Abstract
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 presented. 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.
Highlights
Introduction and motivationsThe persistence of high-level clouds, which normally cover more than 20 % of the globe (Wylie et al, 1994), suggests that they play an important role in the Earth energy budget
It is demonstrated that the differences in mean backscatter profile (BSP) obtained by averaging data of the Final Database (FD) with QC equal to 0 or 1 or alternatively with QC free to assume any possible value are of minor importance
In order to investigate the influence of the cloud geometrical thickness on the shape of the BSP, annual averages of the Cloud-Aerosol LIdar with Orthogonal Polarization (CALIOP) BS are computed for six subsets of the full data set under study (the FD) regarding cirrus clouds with different vertical extensions
Summary
The persistence of high-level clouds, which normally cover more than 20 % of the globe (Wylie et al, 1994), suggests that they play an important role in the Earth energy budget. Maestri: Cirrus backscatter profiles statistics their radiative features (Maestri et al, 2005) and interpretation of remote sensing data At this purpose, Maestri and Holz (2009) show how the assumption on the vertical distribution of IWC might impact cloud properties retrievals (OD and particle effective dimension) from infrared satellite measurements. The present work deals with thin cirrus clouds which, among ice clouds, are difficult to characterize due to their altitude and low IWC It aims at (i) improving the knowledge and characterization of vertical properties of cirri, (ii) contributing to the refinement of the interpretation of remote sensing data and (iii) decreasing the uncertainties related to the computations of radiative fields (which, currently, are normally generated assuming homogeneity in the cloud features along the vertical).
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