I am a meteorologist with research foci in regional Earth system models, weather prediction, data assimilation, and active and passive microwave remote sensing.
Bi-spectral algorithms to estimate cloud properties from reflected solar radiation at an absorbin... more Bi-spectral algorithms to estimate cloud properties from reflected solar radiation at an absorbing and a non-absorbing wavelength are routinely applied to observations of meteorological satellite imagers. The underlying inversion process is highly underconstrained, and is based on the simplified view of 1D radiative transfer theory. It is therefore difficult to quantify the overall accuracy of these retrievals, and validation with
We implemented a PI (Physical Initialization) method in the non hydrostatic limited-area model CO... more We implemented a PI (Physical Initialization) method in the non hydrostatic limited-area model COSMO (version 4.2) of the DWD (German Meteorological Service). The goal is the improvement of quantitative rain nowcasting with a high resolution NWP model. Input radar data is a DWD product: the national radar composite for 16 radars with a spatial resolution of one kilometer and a
Cloud fields from dynamical models often have resolutions that are insufficient for exact 3-dimen... more Cloud fields from dynamical models often have resolutions that are insufficient for exact 3-dimensional radiative transfer calculations. To solve this problem, we have developed a downscaling algorithm that produces higher resolution fields, while preserving the original coarse resolution fields of the mean liquid water content and cloud fraction. Our algorithm extrapolates the power spectrum of the coarse field to small
In a two-part paper, radar rain-rate retrievals using specific attenuation A suggested by Ryzhkov... more In a two-part paper, radar rain-rate retrievals using specific attenuation A suggested by Ryzhkov et al. are thoroughly investigated. Continuous time series of overlapping measurements from two twin polarimetric X-band weather radars in Germany during the summers of 2011-13 are used to analyze various aspects of rain-rate retrieval, including miscalibration correction, mitigation of ground clutter contamination and partial beam blockage (PBB), sensitivity to precipitation characteristics, and the temperature assumptions of the R(A) technique. In this paper, the relations inherent to the R(A) method are used to estimate radar reflectivity Z from A and compare it to the measured Z in order to estimate PBB and calibration offsets for both radars. The fields of Z estimated from A for both radars are consistent, and the differences between Z(A) and measured Z are in good agreement with the ones calculated using either consistency relations between reflectivity at horizontal polarization Z H , differential reflectivity Z DR , and specific differential phase K DP in rain or a digital elevation model in the presence of PBB. In the analysis, the dependence of A on temperature appears to have minimal effects on the overall performance of the method. As expected, the difference between Z(A) and attenuation-corrected measured Z observations varies with rain type and exhibits a weak systematic dependency on rainfall intensity; thus, averaging over several rain events is required to obtain reliable estimates of the Z biases caused by radar miscalibration and PBB.
Journal of Applied Meteorology and Climatology, 2014
Backscatter differential phase d within the melting layer has been identified as a reliably measu... more Backscatter differential phase d within the melting layer has been identified as a reliably measurable but still underutilized polarimetric variable. Polarimetric radar observations at X band in Germany and S band in the United States are presented that show maximal observed d of 8.58 at X band but up to 708 at S band. Dual-frequency observations at X and C band in Germany and dual-frequency observations at C and S band in the United States are compared to explore the regional frequency dependencies of the d signature. Theoretical simulations based on usual assumptions about the microphysical composition of the melting layer cannot reproduce the observed large values of d at the lower-frequency bands and also underestimate the enhancements in differential reflectivity Z DR and reductions in the cross-correlation coefficient r hy . Simulations using a two-layer T-matrix code and a simple model for the representation of accretion can, however, explain the pronounced d signatures at S and C bands in conjunction with small d at X band. The authors conclude that the d signature bears information about microphysical accretion and aggregation processes in the melting layer and the degree of riming of the snowflakes aloft.
