Retrieval of effective ice crystal size in the infrared: Sensitivity study and global measurements from TIROS‐N Operational Vertical Sounder

Abstract

[1] The difference between cirrus emissivities at 8 and 11 mm is sensitive to the mean effective ice crystal size of the cirrus cloud, D e . By using single scattering properties of ice crystals shaped as planar polycrystals, diameters of up to about 70 mm can be retrieved, instead of up to 45 mm assuming spheres or hexagonal columns. The method described in this article is used for a global determination of mean effective ice crystal sizes of cirrus clouds from TOVS satellite observations. A sensitivity study of the D e retrieval to uncertainties in hypotheses on ice crystal shape, size distributions, and temperature profiles, as well as in vertical and horizontal cloud heterogeneities shows that uncertainties can be as large as 30%. However, the TOVS data set is one of few data sets which provides global and long-term coverage. Having analyzed the years 1987-1991, it was found that measured effective ice crystal diameters D e are stable from year to year. For 1990 a global median D e of 53.5 mm was determined. Averages distinguishing ocean/land, season, and latitude lie between 23 mm in winter over Northern Hemisphere midlatitude land and 64 mm in the tropics. In general, larger D e s are found in regions with higher atmospheric water vapor and for cirrus with a smaller effective emissivity.

Bulk microphysical properties of semi-transparent cirrus from AIRS: a six year global climatology and statistical analysis in synergy with geometrical profiling data from CloudSat-CALIPSO

“…This retrieval extends the retrieval of cloud physical properties using AIRS . Compared to earlier retrievals using observations from TOVS (Stubenrauch et al, 1999;Rädel et al, 2003), the use of more spectral channels with AIRS allows an indication of the most frequent ice crystal habit in the cirrus (by distinguishing column-like and aggregate-like ice crystals in the simulations). Simultaneous geometrical profiling (GEOPROF) data (Mace et al, 2009) using combined measurements of the active lidar and radar of the missions CALIPSO (Winker et al, 2003) and CloudSat (Stephens et al, 2002) allow us to characterize the cirrus vertical structure and to study relationships with the bulk microphysical properties.…”

“…CO 2 -sensitive channels of IR vertical sounders allow the determination of cloud height and cloud emissivity, and cloud spectral emissivity differences in the 8-12 µm range allows an estimation of the D e and IWP of semi-transparent cirrus (e.g. Rädel et al, 2003;Kahn et al, 2008;Pavolonis, 2010). IR sounders have continuously observed our planet since 1979, with improvements in spectral resolution: the TIROS-N Operational Vertical Sounders (Smith et al, 1979) onboard the NOAA polar satellites, the Atmospheric InfraRed Sounder onboard Aqua (since 2002) and the InfraRed Atmospheric Sounding Interferometer (Hilton et al, 2012) onboard METOP (since 2006).…”

Impacts of cloud heterogeneities on cirrus optical properties retrieved from space-based thermal infrared radiometry

“…In the thermal infrared atmospheric window, the SWT is one of the most used methods to retrieve the effective diameter and the cloud optical thickness using the difference of brightness temperatures between two thermal infrared channels (Parol et al, 1991;Radel et al, 2003;Dubuisson et al, 2008;Garnier et al, 2012Garnier et al, , 2013. Figure 2 shows brightness temperature difference (BTD) for varying optical thickness (0-50 at 12.05 µm) and eight effective diameters (D eff ) as a function of the 12.05 µm BT (BT 12 ).…”

A concept for a satellite mission to measure cloud ice water path, ice particle size, and cloud altitude

“…The assumed noise level for both IR channels was 0.6 mW m −2 (cm sr −1 ), corresponding to approximately 0.5 K noise at 260 K scene temperature. It should be mentioned that a further IR channel at about 8.3 µm may further improve the performance, especially if ice crystals in cirrus are dominantly aggregates (compare Figure 5 of Rädel et al, 2003), but such a channel was not assumed to be available here. Figure 11 shows the simulated performance for IWP, D me , and z me .…”

“…For these clouds the IR brightness temperatures are higher than the ambient temperature. Cirrus IR emissivities at different wavelengths allow us to retrieve IWP and D eff for such semitransparent cirrus clouds (Rädel et al, 2003;Stubenrauch et al, 2004b).…”