UV Raman lidar measurements of relative humidity for the characterization of cirrus cloud microphysical properties P. Di Girolamo1, D. Summa1, R.-F. Lin2,3, T. Maestri4, R. Rizzi4, and G. Masiello1 1Dipartimento di Ingegneria e Fisica dell'Ambiente, Universitá della Basilicata, viale dell'Ateneo Lucano n. 10, 85100 Potenza, Italy 2Goddard Earth Sci. and Technology Center, Univ. of Maryland Baltimore County, MD, USA 3Mesoscale Atmospheric Processes Branch, NASA GSFC, Code 613.1 Building 33, Greenbelt, MD 20771, USA 4Dipartimento di Fisica, Università di Bologna, viale Berti-Pichat 6/2, 40127 Bologna, Italy
Abstract. Raman lidar measurements performed in Potenza by the Raman lidar system BASIL
in the presence of cirrus clouds are discussed.
Measurements were performed on 6 September 2004 in the frame of the Italian phase
of the EAQUATE Experiment.
The major feature of BASIL is represented by its capability to perform high-resolution
and accurate measurements of atmospheric temperature and water vapour,
and consequently relative humidity, both in daytime and night-time, based on the
application of the rotational and vibrational Raman lidar techniques in the UV.
BASIL is also capable to provide measurements of the particle backscatter and extinction
coefficient, and consequently lidar ratio (at the time of these measurements, only at one wavelength),
which are fundamental to infer geometrical and microphysical properties of clouds.
A case study is discussed in order to assess the capability of Raman lidars to measure
humidity in presence of cirrus clouds, both below and inside the cloud.
While air inside the cloud layers is observed to be always under-saturated with respect to water,
both ice super-saturation and under-saturation conditions are found inside these clouds.
Upper tropospheric moistening is observed below the lower cloud layer.
The synergic use of the data derived from the ground based Raman Lidar and of spectral
radiances measured by the NAST-I Airborne Spectrometer allows the determination of the temporal
evolution of the atmospheric cooling/heating rates due to the presence of the cirrus cloud.
Lidar measurements beneath the cirrus cloud layer have been interpreted using
a 1-D cirrus cloud model with explicit microphysics. The 1-D simulations
indicate that sedimentation-moistening has contributed significantly to the
moist anomaly, but other mechanisms are also contributing. This result
supports the hypothesis that the observed mid-tropospheric humidification is
a real feature which is strongly influenced by the sublimation of
precipitating ice crystals. Results illustrated in this study demonstrate
that Raman lidars, like the one used in this study, can resolve the spatial
and temporal scales required for the study of cirrus cloud microphysical
processes and appear sensitive enough to reveal and quantify upper
tropospheric humidification associated with cirrus cloud sublimation.
Citation: Di Girolamo, P., Summa, D., Lin, R.-F., Maestri, T., Rizzi, R., and Masiello, G.: UV Raman lidar measurements of relative humidity for the characterization of cirrus cloud microphysical properties, Atmos. Chem. Phys., 9, 8799-8811, doi:10.5194/acp-9-8799-2009, 2009.