The use of general descriptive names, registered names, trademarks, etc. in this publication does... more The use of general descriptive names, registered names, trademarks, etc. in this publication does not imply, even in the absence of a specific statement, that such names are exempt from the relevant protective laws and regulations and therefore free for general use Typesetting: Camera ready by editors Printed on acid-free paper 3213 141 -5 4 3 2 1 0 Preface In many aspects science becomes conducted nowadays through technology and preferential criteria of economy. Thus investigation and knowledge is evidently linked to a specific purpose. Especially Earth science is confronted with two major human perspectives concerning our natural environment: sustainability of resources and assessment of risks. Both aspects are expressing urgent needs of the living society, but in the same way those needs are addressing a long lasting fundamental challenge which has so far not been met. Following on the patterns of economy and technology, the key is presumed to be found through a development of feasible concepts for a management of both our natural environment and in one or the other way the realm of life. Although new techniques for observation and analysis led to an increase of rather specific knowledge about particular phenomena, yet we fail now even more frequently to avoid unforeseen implications and sudden changes of a situation. Obviously the improved technological tools and the assigned expectations on a management of nature still exceed our traditional scientific experience and accumulated competence. Earth-and Life-Sciences are nowadays exceedingly faced with the puzzling nature of an almost boundless network of relations, i. e., the complexity of phenomena with respect to their variability. The disciplinary notations and their particular approaches are thus no longer accounting sufficiently for the recorded context of phenomena, for their permanent variability and their unpredictable implications. The large environmental changes of glacial climatic cycles, for instance, demonstrate this complexity of such a typical phenomenology. Ice age cycles involve beside the reorganisation of ice sheets as well changes of the ocean-atmosphere system, the physics and chemistry of the oceans and their sedimentary boundaries. They are linked to the carbon cycle, and the marine and terrestrial ecosystems and last not least the crucial changes in the orbital parameters such as in eccentricity, precession frequency and tilt of the planet during its rotation and movement in space. So far changes of solar radiation through the activity of the sun itself have not yet been adequately incorporated. The entire dynamics of the climate system has therefore the potential to perform abrupt reorganisation as demonstrated by sedimentary records. It becomes quite obvious, in order to reveal the complex nature of phenomena we evidently have to reorganise our own scientific perspectives and our disciplinary bounds as well.
Analysis of the extremes in European precipitation has been performed on the basis of several dat... more Analysis of the extremes in European precipitation has been performed on the basis of several data sets. Daily in-situ data from about 100 European weather stations together with the 6-hourly re-analysis precipitation products were used. We estimated durations of the precipitation periods of a given intensity and analysed 2-dimensional probability distributions of the precipitation totals and the continuity of the
Extremes and GEWEX: The strategy of the GEWEX Hydrometeorology Panel (GHP) has concentrated on as... more Extremes and GEWEX: The strategy of the GEWEX Hydrometeorology Panel (GHP) has concentrated on assembling relevant data sets, addressing long-term water and energy budgets, assessing sources and sinks of moisture, and interacting with the water resource community. GHP, along with the other GEWEX components, is now ready to examine extremes in a systematic manner. The primary objectives of this effort
Existing estimates of climate variability and trends in precipitation extremes are highly uncerta... more Existing estimates of climate variability and trends in precipitation extremes are highly uncertain when quantified from daily and higher resolution rain gauge observations. The major sources of uncertainties are associated with the conceptual definition of extreme precipitation, inhomogeneity of different data types and inaccuracy of statistical methods applied for estimation of precipitation extremes. We assess the impact of these uncertainties on climate variability in extreme precipitation over European continent using different collections of European rain gauge data. We try to discriminate the role of changing precipitation totals and varying characteristics of frequency distributions in forming observed changes in precipitation extremes. These two factors have strong seasonal dependence over Europe with winter growth up to 5% per decade being associated with change in precipitation distribution and summer decrease of 3% per decade primarily implied by changes in total. Change...
