Statistics of vertical backscatter profiles of cirrus clouds Physics Department, Alma Mater Studiorum, University of Bologna, Bologna, Italy
20 Dec 2011
Received: 23 July 2011 – Published in Atmos. Chem. Phys. Discuss.: 16 September 2011 Abstract. A nearly global statistical analysis of vertical backscatter and extinction
profiles of cirrus clouds collected by the CALIOP lidar, on-board of the
Cloud-Aerosol Lidar and Infrared Pathfinder Satellite Observation, is
Revised: 29 November 2011 – Accepted: 08 December 2011 – Published: 20 December 2011
Statistics on frequency of occurrence and distribution of bulk properties of
cirrus clouds in general and, for the first time, of horizontally homogeneous
(on a 5-km field of view) cirrus clouds only are provided. Annual and
seasonal backscatter profiles (BSP) are computed for the horizontally
homogeneous cirri. Differences found in the day/night cases and for
midlatitudes and tropics are studied in terms of the mean physical parameters
of the clouds from which they are derived.
The relationship between cloud physical parameters (optical depth, geometrical
thickness and temperature) and the shape of the BSP is investigated. It is
found that cloud geometrical thickness is the main parameter affecting the
shape of the mean CALIOP BSP. Specifically, cirrus clouds with small
geometrical thicknesses show a maximum in mean BSP curve located near cloud
top. As the cloud geometrical thickness increases the BSP maximum shifts
towards cloud base. Cloud optical depth and temperature have smaller effects
on the shape of the CALIOP BSPs. In general a slight increase in the BSP
maximum is observed as cloud temperature and optical depth increase.
In order to fit mean BSPs, as functions of geometrical thickness and position
within the cloud layer, polynomial functions are provided. The impact on
satellite radiative transfer simulations in the infrared spectrum when using
either a constant ice-content (IWC) along the cloud vertical dimension or an
IWC profile derived from the BSP fitting functions is evaluated. It is, in
fact, demonstrated that, under realistic hypotheses, the mean BSP is linearly
proportional to the IWC profile.
Citation: Veglio, P. and Maestri, T.: Statistics of vertical backscatter profiles of cirrus clouds, Atmos. Chem. Phys., 11, 12925-12943, doi:10.5194/acp-11-12925-2011, 2011.