We present a new mathematical technique for retrieving the temperature profile from a ground base... more We present a new mathematical technique for retrieving the temperature profile from a ground based microwave profiler and ancillary measurements. It is based on a gradient flow approach. We are able to solve the inverse problem of radiative transfer and determine the temperature profile from ten simultaneous brightness temperature measurements in the range from 50.8 to 58.8 GHz. The approach uses no additional statistical information. A physical consistent solution for a temperature profile can be found within five seconds. First results of this technique show promising results with regard to absolute accuracy and inversion height determination, even if the brightness temperature measurements are assumed noisy to 0.5 K. ¥
IEEE Transactions on Geoscience and Remote Sensing, 2007
The potential of a ground-based microwave temperature profiler to combine full tropospheric profi... more The potential of a ground-based microwave temperature profiler to combine full tropospheric profiling with highresolution profiling of the boundary layer is investigated. For that purpose, statistical retrieval algorithms that incorporate observations from different elevation angles and frequencies are derived from long-term radiosonde data. A simulation study shows the potential to significantly improve the retrieval performance in the lowest kilometer by combining angular information from relatively opaque channels with zenith-only information from more transparent channels. Observations by a state-of-the-art radiometer employed during the International Lindenberg Campaign for Assessment of Humidity and Cloud Profiling Systems and Its Impact on High-Resolution Modeling (LAUNCH) in Lindenberg, Germany, are used for an experimental evaluation with observations from a 99-m mast and radiosondes. The comparison not only reveals the high accuracy achieved by combining angular and spectral observations (overall, less than 1 K below 1.5 km), but also emphasizes the need for a realistic description of radiometer noise within the algorithm. The capability of the profiler to observe the height and strength of low-level temperature inversions is highlighted.
We present a method for deriving horizontalhumidity variability from a single-scanning passive mi... more We present a method for deriving horizontalhumidity variability from a single-scanning passive microwave radiometer (MWR). The MWR used has full scanning capabilities in azimuth and elevation and is sensitive to the path of integrated water vapor as well as cloud liquid water. Applying a simple linear-gradient model together with an assumed vertical profile derived from the closest radiosonde ascent, the strength and direction of the horizontal-humidity gradient can be determined with a temporal resolution on the order of 15-20 min. For the case of an approaching frontal system, the derived humidity field can explain up to 88% of the measured humidity variance-the missing variance can most probably be attributed to convective activity.
IEEE Transactions on Geoscience and Remote Sensing, 2009
Ground-based observations from two different radiometers are used to evaluate commonly used micro... more Ground-based observations from two different radiometers are used to evaluate commonly used microwave/ millimeter-wave propagation models at 150 GHz. This frequency has strong sensitivity to changes in precipitable water vapor (PWV) and cloud liquid water. The observations were collected near Hesselbach, Germany, as part of the Atmospheric Radiation Measurement program's support of the General Observing Period and the Convective and Orographic Precipitation Study. The observations from the two radiometers agree well with each other, with a slope of 0.993 and a mean bias of 0.12 K. The observations demonstrate that the relative sensitivity of the different absorption models to PWV in clear-sky conditions at 150 GHz is significant and that four models differ significantly from the observed brightness temperature. These models were modified to get agreement with the 150-GHz observations, where the PWV ranged from 0.35 to 2.88 cm. The models were modified by adjusting the strength of the foreign-and self-broadened water vapor continuum coefficients, where the magnitude was model dependent. In all cases, the adjustment to the two components of the water vapor continuum was in opposite directions (i.e., increasing the contribution from the foreign-broadened component while decreasing contribution from the self-broadened component or vice versa). While the original models had significant disagreements relative to each other, the resulting modified models show much better agreement relative to each other throughout the microwave spectrum. The modified models were evaluated using independent observations at 31.4 GHz.
Bulletin of The American Meteorological Society, 2004
Clouds cause large uncertainties in the determination of climate sensitivity to either natural or... more Clouds cause large uncertainties in the determination of climate sensitivity to either natural or anthropogenic changes. Furthermore, clouds dominate our perception of the weather, and the poor forecast of cloud parameters in numerical weather prediction (NWP) models is striking. In order to improve modeling and forecasting of clouds in climate and NWP models the BALTEX BRIDGE campaign (BBC) was conducted in the Netherlands in August/September 2001. The complex cloud processes, which involve scales from less than a micrometer (condensation nuclei) to thousand kilometers (frontal systems) require an integrated measurement approach. Advanced remote sensing instruments were operated at the central facility in Cabauw, the Netherlands, to derive the vertical cloud structure. A regional network of stations was operated within a 100 x 100 km 2 domain to observe solar radiation, cloud liquid water path, cloud base temperature and height. Aircraft and tethered balloon measurements were used to measure cloud microphysical parameters and solar radiation below, in and above the cloud. Satellite measurements complemented the cloud observations by providing the spatial structure from above. In order to better understand the effect of cloud inhomogeneities on the radiation field, three-dimensional radiative transfer modeling was closely linked to the measurement activities. To evaluate the performance of dynamic atmospheric models for the cloudy atmosphere four operational climate and NWP models were compared to the observations. As a first outcome of BBC we demonstrate that increased vertical resolution can improve the representation of clouds in these models.