Plant physiological properties have a significant influence on the partitioning of radiative forc... more Plant physiological properties have a significant influence on the partitioning of radiative forcing, the spatial and temporal variability of soil water and soil temperature dynamics, and the rate of carbon fixation. Because of the direct impact on latent heat fluxes, these properties may also influence weather-generating processes, such as the evolution of the atmospheric boundary layer (ABL). In this work, crop-specific physiological characteristics, retrieved from detailed field measurements, are included in the biophysical parameterization of the Terrestrial Systems Modeling Platform (TerrSysMP). The physiological parameters for two typical European midlatitudinal crops (sugar beet and winter wheat) are validated using eddy covariance fluxes over multiple years from three measurement sites located in the North Rhine-Westphalia region of Germany. Comparison with observations and a simulation utilizing the generic crop type shows clear improvements when using the crop-specific physiological characteristics of the plant. In particular, the increase of latent heat fluxes in conjunction with decreased sensible heat fluxes as simulated by the two crops leads to an improved quantification of the diurnal energy partitioning. An independent analysis carried out using estimates of gross primary production reveals that the better agreement between observed and simulated latent heat adopting the plant-specific physiological properties largely stems from an improved simulation of the photosynthesis process. Finally, to evaluate the effects of the crop-specific parameterizations on the ABL dynamics, a series of semi-idealized land-atmosphere coupled simulations is performed by hypothesizing three cropland configurations. These numerical experiments reveal different heat and moisture budgets of the ABL using the crop-specific physiological properties, which clearly impacts the evolution of the boundary layer. and weather dynamics ) by modifying the radiation, momentum, water, CO 2 , and energy balance of and the fluxes between the land surface and the atmospheric boundary layer (ABL; Arora 2002). Land-cover change alters albedo, surface runoff, roughness height, and as a consequence the land surface energy partitioning . The large-scale transformation of native lands into agricultural production has increased the interest in exploring the specific role of croplands in the estimation of the energy, water, and carbon budgets ranging from daily to multiyear time scales ).
ABSTRACT Within the research group 'Object-based Analysis and SEamless prediction&#39... more ABSTRACT Within the research group 'Object-based Analysis and SEamless prediction' (OASE) of the Hans Ertel Centre for Weather Research programme (HerZ), a data composite containing weather radar, lightning sensor, and Meteosat Second Generation observations is being developed for the use in object-based weather analysis and nowcasting. At present, a 3D merging scheme combines measurements of the Bonn and Jülich dual polarimetric weather radar systems (data provided by the TR32 and TERENO projects) into a 3-dimensional polar-stereographic volume grid, with 500 meters horizontal, and 250 meters vertical resolution. The merging takes into account and compensates for various observational error sources, such as attenuation through hydrometeors, beam blockage through topography and buildings, minimum detectable signal as a function of noise threshold, non-hydrometeor echos like insects, and interference from other radar systems. In addition to this, the effect of convection during the radar 5-minute volume scan pattern is mitigated through calculation of advection vectors from subsequent scans and their use for advection correction when projecting the measurements into space for any desired timestamp. The Meteosat Second Generation rapid scan service provides a scan in 12 spectral visual and infrared wavelengths every 5 minutes over Germany and Europe. These scans, together with the derived microphysical cloud parameters, are projected into the same polar stereographic grid used for the radar data. Lightning counts from the LINET lightning sensor network are also provided for every 2D grid pixel. The combined 3D radar and 2D MSG/LINET data is stored in a fully documented netCDF file for every 5 minute interval, and is made ready for tracking and object based weather analysis. At the moment, the 3D data only covers the Bonn and Jülich area, but the algorithms are planed to be adapted to the newly conceived DWD polarimetric C-Band 5 minute interval volume scan strategy. An extension of the 3D composite to all of Germany is therefore possible and set as a goal.
