Jens Reichardt

Atmospheric temperature profiling in the presence of clouds with a pure rotational Raman lidar by use of an interference-filter-based polychromator

A lidar polychromator design for the measurement of atmospheric temperature profiles in the presence of clouds with the rotational Raman method is presented. The design utilizes multicavity interference filters mounted sequentially at small angles of incidence. Characteristics of this design are high signal efficiency and adjustable center wavelengths of the filters combined with a stable and relatively simple experimental setup. High suppression of the elastic backscatter signal in the rotational Raman detection channels allows temperature measurements independent of the presence of thin clouds or aerosol layers; no influence of particle scattering on the lidar temperature profile was observed in clouds with a backscatter ratio of at least 45. The minimum integration time needed for temperature profiling with a statistical temperature error of +/-1 K at, e.g., 20-km height and 960-m height resolution is 1.5 h.

Geometrical-optics solution to light scattering by droxtal ice crystals

We investigate the phase matrices of droxtals at wavelengths of 0.66 and 11 microm by using an improved geometrical-optics method. An efficient method is developed to specify the incident rays and the corresponding impinging points on the particle surface necessary to initialize the ray-tracing computations. At the 0.66-microm wavelength, the optical properties of droxtals are different from those of hexagonal ice crystals. At the 11-microm wavelength, the phase functions for droxtals are essentially featureless because of strong absorption within the particles, except for ripple structures that are caused by the phase interference of the diffracted wave.

Relevance of mountain wave cooling for the formation of polar stratospheric clouds over Scandinavia: Mesoscale dynamics and observations for January 1997

The effect of mesoscale mountain wave-induced temperature anomalies on the formation potential of polar stratospheric clouds above northern Scandinavia is analyzed with a one-month mesoscale model integration. The simulation results are contrasted with synoptic-scale analyses and compared with remote sensing and in situ observations. The mesoscale mass flux of air parcels with temperatures below the threshold for cloud formation through a control volume is compared with its synoptic-scale counterpart. A classification of the synoptic-scale flow into periods of large and small mountain wave activity in the stratosphere is proposed. The derived classification will be used for a climatology of stratospheric mountain wave activity above Scandinavia.

Determination of stratospheric aerosol microphysical properties from independent extinction and backscattering measurements with a Raman lidar

An algorithm that permits the retrieval of profiles of particle mass and surface-area concentrations in the stratospheric aerosol layer from independently measured aerosol (particle and Rayleigh) and molecule (Raman or Rayleigh) backscatter signals is developed. The determination is based on simultaneously obtained particle extinction and backscatter profiles and on relations between optical and microphysical properties found from Mie-scattering calculations for realistic stratospheric particle size distributions. The size distributions were measured with particle counters released on balloons from Laramie, Wyoming, between June 1991 and April 1994. Mass and surface-area concentrations can be retrieved with relative errors of 10-20% and 20-40%, respectively, with a laser wavelength of 355 nm and with errors of 20-30% and 30-60%, respectively, with a laser wavelength of 308 nm. Lidar measurements taken within the first three years after the eruption of Mt. Pinatubo in June 1991 are shown. Surface-area concentrations around 20 µm(2) cm(-3) and mass concentrations of 3 to 6 µg m(-3) were found until spring 1993.

Evolution of the Pinatubo Aerosol: Raman Lidar Observations of Particle Optical Depth, Effective Radius, Mass, and Surface Area over Central Europe at 53.4°N

Abstract The Raman lidar technique has been applied to document the evolution and dissipation of the Pinatubo aerosol between 1991 and 1995. For the first time, profiles of the particle extinction coefficient have been determined with lidar in the stratosphere after a major volcanic eruption. From the concurrent observation of particle backscatter and extinction, time series of surface-area and mass concentrations and surface-area-weighted mean (or effective) radius can be determined without having to assume critical aerosol input parameters. Based on these optical and physical parameters, the development of the perturbation of the stratospheric aerosol layer over central Europe is discussed. In terms of particle backscatter and mass the perturbation declined with an e-folding decay time of 14–15 months between April 1992 and April 1994. The monthly mean particle optical depth reached 0.23 in the spring of 1992. Surface-area concentrations of the order of 10–40 mm2 m−3 were observed below 20-km height for...

Unexpectedly low ozone concentration in midlatitude tropospheric ice clouds: A case study

Raman lidar measurements of ozone, water vapor, and cirrus optical properties over northern Germany (53.4°N, 10.4deg;E) in autumn 1995 are presented. In contrast to smooth ozone profiles with values of 50 to 100 ppbv in a cloud-free and dry upper troposphere, pronounced minima in the ozone distribution with values close to zero were found several times in the presence of ice cloud layers.