Physics and Chemistry of The Earth Part B-hydrology Oceans and Atmosphere, 1999
Abatra& A ground based system combining active and passive remote sensing instruments from differ... more Abatra& A ground based system combining active and passive remote sensing instruments from different spectral ranges has been set up at the Meteorological Institute in Bonn to investigate cloud processes. Currently, the sensor package includes a laser ceilometer, an infrared radiometer, an X-Band Radar, and a 4-channel microwave radiometer. Cloud base height and temperature, precipitation, and the integrated quantities of water vapor and cloud liquid water can be mcasured on small time scales (5 15 s). The system will be extended in fall 1998 with a 22-channel microwave to derive profiles of humidity, temperature and liquid water content. A case study using measurements from April I, 1998 demonstrates the ability of the system to characterize various types of clouds. Q
This letter introduces a new approach to characterize small-scale humidity variations using groun... more This letter introduces a new approach to characterize small-scale humidity variations using ground-based microwave radiometry. For that purpose, a microwave profiler routinely performed different scan patterns during its deployment at the Atmospheric Radiation Measurement mobile facility in the Black Forest, Germany. Individual azimuth scans at 30 • elevation revealed spatial variations in integrated water vapor (IWV) up to ±10%. Aircraft observations were used to evaluate the performance of the microwave observations by comparing derived humidity fields with IWV retrieved along individual directions of the microwave radiometer. Distinct humidity signals were reproduced by both observations. Residual uncertainties can be attributed to the temporal variations during the time it took the aircraft to cover the boundary layer and uncertainties in the interpolation. Long-term scanning observations will be further explored to investigate land-surface interaction and to characterize subgrid variability.
1] A large number of all-liquid, nondrizzling stratus clouds (163 hours of measurements) were obs... more 1] A large number of all-liquid, nondrizzling stratus clouds (163 hours of measurements) were observed with a dual-channel microwave radiometer and a colocated 35-GHz cloud radar during the spring and summer months of the Surface Heat Budget of the Arctic Ocean (SHEBA) project. An algorithm developed by Frisch et al. [1995, 1998] to derive the liquid water content (LWC) is applied to these measurements assuming constant cloud drop number density and cloud drop size distribution breadth with height. A second algorithm developed by is specifically adapted for SHEBA clouds using a priori information from a large eddy simulation (LES) model initialized with summertime SHEBA radiosondes; about 50 soundings during nondrizzling, low-level, allliquid water clouds are used. Using model-derived drop size distributions, a relationship between simulated radar reflectivity (Z) and model LWC is derived as well as an a priori LWC profile. Once the theoretical error covariance matrix of the Z-LWC relation is derived and the covariance matrix of the LWC profile is calculated, an optimal estimation method is applied to the SHEBA data. The Frisch et al. and Löhnert et al. methods are also applied to the LES model output, resulting in overall root-mean-square differences on the order of 30 to 60%. Both methods are sensitive to the assumed accuracies of the microwave-radiometer-derived LWP. When applied to LES model output, the Frisch et al. method shows a LWC overestimation in the lower parts of the cloud. These systematic errors are induced by the assumption of constant cloud number concentration with height.
A large number of all-liquid, nondrizzling stratus clouds (163 hours of measurements) were observ... more A large number of all-liquid, nondrizzling stratus clouds (163 hours of measurements) were observed with a dual-channel microwave radiometer and a colocated 35-GHz cloud radar during the spring and summer months of the Surface Heat Budget of the Arctic Ocean (SHEBA) project. An algorithm developed by [1995, 1998] to derive the liquid water content (LWC) is applied to these measurements assuming constant cloud drop number density and cloud drop size distribution breadth with height. A second algorithm developed by [2001] is specifically adapted for SHEBA clouds using a priori information from a large eddy simulation (LES) model initialized with summertime SHEBA radiosondes; about 50 soundings during nondrizzling, low-level, all-liquid water clouds are used. Using model-derived drop size distributions, a relationship between simulated radar reflectivity (Z) and model LWC is derived as well as an a priori LWC profile. Once the theoretical error covariance matrix of the Z-LWC relation is derived and the covariance matrix of the LWC profile is calculated, an optimal estimation method is applied to the SHEBA data. The Frisch et al. and Löhnert et al. methods are also applied to the LES model output, resulting in overall root-mean-square differences on the order of 30 to 60%. Both methods are sensitive to the assumed accuracies of the microwave-radiometer-derived LWP. When applied to LES model output, the Frisch et al. method shows a LWC overestimation in the lower parts of the cloud. These systematic errors are induced by the assumption of constant cloud number concentration with height.
1] This paper investigates the influence of cloud model statistics on the accuracy of statistical... more 1] This paper investigates the influence of cloud model statistics on the accuracy of statistical multiple-frequency liquid water path (LWP) retrievals for a ground-based microwave radiometer. Statistical algorithms were developed from a radiosonde data set in which clouds were modeled by using a relative humidity threshold and a modified adiabatic assumption. Evaluation of the algorithms was then performed by applying the algorithms to four data sets in which clouds were generated in different ways (i.e., threshold method, gradient method, and cloud microphysical model). While classical twochannel algorithms, in this case using frequencies at 22.985 and 28.235 GHz, do not show a significant dependency on the cloud model, the inclusion of an additional 50-GHz channel can introduce significant systematic errors. The addition of a 90-GHz frequency to the two-channel algorithm leads to a larger increase in LWP accuracy than in case of the 50-GHz channel and is less sensitive to the choice of cloud model. A drizzle case from the cloud microphysical model shows no significant loss of accuracy for the microwave radiometer algorithms, in contrast to simple cloud radar retrievals of liquid water. In case of rain, however, the results deteriorate when the total liquid water path is larger than 700 g m À2 .