Bi-spectral algorithms to estimate cloud properties from reflected solar radiation at an absorbin... more Bi-spectral algorithms to estimate cloud properties from reflected solar radiation at an absorbing and a non-absorbing wavelength are routinely applied to observations of meteorological satellite imagers. The underlying inversion process is highly underconstrained, and is based on the simplified view of 1D radiative transfer theory. It is therefore difficult to quantify the overall accuracy of these retrievals, and validation with
We implemented a PI (Physical Initialization) method in the non hydrostatic limited-area model CO... more We implemented a PI (Physical Initialization) method in the non hydrostatic limited-area model COSMO (version 4.2) of the DWD (German Meteorological Service). The goal is the improvement of quantitative rain nowcasting with a high resolution NWP model. Input radar data is a DWD product: the national radar composite for 16 radars with a spatial resolution of one kilometer and a
Cloud fields from dynamical models often have resolutions that are insufficient for exact 3-dimen... more Cloud fields from dynamical models often have resolutions that are insufficient for exact 3-dimensional radiative transfer calculations. To solve this problem, we have developed a downscaling algorithm that produces higher resolution fields, while preserving the original coarse resolution fields of the mean liquid water content and cloud fraction. Our algorithm extrapolates the power spectrum of the coarse field to small
In a two-part paper, radar rain-rate retrievals using specific attenuation A suggested by Ryzhkov... more In a two-part paper, radar rain-rate retrievals using specific attenuation A suggested by Ryzhkov et al. are thoroughly investigated. Continuous time series of overlapping measurements from two twin polarimetric X-band weather radars in Germany during the summers of 2011-13 are used to analyze various aspects of rain-rate retrieval, including miscalibration correction, mitigation of ground clutter contamination and partial beam blockage (PBB), sensitivity to precipitation characteristics, and the temperature assumptions of the R(A) technique. In this paper, the relations inherent to the R(A) method are used to estimate radar reflectivity Z from A and compare it to the measured Z in order to estimate PBB and calibration offsets for both radars. The fields of Z estimated from A for both radars are consistent, and the differences between Z(A) and measured Z are in good agreement with the ones calculated using either consistency relations between reflectivity at horizontal polarization Z H , differential reflectivity Z DR , and specific differential phase K DP in rain or a digital elevation model in the presence of PBB. In the analysis, the dependence of A on temperature appears to have minimal effects on the overall performance of the method. As expected, the difference between Z(A) and attenuation-corrected measured Z observations varies with rain type and exhibits a weak systematic dependency on rainfall intensity; thus, averaging over several rain events is required to obtain reliable estimates of the Z biases caused by radar miscalibration and PBB.
Journal of Applied Meteorology and Climatology, 2014
Backscatter differential phase d within the melting layer has been identified as a reliably measu... more Backscatter differential phase d within the melting layer has been identified as a reliably measurable but still underutilized polarimetric variable. Polarimetric radar observations at X band in Germany and S band in the United States are presented that show maximal observed d of 8.58 at X band but up to 708 at S band. Dual-frequency observations at X and C band in Germany and dual-frequency observations at C and S band in the United States are compared to explore the regional frequency dependencies of the d signature. Theoretical simulations based on usual assumptions about the microphysical composition of the melting layer cannot reproduce the observed large values of d at the lower-frequency bands and also underestimate the enhancements in differential reflectivity Z DR and reductions in the cross-correlation coefficient r hy . Simulations using a two-layer T-matrix code and a simple model for the representation of accretion can, however, explain the pronounced d signatures at S and C bands in conjunction with small d at X band. The authors conclude that the d signature bears information about microphysical accretion and aggregation processes in the melting layer and the degree of riming of the snowflakes aloft.
