Airborne and ground-based measurements of microphysical and radiative properties of inhomogeneous clouds within the lower troposphere (< 4 km altitude) are carried out within the framework of the joint AFO2000 project 4DCLOUDS. In combination with simultaneous ground- based cloud radar and microwave measurements of other experimental 4DCLOUDS project partners, the airborne microphysical measurements are used from the modeling groups within 4DCLOUDS as input for more-dimensional radiative transfer calculations. The results of the models will then be compared with the airborne radiation measurements. Finally the hypothesis, that spatial cloud inhomogeneities are responsible for the often observed serious discrepancy between modeled and calculated cloud radiative properties (albedo, absorption and transmission), will be tested. One intensive measurement campaign is planned (September 2001, Cabauw, The Netherlands), which will be coordinated with the EU-funded project CLIWA-NET.
Vertical profiles as well as constant-altitude flight patterns above, within, and below the clouds are planned. In detail the airborne and ground-based in-situ measurements for this project comprise:
(a) Airborne measurements of microphysical properties (aerosol particles and cloud drops) and meteorological parameters:
- Aerosol particles: Measurements of the size distribution within the optically relevant size range between 0.1 and 10 µm particle diameter, measurements of volume scattering coefficient at three wavelengths (450, 550, 700 nm), of particle absorption coefficient at one wavelength (567 nm) and of the particle number concentration in two size ranges (> 3 nm and > 10 nm). Commercial instruments are used in modified form.
- Cloud drops: Measurements of the drop size distribution, the liquid water content (LWC) and the effective drop radius (Reff). For the measurements of the drop size distribution (< 50 um drop diameter) the so-called Fast FSSP will be used, which is a considerably modified version of the commercial FSSP (Forward Scattering Spectrometer Probe). The LWC and Reff will be obtained by integration of the drop size distribution. Additionally, there will be independent measurements of LWC and Reff with another commercial instrument (PVM, Particle Volume Monitor). In order to characterize the cloud drops with diameters larger than 50 µm an instrument of the type OAP (Opticle Array Probe) will be used in collaboration with GKSS Geesthacht.
(b) Ground-based and airborne spectral measurements of up- and downwelling solar irradiances: The radiation measurements are used to verify the satellite data (project partner Univ. of Dresden) and the results of the radiative transfer calculations (project partners Univ. of Kiel and of Mainz). The airborne up- and downwelling irradiances are measured with a spectral resolution of < 2.4 nm within the spectral range between 250 and 1050 nm. Both components (up- and downwelling) of the spectrometer are kept in horizontal position during the flights by a newly developed horizontal stabilization platform. For the spectral separation of the radiation a grating spectrometer with a diode array (1024 pixel) is used. At the ground a similar spectrometer is employed for the measurement of the downwelling spectral irradiances.
Institute for Tropospheric Research
04318 Leipzig, Germany