Institut für Physik und Meteorologie

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Profiling the molecular destruction rates of temperature and humidity as well as the turbulent kinetic energy dissipation in the convective boundary layer
(2024) Wulfmeyer, Volker; Senff, Christoph; Späth, Florian; Behrendt, Andreas; Lange, Diego; Banta, Robert M.; Brewer, W. Alan; Wieser, Andreas; Turner, David D.
A simultaneous deployment of Doppler, temperature, and water-vapor lidars is able to provide profiles of molecular destruction rates and turbulent kinetic energy (TKE) dissipation in the convective boundary layer (CBL). Horizontal wind profiles and profiles of vertical wind, temperature, and moisture fluctuations are combined, and transversal temporal autocovariance functions (ACFs) are determined for deriving the dissipation and molecular destruction rates. These are fundamental loss terms in the TKE as well as the potential temperature and mixing ratio variance equations. These ACFs are fitted to their theoretical shapes and coefficients in the inertial subrange. Error bars are estimated by a propagation of noise errors. Sophisticated analyses of the ACFs are performed in order to choose the correct range of lags of the fits for fitting their theoretical shapes in the inertial subrange as well as for minimizing systematic errors due to temporal and spatial averaging and micro- and mesoscale circulations. We demonstrate that we achieve very consistent results of the derived profiles of turbulent variables regardless of whether 1 or 10 s time resolutions are used. We also show that the temporal and spatial length scales of the fluctuations in vertical wind, moisture, and potential temperature are similar with a spatial integral scale of ≈160 m at least in the mixed layer (ML). The profiles of the molecular destruction rates show a maximum in the interfacial layer (IL) and reach values of ϵm≃7×10-4 g2 kg-2 s-1 for mixing ratio and ϵθ≃1.6×10-3 K2 s-1 for potential temperature. In contrast, the maximum of the TKE dissipation is reached in the ML and amounts to ≃10-2 m2 s-3. We also demonstrate that the vertical wind ACF coefficient kw∝w′2‾ and the TKE dissipation ϵ∝w′2‾3/2. For the molecular destruction rates, we show that ϵm∝m′2‾w′2‾1/2 and ϵθ∝θ′2‾w′2‾1/2. These equations can be used for parameterizations of ϵ, ϵm, and ϵθ. All noise error bars are derived by error propagation and are small enough to compare the results with previous observations and large-eddy simulations. The results agree well with previous observations but show more detailed structures in the IL. Consequently, the synergy resulting from this new combination of active remote sensors enables the profiling of turbulent variables such as integral scales, variances, TKE dissipation, and the molecular destruction rates as well as deriving relationships between them. The results can be used for the parameterization of turbulent variables, TKE budget analyses, and the verification of large-eddy simulations.
The effect of forest cover changes on the regional climate conditions in Europe during the period 1986–2015
(2024) Breil, Marcus; Schneider, Vanessa K. M.; Pinto, Joaquim G.
Afforestation affects the earth's climate system by changing the biogeochemical and biogeophysical characteristics of the land surface. While the regional effects of afforestation are well understood in the tropics and the high latitudes, its climate impact on the midlatitudes is still the subject of scientific discussions. The general impact of afforestation on the regional climate conditions in Europe during the last decades is investigated in this study. For this purpose, regional climate simulations are performed with different forest cover fractions over Europe. In a first simulation, afforestation in Europe is considered, while this is not the case for a second simulation. We focus on the years 1986–2015, a period in which the forest cover in Europe increased comparatively strongly, accompanied by a strong general warming over the continent. Results show that afforestation has both local and non-local effects on the regional climate system in Europe. Due to an increased transport of turbulent heat (latent + sensible) into the atmosphere, afforestation leads to a significant reduction of the mean local surface temperatures in summer. In northern Europe, mean local surface temperatures were reduced about -0.3 K with afforestation, in central Europe about -0.5 K, and in southern Europe about -0.8 K. During heat periods, this local cooling effect can reach -1.9 K. In winter, afforestation results in a slight local warming in both northern and southern Europe because of the albedo effect of forests. However, this effect is rather small and the mean temperature changes are not significant. In the downwind direction, locally increased evapotranspiration rates with afforestation increase the general cloud cover, which results in a slight non-local warming in winter in several regions of Europe, particularly during cold spells. Thus, afforestation had a discernible impact on the climate change signal in Europe during the period 1986–2015, which may have mitigated the general warming trend in Europe, especially on the local scale in summer.
