Hans Richner

Comparison between Backscatter Lidar and Radiosonde Measurements of the Diurnal and Nocturnal Stratification in the Lower Troposphere

A collection of boundary layer heights has been derived from measurements performed by a groundbased backscatter lidar in Neuchâtel, Switzerland (47.000°N, 6.967°E, 485 m ASL). A dataset of 98 cases have been collected during 2 yr. From these data, 61 are noon and 37 are midnight cases. The following two different schemes were used to retrieve the mixed layer depth and the height of the residual layer from the measurements: the gradient and variance methods. The obtained values were compared with those derived from the potential temperature profiles as computed from radiosonde data. For nocturnal cases, the height of the first aerosol layer above the residual layer was also compared to the corresponding potential temperature value. Correlation coefficients between lidar and radiosonde in both convective and stable conditions are between 0.88 and 0.97.

Understanding and Forecasting Alpine Foehn

This chapter focuses on the history, physics, climatology, forecasting and the broad effects of Alpine foehn on human populations. In the European Alps, foehn winds have been studied since the mid-1800s. The main focus of the investigations was the question of why foehn winds are so warm. While it soon became clear that adiabatic processes provide an explanation, the role of wet adiabatic rising on the upwind side of the Alps continued to be strongly debated. The so-called textbook theory for foehn – heat gain by wet adiabatic, forced lifting on the upwind side followed by dry adiabatic descent in the lee – represents only an extreme situation. Foehn occurs also with partial or complete blocking of the upwind air mass, i.e., with limited or no heat gain by wet adiabatic expansion. The second focus is on processes which lead to descending air masses after passing the mountain ridge. A discussion of the most important processes shows that there seems to be no theory which is applicable in all situations. Forecasting foehn is still a challenge to meteorologists. While the general foehn situation can be predicted reliably, today’s numerical models still often poorly simulate the sudden break in of potentially devastating foehn air in the lee. Efforts to improve this must continue because foehn storms have a significant societal impact (threat to transportation systems and massive increase of fire danger) as several recent incidents show.

Variation of the aerosol stratification over the Rhine Valley during Foehn development: a backscatter lidar study

The vertical aerosol stratification within and above the Rhine Valley was studied with a backscatter lidar during FORM (Foehn in the Rhine Valley during MAP) IOPs 4-5, October 1-3, 1999. The lidar site was in Trubbach, 9° 28 E, 47° 04 N, 490 m asl. Two scintillometers were used to monitor the horizontal and the vertical wind velocity at 600 m above the Rhine Valley floor. A number of surface stations were operational in the valley, as well as a set of radiosounding stations. This provides a possibility to correlate the measurements of the aerosol stratification with the variation of the meteorological parameters defining the Foehn development. The backscatter lidar measurements during the Foehn development show the variation of the aerosol backscatter at different altitudes of the valley PBL. The combination of lidar signal gradient and lidar signal variance presents the cold-pool as an aerosol-rich layer and suggests a likely mechanism for cold-pool erosion by the Foehn air.

Comparison of lidar methods to determine the aerosol mixed layer top

Backscatter lidar measurements were performed in the atmospheric boundary layer and the troposphere above Neuchatel, Switzerland (47.00°N, 6.95°S, 485m asl). The backscatter lidar is based on Nd:YAG laser. The lidar measurements are done in the period from June 2000 till February 2002 as part of the EU project EARLINET (http://lidarb.dkrz.de/earlinet/). From the lidar measurements, we determine the following values vertical profile of the aerosol backscatter coefficients, the gradient of the range-corrected lidar signal and the variance of the range-corrected lidar signal. These values are used to determine the aerosol mixed layer (AML) height in the atmospheric boundary layer (ABL). In this work, we present a comparison of these different lidar methods to determine the AML height. The lidar-obtained values are also compared with the values for ABL top, as determined from upper air weather parameters. This comparison is performed and presented for various seasons and time in the diurnal cycle.

Lidar determination of the frequency of variations of the boundary-layer top

The frequency of thermals up- and downdraft, during the day, and that of gravity waves, during night, are retrieved by applying a fast Fourier transform to the temporal evolution of the boundary-layer height as obtained by lidar measurements. The principal components of each obtained spectrum of frequency are related to the dominant processes occurring at the convective and nocturnal boundary-layer tops. The formation of lee-waves systems during special meteorological conditions determined the oscillation of the boundary-layer height. Fluctuations at the nocturnal boundary-layer height can occur even if the conditions through the layer depth are statically stable. These oscillations are principally due to wind shear and buoyancy (gravity) waves at the altitude of the nocturnal boundary-layer top.

Lidar determination of mixing layer height with high resolution

Ecological monitoring and analysis of the planetary boundary layer (PBL) dynamics require determination of the mixing layer height (MLH) on a continuous basis. In a number of cases it is necessary to determine the MLH with sufficiently high resolution - both altitude and temporal. The backscatter lidar provides a convenient tool for such determination, using the aerosol as tracer and determining its vertical profile and its time-evolution, with the capability for continuous measurements. Although methods already exist, based on the altitude derivative of the backscatter lidar signal (altitude Gradient method) and its time-variance (Variance method), the application of these methods with high resolution is limited by the background noise presence. We report here a further development of backscatter lidar gradient and variance methods for MLH determination, allowing higher resolutions. In it, the MLH determination from the gradient and the variance of the lidar signal is supported by a convenient filter technique. Time scale of increased temporal resolution allows the investigation of the fine atmospheric dynamic structures like convective motion. A number of examples in MLH retrieval are presented. The examples are based on backscatter lidar measurements performed in the PBL above Neuchatel, Switzerland (47.00°N, 6.95°S, 485m asl). The examples show the applicability and the usefulness of the reported technique in measurements of the daily cycle of the MLH dynamics.