Variation of ice crystal size, shape, and asymmetry parameter in tops of tropical deep convective clouds

Abstract

The variation of ice crystal properties in the tops of deep convective clouds off the north coast of Australia is analyzed. Cloud optical thickness, ice effective radius, aspect ratio of ice crystal components, crystal distortion parameter and asymmetry parameter are simultaneously retrieved from combined measurements of the Moderate Resolution Imaging Spectroradiometer (MODIS) and Polarization and Directionality of the Earth's Reflectances (POLDER) satellite instruments. The data are divided into periods with alternating weak and strong convection. Mostly plate‐like particle components with aspect ratios closer to unity and lower asymmetry parameters characterize strongly convective periods, while weakly convective periods generally show lower aspect ratios, relatively more column‐like shapes and somewhat greater asymmetry parameters. Results for strongly convective periods show that, with increasing cloud top temperature, the distortion parameter generally decreases, while the asymmetry parameter and effective radius increase. For one of the strongly convective periods, the rate at which effective radii increase with cloud top temperature is more than double that of the other periods, while the temperature dependence of the other microphysical quantities for this period is substantially weaker. Atmospheric state analysis indicates that these differences are concurrent with differences in middle‐to‐upper tropospheric zonal wind shear. The observed variation of microphysical properties may have significant effects on the shortwave radiative fluxes and cloud absorption associated with deep convection. Additionally, MODIS collection 5 effective radii are estimated to be biased small with an artificially narrow range. Collection 6 products are expected to have less severe biases that depend on cloud top temperature and atmospheric conditions.

The Impact of Two Coupled Cirrus Microphysics–Radiation Parameterizations on the Temperature and Specific Humidity Biases in the Tropical Tropopause Layer in a Climate Model

“…The difference in reflected shortwave flux is at most about 7 W m 22 , assuming an area-averaged incident solar flux of 330 W m 22 , at least for the case considered here. These differences found for the g parameterizations are far less than the range in g found by several studies (Ulanowski et al 2006;Fu 2007;Garrett 2008;Baran 2012;van Diedenhoven et al 2014;Yang et al 2015). Indeed, in the case of Ulanowski et al (2006), experimentally derived g values were found to vary between 0.80 6 0.04 and 0.63 6 0.05 for smooth and rough ice analog rosettes, respectively, and this difference results in a shortwave flux uncertainty of about 256 W m 22 .…”

“…Indeed, in the case of Ulanowski et al (2006), experimentally derived g values were found to vary between 0.80 6 0.04 and 0.63 6 0.05 for smooth and rough ice analog rosettes, respectively, and this difference results in a shortwave flux uncertainty of about 256 W m 22 . Furthermore, the calculated asymmetry parameter values using the two parameterizations compare well against radiometrically derived asymmetry parameter values using POLDER observations from van Diedenhoven et al (2014). The observations from POLDER were located over the north coast of Australia.…”

“…Figure 4c shows «(l) for g and, as can be seen from the figure, «(l) is within 62.5% for about 83% of the database, which is also acceptable. Theoretical and in situ uncertainty in the asymmetry parameter value is far greater than the error in the g parameterization (Ulanowski et al 2006;Fu 2007;Garrett 2008;Baran 2012;van Diedenhoven et al 2014). We compare our parameterization of g to the g parameterization developed by Fu (2007) by assuming q i and T c values of 1.0 3 10 24 kg kg 21 and 190 K, respectively.…”

Ice cloud microphysical trends observed by the Atmospheric Infrared Sounder

“…Surface based and aircraft in situ observations have demonstrated that active periods of deep convection are associated with larger r ei at cloud top (e.g., Protat et al;van Diedenhoven et al, 2014). AIRS r ei are plotted against T ci for transparent 5 and opaque clouds separately; approximately 9.4% of cloud tops are identified as opaque and 90.6% are transparent.…”

“…Using a simultaneous retrieval of ice cloud properties from the MODIS and POLDER 20 instruments, van Diedenhoven et al (2014) found that r ei is larger in stronger convective events compared to others at a given cloud top pressure. van Diedenhoven et al (2016) used in situ aircraft observations to show that r ei decreases with height, reaches a minimum around 14 km, and may subsequently increase at higher altitudes.…”