Hans Gilgen

Baseline Surface Radiation Network (BSRN/WCRP) : New precision radiometry for Climate Research

To support climate research, the World Climate Research Programme (WCRP) initiated a new radiometric network, the Baseline Surface Radiation Network (BSRN). The network aims at providing validation material for satellite radiometry and climate models. It further aims at detecting long-term variations in irradiances at the earth’s surface, which are believed to play an important role in climate change. The network and its instrumentation are designed 1) to cover major climate zones, 2) to provide the accuracy required to meet the objectives, and 3) to ensure homogenized standards for a long period in the future. The limits of the accuracy are defined to reach these goals. The suitable instruments and instrumentations have been determined and the methods for observations and data management have been agreed on at all stations. Measurements of irradiances are at 1 Hz, and the 1-min statistics (mean, standard deviation, and extreme values) with quality flags are stored at a centralized data archive at the WCRP’s World Radiation Monitoring Center (WRMC) in Zurich, Switzerland. The data are quality controlled both at stations and at the WRMC. The original 1-min irradiance statistics will be stored at the WRMC for 10 years, while hourly mean values will be transferred to the World Radiation Data Center in St. Petersburg, Russia. The BSRN, consisting of 15 stations, covers the earth’s surface from 80°N to 90°S, and will soon be joined by seven more stations. The data are available to scientific communities in various ways depending on the communication environment of the users. The present article discusses the scientific base, organizational and technical aspects of the network, and data retrieval methods; shows various application possibilities; and presents the future tasks to be accomplished.

Means and Trends of Shortwave Irradiance at the Surface Estimated from Global Energy Balance Archive Data

Abstract Means and trends of shortwave irradiance at the earth’s surface are calculated from pyranometer measurements stored in the Global Energy Balance Archive (GEBA) database. The GEBA database contains the most comprehensive set of shortwave irradiance monthly means. The relative random error of measurement is approximately 5% of a monthly mean in general and approximately 2% of a yearly mean. The shortwave irradiance yearly means are analyzed in a 2.5° × 2.5° grid. In average example grid cells in Europe (no large altitude differences, no coasts), the difference of shortwave irradiance yearly means measured at different stations (station effect) is less than 5% of the cell mean, and the interannual variability is approximately 4% of the cell mean. On most continents, shortwave irradiance decreases significantly in large regions, and significant positive trends are observed only in four small regions.

Validation of General Circulation Model Radiative Fluxes Using Surface Observations

Abstract The surface radiative fluxes of the ECHAM3 General Circulation Model (GCM) with T2 1, T42, and T 106 resolutions have been validated using observations from the Global Energy Balance Archive (GEBA, World Climate Program-Water Project A7). GEBA contains the most comprehensive dataset now available for worldwide instrumentally measured surface energy fluxes. The GCM incoming shortwave radiation at the surface has been compared with more than 700 long-term monitoring stations. The ECHAM3 models show a clear tendency to overestimate the global annual-mean incoming shortwave radiation at the surface due to an underestimation of atmospheric absorption. The model-calculated global-mean surface shortwave absorption around 165 W m−2 is estimated to be too high by 10–15 W m−2. A similar or higher overestimate is present in several other CYCMS. Deficiencies in the clear-sky absorption of the ECHAM3 radiation scheme are proposed as a contributor to the flux discrepancies. A stand-alone validation of the radi...

Evaluation of Downward Longwave Radiation in General Circulation Models

Abstract The longwave radiation emitted by the atmosphere toward the surface [downward longwave radiation (DLR)] is a crucial factor in the exchange of energy between the earth surface and the atmosphere and in the context of radiation-induced climate change. Accurate modeling of this quantity is therefore a fundamental prerequisite for a reliable simulation and projection of the surface climate in coupled general circulation models (GCM). DLR climatologies calculated in a number of GCMs and in a model in assimilation mode (reanalysis) are analyzed using newly available data from 45 worldwide distributed observation sites of the Global Energy Balance Archive (GEBA) and the Baseline Surface Radiation Network (BSRN). It is shown that substantial biases are present in the GCM-calculated DLR climatologies, with the GCMs typically underestimating the DLR (estimated here to be approximately 344 W m−2 globally). The biases are, however, not geographically homogeneous, but depend systematically on the prevailing ...