Downscaling of ECMWF SEAS5 seasonal forecasts over the Horn of Africa using the WRF model
(2023) Mori, Paolo; Wulfmeyer, Volker
Several studies have shown the potential for downscaling seasonal forecasts on a convection-permitting (CP) scale using limited-area models (LAMs). In most cases, such experiments initial and boundary conditions are derived from atmospheric and surface analyses, which use measurements to constrain the model evolution. For operational use, the boundary conditions are derived from global seasonal forecasts, which only evolve depending on numerical models. This difference will affect the downscaling process and potentially the results’ skill. In this work, the SEAS5 seasonal forecasts are downscaled to address this gap in our understanding. Specifically, the research questions are: What advantages of a CP simulation are present when dynamically downscaling ensemble seasonal forecasts with a LAM? How do boundary conditions and physics parametrization perturbations affect a LAM ensemble in terms of spread and reliability? What perturbations produce more ensemble spread for temperature and precipitation? The study area chosen is the Horn of Africa. The effects of climate change have become much more apparent in East Africa in the last decade: the rainy season has repeatedly failed, which has led to extreme droughts. Therefore, any improvements in this regions seasonal forecasts can help to develop adaptation strategies further. In addition, areas with complex topography benefit the most from increased spatial resolution, and the global models skill is higher in the tropics and subtropics than in middle latitudes. Thus, it is likely that downscaling can extract helpful information in this region. Four global ECWMF SEAS5 ensemble members were dynamically downscaled for summer 2018 over the Horn of Africa using the Weather Research and Forecasting (WRF) model to investigate the potential of a seasonal forecast on convection-permitting resolution (3 km). A total of 16 WRF ensemble members with varied initial and boundary conditions and different physical schemes were used to evaluate the impact of the downscaling. The analysis assessed the effects of perturbations on surface temperature and rainfall in terms of bias, spatial distribution, probability of extreme events, rain belt movements, and ensemble spread. The main findings of this work are the following: the WRF simulations reproduced the spatial distribution of the 2m temperature and precipitation patterns. The bias present in SEAS5 was transferred to the limited-area model, and the signal is even intensified in some areas. For example, while the four SEAS5 members deviated only by +0.2°C on average compared to the ECMWF analyses, the WRF ensemble bias was +1.1°C. The WRF ensemble simulated an average of 264 mm of rain, compared to 248 mm for SEAS5 and 236 mm for the GPM-IMERG satellite product. The convection-permitting resolution reproduced the precipitation probability density function slightly better than the global model and simulated extreme precipitation events missing in SEAS5. However, it overestimated their frequency compared to observations. In addition, WRF can reproduce the daily precipitation cycle well: the peak times coincide with measurements, showing an accurate representation of convection initiation in the area and the potential of dynamical downscaling at convection-permitting resolution. The boundary conditions limited the movement of the rain belt associated with the inter-tropical convergence zone in the downscaling. For example, the north extension of the tropical rain belt decreased in both models by 2 degrees of latitude compared to GPM-IMERG, and the global model timing strongly influenced the movements of the rain belt in WRF. The SEAS5 has shown moderate skill in precipitation forecasts in Ethiopia. Still, a better understanding of the yearly variability of the rain belt position is necessary, as it is a crucial factor in high-resolution downscaling in the region. The downscaling increased the ensemble spread for precipitation by an average of 60%, partially correcting the SEAS5 under-dispersion. In the Ethiopian highlands, perturbed boundary conditions are primarily responsible for the WRF ensemble spread. Their effect is often 50% greater than the variability resulting from the various physics parameterizations. The results show that boundary-conditions perturbations are necessary to generate adequate ensemble dispersion in a limited-area model with complex topography. The analysis partially confirmed the potential to improve seasonal forecasts through downscaling, especially concerning convective precipitation timing and heavy rainfall events. Some advantages of downscaling atmospheric analysis are lost due to the inaccuracies in the forcing derived from SEAS5 and model bias. It also highlights the necessity of further research on physics schemes or combinations suitable to convection-permitting resolutions.
Climate dynamics : the performance of seasonal ensemble forecast for improving food security in Ethiopia
(2023) Ware, Markos Budusa; Wulfmeyer, Volker
Part one of this thesis aims to define homogenous climatic regions using objective clustering methods and characterize seasonal cycles, trends, and anomalies in precipitation and temperature. Climate-based on amplifies inherent spatiotemporal climate variability in the Horn of Africa due to global, regional, coastal, and local processes. The homogeneous climatic regions and synoptic circulation types were defined using Principal Component Analysis (PCA) PCA–K-means and PCA–Ward’s. Using the decision criteria of respective algorithms, four homogenous climatic regions were determined for Ethiopia. These climatic regions were distinctive in their seasonal cycles, trends, and anomalies in annual and seasonal precipitation and temperature. These results highlight that the trends in precipitation and temperature vary not only between climatic regions but also by rainy seasons. The short rains (received between November and December) increased by 50 mm/decade in the southwestern region where the evergreen forest meets with the long rainy season. The mean annual and seasonal temperature increased between 0.3 and 0.6 °C/decade virtually in all climatic regions. Regionalization methods were sensitive to spatial domain size but not to the length of the time series. Climatology of sea-level air pressure showed decreasing northward trend over the study domain, as did the temperature, wind velocity, and relative humidity at 500 hPa. However, geopotential height at 500 hPa and temperature at 850 hPa decreased toward the south over the domain. Circulation types were defined by applying PCA on a composite matrix of the six variables. From the first five Principal Components (PCs), ten circulation types (CTs) were defined over East Africa and then associated with environmental events. CTs clearly distinguished rainy seasons comprising different atmospheric states responsible for varying weathers. The summer season was described by a combination of strong positive anomalies in temperature at 850 hPa, northeasterly winds, and Somali jet at 500 hPa, and weak negative anomalies in temperature at 500 hPa. Trends in the number of days categorized in different CTs showed a significant variation among the groups. The drought events, defined using the consecutive dry days (CDD), correspond with positive anomalies in temperature at 850 hPa, northwesterly and Somali Jet, and negative anomalies in relative humidity at 500 hPa. Flooding, defined using a proxy of 80 mm/day per grid cell, was associated with strong westerly winds at 500 hPa, strong positive anomalies in temperature at the lower troposphere, strong easterlies and southwesterly, and positive anomalies in relative humidity at 500 hPa. Part two of the thesis aims to assess the performance of the seasonal ensemble forecast over the Horn of Africa for improving food security. A seasonal forecast with a horizon of up to seven months offers a great opportunity for agricultural optimization, which results in an improved economy and food security. For this purpose, the Weather Research and Forecasting (WRF) model was applied for dynamical downscaling of the latest seasonal forecasting system version 5 (SEAS5) for summer 2018 with different microphysics parameterizations, and initial and boundary conditions. Downscaling was performed by a horizontal resolution of 3 km over the topographically complex domain of East Africa. The seasonal ensemble forecast was evaluated using probabilistic metrics like the Brier skill score, probability ranking score, continuous probability ranking score, discrimination score, and ignorance score. The results of the WRF showed that the model has a strong warm bias in the 2m temperature and a wet bias in precipitation. The relative operating characteristics (ROC) curve showed a higher predicting probability of 2m temperature in below-normal and above-normal terciles over northern Ethiopia and the Indian Ocean, where the model performed better, highlighting the advantage of high-resolution simulations compared to ERA5. The median and distribution of WRF, SEAS5, and ERA5 showed remarkable variation between the homogenous climatic regions. Especially the summer of 2018 was wetter relative to climatology, and WRF overestimated this condition in the region.
