K. Träumner

Scopes and Challenges of Dual-Doppler Lidar Wind Measurements - An Error Analysis

AbstractPulsed Doppler lidars are powerful tools for long-range, high-resolution measurements of radial wind velocities. With the development of commercial Doppler lidars and the reduction of acquisition costs, dual-Doppler lidar systems will be become increasingly accessible in upcoming years. This study reviews the most common dual-Doppler techniques, describes the setup of a highly synchronized long-range dual-Doppler lidar system, and discusses extensively the different kinds of errors connected with this complex measurement technique. Sources of errors and their interactions are traced through the retrieval algorithm, including errors from single-Doppler lidar and those occurring from a combination of instruments related to various parameters, such as relative beam angles, time and spatial scales of the scan pattern, and atmospheric conditions.

KITcube - a mobile observation platform for convection studies deployed during HyMeX

With the increase of spatial resolution of weather forecast models to order O(1 km), the need for adequate observations for model validation becomes evident. Therefore, we designed and constructed the ‘‘KITcube’’, a mobile observation platform for convection studies of processes on the meso-c scale. The KITcube consists of in-situ and remote sensing systems which allow measuring the energy balance components of the Earth’s surface at different sites; the mean atmospheric conditions by radiosondes, GPS station, and a microwave radiometer; the turbulent characteristics by a sodar and wind lidars; and cloud and precipitation properties by use of a cloud radar, a micro rain radar, disdrometers, rain gauges, and an X-band rain radar. The KITcube was deployed fully for the first time on the French island of Corsica during the HyMeX (Hydrological cycle in the Mediterranean eXperiment) field campaign in 2012. In this article, the components of KITcube and its implementation on the island are described. Moreover, results from one of the HyMeX intensive observation periods are presented to show the capabilities of KITcube.

Mesoscale Eddies Affect Near-Surface Turbulent Exchange: Evidence from Lidar and Tower Measurements

AbstractThe 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 wit...

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, rw 2 , decreased to zero approximately at zi, while rw 2 was nearly constant between 0.5 and 1 z/zi in the cloud-topped CBL. The higher normalised rw 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 rw 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.

Turbulent Structures and Coherence in the Atmospheric Surface Layer

Organized structures in turbulent flow fields are a well-known and still fascinating phenomenon. Although these so-called coherent structures are obvious from visual inspection, quantitative assessment is a challenge and many aspects e.g., formation mechanisms and contribution to turbulent fluxes, are discussed controversially. During the “High Definition Clouds and Precipitation for Advancing Climate Prediction” Observational Prototype Experiment (HOPE) from April to May 2013, an advanced dual Doppler lidar technique was used to image the horizontal wind field near the surface for approximately 300 h. A visual inspection method, as well as a two-dimensional integral length scale analysis, were performed to characterize the observations qualitatively and quantitatively. During situations with forcing due to shear, the wind fields showed characteristic patterns in the form of clearly bordered, elongated areas of enhanced or reduced wind speed, which can be associated with near-surface streaks. During calm situations with strong buoyancy forcing, open cell patterns in the horizontal divergence field were observed. The measurement technique used enables the calculation of integral length scales of both horizontal wind components in the streamwise and cross-stream directions. The individual length scales varied considerably during the observation period but were on average shorter during situations with

A Synergy Approach to Estimate Properties of Raindrop Size Distributions Using a Doppler Lidar and Cloud Radar

z/L<0

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

z/L<0 compared to strongly stable situations. During unstable situations, which were dominated by wind fields with structures, the streamwise length scales increased with increasing wind speed, whereas the cross-stream length scales decreased. Consequently, the anisotropy increased from 1 for calm situations to values of 2–3 for wind speeds of 8–10