1] The ability to determine the cloud boundaries and vertical distribution of cloud liquid water ... more 1] The ability to determine the cloud boundaries and vertical distribution of cloud liquid water for single-layer liquid clouds using zenith-pointing microwave radiometers is investigated. Simulations are used to demonstrate that there is little skill in determining either cloud base or cloud thickness, especially when the cloud thickness is less than 500 m. It is also shown that the different distributions of liquid water content within a cloud with known cloud boundaries results in a maximum change in the brightness temperature of less than 1 K at the surface from 20 to 150 GHz, which is on the order of the instrument noise level. Furthermore, it is demonstrated using the averaging kernel that the number of degrees of freedom for signal (i.e., independent pieces of information) is approximately 1, which implies there is no information on vertical distribution of liquid water in the microwave observations.
A method is presented for deriving physically consistent profiles of temperature, humidity, and c... more A method is presented for deriving physically consistent profiles of temperature, humidity, and cloud liquid water content. This approach combines a ground-based multichannel microwave radiometer, a cloud radar, a lidar-ceilometer, the nearest operational radiosonde measurement, and ground-level measurements of standard meteorological properties with statistics derived from results of a microphysical cloud model. All measurements are integrated within the framework of optimal estimation to guarantee a retrieved profile with maximum information content. The developed integrated profiling technique (IPT) is applied to synthetic cloud model output as a test of accuracy. It is shown that the liquid water content profiles obtained with the IPT are significantly more accurate than common methods that use the microwave-derived liquid water path to scale the radar reflectivity profile. The IPT is also applied to 2 months of the European Cloud Liquid Water Network (CLIWA-NET) Baltic Sea Experiment (BALTEX) BRIDGE main experiment (BBC) campaign data, considering liquidphase, nonprecipitating clouds only. Error analysis indicates root-mean-square uncertainties of less than 1 K in temperature and less than 1 g m Ϫ3 in humidity, where the relative error in liquid water content ranges from 15% to 25%. A comparison of the vertically integrated humidity profile from the IPT with the nearest operational radiosonde shows an acceptable bias error of 0.13 kg m Ϫ2 when the Rosenkranz gas absorption model is used. However, if the Liebe gas absorption model is used, this systematic error increases to Ϫ1.24 kg m Ϫ2 , showing that the IPT humidity retrieval is significantly dependent on the chosen gas absorption model.
A method for combining ground based multi-channel microwave radiometer measurements (TB), cloud r... more A method for combining ground based multi-channel microwave radiometer measurements (TB), cloud radar reflectivity (Z) and statistics of a cloud model is proposed for deriving cloud liquid water density profiles (LWC). Cloud radars can measure the radar reflectivity (Z) in a specified height and thus (in combination with lidar ceilometers) imply information on the vertical extension of a cloud. However,
Journal of Atmospheric and Oceanic Technology, 2008
This paper describes advances in ground-based thermodynamic profiling of the lower troposphere th... more This paper describes advances in ground-based thermodynamic profiling of the lower troposphere through sensor synergy. The well-documented integrated profiling technique (IPT), which uses a microwave profiler, a cloud radar, and a ceilometer to simultaneously retrieve vertical profiles of temperature, humidity, and liquid water content (LWC) of nonprecipitating clouds, is further developed toward an enhanced performance in the boundary layer and lower troposphere. For a more accurate temperature profile, this is accomplished by including an elevation scanning measurement modus of the microwave profiler. Heightdependent RMS accuracies of temperature (humidity) ranging from ϳ0.3 to 0.9 K (0.5-0.8 g m Ϫ3 ) in the boundary layer are derived from retrieval simulations and confirmed experimentally with measurements at distinct heights taken during the 2005 International Lindenberg Campaign for Assessment of Humidity and Cloud Profiling Systems and its Impact on High-Resolution Modeling (LAUNCH) of the German Weather Service. Temperature inversions, especially of the lower boundary layer, are captured in a very satisfactory way by using the elevation scanning mode. To improve the quality of liquid water content measurements in clouds the authors incorporate a sophisticated target classification scheme developed within the European cloud observing network CloudNet. It allows the detailed discrimination between different types of backscatterers detected by cloud radar and ceilometer. Finally, to allow IPT application also to drizzling cases, an LWC profiling method is integrated. This technique classifies the detected hydrometeors into three different size classes using certain thresholds determined by radar reflectivity and/or ceilometer extinction profiles. By inclusion into IPT, the retrieved profiles are made consistent with the measurements of the microwave profiler and an LWC a priori profile. Results of IPT application to 13 days of the LAUNCH campaign are analyzed, and the importance of integrated profiling for model evaluation is underlined.