Sensitivity of land-atmosphere coupling strength in dependence of land cover and atmospheric thermodynamics over Europe
(2023) Jach, Lisa; Wulfmeyer, Volker
Biogeophysical feedbacks between the land surface and the atmosphere have been identified to heavily control the climate system. Land-atmosphere (L-A) coupling strength is a concept to quantify the feedback processes. However, the quantification is still subject to uncertainties, in particular, in the context of land surface influences on local convective precipitation. On the one hand, feedback processes are the result of a chain of complex interactions between various components in the L-A system all exhibiting spatiotemporal variability. On the other hand, L-A coupling strength is not a directly measurable quantity. It can be assessed with different scientific approaches, which makes the quantification dependent on the methodology and the availability of suitable data sets. The aim of this doctoral thesis is to investigate the impact of changes in the vegetation cover and the atmospheric thermodynamic conditions on the long-term coupling signal between the land surface and the triggering of deep moist convection during the European summer. The ‘convective triggering potential – low-level humidity index’ framework, which is a commonly used L-A coupling metric, classifies a day in favor for L-A coupling or not, based on the prevailing thermodynamic conditions in the atmosphere. The daily classifications are used to measure the frequency of days with favorable conditions during the study period, and to identify regions with high frequencies of favorable conditions as coupling hot spots. The framework is applied to model output from regional climate model (RCM) simulations with WRF-NoahMP with diverging land cover conducted over the historical period 1986-2015 for the Euro-CORDEX domain. Impacts of changes in vegetation cover are analyzed by comparing the L-A coupling strength from two sensitivity experiments with idealized extreme land use and land cover changes (LULCCs) against a simulation with realistic land cover. A posteriori modifications to the temperature and moisture output fields of the simulation with realistic land cover were implemented to analyze impacts of systematic changes in the atmospheric thermodynamic conditions. A potential coupling hot spot with predominantly positive feedbacks was identified over Eastern Europe. In Southern Europe and Europe’s coastal areas, the coupling is regularly inhibited by very dry, very wet or stable conditions in the atmosphere. The location of the hot spot appeared insensitive to LULCCs and changes in the thermodynamic conditions. None of the sensitivity tests within a realistic range of temperature and moisture modifications for a recent climate period, led to a disappearance of the hot spot or to overcome the causes for inhibiting coupling in the respective areas in summer. Nevertheless, the experiments demonstrated also considerable variance of the coupling strength within the hot spot region. LULCCs changed the turbulent heat fluxes from the land surface, and thus the atmospheric boundary layer (ABL) heating and moistening. This impacted the boundary layer development of each day. It also caused changes in the average thermodynamic characteristics during the study period, which changed the frequency of favorable pre-conditioning for convection triggering and enhanced the variance in the coupling strength in the hot spot. Both effects were identified to influence the land surface control on the occurrence of convective precipitation. Furthermore, the sensitivity tests with a posteriori modifications revealed uncertainties in the predominant atmospheric response to differently wet surfaces around the Black Sea, shown by a disagreement in the predominant coupling pathway between the modification cases. The findings further indicate uncertainty in whether the hot spot expands over Central Europe, as the feedback signal was sensitive to changes in temperature and moisture. Additionally, the model has a warm and dry bias in this area, which suggests an overestimation of the humidity deficit. The large humidity deficit, in turn, was the inhibiting factor for a high frequency of occurrence of favorable pre-conditions for deep moist convection. The analyses reveal a sensitivity of the L-A coupling strength and atmospheric response to the prevailing land surface and atmospheric conditions in the hot spot. This highlights the need to consider both the land surface state and its impact on L-A coupling strength with respect to predictions of convective precipitation events in strongly coupled regions (and periods). Given that L-A coupling provides predictive skill for climate projections and seasonal forecasts, improved understanding about causes of variability in L-A coupling strength is crucial for improvements therein.
Convective-scale data assimilation of thermodynamic lidar data into the weather research and forecasting model
(2022) Thundathil, Rohith Muraleedharan; Wulfmeyer, Volker
This thesis studies the impact of assimilating temperature and humidity profiles from ground-based lidar systems and demonstrates its value for future short-range forecast. Thermodynamic profile obtained from the temperature Raman lidar and the water-vapour differential absorption lidar of the University of Hohenheim during the High Definition of Clouds and Precipitation for advancing Climate Prediction (HD(CP)2) project Observation Prototype Experiment (HOPE) are assimilated into the Weather Research and Forecasting model Data Assimilation (WRFDA) system by means of a new forward operator. The impact study assimilating the high-resolution thermodynamic lidar data was conducted using variational and ensemble-based data assimilation methods. The first part of the thesis describes the development of the thermodynamic lidar operator and its implementation through a deterministic DA impact study. The operator facilitates the direct assimilation of water vapour mixing ratio (WVMR), a prognostic variable in the WRF model, without conversion to relative humidity. Undesirable cross sensitivities to temperature are avoided here so that the complete information content of the observation with respect to the water vapour is provided. The assimilation experiments were performed with the three-dimensional variational (3DVAR) DA system with a rapid update cycle (RUC) with hourly frequency over ten hours. The DA experiments with the new operator outperformed the previously used relative humidity operator, and the overall humidity and temperature analyses improved. The simultaneous assimilation of temperature and WVMR resulted in a degradation of the temperature analysis compared to the improvement observed in the sole temperature assimilation experiment. The static background error covariance matrix (B) in the 3DVAR was identified as the reason behind this behaviour. The correlation between the temperature and WVMR variables in the background error covariance matrix of the 3DVAR, which is static and not flow-dependent, limited the improvement in temperature. The second part of the thesis provides a solution for overcoming the static B matrix issue. A hybrid, ensemble-based approach was applied using the Ensemble Transform Kalman Filter (ETKF) and the 3DVAR to add flow dependency to the B matrix. The hybrid experiment resulted in a 50% lower temperature and water vapour root mean square error (RMSE) than the 3DVAR experiment. Comparisons against independent radiosonde observations showed a reduction of RMSE by 26% for water vapour and 38% for temperature. The planetary boundary layer (PBL) height of the analyses also showed an improvement compared to the available ceilometer. The impact of assimilating a single lidar vertical profile spreads over a 100 km radius, which is promising for future assimilation of water vapour and temperature data from operational lidar networks for short-range weather forecasting. A forecast improvement was observed for 7 hours lead time compared with the ceilometer derived planetary boundary layer height observations and 4 hours with Global Navigation Satellite System (GNSS) derived integrated water vapour observations. With the help of sophisticated DA systems and a robust network of lidar systems, the thesis throws light on the future of short-range operational forecasting.
