Werner Wergen

The atmospheric dynamics mission for global wind field measurement

The prime aim of the Atmospheric Dynamics Mission is to demonstrate measurements of vertical wind profiles from space. Extensive studies conducted by the European Space Agency over the past 15 years have culminated in the selection of a high-performance Doppler wind lidar based on direct-detection interferometric techniques. Such a system, with a pulsed laser operating at 355-nm wavelength, would utilize both Rayleigh scattering from molecules and Mie scattering from thin cloud and aerosol particles; measurement of the residual Doppler shift from successive levels in the atmosphere provides the vertical wind profiles. The lidar would be accommodated on a satellite flying in a sun-synchronous orbit, at an altitude of ~400 km, providing near-global coverage; target date for launch is in 2007. Processing of the backscatter signals will provide about 3000 globally distributed wind profiles per day, above thick clouds or down to the surface in clear air, at typically 200-km separation along the satellite track...

The WGNE Assessment of Short-term Quantitative Precipitation Forecasts

Abstract Twenty-four-hour and 48-h quantitative precipitation forecasts (QPFs) from 11 operational numerical weather prediction models have been verified for a 4-yr period against rain gauge observations over the United States, Germany, and Australia to assess their skill in predicting the occurrence and amount of daily precipitation. Model QPFs had greater skill in winter than in summer, and greater skill in midlatitudes than in Tropics, where they performed only marginally better than “ persistence.” The best agreement among models, as well as the best ability to discriminate raining areas, occurred for a low rain threshold of 1–2 mm d−1. In contrast, the skill for forecasts of rain greater than 20 mm d−1 was generally quite low, reflecting the difficulty in predicting precisely when and where heavy rain will fall. The location errors for rain systems, determined using pattern matching with the observations, were typically about 100 km for 24-h forecasts, with smaller errors occurring for the heaviest r...

The use of GPS measurements for water vapor determination

Abstract A workshop on the use of Global Positioning System (GPS) measurements in weather and climate with emphasis on water vapor determination, was organized by the National Environmental Research Councils (NERC) Environmental Systems Science Centre (ESSC), at the University of Reading, Reading, United Kingdom, and took place there 29–31 August 2001. This paper gives a broad overview and general background of the use of GPS data for weather and climate. It outlines the objectives of the workshop and presents ongoing national, regional, and international activities both for ground-based and satellite-based systems. This includes work in the United States, China, and Japan, and different European efforts, including activities under European Community programs. Data assimilation of GPS data for weather prediction and climate is discussed as are ways in which to develop GPS-based systems to become an integrated part of the World Weather Watch. This includes ways of systematically using GPS data from the in...

Impact of profile observations on the German Weather Service's NWP system

In preparation for a study on the potential impact of a space-borne Doppler wind lidar on the quality of NWP products, a series of assimilations and forecasts were conducted to estimate the potential benefit of conventional wind and temperature profile measurements over North America to numerical weather forecasts for the Northern Hemisphere and specifically, Europe. A comparison of the forecast quality of a control run, using all available observations, to experiments omitting wind and temperature data from specific instruments (radiosondes, pilot stations and aircraft) makes it possible to estimate the importance of the omitted data, and clarify whether winds derived from the geostrophic relation are sufficient or whether observed wind profiles result in a more realistic definition of the initial state for numerical weather prediction systems in the extratropic regions. Very little impact on forecast quality was noted when wind or temperature observations from radiosondes and pilots were excluded from the assimilation process. However, a clear deterioration in forecast quality was observed when additionally all available wind or temperature measurements from aircraft were also withheld. Comparisons of the relative utility of wind and temperature observations over North America show that assimilations and forecasts derive more benefit from wind data than from temperature data. The greatest deterioration could be observed if both wind and temperature observations were omitted from the assimilation cycle. By tracing the differences between the control forecasts and the experimental forecasts to their initial difference, the regions around Hudson Bay. Novia Scotia, Buffin Bay and Northern Canada could be identified as sensitive areas, i.e. those where a missing observation could have a substantial effect on the forecast for the Northern Hemisphere and Europe. Comparisons of the relative utility of radiosonde wind and temperature observations over Canada and Alaska to numerical weather forecast quality, in contrast to the sonde and aircraft network over the United States, reveal the importance of the conventional radiosonde network in the higher northern latitudes.