A 22-channel Microwave Radiometer for Cloud Carthography (MICCY) for the profiling of tropospheri... more A 22-channel Microwave Radiometer for Cloud Carthography (MICCY) for the profiling of tropospheric temperature, humidity, and cloud liquid water has been developed. The radiometer has 10 channels along the high-frequency wing of the 22.235 GHz water vapor line, 10 channels along the low-frequency side of the 60 GHz oxygen complex, and 2 channels at 90 GHz. Two features make the radiometer a unique tool for the observation of small-scale structures related to cloud processes: (1) Atmospheric brightness temperatures are measured simultaneously at all channels with an integration time of 1 s. (2) A Cassegrain system including a 90 cm off-axis parabolic mirror leads to a spatial resolution better than 1 ø full width at half maximum for all frequencies. The necessity of these features for cloud observations is demonstrated. Algorithms for the retrieval of integrated water vapor, integrated cloud liquid water, and the profiles of temperature and humidity were developed on the basis of artificial neural networks. Measurement examples, including comparisons with radiosondes, demonstrate the systems capabilities for high-resolution atmospheric monitoring.
The implementation of an operational network of microwave radiometers is presently hampered by th... more The implementation of an operational network of microwave radiometers is presently hampered by the cost and complexity of the available instruments. For this reason, the definition and design of a low-cost microwave radiometer suitable for automatic, high-quality observations of liquid water path (LWP) were one objective of the BALTEX cloud liquid water network: CLIWA-NET. In the course of the project, it turned out that a full profiling radiometer with 14 channels can be produced at only about 30% higher cost than a classical dual-channel IWV/LWP radiometer. The profiling capability allows simultaneous observations of LWP and the lower tropospheric (0-5 km) humidity and temperature profiles with a temporal resolution of less than 10 s and a vertical resolution from 100 m to 1 km in the planetary boundary layer depending on height and atmospheric conditions. The latter is possible due to an elevation scan capability and by the implementation of a new filter bank design. The radiometer has several additional sensors 0169-8095/$ -see front matter D (temperature, humidity, pressure, rain detector and GPS) which guarantee, together with a flexible software package, the operational performance of the system with maintenance intervals of about every 3 months. The performance of the first prototype has been verified during a 3-week campaign at Cabauw, The Netherlands. D
1] Using high-resolution oxygen A band spectrometry (l/Dl = 60000) in the 767.7-770.7 nm waveleng... more 1] Using high-resolution oxygen A band spectrometry (l/Dl = 60000) in the 767.7-770.7 nm wavelength range, we investigate the first and second moments of the distributions of path lengths of photons in transmitted skylight for different cloud conditions. Our observations are supported by measurements of column liquid water path by multichannel microwave radiometry, cloud structure by millimeter cloud radar observations, and cloud base by a laser ceilometer. For the investigated multilayer cloud covers (decks of stratus, cumulus, altostratus, and cirrus), our measurements indicate that the photon path statistics are mostly governed by anomalous diffusion, whereby classical diffusion occurs in the limiting case of a single compact (plane parallel) cloud layer. The ratio for the inferred second and first moments of the path lengths confirms the relation recently derived by for photon diffusion in single optically thick cloud layers and extends it to more complex cloud geometry. (2006), Path length distributions for solar photons under cloudy skies: Comparison of measured first and second moments with predictions from classical and anomalous diffusion theories,
The ldquoatmospheric propagation and profiling systemrdquo (ATPROP) advanced ground-based microwa... more The ldquoatmospheric propagation and profiling systemrdquo (ATPROP) advanced ground-based microwave radiometer, for radiowave propagation assessment at Ku, Ka, Q/V and W bands has been developed. The design of ATPROP is based on the requirements of SatCom, SatNav systems and Space Science Missions. ATPROP consists of two independent subsystems, operating at Ka band, near the 60 GHz oxygen absorption band and at 15/90 GHz. ATPROP has a full non-GEO satellite tracking capability and uses switched Dicke references to improve stability. Its performance allows to accurately derive atmospheric attenuation, sky noise, wet delay, and cloud, vapor and air temperature profiles.
Journal of Atmospheric and Oceanic Technology, 2001
A method for combining ground-based passive microwave radiometer retrievals of integrated liquid ... more A method for combining ground-based passive microwave radiometer retrievals of integrated liquid water (LWP), radar reflectivity profiles (Z ), and statistics of a cloud model is proposed for deriving cloud liquid water profiles (LWC). A dynamic cloud model is used to determine Z-LWC relations and their errors as functions of height above cloud base. The cloud model is also used to develop an LWP algorithm based on simulations of brightness temperatures of a 20-30-GHz radiometer. For the retrieval of LWC, the radar determined Z profile, the passive microwave retrieved LWP, and a model climatology are combined by an inverse error covariance weighting method. Model studies indicate that LWC retrievals with this method result in rms errors that are about 10%-20% smaller in comparison to a conventional LWC algorithm, which constrains the LWC profile exactly to the measured LWP. According to the new algorithm, errors in the range of 30%-60% are to be anticipated when profiling LWC. The algorithm is applied to a time series measurement of a stratocumulus layer at GKSS in Geesthacht, Germany. The GKSS 95-GHz cloud radar, a 20-30-GHz microwave radiometer, and a laser ceilometer were collocated within a 5-m radius and operated continuously during the measurement period. The laser ceilometer was used to confirm the presence of drizzle-sized drops.