Large eddy simulations of the thermodynamic budgets in a small catchment
(2020) Adam, Stephan; Wulfmeyer, Volker
In this study, the regional water and energy budgets at the land surface and the planetary boundary layer (PBL) were simulated for three different real case studies. Nested large eddy simulation (LES) experiments with the WRF-NOAHMP-HYDRO model were conducted for real cases. The LES was operated for two different sites, the Kraichgau and the Ammer catchment which are both located in the southwest of Germany, covering one full diurnal cycle for each case study.
A backscatter lidar forward operator for aerosol-representing atmospheric chemistry models
(2020) Geisinger, Armin; Wulfmeyer, Volker
State-of-the-art atmospheric chemistry models are capable of simulating the transport and evolution of aerosols and trace gases but there is a lack of reliable methods for model validation and data assimilation. Networks of automated ceilometer lidars (ACLs) could be used to fill this gap. These networks are already used for the detection of clouds and aerosols, providing a 3D dataset of atmospheric backscatter profiles. But as the aerosol number concentration cannot be obtained from the ACL data alone; one needs a backscatter-lidar forward model to simulate lidar profiles from the model variables. Such an operator allows then for a qualitative and quantitative model validation based on ACL data. In this work, a newly developed backscatter-lidar forward operator and the related sensitivity studies are presented and results of the forward operator applied on model output data are compared to measured ACL profiles in the frame of a case study. As case study, the eruption of the Icelandic volcano Eyjafjallajökull in 2010 was chosen and extensively analyzed. The Consortium for Small-scale Modeling - Aerosols and Reactive Trace gases (COSMO-ART) model of DWD (Deutscher Wetterdienst) was operated during this event for ash-transport simulations over Europe. For the forward model, the attenuated backscatter coefficient is used as lidar-independent variable, which only relies on the laser wavelength. To calculate the attenuated backscatter coefficient, the size-dependent aerosol number concentration and the scattering properties of each aerosol type and size have to be simulated. While the aerosol number concentration is a model output variable, the scattering properties were determined by extensive scattering calculations. As these scattering calculations require assumptions about the aerosol refractive indices and shapes, sensitivity studies were performed to estimate the uncertainties related to the particle properties as represented by the model system. An analysis of the particle shape effect for the extinction and backscatter coefficients resulted in huge differences of the scattering properties between spherical, ellipsoidal and cylindrical particle shapes. Due to a particle shape mixture in typical volcanic ash plumes, the application of non-spherical scattering calculation methods for estimating the effective optical properties requires more information related to the particle shape distribution (specifically: a particle size and shape distribution). As such information was not available for the present case study, it was necessary to assume spherical shaped volcanic ash particles but estimate the uncertainty related to this assumption within the frame of additional sensitivity studies. Finally, the forward modeled lidar profiles were compared to ACL measurements from stations of the German ACL network. The comparison required an extraction of common time and height intervals of the ACL and forward modeled COMSO-ART data as well as reshaping the datasets to the same vertical and temporal resolution. Significant differences between ACL profiles and the output of the forward operator applied to the COSMO-ART data were found. Some ash layer structures were at similar coordinates which is remarkable due to the uncertainties related to the model dynamics and the limited amount of measurement data that could be used for model validation. In detail, however, the major fraction of the compared time and height interval differed both in the relative signal intensity and the layer structures of the volcanic ash plume. Based on such quantitative comparison, a future data assimilation system could correct the model prediction of the forward modeled attenuated backscatter coefficient, the time of arrival, as well as the vertical structure of the volcanic ash plume. In summary, the continuous and distributed data stream provided by ACL stations was found to deliver valuable verification information for dispersion simulations of aerosol events. But major issues have been determined which limit current realizations of backscatter-lidar forward operators for aerosol transport simulations: First, it is suggested that the ACL systems improve their dynamic range and perform automatic calibration to increase the precision of ACL data and for calculating the measured attenuated backscatter coefficient with a minimum leftover of uncertainties. This will allow for the calculation of the attenuated backscatter coefficient in the presence of clouds as well as of faint aerosol signals. Second, the aerosols scattering properties have to be analyzed even more extensively which includes both the variety of aerosol sizes or types as well as the size distribution information. From the findings within this study, the particle size distribution was indentified to be a critical component when using monodisperse size classes.
