GACP Projects
GACP ACCOMPLISHMENT REPORT
Beate G. Liepert
Lamont-Doherty Earth Observatory of Columbia University
Palisades, New York
Surface Solar Radiation Database
Abstract: The main goal of this study is to address the variability of the direct and indirect aerosol effect for the time period of space-based monitoring. We analyze historic high-quality records of broad band solar radiation measurements worldwide. The data are on an hourly basis, which enables us to separate clear, partly cloudy and overcast sky conditions and stratify the data by solar zenith angle categories to get the diurnal cycle. Most records that are analyzed span the time interval from mid 1960s to 1990. The direct aerosol effect and its temporal variability are investigated with clear sky climatologies from diurnal to decadal time scale. The indirect aerosol effect is analyzed with cloud cover, precipitation and cloudy sky radiation climatologies. Comparisons with modeled solar radiation data based on different aerosol scenarios provide a tool for validating aerosol climatologies of the past 30 years. Together with the improved satellite retrievals and model calculations the investigations will enable more realistic aerosol climatologies which are needed for global climate change studies.
Goals: We assess the direct and indirect aerosol effect on global climate and its temporal variability by studying surface solar radiation of selected historic records all over the world. We produce climatologies of broadband solar radiation for various cloud conditions for the time period of three decades and investigate their temporal changes in respect to changing aerosol concentrations. These climatologies will be compared with results from GCM experiments and satellite data, though they provide a ground based independent tool for the validation of candidate aerosol climatologies and the temporal changes within the last two to three decades.
Objectives: The objective of GACP is the quantitative assessment of radiative forcing caused by atmospheric aerosols during the 20-year period of satellite observations. This requires global satellite retrievals of sufficient accuracy as well as model calculations of key aerosol properties. Additional independent information on aerosol radiative forcing is needed to constrain the retrieved and calculated aerosol properties and its variability especially over land. To serve this need we develop and analyze a broadband solar radiation database specifically designed for comparisons with global climate models and satellite data. The indirect and direct aerosol radiative effect on a 30 years time scale will be addressed. We focus on comparisons of climatologies in contrast to single field experiments. Aerosol transport models and temporarily changing emission scenarios will be tested with these analyzed data resulting in more realistic aerosol climatologies which are needed for global climate change studies.
Approach: We assess the aerosol radiative effect and its changes over the last three decades by analyzing historic ground based solar radiation records over land.
Data Acquisition: Total broadband solar radiation measurements exist since the early 1960s. The World Climate Research Program has established the Baseline Surface Radiation Network (BSRN) which collects current high quality solar radiation measurements. Historical data are also available on a monthly resolution from the Global Energy Balance Archive (GEBA) worldwide. Long-term recordings with continuous calibration histories exist for several sites worldwide, which are selected for our applications. The existing surface solar radiation network has a poor density but on the other hand, covers the land areas where satellite retrievals of aerosol properties are still inadequate. Some of these records are available on an hourly basis, which enables us to produce solar radiation climatologies of clear, partly cloudy and overcast sky conditions for the selected sites. Statistical analyses will be performed with these data.
Model vs. Observation Comparisons: We compare solar radiation climatologies with various GCMs. One comparison is performed with the ECHAM4 model utilized by Lohmann et al. who investigates the indirect aerosol effect. In another study we compare the change over time of the direct aerosol effect modeled with the GISS-GCM. Various emission scenarios of the last 30 years will be investigated. The temporal varying aerosol climatologies are produced by Tegen et al.
Comparisons of the Derived Climatologies with Intensive Field Campaigns: Field campaigns provide a large variety of high-resolution measurements of aerosol and radiative parameters, whereas the climatological database of solar radiation constrains the overall long-term variability of a few key elements. Therefore we use intensive field campaigns to explain possible variations uncovered in the solar radiation climatologies. The strategy is to test the hypothesis drawn from the climatologies with specific case studies from intensive field campaigns.
Comparisons with Satellite Data: Satellite derived climatologies of TOA solar fluxes for clear and overcast conditions will be compared with surface solar radiative fluxes to assess the atmospheric solar absorption. Potential other comparison strategies will be developed in cooperation with other members of GACP.
Tasks Completed: The surface solar radiation database for the US has been analyzed. These observations were used to validate the indirect aerosol effect modeled with the ECHAM4-GCM. Results of two model experiments - one with a pre-industrial and one with a doubled present-day sulfate load - were compared with long-term climatologies of cloud amount, precipitation rates and surface solar radiation for nine regions in the US. The results have been summarized in a paper (see bibliography) accepted in J. Climate.
The GEBA archive has been searched for long-term surface solar radiation observations worldwide. 95 records spanning the interval from 1960 to 1990 have been statistically analyzed. The results will be published together with more detailed analysis of the US solar radiation database.
Future Plans:
In the third year we will finish trend analysis of the US solar radiation climatologies and publish the results. We also plan to analyze the long-term Canadian solar radiation and cloud cover recordings.
More analytical studies of GCM experiments are planned. We want to focus on the modeled solar fluxes and cloud coverage for a variety of aerosol climatologies. Comparisons of our surface radiation data set, ERBE TOA solar irradiance and model climatologies of solar fluxes are therefore planned. The experiments used to study the indirect aerosol effect are performed with ECHAM4 provided by U. Lohmann. The investigation of the change of the aerosol effect over the last 50 year will be performed with the GISS-GCM.