We present a new mathematical technique for retrieving the temperature profile from a ground base... more We present a new mathematical technique for retrieving the temperature profile from a ground based microwave profiler and ancillary measurements. It is based on a gradient flow approach. We are able to solve the inverse problem of radiative transfer and determine the temperature profile from ten simultaneous brightness temperature measurements in the range from 50.8 to 58.8 GHz. The approach uses no additional statistical information. A physical consistent solution for a temperature profile can be found within five seconds. First results of this technique show promising results with regard to absolute accuracy and inversion height determination, even if the brightness temperature measurements are assumed noisy to 0.5 K. ¥
IEEE Transactions on Geoscience and Remote Sensing, 2007
The potential of a ground-based microwave temperature profiler to combine full tropospheric profi... more The potential of a ground-based microwave temperature profiler to combine full tropospheric profiling with highresolution profiling of the boundary layer is investigated. For that purpose, statistical retrieval algorithms that incorporate observations from different elevation angles and frequencies are derived from long-term radiosonde data. A simulation study shows the potential to significantly improve the retrieval performance in the lowest kilometer by combining angular information from relatively opaque channels with zenith-only information from more transparent channels. Observations by a state-of-the-art radiometer employed during the International Lindenberg Campaign for Assessment of Humidity and Cloud Profiling Systems and Its Impact on High-Resolution Modeling (LAUNCH) in Lindenberg, Germany, are used for an experimental evaluation with observations from a 99-m mast and radiosondes. The comparison not only reveals the high accuracy achieved by combining angular and spectral observations (overall, less than 1 K below 1.5 km), but also emphasizes the need for a realistic description of radiometer noise within the algorithm. The capability of the profiler to observe the height and strength of low-level temperature inversions is highlighted.
We present a method for deriving horizontalhumidity variability from a single-scanning passive mi... more We present a method for deriving horizontalhumidity variability from a single-scanning passive microwave radiometer (MWR). The MWR used has full scanning capabilities in azimuth and elevation and is sensitive to the path of integrated water vapor as well as cloud liquid water. Applying a simple linear-gradient model together with an assumed vertical profile derived from the closest radiosonde ascent, the strength and direction of the horizontal-humidity gradient can be determined with a temporal resolution on the order of 15-20 min. For the case of an approaching frontal system, the derived humidity field can explain up to 88% of the measured humidity variance-the missing variance can most probably be attributed to convective activity.
IEEE Transactions on Geoscience and Remote Sensing, 2009
Ground-based observations from two different radiometers are used to evaluate commonly used micro... more Ground-based observations from two different radiometers are used to evaluate commonly used microwave/ millimeter-wave propagation models at 150 GHz. This frequency has strong sensitivity to changes in precipitable water vapor (PWV) and cloud liquid water. The observations were collected near Hesselbach, Germany, as part of the Atmospheric Radiation Measurement program's support of the General Observing Period and the Convective and Orographic Precipitation Study. The observations from the two radiometers agree well with each other, with a slope of 0.993 and a mean bias of 0.12 K. The observations demonstrate that the relative sensitivity of the different absorption models to PWV in clear-sky conditions at 150 GHz is significant and that four models differ significantly from the observed brightness temperature. These models were modified to get agreement with the 150-GHz observations, where the PWV ranged from 0.35 to 2.88 cm. The models were modified by adjusting the strength of the foreign-and self-broadened water vapor continuum coefficients, where the magnitude was model dependent. In all cases, the adjustment to the two components of the water vapor continuum was in opposite directions (i.e., increasing the contribution from the foreign-broadened component while decreasing contribution from the self-broadened component or vice versa). While the original models had significant disagreements relative to each other, the resulting modified models show much better agreement relative to each other throughout the microwave spectrum. The modified models were evaluated using independent observations at 31.4 GHz.
Bulletin of The American Meteorological Society, 2004
Clouds cause large uncertainties in the determination of climate sensitivity to either natural or... more Clouds cause large uncertainties in the determination of climate sensitivity to either natural or anthropogenic changes. Furthermore, clouds dominate our perception of the weather, and the poor forecast of cloud parameters in numerical weather prediction (NWP) models is striking. In order to improve modeling and forecasting of clouds in climate and NWP models the BALTEX BRIDGE campaign (BBC) was conducted in the Netherlands in August/September 2001. The complex cloud processes, which involve scales from less than a micrometer (condensation nuclei) to thousand kilometers (frontal systems) require an integrated measurement approach. Advanced remote sensing instruments were operated at the central facility in Cabauw, the Netherlands, to derive the vertical cloud structure. A regional network of stations was operated within a 100 x 100 km 2 domain to observe solar radiation, cloud liquid water path, cloud base temperature and height. Aircraft and tethered balloon measurements were used to measure cloud microphysical parameters and solar radiation below, in and above the cloud. Satellite measurements complemented the cloud observations by providing the spatial structure from above. In order to better understand the effect of cloud inhomogeneities on the radiation field, three-dimensional radiative transfer modeling was closely linked to the measurement activities. To evaluate the performance of dynamic atmospheric models for the cloudy atmosphere four operational climate and NWP models were compared to the observations. As a first outcome of BBC we demonstrate that increased vertical resolution can improve the representation of clouds in these models.