10 W-Average-Power Single-Frequency Ti:sapphire Laser with Tuning Agility – A Breakthrough in High-Resolution 3D Water-Vapor Measurement
(2018) Metzendorf, Simon; Wulfmeyer, Volker
The differential absorption lidar (DIAL) technique is well suited for measuring the humidity field of the atmosphere with high spatial and temporal resolution as well as accuracy. The water-vapor DIAL of the University of Hohenheim is a mobile, ground-based, scanning system. The DIAL methodology and the application in the Hohenheim-DIAL impose stringent requirements on the laser transmitter. In this thesis, a new laser transmitter was realized and employed. It is a pulsed, actively frequency-stabilized titanium-sapphire laser system, pumped with a Nd:YAG master-oscillator power-amplifier (MOPA) and alternately seeded by two diode lasers. As pump source, two commercially custom-made, diode-pumped, Q-switched, and frequency-doubled Nd:YAG lasers in MOPA architecture were employed. The relevant properties for pumping the Ti:sapphire laser were studied. The second Nd:YAG MOPA provides a considerably higher average output power (up to P = 63 W at 532 nm, or a pulse energy of up to E = 210 mJ at a repetition rate of f = 300 Hz) and an almost ideal top-hat beam profile. Thus, efficient end-pumping of the Ti:sapphire crystal was enabled without any optical damage. The components for injection seeding of the titanium-sapphire laser, making narrowband operation at two alternating frequencies (online and offline) possible, were substantially improved. Now, advanced commercial external-cavity diode lasers (ECDL) are applied. With an analog regulation signal of a wavelength meter, the frequency of an ECDL can be stabilized precisely to a defined value (standard deviation < 1 MHz). Optionally, the frequency can be tuned according to various mathematical functions. The online-offline-switching is accomplished with a fiber switch. The crosstalk is extraordinarily low (< -61 dB), the switching time sufficiently short (~ 1.5 ms), and the spatial overlap of the signals, due to the waveguide, almost perfect. The power of the seeders in front of the resonator is more than sufficient, 17-20 mW. The Ti:sapphire laser consists of a ring resonator with four mirrors in a bow-tie layout. With adequate components, the operation wavelength at 818 nm is pre-selected and unidirectional propagation is ensured. The laser crystal is installed in an in-house-manufactured cooling mount, of which two designs were utilized and compared. The gain-switched Ti:sapphire laser was developed to operate in a dynamically stable state of the thermal lens, which arises in the crystal at high powers. To this end, the resonator was theoretically analyzed beforehand and the focal length of the thermal lens measured. The implementation of a cylindrical lens compensates the stronger contraction of the eigenmode in the tangential plane. By these means, a stable operation with an average output power of P = 10 W (corresponding to E = 33.3 mJ at f = 300 Hz; pulse duration ~ 30 ns) was realized. With a modified configuration of the cylindrical lens a maximum output power of P_max = 11.8 W (E_max = 39.3 mJ) was achieved. These values are the highest which were obtained so far for a laser of this kind, i.e., a laser transmitter whose power originates from a single radiation source (without further amplification or conversion). The laser cavity is actively stabilized to the frequency of the seeder, following a Pound-Drever-Hall technique. This yields permanent single-frequency operation with very high frequency stability (standard deviation < 2 MHz) and a narrow linewidth (< 63 MHz). These results correspond to the resolution limit of the characterizing wavelength meter. Laser emission occurs in the fundamental transverse mode, TEM_00 (M² <= 1.06). The laser system of the Hohenheim-DIAL has been successfully operated on several field campaigns. Its robustness has been demonstrated, for instance, during an uninterrupted operation for over 30 hours and an overseas transport to the USA which the system endured without damage. This work presents a vertical pointing and two scanning water-vapor DIAL measurements, confirming a high resolution and accuracy. The vertical measurement was executed for the first time at 10 W laser operation. Furthermore, two special DIAL measurements are discussed: The measurements on a strongly backscattering target demonstrate a high spectral purity >= 99.97% of the laser transmitter. Finally, an atmospheric measurement with a tuning online wavelength shows the frequency-agility of the laser and allows to determine the water-vapor absorption line experimentally. The comparison with the spectrum of a database shows a very good agreement (~ 5-10 % deviation in the absorption cross sections absolute value).
Studies of soil-vegetation-atmosphere feedback processes with WRF on the convection permitting scale
(2017) Milovac, Josipa; Wulfmeyer, Volker
Land system models which can incorporate land-atmosphere and human-environment interactions are vital for reliable climate projections in heterogeneous agricultural landscapes. At resolutions fine enough to resolve detailed land use, models need a sophisticated representation of planetary boundary layer (PBL) and land surface processes in order to predict changes in key quantities like precipitation or temperatures. Assessment of turbulence schemes and land surface models (LSM) is fundamental therefore not only to advance model development, but also to understand important phenomena like feedbacks within the soil-vegetation-atmosphere (SVA) continuum. Up until now however, a lack of appropriate observations has impeded any comprehensive assessments. Here, through comparisons with so far unique profile measurements, the study investigates the impact of using different PBL schemes and LSMs, and explores how SVA feedbacks are simulated by the model. Using the Weather Research and Forecasting (WRF) model, a six member ensemble was run, at a convection permitting resolution, with varying combinations of LSMs (NOAH and NOAH-MP) and PBL schemes (two local and two non-local approaches). The analysis was performed for two case studies – a dry and a convective weather situation – in three different locations in Germany. During the dry case, key convective PBL (CBL) features were analysed, and the simulations were compared with high resolution water vapour differential absorption lidar measurements. For the convective case, the focus was on exploring the model representation of the pre-convective environment and the ensuing convection and precipitation. In both cases, the nature of the simulated SVA feedback processes was assessed through an innovative “mixing diagram” approach. Results show that the nonlocal PBL schemes produce a drier and higher CBL than the local schemes. These results are sensitive to parameters calculated in the surface layer schemes, which are themselves often paired with PBL schemes. Furthermore, the NOAH‑MP LSM produces drier atmospheric conditions than NOAH, with a difference in mixing ratio profiles ranging up to 1.4 gkg-1. These variations are more pronounced in the upper CBL than close to the ground. The mixing diagrams indicate that these deviations are mainly related to entrainment fluxes. In the dry case, NOAH-MP’s dry air entrainment is up to 6 times higher than with NOAH, while in the convective case the difference is not as pronounced (up to 1.5 higher with NOAH-MP). This suggests that the difference in the simulation of the CBL between the two LSMs is strongly linked to the surface energy partitioning – the higher the Bowen ratio, the greater the difference between the LSMs. Thus, WRF appears to be more sensitive to the choice of LSM at higher Bowen ratios. NOAH and NOAH-MP exhibit marked differences in representing atmospheric variables such as moisture. Those differences are not constrained to the lower atmosphere close to the land surface, but extended to the lower troposphere. The variations in free tropospheric moisture between the LSMs strongly affects the nature of the simulated convection, and associated precipitation. The degree of sensitivity of the spatial variability and amount of the precipitation with respect to the selection of LSM and PBL scheme shows a strong dependence on the analysed region. A distinct finding of this thesis is the greater sensitivity of WRF with respect to the PBL development to the selection of the LSM, than to the PBL scheme. Furthermore, the impact of this sensitivity is not constrained to the lower CBL, but extends up to the interfacial layer and the lower troposphere - for both dry and convective weather conditions. On the other hand, it is clear that the simulated coupling strength between the land surface and atmosphere is very sensitive to the surface Bowen ratio. The synergies between high resolution measurements and model simulations, with an advanced representation of the land surface processes, will facilitate not only further development of parameterization schemes, but also an improvement in our understanding of land-atmosphere interactions.
