Showing papers by "Matthias Wiegner published in 2004"

Aerosol lidar intercomparison in the framework of the EARLINET project. 2.Aerosol backscatter algorithms

Abstract: An intercomparison of aerosol backscatter lidar algorithms was performed in 2001 within the framework of the European Aerosol Research Lidar Network to Establish an Aerosol Climatology (EARLINET) The objective of this research was to test the correctness of the algorithms and the influence of the lidar ratio used by the various lidar teams involved in the EARLINET for calculation of backscatter-coefficient profiles from the lidar signals The exercise consisted of processing synthetic lidar signals of various degrees of difficulty One of these profiles contained height-dependent lidar ratios to test the vertical influence of those profiles on the various retrieval algorithms Furthermore, a realistic incomplete overlap of laser beam and receiver field of view was introduced to remind the teams to take great care in the nearest range to the lidar The intercomparison was performed in three stages with increasing knowledge on the input parameters First, only the lidar signals were distributed; this is the most realistic stage Afterward the lidar ratio profiles and the reference values at calibration height were provided The unknown height-dependent lidar ratio had the largest influence on the retrieval, whereas the unknown reference value was of minor importance These results show the necessity of making additional independent measurements, which can provide us with a suitable approximation of the lidar ratio The final stage proves in general, that the data evaluation schemes of the different groups of lidar systems work well

Air mass modification over Europe: EARLINET aerosol observations from Wales to Belarus

Abstract: [1] For the first time, the vertically resolved aerosol optical properties of western and central/eastern European haze are investigated as a function of air mass transport. Special emphasis is put on clean maritime air masses that cross the European continent from the west and become increasingly polluted on their way into the continent. The study is based on observations at seven lidar stations (Aberystwyth, Paris, Hamburg, Munich, Leipzig, Belsk, and Minsk) of the European Aerosol Research Lidar Network (EARLINET) and on backward trajectory analysis. For the first time, a lidar network monitored continent-scale haze air masses for several years (since 2000). Height profiles of the particle backscatter coefficient and the particle optical depth of the planetary boundary layer (PBL) at 355-nm wavelength are analyzed for the period from May 2000 to November 2002. From the observations at Aberystwyth, Wales, the aerosol reference profile for air entering Europe from pristine environments was determined. A mean 355-nm optical depth of 0.05 and a mean PBL height of 1.5 km was found for clean maritime summer conditions. The particle optical depth and PBL height increased with increasing distance from the North Atlantic. Mean summer PBL heights were 1.9–2.8 km at the continental sites of Leipzig, Belsk, and Minsk. Winter mean PBL heights were mostly between 0.7 and 1.3 km over the seven EARLINET sites. Summer mean 355-nm optical depths increased from 0.17 (Hamburg, northwesterly airflow from the North Sea) and 0.21 (Paris, westerly flow from the Atlantic) over 0.33 (Hamburg, westerly flow) and 0.35 (Leipzig, westerly flow) to 0.59 (Belsk, westerly flow), and decreased again to 0.37 (westerly flow) at Minsk. Winter mean optical depths were, on average, 10–30% lower than the respective summer values. PBL-mean extinction coefficients were of the order of 200 Mm−1 at 355 nm at Hamburg and Leipzig, Germany, and close to 600 Mm−1 at Belsk, Poland, in winter for westerly flows. Whereas the optical depth for westerly flows was typically <0.35 during the summer halfyear, it increased to values of 0.5–0.7 over most of the central European sites during easterly flows. Compared to aerosol sources in Poland and southeastern Europe, the highly industrialized and populated western European region was found to contribute only moderately to the European aerosol burden. Comparably clean conditions (low particle optical depth) prevailed at the Munich site, indicating a sensitive influence of the orography on haze conditions. Estimates of the mean effective (upward minus downward) aerosol mass flux into the atmosphere along the way from the Atlantic Ocean to central Europe (Leipzig), and from Leipzig to Belsk, that are consistent with the optical depth increase, yield values of 0.11–0.17 μg/(m2s) and 0.25 μg/(m2s), respectively. These values correspond to mean effective aerosol mass fluxes for 50 km × 50 km grid cells of the order of 10000 Mg/year and 20000 Mg/year, respectively. The estimates are in reasonable agreement with EMEP emission data.

Saharan Dust Outbreaks Towards Europe: 3 Years of Systematic Observations by the European LIDAR Network in the Frame of the Earlinet Project (2000-2003)

Abstract: During a three-year period (2000-2003) 90 Saharan dust outbreaks over Europe were monitored by a coordinated aerosol lidar Network in the frame of the EARLINET project. Multiple aerosol layers of variable thickness (200-2500 m) were systematically observed in the altitude region 1.5-8 km height asl., while traces of dust particles reached heights of 8-10 km, after a 2-5 days transport from the source region. Aerosol optical depths, depolarization ratios and extinction-to-backscatter ratios (lidar ratios) of aerosols ranged from 0.3-0.8 (at 355 nm or 351 nm), 10 to 25 % and 35 to 80 sr, respectively, within the lofted dust plumes. Air mass back-trajectory analysis and model calculations from the DREAM model, in conjunction with satellite data analysis (MODIS, TOMS, SeaWiFS), related the measurements during these dust events to the Saharan region. The vertical and horizontal extent of these outbreaks is discussed and their seasonal distribution is given. 1. INTRODUCTION Atmospheric particles and mainly the mineral dust particles, play an important role in the earth’s radiation balance and climate, by scattering and absorbing, both incoming and outgoing radiation [1,2]. Every year very large (million of tons) quantities of desert dust from the Sahara and surrounding regions are exported to the North Atlantic Ocean and the Mediterranean Sea [3-5]. Up to 2000 the Saharan dust events had been more systematically studied over the Mediterranean region using satellite data [6], thus up to that year, only a few publications existed on the vertical distribution of desert dust particles over Europe [7-8]. Since 2000, the EARLINET project provided the first systematic aerosol lidar observations of Saharan dust aerosols over the European continent [9-13], on a coherent network basis. Dust model simulations were found to be consistent with the lidar network observations [11,14].

Long-Range Transport of Saharan Dust Over Europe Observed by LIDAR and Sun Photometers (earlinet, Aeronet) and Satellite Observations

Abstract: A strong dust outbreak over western Europe has been documented by means of lidar and photometer networks and satellite observations (TOMS, METEOSAT, MODIS). We observe distinct layers versus height above different lidar stations with high optical thickness. Depending on their height, these layers present very different properties. A main layer owed a high lidar depolarisation ratio up to 20% and very low angstrom coefficient near 0. Other layers up to 6km showed more mixed aerosol characteristics. We linked this layers and characteristics with the source regions south of Atlas mountain and in the Hoggar area, found from backtrajectories analysis,

Potential of lidars for aerosol remote sensing

Abstract: Measurements of aerosols are urgently required for understanding and modelling their role in the climate system and to investigate interactions between aerosols, clouds and radiation. Lidar is an excellent tool for aerosol observations, in particular, as it provides range resolved data. In this paper we briefly describe the different lidar configurations useful for aerosol observations and discuss in particular the limitations due to the unknown lidar ratio and strategies to overcome this problem. As a result, extinction coefficient as a function of height can be obtained. Recent approaches to derive microphysical parameters are introduced as well. The potential of lidars for aerosol remote sensing is illustrated by highlighting a few of the most important lidar activities of the last years and the upcoming spaceborne experiments.