Physics and Chemistry of The Earth Part B-hydrology Oceans and Atmosphere, 1999
Abatra& A ground based system combining active and passive remote sensing instruments from differ... more Abatra& A ground based system combining active and passive remote sensing instruments from different spectral ranges has been set up at the Meteorological Institute in Bonn to investigate cloud processes. Currently, the sensor package includes a laser ceilometer, an infrared radiometer, an X-Band Radar, and a 4-channel microwave radiometer. Cloud base height and temperature, precipitation, and the integrated quantities of water vapor and cloud liquid water can be mcasured on small time scales (5 15 s). The system will be extended in fall 1998 with a 22-channel microwave to derive profiles of humidity, temperature and liquid water content. A case study using measurements from April I, 1998 demonstrates the ability of the system to characterize various types of clouds. Q
This letter introduces a new approach to characterize small-scale humidity variations using groun... more This letter introduces a new approach to characterize small-scale humidity variations using ground-based microwave radiometry. For that purpose, a microwave profiler routinely performed different scan patterns during its deployment at the Atmospheric Radiation Measurement mobile facility in the Black Forest, Germany. Individual azimuth scans at 30 • elevation revealed spatial variations in integrated water vapor (IWV) up to ±10%. Aircraft observations were used to evaluate the performance of the microwave observations by comparing derived humidity fields with IWV retrieved along individual directions of the microwave radiometer. Distinct humidity signals were reproduced by both observations. Residual uncertainties can be attributed to the temporal variations during the time it took the aircraft to cover the boundary layer and uncertainties in the interpolation. Long-term scanning observations will be further explored to investigate land-surface interaction and to characterize subgrid variability.
1] A large number of all-liquid, nondrizzling stratus clouds (163 hours of measurements) were obs... more 1] A large number of all-liquid, nondrizzling stratus clouds (163 hours of measurements) were observed with a dual-channel microwave radiometer and a colocated 35-GHz cloud radar during the spring and summer months of the Surface Heat Budget of the Arctic Ocean (SHEBA) project. An algorithm developed by Frisch et al. [1995, 1998] to derive the liquid water content (LWC) is applied to these measurements assuming constant cloud drop number density and cloud drop size distribution breadth with height. A second algorithm developed by is specifically adapted for SHEBA clouds using a priori information from a large eddy simulation (LES) model initialized with summertime SHEBA radiosondes; about 50 soundings during nondrizzling, low-level, allliquid water clouds are used. Using model-derived drop size distributions, a relationship between simulated radar reflectivity (Z) and model LWC is derived as well as an a priori LWC profile. Once the theoretical error covariance matrix of the Z-LWC relation is derived and the covariance matrix of the LWC profile is calculated, an optimal estimation method is applied to the SHEBA data. The Frisch et al. and Löhnert et al. methods are also applied to the LES model output, resulting in overall root-mean-square differences on the order of 30 to 60%. Both methods are sensitive to the assumed accuracies of the microwave-radiometer-derived LWP. When applied to LES model output, the Frisch et al. method shows a LWC overestimation in the lower parts of the cloud. These systematic errors are induced by the assumption of constant cloud number concentration with height.
A large number of all-liquid, nondrizzling stratus clouds (163 hours of measurements) were observ... more A large number of all-liquid, nondrizzling stratus clouds (163 hours of measurements) were observed with a dual-channel microwave radiometer and a colocated 35-GHz cloud radar during the spring and summer months of the Surface Heat Budget of the Arctic Ocean (SHEBA) project. An algorithm developed by [1995, 1998] to derive the liquid water content (LWC) is applied to these measurements assuming constant cloud drop number density and cloud drop size distribution breadth with height. A second algorithm developed by [2001] is specifically adapted for SHEBA clouds using a priori information from a large eddy simulation (LES) model initialized with summertime SHEBA radiosondes; about 50 soundings during nondrizzling, low-level, all-liquid water clouds are used. Using model-derived drop size distributions, a relationship between simulated radar reflectivity (Z) and model LWC is derived as well as an a priori LWC profile. Once the theoretical error covariance matrix of the Z-LWC relation is derived and the covariance matrix of the LWC profile is calculated, an optimal estimation method is applied to the SHEBA data. The Frisch et al. and Löhnert et al. methods are also applied to the LES model output, resulting in overall root-mean-square differences on the order of 30 to 60%. Both methods are sensitive to the assumed accuracies of the microwave-radiometer-derived LWP. When applied to LES model output, the Frisch et al. method shows a LWC overestimation in the lower parts of the cloud. These systematic errors are induced by the assumption of constant cloud number concentration with height.
1] This paper investigates the influence of cloud model statistics on the accuracy of statistical... more 1] This paper investigates the influence of cloud model statistics on the accuracy of statistical multiple-frequency liquid water path (LWP) retrievals for a ground-based microwave radiometer. Statistical algorithms were developed from a radiosonde data set in which clouds were modeled by using a relative humidity threshold and a modified adiabatic assumption. Evaluation of the algorithms was then performed by applying the algorithms to four data sets in which clouds were generated in different ways (i.e., threshold method, gradient method, and cloud microphysical model). While classical twochannel algorithms, in this case using frequencies at 22.985 and 28.235 GHz, do not show a significant dependency on the cloud model, the inclusion of an additional 50-GHz channel can introduce significant systematic errors. The addition of a 90-GHz frequency to the two-channel algorithm leads to a larger increase in LWP accuracy than in case of the 50-GHz channel and is less sensitive to the choice of cloud model. A drizzle case from the cloud microphysical model shows no significant loss of accuracy for the microwave radiometer algorithms, in contrast to simple cloud radar retrievals of liquid water. In case of rain, however, the results deteriorate when the total liquid water path is larger than 700 g m À2 .