Correlations among the optical properties of cirrus‐cloud particles: Implications for spaceborne remote sensing

[1] Lidar measurements of Arctic (67.9� N) cirrus clouds reveal a strong positive correlation between particle depolarization ratio and extinction-to-backscatter (lidar) ratio for ambient cloud temperatures above 45� C, and an anti-correlation for colder temperatures. Similar correlations are evident in a 2-year midlatitude (53.5� N) cirrus data set. These data suggest that robust relationships may exist between these particle optical properties that will facilitate the retrieval of cirrus extinction profiles from polarization-sensitive (spaceborne) elastic-backscatter lidars. INDEX TERMS: 0360 Atmospheric Composition and Structure: Transmission and scattering of radiation; 1640 Global Change: Remote sensing; 0305 Atmospheric Composition and Structure: Aerosols and particles (0345, 4801); 0320 Atmospheric Composition and Structure: Cloud physics and chemistry

Three-signal method for accurate measurements of depolarization ratio with lidar

A method is presented that permits the determination of atmospheric depolarization-ratio profiles from three elastic-backscatter lidar signals with different sensitivity to the state of polarization of the backscattered light. The three-signal method is far less sensitive to experimental errors and does not require calibration of the measurement, as is the case of the two-signal lidar technique conventionally used for the observation of depolarization ratios. The three-signal method is applied to a polar stratospheric cloud observation. In the analysis we show that, depending on the statistical error of the measurement and on the lidar system parameters, the new method requires minimum cloud volume depolarization ratios to be applicable; in the case study presented, this threshold is approximately 0.2. Depolarization ratios determined with the three-signal method can be used to accurately calibrate measurements with the conventional two-signal technique.

High accuracy humidity measurements using the standardized frequency method with a research upper-air sounding system

Water vapour is the most important greenhouse gas in the atmosphere. The knowledge of water vapour distribution is of great importance for both operational purposes and climatological investigations. Several years ago the method of standardized frequencies was developed at Lindenberg Observatory of Deutscher Wetterdienst for the calculation of relative humidity (hereinafter RH) profiles measured with a H-Humicap research sonde type based on Vaisalas RS90 operational radiosonde. Firstly, this paper presents the further improvement of that method and gives a description of the method and of the calibration technique. RH profiles in the troposphere can be determined using the method of standardized frequencies with an absolute accuracy of 1% RH. The resolution of those humidity measurements is 0.01% RH. Results of about 70 radiosonde ascents are presented and as an example one ascent is compared to Lidar RH measurements. Secondly, a correction function for humidity profiles measured with RS80 A-Humicap (world-wide used operational radiosonde) is presented in this paper. It can also be applied to historical RS80 data sets. The third aim of this paper is to give some prospects/possibilities for relative humidity measurements in the stratosphere. Further investigations are necessary to improve our calibration model for humidity measurements in the upper stratosphere, especially at pressure levels lower than 200 hPa.

The impact of ice crystal shapes, size distributions and spatial structures of cirrus clouds on solar radiative fluxes

The solar radiative properties of cirrus clouds depend on ice particle shape, size, and orientation, as well as on the spatial cloud structure. Radiation schemes in atmospheric circulation models rely on estimates of cloud optical thickness only. In the present work, a Monte Carlo radiative transfer code is applied to various cirrus cloud scenarios to obtain the radiative response of uncertainties in the above-mentioned microphysical and spatial cloud properties (except orientation). First, plane-parallel homogeneous (0D) clouds with different crystal shapes (hexagonal columns, irregular polycrystals) and 114 different size distributions have been considered. The resulting variabilities in the solar radiative fluxes are in the order of a few percent for the reflected and about 1% for the diffusely transmitted fluxes. Largest variabilities in the order of 10% to 30% are found for the solar broadband absorptance. However, these variabilities are smaller than the flux differences caused by the choice of ice particle geometries. The influence of cloud inhomogeneities on the radiative fluxes has been examined with the help of time series of Raman lidar extinction coefficient profiles as input for the radiative transfer calculations. Significant differences between results for inhomogeneous and plane-parallel clouds were found. These differences are in the same order of magnitude as those arising from using extremely different crystal shapes for the radiative transfer calculations. From this sensitivity study, the ranking of cirrus cloud properties according to their importance in solar broadband radiative transfer is optical thickness, ice crystal shape, ice particle size, and spatial structure.