0000000000124155
AUTHOR
Ulrike Lohmann
showing 13 related works from this author
HoloGondel: in situ cloud observations on a cable car in the Swiss Alps using a holographic imager
2017
In situ observations of cloud properties in complex alpine terrain where research aircraft cannot sample are commonly conducted at mountain-top research stations and limited to single-point measurements. The HoloGondel platform overcomes this limitation by using a cable car to obtain vertical profiles of the microphysical and meteorological cloud parameters. The main component of the HoloGondel platform is the HOLographic Imager for Microscopic Objects (HOLIMO 3G), which uses digital in-line holography to image cloud particles. Based on two-dimensional images the microphysical cloud parameters for the size range from small cloud particles to large precipitation particles are obtained for th…
General overview: European Integrated project on Aerosol Cloud Climate and Air Quality interactions (EUCAARI)-integrating aerosol research from nano …
2011
In this paper we describe and summarize the main achievements of the European Aerosol Cloud Climate and Air Quality Interactions project (EUCAARI). EUCAARI started on 1 January 2007 and ended on 31 December 2010 leaving a rich legacy including: (a) a comprehensive database with a year of observations of the physical, chemical and optical properties of aerosol particles over Europe, (b) comprehensive aerosol measurements in four developing countries, (c) a database of airborne measurements of aerosols and clouds over Europe during May 2008, (d) comprehensive modeling tools to study aerosol processes fron nano to global scale and their effects on climate and air quality. In addition a new Pan…
HOLIMO II: a digital holographic instrument for ground-based in situ observations of microphysical properties of mixed-phase clouds
2013
Measurements of the microphysical properties of mixed-phase clouds with high spatial resolution are important to understand the processes inside these clouds. This work describes the design and characterization of the newly developed ground-based field instrument HOLIMO II (HOLographic Imager for Microscopic Objects II). HOLIMO II uses digital in-line holography to in situ image cloud particles in a well-defined sample volume. By an automated algorithm, two-dimensional images of single cloud particles between 6 and 250 μm in diameter are obtained and the size spectrum, the concentration and water content of clouds are calculated. By testing the sizing algorithm with monosized beads a system…
Mixed-Phase Clouds: Progress and Challenges
2017
Mixed-phase clouds represent a three-phase colloidal system consisting of water vapor, ice particles, and coexisting supercooled liquid droplets. Mixed-phase clouds are ubiquitous in the troposphere, occurring at all latitudes from the polar regions to the tropics. Because of their widespread nature, mixed-phase processes play critical roles in the life cycle of clouds, precipitation formation, cloud electrification, and the radiative energy balance on both regional and global scales. Yet, in spite of many decades of observations and theoretical studies, our knowledge and understanding of mixed-phase cloud processes remains incomplete. Mixed-phase clouds are notoriously difficult to represe…
A statistical subgrid-scale algorithm for precipitation formation in stratiform clouds in the ECHAM5 single column model
2011
Abstract. Cloud properties are usually assumed to be homogeneous within the cloudy part of the grid-box, i.e. subgrid-scale inhomogeneities in cloud cover and/or microphysical properties are often neglected. However, precipitation formation is initiated by large particles. Thus mean values are not representative and could lead to a delayed onset of precipitation. For a more physical description of the subgrid-scale structure of clouds we introduce a new statistical sub-column algorithm to study the impact of cloud inhomogeneities on stratiform precipitation. Each model column is divided into N independent sub-columns with sub-boxes in each layer, which are completely clear or cloudy. The cl…
Explicit cloud-top entrainment parameterization in the global climate model ECHAM5-HAM
2011
New developments in the turbulence parameterization in the general circulation model ECHAM5-HAM are presented. They consist mainly of an explicit entrainment closure at the top of stratocumulus-capped boundary layers and the addition of an explicit contribution of the radiative divergence in the buoyancy production term. The impact of the new implementations on a single column model study and on the global scale is presented here. The parameterization has a "smoothing" effect: the abnormally high values of turbulence kinetic energy are reduced, both in the single column and in the Californian stratocumulus region. A sensitivity study with prescribed droplet concentration shows a reduction i…
Microphysical and radiative changes in cirrus clouds by geoengineering the stratosphere
2013