1] The ability to determine the cloud boundaries and vertical distribution of cloud liquid water ... more 1] The ability to determine the cloud boundaries and vertical distribution of cloud liquid water for single-layer liquid clouds using zenith-pointing microwave radiometers is investigated. Simulations are used to demonstrate that there is little skill in determining either cloud base or cloud thickness, especially when the cloud thickness is less than 500 m. It is also shown that the different distributions of liquid water content within a cloud with known cloud boundaries results in a maximum change in the brightness temperature of less than 1 K at the surface from 20 to 150 GHz, which is on the order of the instrument noise level. Furthermore, it is demonstrated using the averaging kernel that the number of degrees of freedom for signal (i.e., independent pieces of information) is approximately 1, which implies there is no information on vertical distribution of liquid water in the microwave observations.
A method is presented for deriving physically consistent profiles of temperature, humidity, and c... more A method is presented for deriving physically consistent profiles of temperature, humidity, and cloud liquid water content. This approach combines a ground-based multichannel microwave radiometer, a cloud radar, a lidar-ceilometer, the nearest operational radiosonde measurement, and ground-level measurements of standard meteorological properties with statistics derived from results of a microphysical cloud model. All measurements are integrated within the framework of optimal estimation to guarantee a retrieved profile with maximum information content. The developed integrated profiling technique (IPT) is applied to synthetic cloud model output as a test of accuracy. It is shown that the liquid water content profiles obtained with the IPT are significantly more accurate than common methods that use the microwave-derived liquid water path to scale the radar reflectivity profile. The IPT is also applied to 2 months of the European Cloud Liquid Water Network (CLIWA-NET) Baltic Sea Experiment (BALTEX) BRIDGE main experiment (BBC) campaign data, considering liquidphase, nonprecipitating clouds only. Error analysis indicates root-mean-square uncertainties of less than 1 K in temperature and less than 1 g m Ϫ3 in humidity, where the relative error in liquid water content ranges from 15% to 25%. A comparison of the vertically integrated humidity profile from the IPT with the nearest operational radiosonde shows an acceptable bias error of 0.13 kg m Ϫ2 when the Rosenkranz gas absorption model is used. However, if the Liebe gas absorption model is used, this systematic error increases to Ϫ1.24 kg m Ϫ2 , showing that the IPT humidity retrieval is significantly dependent on the chosen gas absorption model.
A method for combining ground based multi-channel microwave radiometer measurements (TB), cloud r... more A method for combining ground based multi-channel microwave radiometer measurements (TB), cloud radar reflectivity (Z) and statistics of a cloud model is proposed for deriving cloud liquid water density profiles (LWC). Cloud radars can measure the radar reflectivity (Z) in a specified height and thus (in combination with lidar ceilometers) imply information on the vertical extension of a cloud. However,
Journal of Atmospheric and Oceanic Technology, 2008
This paper describes advances in ground-based thermodynamic profiling of the lower troposphere th... more This paper describes advances in ground-based thermodynamic profiling of the lower troposphere through sensor synergy. The well-documented integrated profiling technique (IPT), which uses a microwave profiler, a cloud radar, and a ceilometer to simultaneously retrieve vertical profiles of temperature, humidity, and liquid water content (LWC) of nonprecipitating clouds, is further developed toward an enhanced performance in the boundary layer and lower troposphere. For a more accurate temperature profile, this is accomplished by including an elevation scanning measurement modus of the microwave profiler. Heightdependent RMS accuracies of temperature (humidity) ranging from ϳ0.3 to 0.9 K (0.5-0.8 g m Ϫ3 ) in the boundary layer are derived from retrieval simulations and confirmed experimentally with measurements at distinct heights taken during the 2005 International Lindenberg Campaign for Assessment of Humidity and Cloud Profiling Systems and its Impact on High-Resolution Modeling (LAUNCH) of the German Weather Service. Temperature inversions, especially of the lower boundary layer, are captured in a very satisfactory way by using the elevation scanning mode. To improve the quality of liquid water content measurements in clouds the authors incorporate a sophisticated target classification scheme developed within the European cloud observing network CloudNet. It allows the detailed discrimination between different types of backscatterers detected by cloud radar and ceilometer. Finally, to allow IPT application also to drizzling cases, an LWC profiling method is integrated. This technique classifies the detected hydrometeors into three different size classes using certain thresholds determined by radar reflectivity and/or ceilometer extinction profiles. By inclusion into IPT, the retrieved profiles are made consistent with the measurements of the microwave profiler and an LWC a priori profile. Results of IPT application to 13 days of the LAUNCH campaign are analyzed, and the importance of integrated profiling for model evaluation is underlined.