Regional climate simulation with a high resolution GCM: surface radiative fluxes

The ability of a high resolution (T106) version of the ECHAM3 general circulation model to simulate regional scale surface radiative fluxes has been assessed using observations from a new compilation of worldwide instrumentally-measured surface fluxes (Global Energy Balance Archive, GEBA). The focus is on the European region where the highest density of observations is found, and their use for the validation of global and regional climate models is demonstrated. The available data allow a separate assessment of the simulated fluxes of surface shortwave, longwave, and net radiation for this region. In summer, the incoming shortwave radiation calculated by the ECHAM3/T106 model is overestimated by 45 W m−2 over most of Europe, which implies a largely unrealistic forcing on the model surface scheme and excessive surface temperatures. In winter, too little incoming shortwave radiation reaches the model surface. Similar tendencies are found over large areas of the mid-latitudes. These biases are consistent with deficiencies in the simulation of cloud amount, relative humidity and clear sky radiative transfer. The incoming longwave radiation is underestimated at the European GEBA stations predominantly in summer. This largely compensates for the excessive shortwave flux, leading to annual mean net radiation values over Europe close to observations due to error cancellation, a feature already noted in the simulated global mean values in an earlier study. Furthermore, the annual cycle of the simulated surface net radiation is strongly affected by the deficiencies in the simulated incoming shortwave radiation. The high horizontal resolution of the GCM allows an assessment of orographically induced flux gradients based on observations from the European Alps. Although the model-calculated and observed flux fields substantially differ in their absolute values, several aspects of their gradients are realistically captured. The deficiencies identified in the model fields are generally consistent at most stations, indicating a high degree of representativeness of the measurements for their larger scale setting.

Gridded temperature and accumulation distributions for Greenland for use in cryospheric models

Abstract Gridded distributions of the annual mean temperature and annual total accumulation for Greenland are presented. They are objectively derived by deterministic and statistical interpolation from measurements at coastal stations and at locations on the ice sheet. They can be used not only for driving cryospheric models, but also for verifying atmospheric simulations.

The distribution of solar energy at the Earth's surface as calculated in the ECMWF re‐analysis

Knowledge of the distribution of solar energy at the earths surface is required for various practical and theoretical purposes. The solar fluxes calculated in Re-Analysis projects are promising since their radiative transfer calculations take into account the best current estimates of the humidity and temperature structure in the atmosphere. The present study assesses the quality of the insolation climatology of the ECMWF Re-Analysis (ERA) using a comprehensive set of surface observations. It is shown that the ERA surface insolation climatology is in good agreement with more than 700 long-term monitoring sites. The biases are substantially smaller than generally found in GCMs, which typically overestimate the surface insolation due to an underestimated atmospheric shortwave absorption. Contributing to the quality of the ERA insolation climatology is not only the realistic structure of the overlying atmosphere, but also the accurate performance of the radiation scheme under clear-sky conditions. This is shown in off-line calculations with the ERA radiation scheme using prescribed atmospheric profiles from radiosondes as input. Overall, the paper suggests that the ERA reproduces the global distribution of solar energy at the surface with considerable skill and that the ERA insolation climatology may be useful for various applications.

Spatial data reallocation based on multidimensional range queries. A contribution to data management for the earth sciences

Earth scientists, by definition, work in an interdisciplinary environment and therefore collect and disseminate data using distinct methods, depending on whether the associated information arises during field measurements, arrives via remote sensing, or represents simulation results. When calculating the global radiation over one of the oceans, for instance, it is essential that cloud data from at least three different sources can be accessed based on one and the same geographic grid. Differently gridded source data must therefore be reorganized through interpolation for example. This can be done in different ways and we investigate a method whose practicality depends on fast spatial range queries, but in turn provides a flexibility that makes it particularly easy to accommodate differently organized data sets-a valuable feature when unanticipated grid organizations show up.<<ETX>>

Spatial Reallocation of Global Gridded Climate Datasets

Global climatological data are often evaluated in grids with perpendicular boundaries in a cylinder projection of the longitude and latitude circles of the earth. Typical datasets contain 106 to 108 values. Each dataset is organized in a different grid, according to the authorss preferences. When calculating the global radiation over the ocean, for example, it is essential that cloud data from at least three different sources can be accessed based on one grid. One approach is to interpolate data from several datasets, each with a different grid, and combine them into a single dataset with an arbitrary grid.