We also plan to establish and intensify cooperation with the other GACP members who might want to use our processed database.
Results: The results of the second year of this project are summarized in the paper Liepert and Lohmann, "A comparison of surface observations and ECHAM4-GCM experiments and its relevance to the indirect aerosol effect" accepted in J. Climate. Model results and the observations of solar irradiance, precipitation and cloud cover for nine regions of the US and Germany has been compared. Two model experiments with a pre-industrial and a present-day sulfate aerosol load were used to simulate the aerosol effect on clouds and radiation. The study shows underestimated solar radiation mainly under cloudy cases whereas the clear sky solar radiation is adequately simulated. Deficiencies in the land parameterization scheme have been detected and the importance of an adequate parameterization of the seasonality of the land surface has been stressed.
The statistical analysis of long-term global radiation records from the GEBA archive shows a decreasing tendency over the time period from the 1960s to the 1980s. A more detailed analysis has been performed with the US solar radiation database. The data have been separated into clear, cloudy and overcast sky conditions. Clear sky solar radiation decreased by 8W/m2 from the 1960s to the 1980s whereas the overcast reduction was 18W/m2. These results will be compared with the GISS-GCM simulations of the past 30 years.
GACP Significant Highlights
Beate G. Liepert and Ulrike Lohmann
A COMPARISON OF SURFACE OBSERVATIONS AND ECHAM4-GCM EXPERIMENTS AND ITS RELEVANCE TO THE INDIRECT AEROSOL EFFECT
Abstract: The observations of solar irradiance at the surface, total cloud cover and precipitation rates have been used to evaluate aerosol-cloud-interactions in a GCM. Records from Germany and US were available for the time period from 1985 to 1990 and 1960 to 1990. The model used here is the ECHAM4 GCM run for a 5-year period with a fully coupled sulfur chemistry - cloud scheme (Lohmann and Feichter, 1997). We studied two experiments - one with an annual mean sulfate load of 0.36Tg S for the pre-industrial simulation and one with 1.05Tg S for the present day simulation.
Our goal was to indirectly confirm the existence of the indirect aerosol effect by finding indices for a better agreement of observations with the present day experiment compared to the pre-industrial experiment. We were able to draw such a conclusion only for the German data but not for the United States. The model correctly predicts the annual mean total cloud cover in Germany and the US, whereas global solar radiation is underestimated by 13W/m2. This deficiency stems from cloudy conditions. Clouds are either optically too thick or the vertical distribution of clouds is erroneous. This is confirmed by the modeled overcast solar irradiance, which is 27W/m2 lower than observed whereas for the clear sky model the observations agree. Precipitation rates are underestimated by 42% in the United States. The seasonal cycle of the precipitation rate is incorrect in all US regions. The modeled cloud cover is too low over the Central United States in July and August and consequently the solar irradiance exceeds the observations during these months. The opposite occurs in winter when the model overestimates the cloud cover and thus underestimates solar irradiance. We suggest the non-seasonality of vegetation and soil parameters as possible causes for these deficiencies. The convective precipitation formation might also contribute to these discrepancies.
On the other hand, this drying out effect of the inner continent is not as pronounced in coastal regions and in particular, the comparisons for the German grid-box provide indications for the validity of the indirect aerosol effect. The modeled annual cloud cover and solar radiation cycles for the present day aerosol load are in better agreement with observations. Furthermore, the model shows an interesting shift from low cloud reduction to cirrus formation in spring as a consequence of the indirect aerosol effect, a result, which is confirmed by observational data.
Future Plans
Beate G. Liepert
Lamont-Doherty Earth Observatory of Columbia University
Palisades, New York
Research Plan for the Third Year For the third year, further comparisons of surface observations and model results are planned to investigate the aerosol radiative effect.
We want to focus on explaining the observed decrease in solar radiation under overcast cloudiness (see results). Climatologies of the ECHAM simulations of the indirect aerosol effect will be produced and compared to observations. U. Lohmann, who will provide the data, will perform the new ECHAM experiments. These runs include experiments with organic and black carbon aerosols, sea salt besides sulfate aerosols. We anticipate that the new data are closer to the observed climatology.
Secondly, an analysis of the GISS-GCM model run simulating the climate change over the past 30 years is planned. We want to focus on the aerosol effect of these model runs. These experiments are performed with several different aerosol scenarios, meaning differently temporally varying aerosol climatologies. The surface solar radiation database will provide a basis for this study. One goal will be to indirectly assess the right tendency of the aerosol effect on solar radiation. Furthermore we anticipate using the TOA solar fluxes from ERBE and the surface solar fluxes at clear skies to restrain aerosol absorption in the atmosphere.
Cooperation with P. Stackhouse who is the PI of a similar project of the GACP will be established to deliver the data to NASA archives and make them publicly accessible.
GACP BIBLIOGRAPHY
List of Publications
Liepert, B., and U. Lohmann, 2000: A comparison of surface observations and ECHAM4-GCM experiments and its relevance to the indirect aerosol effect. J. Climate, accepted.
b) List of Presentations
Liepert, B., and U. Lohmann, 1999: A comparison of surface observations and ECHAM4-GCM experiments and its relevance to the indirect aerosol effect. AGU 1999 Fall Meeting, December 13-17, 1999, San Francisco, CA.