3-D observations of absolute humidity from the land surface to the lower troposphere with scanning differential absorption lidar
(2016) Späth, Florian Heiko; Wulfmeyer, Volker
The water vapor (WV) distribution in the atmospheric boundary layer (ABL) is spatially and temporally highly variable. To investigate this behavior, the Institute of Physics and Meteorology at the University of Hohenheim (UHOH) developed a unique scanning differential absorption lidar (DIAL). This instrument allows for water vapor measurements with high temporal and spatial resolutions of the orders of seconds and tens of meters in the range of several kilometers from the surface up to the lower troposphere. Additionally, the UHOH DIAL system can perform scanning measurements which allows for observations down to the surface as well as for observations of the horizontal moisture variability. Within this thesis, three aspects regarding high-resolution observations of moisture in the ABL with scanning DIAL are demonstrated: 1) the development of a new seeder system for the laser transmitter, 2) the presentation of three scan modes, and 3) applications of 2-D to 3-D WV DIAL data. The newly developed seeder system is based on distributed feedback (DFB) laser diodes as seed lasers and an electro-optical deflector as optical switch. The setup and its specifications are presented. Scanning measurements were performed to capture the spatial WV structures. For this purpose, three scan modes with measurement examples are presented: 1) Range-height indicator (RHI) scans provide vertical cross-section images of the atmospheric humidity distribution. The presented series of four measurements show several humidity layers with different WV content and their evolution. Clouds appear in the last scan. 2) A volume scan captures the whole three-dimensional WV structure made out of several conical scans of different elevation angles. The horizontal variation of the layer heights can be related to the terrain profile with a small hill near the DIAL site. 3) Low elevation scans observe the WV distribution directly above the surface. Thus, relationships of the ground characteristics and vegetation with the humidity content above can be investigated. It is shown that there was more moisture above a maize field and above a forest than above grassland. For the analysis of scanning measurements, new analysis and visualization routines as well as new methods for the error estimation were developed. More scientific applications of high-resolution WV data from DIAL measurements are presented in three publications. A evaluation study compared humidity profiles from model simulations with different land-surface schemes with horizontal mean profiles of scanning DIAL measurements. High-resolution humidity fluctuations from vertical measurements were used to determine higher-order moments up to the fourth-order as well as skewness and kurtosis. Furthermore, such WV profiles were combined with profiles of temperature and vertical wind velocities and used for the development of new turbulence parameterizations and for model validation.
High-resolution measurements of temperature and humidity fields in the atmospheric boundary layer with scanning rotational Raman lidar
(2016) Hammann, Eva; Wulfmeyer, Volker
The Institute of Physics and Meteorology of the University of Hohenheim (UHOH) operates a scanning rotational Raman lidar (RRL) for high-resolution temperature and water vapor measurements. The measurement performance of the RRL was improved in several aspects. The statistical error of temperature measurements was reduced by up to 70% through optimization of the filter passbands for various solar background conditions. The optimization method, based on detailed simulations, was written for one specific wavelength and was not applicable to other Raman lidar systems. Therefore the simulation results were parametrized in respect to temperature and background level and expressed in units of wavenumbers. A new interference filter transmitting rotational Raman lines near the excitation wavelength was installed, resulting in a higher transmission and eliminating possible leakage signal. A detection channel for the vibrational Raman line of water vapor was added for the retrieval of water vapor mixing ratios during day-and nighttime. More than 300 hours of temperature and more than 200 hours of water vapor measurements were performed and the acquired profiles used in several publications. Atmospheric variance and higher order moment profiles of the daytime atmospheric boundary layer were derived.
Microwave forward model for land surface remote sensing
(2015) Park, Chang-Hwan; Wulfmeyer, Volker
In order to improve hydro-meteorological model prediction using remote-sensing measurements the difference between the model world and the observed world should be identified. The forward model proposed in this study allows us to simulate the BT (brightness temperature) from the land surface model to compare with the observed microwave BT. The proposed dielectric mixing model is the key part of the forward model to properly link the model parameters and the BT observed by remote sensing. In this study, it was established that the physically valid computation of the effective dielectric constant should be based on the arithmetic average with consideration of the proposed universal damping factor. This physically based dielectric mixing model is superior to the refractive mixing model or semi-empirical/calibration model with RMSE values of 0.96 and 0.63 for the predicted real and imaginary parts, respectively, compared to the measured values. The RMSE obtained with the new model is smaller than those obtained by other researchers using refractive mixing models for operational microwave remote sensing. Once we determine the model uncertainty using this forward model, we can update the model state using the values obtained from the remote-sensing measurement. The challenging task in this process is to resolve the ill-posed inversion problem (estimation of multiple model parameters from a single BT measurement). This study proposes a simple partitioning factor based on model physics. Again, the forward model is crucial because these factors are required to be computed in BT space. In the case study involving the Schäfertal catchment area, the proposed forward model, including the new dielectric mixing model, and the proper partitioning factors computed from land surface model physics was able to successfully extract the refined soil texture information from the microwave BT measurements. The highly resolved soil moisture variability based on the refined soil texture will allow us to predict convective precipitation with higher spatial and temporal accuracy in the numerical weather forecasting model. Moreover, microwave remote sensing using the developed forward model, which provides the soil texture, soil moisture, and soil temperature with a fine scale resolution, is expected to open up new possibilities to examine the energy balance closure problem with unprecedented realism.
