OBSERVATION OF AEROSOL−CLOUD−TURBULENCE INTERACTION WITH INTEGRATED REMOTE-SENSING INSTRUMENTATION

Ulla Wandinger (1), Patric Seifert (1), Ronny Engelmann (1), Johannes Bühl (1), Janet Wagner (1), Jörg Schmidt (1), Bernhard Pospichal (2), Holger Baars (1), Anja Hiebsch (1), Thomas Kanitz (1), Annett Skupin (1), Lukas

Pfitzenmaier (1), Birgit Heese (1), Dietrich Althausen (1), Albert Ansmann (1)

(1) Leibniz Institute for Tropospheric Research, Permoserstr. 15, 04318 Leipzig, Germany, Email: ulla@tropos.de (2) Leipzig Institute for Meteorology, University of Leipzig, Stephanstr. 3, 04103 Leipzig

ABSTRACT

LACROS, the Leipzig Aerosol and Cloud Remote Observations System, comprises a set of state-of-the-art active and passive remote-sensing instruments, including multiwavelength Raman polarization lidar, 2-µm Doppler lidar, 35-GHz cloud radar, Sun photometer, 1064-nm ceilometer, and microwave radiometer. LACROS is designed as a mobile facility for temporary field experiments and for long-term observations at its home site in Leipzig. The focus of LACROS applications is on the study of aerosols, clouds, and aerosol-cloud-turbulence interaction. Exemplary applications are the heterogeneous formation of ice in mixed-phase clouds, the influence of atmospheric turbulence on cloud microphysical properties, or the modification of cloud parameters in dependence on aerosol particle type and concentration. 1. INTRODUCTION

The understanding of the role of aerosols, clouds, and in particular of their interaction under specific meteorological conditions is one of the challenging tasks to better understand the Earth’s atmospheric system and to predict future climate. Because of the complexity of the processes and the multitude of involved parameters, research in this field requires the synergistic application of sophisticated instrumentation for dedicated process studies as well as long-term and network observations in order to obtain statistically significant results and to exploit regional diversity. Coordinated activities have been setup in this context. In Europe, the integration of observational capabilities is facilitated in ACTRIS (Aerosols, Clouds and Trace Gases Research InfraStructure Network). The project, which is funded by the European Union in the Seventh Framework Programme, comprises remote-sensing as well as in-situ infrastructure. The remote-sensing part of ACTRIS incorporates EARLINET (European Aerosol Research Lidar Network, www.earlinet.org), Cloudnet http://www.cloudnet.org), and the European part of AERONET (Aerosol Robotic Network, http://aeronet.gsfc.nasa.gov). These networks developed more or less independently over the past 10−15 years. Therefore, up to now, only a handful of combined stations exist with the complete suite of aerosol and

cloud remote-sensing instruments. One of them is the Leipzig Aerosol and Cloud Remote Observations System LACROS, which is described in more detail in Sec. 2. Two joint research activities in ACTRIS deal with the development of integrated remote-sensing techniques, one is on lidar and Sun-photometer integration and one on the development of a framework for cloud−aerosol interaction studies (see www.actris.net). Respective measurement capabilities of LACROS based on instrument synergy are discussed in Sec. 3. 2. INSTRUMENTATION

LACROS is a multi-sensor facility stationed at the Leibniz Institute for Tropospheric Research (IfT) in Leipzig, Germany (see Fig. 1). It comprises active and passive remote-sensing instruments including multiwavelength Raman polarization lidar, 35-GHz cloud radar, 2-µm Doppler lidar, 1064-nm ceilometer, Sun photometer, and microwave radiometer. Almost all instruments are mobile or exist in a mobile version, i.e. LACROS is designed for field deployment as well as long-term observations at its home site.

Figure 1. Top view of IfT main building with indicated positions of LACROS instrumentation. All sensors are located within an area of 100 m ×70 m. The multiwavelength Raman polarization lidar MARTHA (Multiwavelength Atmospheric Raman lidar for Temperature, Humidity, and Aerosol profiling) is a stationary system applied for long-term observations of

aerosols in the frame of EARLINET [1]. In field experiments, the containerized multiwavelength lidar BERTHA (Backscatter, Extinction, lidar-Ratio, Temperature, and Humidity profiling Apparatus) with equivalent observing capabilities is used instead [2]. Currently, BERTHA is being upgraded with a high-spectral-resolution channel in order to improve daytime aerosol measurements. Alternatively, the fully autonomous and portable multiwavelength Raman lidar PollyXT can be deployed [3]. In contrast to MARTHA and BERTHA, it allows continuous, unattended operation. The lidar instrumentation is completed with the wind lidar WiLi, a containerized scanning 2-µm coherent Doppler instrument [4]. IfT Leipzig is an AERONET site [5] with a stationary Sun photometer. Additional instruments, among them a dual-wheel polarization photometer, are available for field experiments. The core system for cloud observations is the 35-GHz scanning cloud radar MIRA-35s with Doppler and polarization capabilities (www.metek.de). It is implemented in a container, together with the microwave radiometer HATPRO which is additionally equipped with a two-channel infrared radiometer [6]. For the determination of precipitation properties an optical disdrometer records the velocity and size distribution of falling hydrometeors in the size range from 0.1 to 10 mm on the container roof. The ceilometer CHM 15kx (http://www.jenoptik.com) complements the series of instruments necessary to apply the standard Cloudnet algorithms for the retrieval of cloud properties [7]. The LACROS core instrumentation is supplemented with a radiosonde station, an all-sky imager, and meteorological sensors. At the IfT roof, aerosol in-situ probes are taken routinely. A Spectral Aerosol Extinction Measurement System (SAEMS) based on a DOAS (Differential Optical Absorption Spectroscopy) measurement device provides spectrally resolved extinction coefficients of aerosols at 15 m above ground. The implementation of a Baseline Surface Radiation Network (BSRN) station for surface radiation measurements is foreseen in the near future. 3. MEASUREMENT CAPABILITIES BASED ON

INSTRUMENT SYNERGY

3.1. Aerosol

The vertically resolved characterization of aerosol particles in terms of their optical and microphysical properties is based on the multiwavelength Raman polarization lidar technique. Whereas spectrally resolved measurements of backscatter and extinction coefficients provide information on particle size and absorption properties, the measurement of the particle depolarization ratio allows the identification and quantification of coarse non-spherical particles, e.g., desert dust or volcanic ash [8].