[1] In the absence of tangible progress in reducing greenhouse gas emissions, the implementation of solar radiation management has been suggested as measure to stop global warming. Here we investigate the impacts on northern midlatitude cirrus from continuous SO2emissions of 2–10 Mt/a in the tropical stratosphere. Transport of geoengineering aerosols into the troposphere was calculated along trajectories based on ERA Interim reanalyses using ozone concentrations to quantify the degree of mixing of stratospheric and tropospheric air termed “troposphericity”. Modeled size distributions of the geoengineered H2SO4-H2O droplets have been fed into a cirrus box model with spectral microphysics. Th…
Impact of surface and near-surface processes on ice crystal concentrations measured at mountain-top research stations
2017
In situ cloud observations at mountain-top research stations regularly measure ice crystal number concentrations (ICNCs) orders of magnitudes higher than expected from measurements of ice nucleating particle (INP) concentrations. Thus, several studies suggest that mountain-top in situ cloud microphysical measurements are influenced by surface processes, e.g., blowing snow, hoar frost or riming on snow-covered trees, rocks and the snow surface. This limits the relevance of such measurements for the study of microphysical properties and processes in free-floating clouds. This study assesses the impact of surface processes on in situ cloud observations at the Sonnblick Observatory in the Hohen…
Persistence of orographic mixed‐phase clouds
2016
Mixed-phase clouds (MPCs) consist of ice crystals and supercooled water droplets at temperatures between 0 and approximately −38°C. They are thermodynamically unstable because the saturation vapor pressure over ice is lower than that over supercooled liquid water. Nevertheless, long-lived MPCs are ubiquitous in the Arctic. Here we show that persistent MPCs are also frequently found in orographic terrain, especially in the Swiss Alps, when the updraft velocities are high enough to exceed saturation with respect to liquid water allowing simultaneous growth of supercooled liquid droplets and ice crystals. Their existence is characterized by holographic measurements of cloud particles obtained …
Inadvertent climate modification due to anthropogenic lead
2009
Atmospheric particles are thought to initiate the majority of terrestrial precipitation. Field-based measurements of ice-crystal residues, together with controlled environment experiments on artificial clouds, suggest that anthropogenic lead-containing particles are among the most efficient ice-forming substances in the atmosphere.
The influence of absorbed solar radiation by Saharan dust on hurricane genesis
2015
To date, the radiative impact of dust and the Sahar an air layer (SAL) on North Atlantic hurricane activity is not yet known. According to previous studies, dust stabilizes the atmosphere due to absorption of solar radiation but thus shifts convection to regions more conducive for hurricane genesis. Here we analyze differences in hurricane genesis and frequency from ensemble sensitivity simulations with radiatively active and inactive dust in the aerosol-climate model ECHAM6-HAM. We investigate dust burden and other hurricane-related variables and determine their influence on disturbances which develop into hurricanes (developing disturbances, DDs) and those which do not (nondeveloping dist…
Vertical grid refinement for stratocumulus clouds in the radiation scheme of a global climate model
2021
Abstract. In this study, we implement a vertical grid refinement scheme in the radiation routine of the global aerosol-climate model ECHAM-HAM, aiming to improve the representation of stratocumulus clouds and address the underestimation of their cloud cover. The scheme is based on a reconstruction of the temperature inversion as a physical constraint for the cloud top. On the refined grid, the boundary layer and the free troposphere are separated and the cloud's layer is made thinner. The cloud cover is re-calculated either by conserving the cloud volume (SC-VOLUME) or by using the Sundqvist cloud cover routine on the new grid representation (SC-SUND). In global climate simulations, we find…
HoloGondel: in-situ cloud observations on a cable car in the Swiss Alps using a holographic imager
2016
Abstract. In-situ observations of cloud properties in complex alpine terrain where research aircraft cannot sample are commonly conducted at mountain-top research stations and limited to single-point measurements. The HoloGondel platform overcomes this limitation by using a cable car to obtain vertical profiles of the microphysical and meteorological cloud parameters. The main component of the HoloGondel platform is the HOLographic Imager for Microscopic Objects (HOLIMO 3G), which uses digital 5 in-line holography to image cloud particles. Based on a two dimensional shadow-graph the phase resolved microphysical cloud parameters for the size range from small cloud particles to large precipit…