The impact of irrigated biomass plantations on mesoscale climate in coastal arid regions
(2015) Branch, Oliver; Wulfmeyer, Volker
Large-scale agroforestry in coastal arid and semi-arid regions could provide a geoengineering solution to anthropogenic climate change. Since agroforestry may impact on mesoscale climate in unknown ways, urgent research into potential impacts of large-plantations is needed to fully assess the viability and optimal placement for such schemes. Validated mesoscale simulations provide insights into feedbacks between land surface and atmosphere, particularly with respect to convective processes. Simulations of irrigated Simmondsia chinensis (jojoba) plantations were carried out with the WRF-NOAH atmosphere-land surface model using prescribed land surface and plant parameters. A sub-surface irrigation algorithm was developed based on critical soil moisture stress levels and implemented into the model code. The simulation of desert and plantation land surfaces was validated with field data from two sites in the Negev Desert - an arid desert site and a 400 ha jojoba plantation. For desert and vegetated surfaces, the model output of diurnal meteorological quantities and energy fluxes generally match well with the respective observations. Diurnal 2m-temperatures over the desert and plantation are matched by the model to within ± 0.2 °C and ± 1.5 °C, respectively. Wind speeds for both surfaces match to within 0.5 ms−1 and plantation latent heat is reproduced to within ± 20 Wm−2. Subsequent to validation, larger plantations of 100 km × 100 km were then simulated in two coastal arid regions, Israel and Oman over a period of one month and compared with control runs, without plantations. In Oman, convection and precipitation were triggered or enhanced by the plantation over multiple days whereas in Israel almost no impacts were observed. Two mechanisms were responsible for observed convection initiation: turbulent vertical transport of scalars due to increased surface heating and roughness as well as a low pressure-induced convergence at the canopy leeside. The main contributors to the surface heating effect were reduced albedo and the high water-use efficiency exhibited by specialist desert species. The combination of increased net surface radiation and high stomatal resistances significantly limited transpiration and led to a surplus in sensible heat flux compared with the surrounding soils (> 100 Wm−2). In Oman, convection initiation triggered by the plantation tended to occur on days when a high mid-tropospheric temperature lapse rate and significant surface air humidity were present. Israel exhibits more stable lapse rates during summer and drier conditions aloft, both of which suppressed convection significantly, even with a similar land surface perturbation. The initiation of moist convection at the mesoscale is therefore strongly controlled by prevailing synoptic conditions. A regional climatological analysis of temperature and humidity ECMWF reanalysis data and station precipitation data indicate that the south-west of North America has particularly suitable conditions for impacts. Coastal locations in Baja California and the Sonoran Desert exhibit a seasonal concurrence of monsoonal instability, high surface humidity and integrated column water vapor, but at the same time low precipitation. Therefore plantation impacts on convection there are likely and could be beneficial in terms of higher amounts of precipitation. These findings indicate that mesoscale convective events can be triggered by large plantations within arid and semi-arid regions and that these effects may be controllable via judicious placement of such schemes. Thus arid agroforestry has the potential not only to increase precipitation and reverse desertification within arid and semi-arid regions, but also to mitigate climate change if implemented on very large scales.
Aircraft air data system based on the measurement of Raman and elastic backscatter via active optical remote-sensing
(2012) Fraczek, Michael Darius; Wulfmeyer, Volker
Flight safety in all weather conditions demands exact and reliable determination of flight-critical air parameters. Conventional aircraft air data systems can be impacted by probe failure caused by mechanical damage or impairment due to different environmental influences. In this thesis, a novel measurement concept for optically measuring the air temperature, density, pressure, moisture and particle backscatter for aircrafts is presented. The detection of volcanic ash is possible as well. This concept is independent from assumptions about the atmospheric state and eliminates the drawbacks of conventional aircraft probes. The measurement principle is based on a laser emitting pulses into the atmosphere from inside the aircraft and a receiver detecting the light signals backscattered from a defined region just outside the disturbed area of the fuselage air flow. With four receiver channels, different spectral portions of the Raman backscatter of dry air and water vapor, as well as the elastic backscatter are extracted. Measurements at daytime and in any atmospheric condition, including very dense clouds, are possible. In the framework of this thesis, a first laboratory prototype of such a measurement system using 532 nm laser radiation was developed, comprising all relevant theoretical and experimental studies. These were notably the comparative feasibility assessment of the measurement methodology, the computational modeling of the measurement concept, the laboratory setup and the experimental validation. Detailed and realistic performance and optimization calculations were made based on the parameters of the first prototype. The impact and the correction of systematic errors due to solar background and elastic signal cross-talk appearing in optically dense clouds were analyzed in computational simulations. The simulations supplement the experimental results for measurement scenarios that are not generable in the laboratory. The laboratory experiments validate the predictions from the simulations with regard to systematic errors and statistical measurement uncertainties. Where possible, the experimental setup and the signal and data analysis were optimized. Residual differences between the experimental and the model results were analyzed in detail. Concrete further hardware optimizations were suggested. The resulting experimental systematic measurement errors at air temperatures varying from 238 K to 308 K under constant air pressure are < 0.05 K, < 0.07 % and < 0.06 % for temperature, density and pressure, respectively. The systematic errors for measurements at air pressures varying from 200 hPa to 950 hPa under constant air temperature are < 0.22 K, < 0.36 % and < 0.31 %, respectively. The experimentally achieved 1-σ statistical measurement uncertainties for the analysis of each single detected signal pulse range from 0.75 K to 2.63 K for temperature, from 0.43 % to 1.21 % for density, and from 0.51 % to 1.50 % for pressure, respectively, for measurement altitudes from 0 m to 13400 m. In order to meet measurement error requirements specified in aviation standards, minimum laser pulse energies were experimentally determined to be used with the designed measurement system. With regard to 100-pulse-averaged temperature measurements, the pulse energy at 532 nm has to be larger than 11 mJ (35 mJ), when regarding 1-σ (3-σ) uncertainties at all measurement altitudes. For 100-pulse-averaged pressure measurements, the laser pulse energy has to be respectively larger than 95 mJ (355 mJ). Based on these experimental results, the laser pulse energy requirements were extrapolated to the ultraviolet wavelength region as well, resulting in much lower laser pulse energy demand. The successful results of this thesis do not only prove the viability of the concept implementation, but also demonstrate its high potential for aircraft air data system application.