Virtual Doppler Lidar Instrument

Abstract Doppler lidars measure the range-resolved line-of-sight wind component by extracting the Doppler shift of radiation backscattered from atmospheric aerosols and molecules. A virtual instrument was developed to simulate wind measurements by flying virtually over the atmosphere. The atmosphere contains all components that influence the lidar, that is, wind, turbulence, aerosols, clouds, etc. For a selected time period, a dataset of the atmospheric conditions from the global model and the local model was provided by the German Weather Service. Three different Doppler lidar systems were simulated for this report: a coherent airborne conical scanning 10-μm Doppler lidar, a 10-μm and a 2-μm spaceborne system, and a spaceborne incoherent ultraviolet Doppler lidar.

Evaluation of European Land Data Assimilation System (ELDAS) products using in situ observations

Three land-surface models with land-data assimilation scheme (DA) were evaluated for one growing season using in situ observations obtained across Europe. To avoid drifts in the land-surface state in the models, soil moisture corrections are derived from errors in screen-level atmospheric quantities.With the in situ data it is assessed whether these land-surface schemes produce adequate results regarding the annual range of the soil water content, the monthly mean soil moisture content in the root zone and evaporative fraction (the ratio of evapotranspiration to energy available at the surface). DA considerably reduced bias in net precipitation, while slightly reducing RMSE as well. Evaporative fraction was improved in dry conditions but was hardly affected in moist conditions. The amplitude of soil moisture variations tended to be underestimated. The impact of improved land-surface properties like Leaf Area Index, water holding capacity and rooting depth may be as large as corrections of the DA systems. Because soil moisture memorizes errors in the hydrological cycle of the models, DA will remain necessary in forecast mode. Model improvements should be balanced against improvements of DA per se. Model bias appearing from persistent analysis increments arising from DA systems should be addressed by model improvements.

Lidar research network water vapor and wind

This paper reports on a workshop which took place on April 08/09, 2002 at the University of Hohenheim, Stuttgart, Germany. During this workshop, laser and lidar experts as well as experts in atmospheric sciences came together. The goals of this workshop were to establish an interdisciplinary network on lidar research, to develop operational reference systems for water vapor and wind lidar measurements and to demonstrate the significant impact of water vapor and wind lidar systems on climate and weather research. The main result was the setup of an interdisciplinary expert network Lidar research water vapor and wind. This network is responding to large gaps in our knowledge of the global water vapor and wind distribution and shall improve weather and climate research. The network has mainly the following objectives: 1) The development of high-performance, high-power laser transmitters for wind and water vapor measurements based on novel pump laser designs, 2) research on efficient frequency conversion techniques of the high-power pump light into wavelengths required for water vapor and wind lidar systems, 3) the development of an operational reference water vapor differential absorption lidar system, also as a demonstrator for space borne missions like the Water Vapor Lidar Experiment in Space (WALES), 4) the performance of observation system simulation experiments (OSSEs) in order to demonstrate the impact of lidar data on nowcasting and short-range weather forecast, 5) the extension of the application of current and future lidar data in combination with other instruments for process studies, 6) the design of future field studies including a new generation of remote sensing systems. The network shall investigate key questions in atmospheric sciences, particularly, the role of water vapor in the Earths weather and climate system by the application of lidar systems with unprecedented accuracy, resolution and range.

Boundary-layer diffusion modelling: The Gaussian plume approach versus the spectral solution

The equation of turbulent diffusion is solved for a vertical area source within the planetary boundary layer. The traditional Gaussian-plume approach is compared with the spectral solution of the diffusion equation used together with the barotropic boundary-layer model of Lettau and Dabberdt (1970). The results of the numerical computations are presented and the differences between the solutions are discussed.

Spaceborne Doppler lidar perspectives

Lidar technology and applications are well-established (Kirchbaumer et al. 1993). A backscatter lidar technology experiment was tested in space in 1994 (Winker et al. 1994). Scientists need global information on wind, clouds, and aerosol layers. On a space-borne platform, only a limited amount of power is available for a lidar system. Therefore, a compromise is necessary between the possibilities and the requirements.

Assimilation of CHAMP and GRACE-A Radio Occultation Data in the GME Global Meteorological Model of the German Weather Service

The assimilation of GPS radio occultations within the three-dimensional variational data assimilation system of the German Weather Service requires GPS radio occultation bending angle forward operators. To optimize the forward operator setup, different one- and three-dimensional bending angle forward operators are evaluated. The innovation statistics for radio occultation data from the CHAMP, GRACE-A and FORMOSAT-3/COSMIC satellites are compared with estimates based on the background and observation errors specified in the assimilation scheme. Numerical experiments are performed to assess the impact of assimilated radio occultation data on the weather forecast scores.