Marc Schröder

Airborne measurements of areal spectral surface albedo over different sea and land surfaces

[1]xa0Airborne measurements of the ratio of spectral upward and downward irradiances (so-called spectral albedo) are used to derive the areal spectral surface albedo in the wavelength range from 330 to 1670 nm. The data were collected over different sea and land surfaces in cloudless atmospheric conditions during three field campaigns. Measurements from the Albedometer (developed at IfT) and the NASA Solar Spectral Flux Radiometer (SSFR) are employed. Spectral radiative transfer calculations show that atmospheric scattering and absorption within the layer beneath the flight level considerably contribute to the airborne albedo measurements reported here, even for low flight altitudes (0.2–0.5 km). To remove this atmospheric masking, a nonlinear extrapolation of the airborne albedo measurements to the ground is performed. The nonlinearity is due to the vertically inhomogeneous distribution of the particle microphysical properties. This fact underlines the importance of aerosol profile measurements for the proper correction of atmospheric masking. Examples of the extrapolated areal spectral surface albedos are discussed in terms of their solar zenith angle dependence, their small-scale, and general variability. Finally, typical areal spectral surface albedos for different sea and land surfaces, as derived from the three measurement campaigns, are supplied in parameterized form for use in radiative transfer applications.

THE BALTEX BRIDGE CAMPAIGN: An Integrated Approach for a Better Understanding of Clouds

Clouds affect our daily life in many ways. They dominate our perception of weather and, thus, have an enormous influence on our everyday activities and our health. This fact is completely at odds with our knowledge about clouds, their representation in climate and weather forecast models, and our ability to predict clouds. It is their high variability in time and space that makes clouds both hard to monitor and to model. Clouds are the major concern in the climate modeling community, as stated by the Intergovernmental Panel on Climate Change (IPCC; information available online at www.ipcc.ch) x93the most urgent scientific problems requiring attention to determine the rate and magnitude of climate change and sea level rise are the factors controlling the distribution of clouds and their radiative characteristics.x94 A similar conclusion was obtained within the Atmospheric Model Intercomparison Project (AMIP; e.g., Gates et al. 1999). The great challenge of climate research is to correctly account for the fact that the global state of our climate system is largely driven by various small-scale processes and their interaction with each other. Clouds are the most visible examples of this situation. On a global scale, clouds have a strong cooling effect on our climate: more solar radiation is reflected back to space than thermal surface radiation is trapped in the atmosphere. However, because radiation reacts on the instantaneous cloudy atmosphere and not on some climatological mean, the physical processes leading to the overall radiative effect strongly depend on the spatial distribution and structure of clouds.

Determination of three‐dimensional cloud structures from high‐resolution radiance data

[1]xa0The three-dimensional structure and the inhomogeneity of clouds pose a field of challenges. The characterization of their spatial structure, their microphysical properties, and their variability is difficult. This kind of knowledge is crucial to any investigation on the impact of clouds on the radiation budget or on the reliability of cloud remote sensing data. In this article the characteristics of radiation transport in inhomogeneous clouds are studied using three-dimensional (3-D) simulations of radiative transport and the independent pixel approximation (IPA). The opposing effects of radiative smoothing and sharpening due to horizontal photon transport are examined in terms of the Greens function, which describes the interrelation of the radiance fields calculated using IPA and 3-D radiative transport. On the basis of these considerations a novel method was developed for the retrieval of realistic 3-D stratocumulus structures from high-spatial-resolution radiance fields observed by a compact airborne spectrographic imager (CASI, 15 m resolution). An initial distribution of liquid water content and effective droplet size retrieved using the IPA assumption and an adiabatic microphysical model is iteratively adjusted with the objective of matching the observation by the 3-D forward radiative transfer simulation for the derived cloud. For the iterative adjustment an approximate Greens function is utilized to remove 3-D effects from the observation. The performance of the method is characterized by application to a known cloud structure and by comparison of the derived cloud properties to in situ data from various field campaigns. The method provides the ideal basis for our studies on the remote sensing of inhomogeneous clouds.

The GEWEX Water Vapor Assessment: Results from Intercomparison, Trend, and Homogeneity Analysis of Total Column Water Vapor

AbstractThe Global Energy and Water Cycle Exchanges project (GEWEX) water vapor assessment’s (G-VAP) main objective is to analyze and explain strengths and weaknesses of satellite-based data records of water vapor through intercomparisons and comparisons with ground-based data. G-VAP results from the intercomparison of six total column water vapor (TCWV) data records are presented. Prior to the intercomparison, the data records were regridded to a common regular grid of 2° × 2° longitude–latitude. All data records cover a common period from 1988 to 2008. The intercomparison is complemented by an analysis of trend estimates, which was applied as a tool to identify issues in the data records. It was observed that the trends over global ice-free oceans are generally different among the different data records. Most of these differences are statistically significant. Distinct spatial features are evident in maps of differences in trend estimates, which largely coincide with maxima in standard deviations from t...

Comparison of precipitation in the regional climate model BALTIMOS to radar observations

Observational data and simulations of the regional climate system Baltic integrated model system (BALTIMOS) were used to study precipitation in the Baltic Sea and its drainage basin with a special focus on the diurnal cycle. The study includes a general evaluation of BALTIMOS precipitation, showing that BALTIMOS has too many light rain events causing an overestimation of the total annual precipitation amount. The diurnal cycle as well as its spatial distribution was analysed. BALTIMOS captures the broad characteristics: a significant diurnal variability with an afternoon peak above land and weak variability with a nocturnal peak above sea. An algorithm to distinguish between frontal and convective precipitation was applied to examine the diurnal cycle more thoroughly. The local solar time of maximum rain in summer is about 1 to 2xa0h earlier in BALTIMOS than in radar observations of precipitation.