Model evaluation and data assimilation impact studies in the framework of COPS
(2012) Schwitalla, Thomas; Wulfmeyer, Volker
The goal of this thesis was the study of new approaches for improving and investigating quantitative precipitation forecasting (QPF), e.g., by optimizing model resolution, physics combination, and data assimilation. A forecasting system based on the Mesoscale Model 5 (MM5) was compared against other operational numerical weather prediction models from Meteo France, MeteoSwiss and the German Weather Service primarily with respect to daytime precipitation. First, a notable daytime dry bias was observed. It appears to be the result of a too small high-resolution domain and the switched-off convection parameterization from the second to the innermost domain. Even the application of a 4-dimensional variational data assimilation (4DVAR) with GPS slant total delays (STD) does not solve this problem due to inconsistent model physics between the 4DVAR and the forecasting model. Nevertheless, the MM5 is in good agreement with the shape of the observed diurnal cycle after the spin-up phase. As the development of the MM5 was suspended, a transition to the new Weather Research and Forecasting (WRF) model system was made after the D-PHASE period (end of 2007). This system features state-of-the-art physics packages and also a variational data assimilation system. As a new observing system, GPS Zenith Total Delay (ZTD) data from Central Europe were incorporated into the 3-dimensional variational data assimilation (3DVAR) system to further improve the initial water vapor field. A first study with this system revealed an improvement of the integrated water vapor RMSE of about 15% and a small but positive impact on the spatial and quantitative precipitation forecast. Additionally, the importance of assimilating upper air observations and the necessity to select a large, convection permitting model domain emerged. Finally a rapid update cycle (RUC) approach, comparable to operational forecast centers, has been developed for a convection-permitting configuration of the WRF model. The system is capable to assimilate radar observations from Germany and France, GPS-ZTD data and satellite radiances and can be applied even for near real-time applications. First experiments with this system show promising results in comparison to other operational models.
Assimilation of ground-based and airborne lidar data into the MM5 4D-Var system
(2010) Grzeschik, Matthias; Wulfmeyer, Volker
This work investigates the impact of assimilating water vapor Light Detection and Ranging (lidar) data into mesoscale Numerical Weather Prediction (NWP) models. Two cases from the field campaigns International H20 Project 2002 (IHOP_2002) and International Lindenberg Campaign for Assessment of Humidity- and Cloud-Profiling Systems and its Impact on High-Resolution Modelling 2005 (LAUNCH-2005) are presented. In the first case, airborne water vapor Differential Absorption Lidar (DIAL) data are used for an assimilation for 24 May 2002, where convection occurred along an eastward moving dryline in western Texas and Oklahoma south of a triple point that formed in western Oklahoma. In the second case, a network of three ground based water vapor Raman lidars, operated behind a sharp frontal rain band with a northwesterly flow, are used. The method employed, Four-Dimensional Variational Data Assimilation (4D-Var), is described in relation to other methods and the implementation is given in detail. The data assimilation results in a large modification of the initial fields. The assimilation into the preconvective conditions changed not only the water vapor field but also the location of convergence lines, causing positive modification of Convective Initiation (CI). In the LAUNCH-2005 case a strong correction of the vertical structure and the absolute values of the initial water-vapor field of the order of 1g/kg was found. This occurred mainly upstream of the lidar systems within an area that was comparable with the domain covered by the lidar systems. The correction of the water-vapor field was validated using independent Global Positioning System (GPS) sensors. Much better agreement with GPS zenith wet path delay was achieved with the initial water-vapor field after 4D-Var. Furthermore, the impact of the assimilation and its temporal evolution was investigated with introduced measures. The results demonstrate the high value of accurate vertically resolved mesoscale water vapor observations and advanced data assimilation systems for short-range weather forecasting.
Application of Global Positioning System slant path delay data for mesoscale modelverification and four-dimensional variational assimilation
(2010) Zus, Florian; Wulfmeyer, Volker
Observation modeling is required in order to make use of slant path delay data, processed from ground-based Global Positioning System (GPS) measurements, for verification purposes and numerical weather prediction. A rigorous ray-tracing algorithm based on the Euler-Lagrange equation derived from Fermat's principle is developed to simulate the propagation of GPS radio signals in a mesoscale model. The ray-tracing algorithm is based on a finite difference scheme and allows the direct numerical simulation of GPS slant path delays.
A high-power laser transmitter for ground-based and airborne water-vapor measurements in the troposphere
(2009) Schiller, Max; Wulfmeyer, Volker
A gain-switched high-power single-frequency Ti:sapphire laser was developed. It is pumped with a frequency-doubled diode-pumped Nd:YAG laser. The laser fulfills the requirements for a transmitter of a water-vapor differential absorption lidar (DIAL), intended for accurate high temporally- and spatially-resolved measurements from the ground to the upper troposphere. The laser was developed using thermal, resonator-design, spectral, and pulse-evolution models. There were layouts assembled for operation at 935 nm and 820 nm optimized for airborne and groundbased measurements, respectively. A birefringent filter and an external-cavity diode laser as an injection seeder are controlling the spectral properties of the transmitter. With a frequency stability of < 60 MHz rms, an emission bandwidth of < 160 MHz, and a spectral purity of > 99.7 %, the total error from the laser properties is smaller than 5 % for water-vapor measurements in the troposphere. The laser beam profile is near-Gaussian with M2 < 2. The achieved laser power was 4.5 W at 935 nm and 7 W at 820 nm at repetition rate of 250 Hz. These values are the highest reported for a single-frequency Ti:sapphire laser. As a part of a ground-based water-vapor DIAL system, the transmitter was deployed during the measurement campaign COPS (Convective and Orographically-induces Precipitation Study). Comparisons with radiosondes confirmed a high precision of the acquired water-vapor day- and nighttime measurements.
Theoretical analysis and design of high-performance frequency converters for LIDAR systems
(2009) Wagner, Gerd; Wulfmeyer, Volker
Frequency converters based on parametric and nonparametric frequency conversion are analyzed with respect to the specifications for high-average power water-vapor DIAL transmitters (DIAL: Differential Absorption LIDAR; LIDAR: Light Detection and Ranging). A Ti:Sapphire laser was selected as a suitable frequency converter to fulfill simultaneously all the requirements in the wavelength range of 935 nm and 820 nm. As thermal effects have a decisive influence on the overall performance and laser resonator design, they were simulated on Ti:Sapphire laser crystals in detail for different crystals, pump, and cooling configurations using finite element analysis (FEA). The performance and spectral properties of the Ti:Sapphire laser transmitter were modeled with a rate-equation approach for stable and unstable resonators. First theoretical results of an end-pumped Ti:Sapphire laser based on an optimized, asymmetric confocal unstable ring resonator design are presented. The obtained results can especially be used for the further development of a Ti:Sapphire laser to serve as a demonstrator for a future space-borne DIAL system transmitter according to the WALES (Water Vapor Lidar Experiment in Space) specifications. Furthermore, the adaptation of the developed theory modules to other lasing materials and configurations is straightforward.

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