JOYCE: Jülich Observatory for cloud evolution

The Jülich Observatory for Cloud Evolution (JOYCE), located at Forschungszentrum Jülich in the most western part of Germany, is a recently established platform for cloud research. The main objective of JOYCE is to provide observations, which improve our understanding of the cloudy boundary layer in a midlatitude environment. Continuous and temporally highly resolved measurements that are specifically suited to characterize the diurnal cycle of water vapor, stability, and turbulence in the lower troposphere are performed with a special focus on atmosphere–surface interaction. In addition, instruments are set up to measure the micro- and macrophysical properties of clouds in detail and how they interact with different boundary layer processes and the large-scale synoptic situation. For this, JOYCE is equipped with an array of state-of-the-art active and passive remote sensing and in situ instruments, which are briefly described in this scientific overview. As an example, a 24-h time series of the evolution of a typical cumulus cloud-topped boundary layer is analyzed with respect to stability, turbulence, and cloud properties. Additionally, we present longer-term statistics, which can be used to elucidate the diurnal cycle of water vapor, drizzle formation through autoconversion, and warm versus cold rain precipitation formation. Both case studies and long-term observations are important for improving the representation of clouds in climate and numerical weather prediction models

Emission fluxes and atmospheric degradation of monoterpenes above a boreal forest: field measurements and modelling

The contribution of monoterpenes to aerosol formation processes within and above forests is not well understood. This is also true for the particle formation events observed during the BIOFOR campaigns in Hyytiälä, Finland. Therefore, the diurnal variation of the concentrations of several biogenic volatile organic compounds (BVOCs) and selected oxidation products in the gas and particle phase were measured on selected days during the campaigns in Hyytiälä, Finland. α-pinene and Δ3-carene were found to represent the most important monoterpenes above the boreal forest. A clear vertical gradient of their concentrations was observed together with a change of the relative monoterpene composition with height. Based on concentration profile measurements of monoterpenes, their fluxes above the forest canopy were calculated using the gradient approach. Most of the time, the BVOC fluxes show a clear diurnal variation with a maximum around noon. The highest fluxes were observed for α-pinene with values up to 20 ng m−2 s−1 in summer time and almost 100 ng m−2 s−1 during the spring campaign. Furthermore, the main oxidation products from α-pinene, pinonaldehyde, and from β-pinene, nopinone, were detected in the atmosphere above the forest. In addition to these more volatile oxidation products, pinic and pinonic acid were identified in the particle phase in a concentration range between 1 and 4 ng m−3. Beside these direct measurement of known oxidation products, the chemical sink term in the flux calculations was used to estimate the amount of product formation of the major terpenes (α-pinene, β-pinene, Δ3-carene). A production rate of very low volatile oxidation products (e.g., multifunctional carboxylic) from ·OH- and O3-reaction of monoterpenes of about 1.3·104 molecules cm−3 s−1 was estimated for daylight conditions during summer time. Additionally, model calculations with the one-dimensional multilayer model CACHE were carried out to investigate the diurnal course of BVOC fluxes and chemical degradation of terpenes

Mixing-layer height retrieval with ceilometer and Doppler lidar: from case studies to long-term assessment

Aerosol signatures observed by ceilometers are frequently used to derive mixing-layer height (MLH) which is an essential variable for air quality modelling. However, Doppler wind lidar measurements of vertical velocity can provide a more direct estimation of MLH via simple thresholding. A case study reveals difficulties in the aerosol-based MLH retrieval during transition times when the mixing layer builds up in the morning and when turbulence decays in the afternoon. The difficulties can be explained by the fact that the aerosol distribution is related to the history of the mixing process and aerosol characteristics are modified by humidification. The results of the case study are generalized by evaluating one year of joint measurements by a Vaisala CT25K and a HALO Photonics Streamline wind lidar. On average the aerosol-based retrieval gives higher MLH than the wind lidar with an overestimation of MLH by about 300 m (600 m) in the morning (late afternoon). Also, the daily aerosol-based maximum MLH is larger and occurs later during the day and the average morning growth rates are smaller than those derived from the vertical wind. In fair weather conditions classified by less than 4 octa cloud cover the mean diurnal cycle of cloud base height corresponds well to the mixing-layer height showing potential for a simplified MLH estimation