A 22-channel Microwave Radiometer for Cloud Carthography (MICCY) for the profiling of tropospheri... more A 22-channel Microwave Radiometer for Cloud Carthography (MICCY) for the profiling of tropospheric temperature, humidity, and cloud liquid water has been developed. The radiometer has 10 channels along the high-frequency wing of the 22.235 GHz water vapor line, 10 channels along the low-frequency side of the 60 GHz oxygen complex, and 2 channels at 90 GHz. Two features make the radiometer a unique tool for the observation of small-scale structures related to cloud processes: (1) Atmospheric brightness temperatures are measured simultaneously at all channels with an integration time of 1 s. (2) A Cassegrain system including a 90 cm off-axis parabolic mirror leads to a spatial resolution better than 1 ø full width at half maximum for all frequencies. The necessity of these features for cloud observations is demonstrated. Algorithms for the retrieval of integrated water vapor, integrated cloud liquid water, and the profiles of temperature and humidity were developed on the basis of artificial neural networks. Measurement examples, including comparisons with radiosondes, demonstrate the systems capabilities for high-resolution atmospheric monitoring.
The implementation of an operational network of microwave radiometers is presently hampered by th... more The implementation of an operational network of microwave radiometers is presently hampered by the cost and complexity of the available instruments. For this reason, the definition and design of a low-cost microwave radiometer suitable for automatic, high-quality observations of liquid water path (LWP) were one objective of the BALTEX cloud liquid water network: CLIWA-NET. In the course of the project, it turned out that a full profiling radiometer with 14 channels can be produced at only about 30% higher cost than a classical dual-channel IWV/LWP radiometer. The profiling capability allows simultaneous observations of LWP and the lower tropospheric (0-5 km) humidity and temperature profiles with a temporal resolution of less than 10 s and a vertical resolution from 100 m to 1 km in the planetary boundary layer depending on height and atmospheric conditions. The latter is possible due to an elevation scan capability and by the implementation of a new filter bank design. The radiometer has several additional sensors 0169-8095/$ -see front matter D (temperature, humidity, pressure, rain detector and GPS) which guarantee, together with a flexible software package, the operational performance of the system with maintenance intervals of about every 3 months. The performance of the first prototype has been verified during a 3-week campaign at Cabauw, The Netherlands. D
1] Using high-resolution oxygen A band spectrometry (l/Dl = 60000) in the 767.7-770.7 nm waveleng... more 1] Using high-resolution oxygen A band spectrometry (l/Dl = 60000) in the 767.7-770.7 nm wavelength range, we investigate the first and second moments of the distributions of path lengths of photons in transmitted skylight for different cloud conditions. Our observations are supported by measurements of column liquid water path by multichannel microwave radiometry, cloud structure by millimeter cloud radar observations, and cloud base by a laser ceilometer. For the investigated multilayer cloud covers (decks of stratus, cumulus, altostratus, and cirrus), our measurements indicate that the photon path statistics are mostly governed by anomalous diffusion, whereby classical diffusion occurs in the limiting case of a single compact (plane parallel) cloud layer. The ratio for the inferred second and first moments of the path lengths confirms the relation recently derived by for photon diffusion in single optically thick cloud layers and extends it to more complex cloud geometry. (2006), Path length distributions for solar photons under cloudy skies: Comparison of measured first and second moments with predictions from classical and anomalous diffusion theories,
The ldquoatmospheric propagation and profiling systemrdquo (ATPROP) advanced ground-based microwa... more The ldquoatmospheric propagation and profiling systemrdquo (ATPROP) advanced ground-based microwave radiometer, for radiowave propagation assessment at Ku, Ka, Q/V and W bands has been developed. The design of ATPROP is based on the requirements of SatCom, SatNav systems and Space Science Missions. ATPROP consists of two independent subsystems, operating at Ka band, near the 60 GHz oxygen absorption band and at 15/90 GHz. ATPROP has a full non-GEO satellite tracking capability and uses switched Dicke references to improve stability. Its performance allows to accurately derive atmospheric attenuation, sky noise, wet delay, and cloud, vapor and air temperature profiles.
Journal of Atmospheric and Oceanic Technology, 2001
A method for combining ground-based passive microwave radiometer retrievals of integrated liquid ... more A method for combining ground-based passive microwave radiometer retrievals of integrated liquid water (LWP), radar reflectivity profiles (Z ), and statistics of a cloud model is proposed for deriving cloud liquid water profiles (LWC). A dynamic cloud model is used to determine Z-LWC relations and their errors as functions of height above cloud base. The cloud model is also used to develop an LWP algorithm based on simulations of brightness temperatures of a 20-30-GHz radiometer. For the retrieval of LWC, the radar determined Z profile, the passive microwave retrieved LWP, and a model climatology are combined by an inverse error covariance weighting method. Model studies indicate that LWC retrievals with this method result in rms errors that are about 10%-20% smaller in comparison to a conventional LWC algorithm, which constrains the LWC profile exactly to the measured LWP. According to the new algorithm, errors in the range of 30%-60% are to be anticipated when profiling LWC. The algorithm is applied to a time series measurement of a stratocumulus layer at GKSS in Geesthacht, Germany. The GKSS 95-GHz cloud radar, a 20-30-GHz microwave radiometer, and a laser ceilometer were collocated within a 5-m radius and operated continuously during the measurement period. The laser ceilometer was used to confirm the presence of drizzle-sized drops.
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Papers by Ulrich Löhnert