The use of general descriptive names, registered names, trademarks, etc. in this publication does... more The use of general descriptive names, registered names, trademarks, etc. in this publication does not imply, even in the absence of a specific statement, that such names are exempt from the relevant protective laws and regulations and therefore free for general use Typesetting: Camera ready by editors Printed on acid-free paper 3213 141 -5 4 3 2 1 0 Preface In many aspects science becomes conducted nowadays through technology and preferential criteria of economy. Thus investigation and knowledge is evidently linked to a specific purpose. Especially Earth science is confronted with two major human perspectives concerning our natural environment: sustainability of resources and assessment of risks. Both aspects are expressing urgent needs of the living society, but in the same way those needs are addressing a long lasting fundamental challenge which has so far not been met. Following on the patterns of economy and technology, the key is presumed to be found through a development of feasible concepts for a management of both our natural environment and in one or the other way the realm of life. Although new techniques for observation and analysis led to an increase of rather specific knowledge about particular phenomena, yet we fail now even more frequently to avoid unforeseen implications and sudden changes of a situation. Obviously the improved technological tools and the assigned expectations on a management of nature still exceed our traditional scientific experience and accumulated competence. Earth-and Life-Sciences are nowadays exceedingly faced with the puzzling nature of an almost boundless network of relations, i. e., the complexity of phenomena with respect to their variability. The disciplinary notations and their particular approaches are thus no longer accounting sufficiently for the recorded context of phenomena, for their permanent variability and their unpredictable implications. The large environmental changes of glacial climatic cycles, for instance, demonstrate this complexity of such a typical phenomenology. Ice age cycles involve beside the reorganisation of ice sheets as well changes of the ocean-atmosphere system, the physics and chemistry of the oceans and their sedimentary boundaries. They are linked to the carbon cycle, and the marine and terrestrial ecosystems and last not least the crucial changes in the orbital parameters such as in eccentricity, precession frequency and tilt of the planet during its rotation and movement in space. So far changes of solar radiation through the activity of the sun itself have not yet been adequately incorporated. The entire dynamics of the climate system has therefore the potential to perform abrupt reorganisation as demonstrated by sedimentary records. It becomes quite obvious, in order to reveal the complex nature of phenomena we evidently have to reorganise our own scientific perspectives and our disciplinary bounds as well.
Analysis of the extremes in European precipitation has been performed on the basis of several dat... more Analysis of the extremes in European precipitation has been performed on the basis of several data sets. Daily in-situ data from about 100 European weather stations together with the 6-hourly re-analysis precipitation products were used. We estimated durations of the precipitation periods of a given intensity and analysed 2-dimensional probability distributions of the precipitation totals and the continuity of the
Extremes and GEWEX: The strategy of the GEWEX Hydrometeorology Panel (GHP) has concentrated on as... more Extremes and GEWEX: The strategy of the GEWEX Hydrometeorology Panel (GHP) has concentrated on assembling relevant data sets, addressing long-term water and energy budgets, assessing sources and sinks of moisture, and interacting with the water resource community. GHP, along with the other GEWEX components, is now ready to examine extremes in a systematic manner. The primary objectives of this effort
Existing estimates of climate variability and trends in precipitation extremes are highly uncerta... more Existing estimates of climate variability and trends in precipitation extremes are highly uncertain when quantified from daily and higher resolution rain gauge observations. The major sources of uncertainties are associated with the conceptual definition of extreme precipitation, inhomogeneity of different data types and inaccuracy of statistical methods applied for estimation of precipitation extremes. We assess the impact of these uncertainties on climate variability in extreme precipitation over European continent using different collections of European rain gauge data. We try to discriminate the role of changing precipitation totals and varying characteristics of frequency distributions in forming observed changes in precipitation extremes. These two factors have strong seasonal dependence over Europe with winter growth up to 5% per decade being associated with change in precipitation distribution and summer decrease of 3% per decade primarily implied by changes in total. Change...
