and tower measurements

The eddy-covariance technique tends to underestimate turbulent heat fluxes, which results in nonclosure of the surface energy balance. This study shows experimental evidence that mesoscale turbulent organized structures, which are inherently not captured by the standard eddy-covariance technique, can affect near-surface turbulent exchange. By using a combined setup of three Doppler wind lidars above a cropland-dominated area in Germany, low-frequency turbulent structures were detected in the surface layer down to a few meters above ground. In addition, data from two micrometeorological stations in the study area were analyzed with respect to energy balance closure. In accordance with several previous studies, the data confirm a strong friction velocity dependence of the energy balance residual. At both stations, the energy balance residual was found to be positively correlated with the vertical moisture gradient in the lower atmospheric boundary layer, but at only one station was it correlated with the temperature gradient. This result indicates that mesoscale transport probably contributes more to the latent heat flux than to the sensible heat flux, but this conclusion depends largely on the measurement site. Moreover, flow distortion due to tower mountings and measurement devices affects the energy balance closure considerably for certain wind directions

Dry and moist convection in the boundary layer over the Black Forest - a combined analysis of in situ and remote sensing data

During the COPS experiment performed in south-western Germany and eastern France in 2007, several insitu and remote sensing systems were operated at Hornisgrinde - the highest summit of the northern Black Forest mountains. For this case study, data from a surface flux station, radiosondes, cloud camera, cloud radar, wind lidar, water vapour differential absorption lidar, and microwave profiler were used to investigate turbulence characteristics in cloud-free and cloud-topped convective boundary layers (CBLs). Short time intervals were analysed, during which dry and moist convective cells occurred, in order to obtain insight of the processes that determine the turbulent characteristic in the CBL. The frequently-used aerosol concentration was used to calculate the CBL height, zi . It was found that active CBL clouds penetrated deeper into the free troposphere than dry convective cells. In the cloud-free CBL the normalised variance of the vertical velocity, ?w 2, decreased to zero approximately at zi , while ?w 2 was nearly constant between 0.5 and 1 z/zi in the cloud-topped CBL. The higher normalised ?w 2 values in the cloud layer could be attributed to the additional elevated heat source due to condensation. In the cloud-free CBL the latent heat flux showed a strong decrease between 0.7 and 1.1 z/zi , i.e., it considerably moistened the upper part of the CBL and entrainment zone. In the cloud-topped CBL the latent heat flux decreased significantly above the CBL top only and became zero at about 1.4 z/zi . CBL height calculations, which took measures of the turbulence into account, resulted in normalised ?w 2 and E profiles, which became zero at the CBL top and appeared more appropriate for CBL height scaling over complex terrain. The case studies demonstrated that only the combined use of different monitoring systems allowed for the recording of the entire structure of the convective cells and that synergetic measurements in cloud-topped CBLs were indispensable to capture the latters' turbulent characteristics. Also, significant differences between turbulent characteristics in cloud-free and cloud-topped CBLs became evident

On the relevance of mesoscale transport for in-situ energy balance measurements and its partitioning between sensible and latent heat

Mesoscale transport of energy and matter between the surface and the atmosphere often occurs in form of non-propagating organized structures or thermally-induced circulations. Spatially resolving measurements are required to capture such fluxes and, thus far, airborne measurements are the only means to accomplish this. In contrast, tower-based eddy-covariance measurements are conducted at one point and therefore inherently cannot capture the total atmospheric exchange, which is recognized as a major contributor to energy balance closure problems. As long as there are mean vertical thermal and humidity gradients in the Atmospheric Boundary-Layer, with higher potential temperatures and specific humidities in the surface layer as compared with the outer-layer, such organized structures will lead to a systematic underestimation of turbulent energy fluxes from eddy-towers. Firstly, we address the question of how deep such meso-γ scale motions penetrate into the surface layer. We present indications from Doppler-LiDAR, airborne and tower-based measurements, which show that mesoscale motion can indeed be found quite close to the surface, but the mesoscale effect vanishes when measurements are actually conducted within the roughness sublayer and when shear stress is sufficiently large to break up mesoscale contributions into smaller eddies. This will be illustrated by observations from Germany and Israel. Secondly, we investigate whether the common practice of adjusting the measured eddy tower fluxes for energy balance closure by conserving the Bowen ratio is supported by experimental evidence. Mesoscale and small-scale turbulent fluxes from four different flight campaigns are presented, which were carried out on board of the Canadian Twin Otter (National Research Council of Canada) and the German Polar 5 (Alfred-Wegener Institute) research aircraft over different landscapes in Canada and Alaska