Sun photometers provide column-integrated aerosol optical, microphysical, and radiative properties. So far, additional information from Sun photometer measurements has mainly been used to constrain lidar retrievals. Only recently, the development of combined lidar and Sun photometer inversion algorithms has started aiming at optimum use of both techniques and the derivation of vertically resolved information on microphysical particle parameters. These activities are supported in ACTRIS Workpackage 20. An example of a combined lidar and Sun photometer retrieval obtained with LIRIC (Lidar-Radiometer Inversion Code, [9]) is shown in Fig. 2. The measurement was performed at Leipzig during a strong Saharan dust outbreak in May 2008 [10].

Figure 2. Backscatter and extinction coefficients calculated with LIRIC for a deep Saharan dust layer over Leipzig on 30 May 2008, 09:10−10:31 UTC, and derived concentrations profiles of fine and coarse non-spherical particles. The LIRIC retrieval is compared with the backscatter coeffcient at 532 nm derived using the Klett method.

Input parameters for this retrieval are the lidar elastic-backscatter signals at the three wavelengths 355, 532, 1064 nm and the cross-polarized signal at 532 nm together with Sun photometer retrieval products, i.e. aerosol optical thickness, columnar volume concentration and size distribution of fine-mode (radius <0.5 µm) and coarse-mode particles (radius >0.5 µm). The algorithm provides volume concentration profiles of fine-mode and spherical and non-spherical coarse-mode particles. Non-sphericity is considered in the scattering calculations through an ensemble of spheroids. In the example shown in Fig. 2, non-spherical Saharan dust particles dominate the particle volume concentration in the atmosphere. It has been shown that dust particles act as efficient ice nuclei and enhance ice formation at comparably high temperatures of around −10 °C [11]. Thus combined retrievals of aerosol concentration profiles may help investigating such kind of aerosol−cloud interaction in future.

3.2. Clouds

The synergistic use of radar and lidar, together with passive microwave sounding, provides the opportunity to retrieve microphysical properties of hydrometeors and to study cloud and precipitation processes. Up to now, cloud radars have been coupled mainly with ceilometers or simple backscatter lidars [12,13,14]. The full potential of combining multi-wavelength, polarization, and multi-field-of-view lidars [15] and radar systems with Doppler and polarization measurement capabilities has not been exploited yet.

Figure 3. Combined lidar and radar observation of an ice-precipitating altocumulus cloud over Leipzig on 26 August 2011. The signal intensities in terms of the lidar range-corrected (RC) signal and the radar signal-to-noise ratio (SNR) are shown. Fig. 3 shows an example for a combined cloud observation. An ice-precipitating altocumulus layer was observed with LACROS between 4.5 and 6 km height from 0100 to 0140 UTC on 26 August 2011. The lidar measurements of the 1064-nm range-corrected signal and the 532-nm depolarization ratio (not shown) indicated that the cloud top consisted of an optically thick, about 200 m deep layer with super-cooled liquid-water droplets. Ice formation took place, and the large crystals fell into a dryer atmospheric layer below, where they evaporated. The instrument synergy is clearly visible. The small liquid-water droplets strongly scatter and attenuate the laser light, but are practically invisible for the radar. In contrast, the radar is much more sensitive to the larger ice crystals due to the long wavelength of 0.8 cm of the emitted signal. Few ice crystals in the lower part of the virga still cause a strong radar reflectivity, whereas in the lidar signal they appear with the same intensity as the surrounding aerosol. 3.3. Turbulence and terminal velocity

The Doppler capabilities of wind lidar and cloud radar allow the investigation of turbulent motion as well as terminal fall velocity of hydrometeors. The latter depends on the size of the falling droplets or ice crystals and may thus be used to derive size information. Again,

the lidar is more sensitive to small particles which move with the speed of the air parcel. Therefore, the lidar is preferably applied to study atmospheric turbulence. The differences can be clearly seen in Fig. 4, where the Doppler velocities for the cloud presented in Fig. 3 are shown. The lidar resolves up- and downdrafts in the layer with the super-cooled droplets, whereas the radar mainly indicates the terminal velocity of the falling ice crystals. Synergistic effects through coupling of Doppler lidar and radar measurements are also not explored yet. They will be subject of studies with LACROS in the near future.

Figure 4. Combined lidar and radar observation of vertical velocities in the altocumulus cloud shown in Fig. 3. 4. SUMMARY

We presented an overview of the instrumentation and measurement capabilities of LACROS, the Leipzig Aerosol and Cloud Remote Observations System. This mobile facility is equipped with state-of-the-art aerosol and cloud remote-sensing instruments. It is applied for long-term observations to study the interaction of clouds, aerosols, and atmospheric turbulence. LACROS will also be deployed in field observations such as the HD(CP)2 (High definition clouds and precipitation for advancing climate prediction) campaign in Jülich in 2013. LACROS observations are available online via internet (http://polly.tropos.de/martha/index.html). Acknowledgment Part of the research leading to these results has received funding from the European Union Seventh Framework Programme (FP7/2007-2013) under grant agreement n° 262254. 5. REFERENCES

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