Validating precipitation forecasts using remote sensor synergy: A case study approach

Several types of remote sensing data are applied synergistically to evaluate the chain of microphysical processes leading to precipitation in a high-resolution numerical weather prediction model. The data provides information relating to (i) cloud-top temperature and optical depth (SEVERI), (ii) ice cloud amount (AMSUB), (iii) type and amount of precipitation particles (polarimetric radar), and (iv) surface precipitation (raingauge-calibrated radar data). Forecasts are produced by the COSMO-DE model of the German weather service, with a horizontal resolution of 2.8 km. The comparison with data is done in a model-to-observation framework, that is, forward operators are applied to the model output to produce synthetic data sets that can be directly compared to the observations. Additional diagnostics based on diurnal cycle and system tracking are also considered. Two case studies over Germany from the summer of 2006 are examined. The first case is dominated by widespread stratiform precipitation. Together the various data sets show that the model overestimates the amount of high cloud, while underestimating the concentration of ice scatterers and overestimating reflectivity and differential reflectivity (ZDR). This indicates errors in both the amount and the size distributions of cloud and precipitation particles in the model’s microphysical parameterization. In the second case a narrow band of convective precipitation is embedded in a cold front, with significant modulation by the diurnal cycle. The model fails to show a significant diurnal cycle in cloud amount, and the timing and duration of convective cells is incorrect. In this case, both the microphysical parameterization, and errors in the interaction of the simulated front with the orography of the Alps appear to contribute. These results demonstrate the potential of combinations of remote sensing data for model evaluation, although a long-term trial will be required to determine whether the errors seen in the case studies are characteristic for COSMO-DE. Zusammenfassung Verschiedene Fernerkundungsdaten werden synergetisch zur Evaluierung der mikrophysikalischen Prozesskette der Niederschlagsgenerierung in einem hochaufgelosten numerischen Wettervorhersagemodell genutzt. Die Daten beinhalten Informationen uber (i) Wolkenobergrenzentemperatur und optische Dicke (SEVIRI), (ii) Vorkommen von Eis und Schnee (AMSU), (iii) Art und Gehalt an Niederschlagspartikeln (polarimetrisches Radar) und (iv) Bodenniederschlag (Radar mit Niederschlagssammler kalibriert). Die Vorhersagen stammen vom COSMO-DE des Deutschen Wetterdiensts mit einer horizontalen Auflosung von 2,8 km. Der Vergleich wird mittels eines “Modell zu Beobachtung”-Ansatzes durchgefuhrt, d. h. Vorwartsoperatoren werden auf den Modelloutput angewendet, um synthetische Beobachtungen zu generieren, die direkt mit den Beobachtungen verglichen werden konnen. Zusatzliche Diagnostiken basierend auf Tagesgang und Verfolgung einzelner Konvektionszellen werden ebenfalls angewendet. Zwei Fallstudien uber Deutschland aus dem Sommer 2006 werden untersucht. Die erste Fallstudie wird von ausgedehntem Niederschlag dominiert. Die verschiedenen Daten zeigen, dass das Modell den Anteil hoher, stratiformer Wolken uberschatzt, wahrend die Konzentration groserer streuender Eispartikel unter- und die bodennahe Reflektivitat und die differentielle Reflektivitat (ZDR) uberschatzt werden. Dies deutet auf Fehler in den mikrophysikalischen Parameterisierungen sowie in den Annahmen zur Grosenverteilung von Wolken- und Niederschlagspartikeln im Modell hin. In der zweiten Fallstudie ist ein enges Band konvektiven Niederschlages eingebettet in eine Kaltfront, mit signifikanter Modulation durch den Tagesgang. Dem Modell gelingt es nicht einen deutlichen Tagesgang in der Wolkenbedeckung zu reproduzieren und Einsetzen sowie Lebensdauer der konvektiven Zellen werden nicht getroffen. In diesem Fall scheinen sowohl die mikrophysikalischen Parametrisierungen als auch die Wechselwirkung der Front mit der Orographie der Alpen zum Fehler der Vorhersage beizutragen. Diese Resultate zeigen das Potential der Kombination verschiedener Fernerkundungsdaten fur die Modellevaluierung. Um zu uberprufen, dass diese Fehler charakteristisch fur das COSMO-DE sind, ist jedoch eine langfristige Vergleichsstudie notig.

ESA DUE GlobVapour water vapor products: Validation

The main objective of the European Space Agency (ESA) Data User Element (DUE) GlobVapour project was the development of multi-annual global water vapor data sets. Since water vapour is a key climate variable it is important to have a good understanding of its behavior in the climate system. The ESA DUE GlobVapour project provides water vapor data, including error estimates, based on carefully calibrated and inter-calibrated satellite radiances in response to user requirements for long time series satellite observations. ESA DUE GlobVapour total columnar water vapor (TCWV) products derived from GOME/SCIA/GOME-2 (1996-2008) and SSM/I+MERIS (2003-2008) have been validated for the mentioned period, using satellite-based (AIRS, ATOVS) and ground-based measurements (radiosondes and microwave radiometer). The validation results are discussed in the following. The technical specifications on bias (1 kg/m2 for SSMI+MERIS and 2 kg/m2 for GOME/SCIA/GOME-2) are generally met. For more information, documents and data ...