Plant physiological properties have a significant influence on the partitioning of radiative forc... more Plant physiological properties have a significant influence on the partitioning of radiative forcing, the spatial and temporal variability of soil water and soil temperature dynamics, and the rate of carbon fixation. Because of the direct impact on latent heat fluxes, these properties may also influence weather-generating processes, such as the evolution of the atmospheric boundary layer (ABL). In this work, crop-specific physiological characteristics, retrieved from detailed field measurements, are included in the biophysical parameterization of the Terrestrial Systems Modeling Platform (TerrSysMP). The physiological parameters for two typical European midlatitudinal crops (sugar beet and winter wheat) are validated using eddy covariance fluxes over multiple years from three measurement sites located in the North Rhine-Westphalia region of Germany. Comparison with observations and a simulation utilizing the generic crop type shows clear improvements when using the crop-specific physiological characteristics of the plant. In particular, the increase of latent heat fluxes in conjunction with decreased sensible heat fluxes as simulated by the two crops leads to an improved quantification of the diurnal energy partitioning. An independent analysis carried out using estimates of gross primary production reveals that the better agreement between observed and simulated latent heat adopting the plant-specific physiological properties largely stems from an improved simulation of the photosynthesis process. Finally, to evaluate the effects of the crop-specific parameterizations on the ABL dynamics, a series of semi-idealized land-atmosphere coupled simulations is performed by hypothesizing three cropland configurations. These numerical experiments reveal different heat and moisture budgets of the ABL using the crop-specific physiological properties, which clearly impacts the evolution of the boundary layer. and weather dynamics ) by modifying the radiation, momentum, water, CO 2 , and energy balance of and the fluxes between the land surface and the atmospheric boundary layer (ABL; Arora 2002). Land-cover change alters albedo, surface runoff, roughness height, and as a consequence the land surface energy partitioning . The large-scale transformation of native lands into agricultural production has increased the interest in exploring the specific role of croplands in the estimation of the energy, water, and carbon budgets ranging from daily to multiyear time scales ).
ABSTRACT Within the research group 'Object-based Analysis and SEamless prediction&#39... more ABSTRACT Within the research group 'Object-based Analysis and SEamless prediction' (OASE) of the Hans Ertel Centre for Weather Research programme (HerZ), a data composite containing weather radar, lightning sensor, and Meteosat Second Generation observations is being developed for the use in object-based weather analysis and nowcasting. At present, a 3D merging scheme combines measurements of the Bonn and Jülich dual polarimetric weather radar systems (data provided by the TR32 and TERENO projects) into a 3-dimensional polar-stereographic volume grid, with 500 meters horizontal, and 250 meters vertical resolution. The merging takes into account and compensates for various observational error sources, such as attenuation through hydrometeors, beam blockage through topography and buildings, minimum detectable signal as a function of noise threshold, non-hydrometeor echos like insects, and interference from other radar systems. In addition to this, the effect of convection during the radar 5-minute volume scan pattern is mitigated through calculation of advection vectors from subsequent scans and their use for advection correction when projecting the measurements into space for any desired timestamp. The Meteosat Second Generation rapid scan service provides a scan in 12 spectral visual and infrared wavelengths every 5 minutes over Germany and Europe. These scans, together with the derived microphysical cloud parameters, are projected into the same polar stereographic grid used for the radar data. Lightning counts from the LINET lightning sensor network are also provided for every 2D grid pixel. The combined 3D radar and 2D MSG/LINET data is stored in a fully documented netCDF file for every 5 minute interval, and is made ready for tracking and object based weather analysis. At the moment, the 3D data only covers the Bonn and Jülich area, but the algorithms are planed to be adapted to the newly conceived DWD polarimetric C-Band 5 minute interval volume scan strategy. An extension of the 3D composite to all of Germany is therefore possible and set as a goal.
In recent years, a number of simulation platforms with varying complexity which couple groundwate... more In recent years, a number of simulation platforms with varying complexity which couple groundwater, land surface, and atmospheric models have emerged. These platforms are designed to include processes affecting energy fluxes and soil moisture variations at the land surface such as shallow groundwater, overland flow, and subsurface lateral flow. Previous studies demonstrate the sensitivity of atmospheric boundary layer dynamics and precipitation to land surface energy fluxes and groundwater dynamics, as well as the importance of capturing these interactions through coupled models. This study compares two distributed, physically-based, state-of-the-art hydrological modelling platforms: The ParFlow-CLM-COSMO platform TerrSysMP (Terrestrial System Modelling Platform), developed within the Transregional Collaborative Research Centre 32 (TR32), and the HIRHAM-MIKE SHE platform developed within the HOBE Centre for Hydrology and the HYdrological Modelling for Assessing Climate Change Impact...
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Papers by Clemens Simmer