The variability of tropical ice cloud properties as a function of the large-scale context from ground-based radar-lidar observations over Darwin, Australia

Abstract

Abstract. The high complexity of cloud parameterizations now held in models puts more pressure on observational studies to provide useful means to evaluate them. One approach to the problem put forth in the modelling community is to evaluate under what atmospheric conditions the parameterizations fail to simulate the cloud properties and under what conditions they do a good job. It is the ambition of this paper to characterize the variability of the statistical properties of tropical ice clouds in different tropical "regimes" recently identified in the literature to aid the development of better process-oriented parameterizations in models. For this purpose, the statistical properties of non-precipitating tropical ice clouds over Darwin, Australia are characterized using ground-based radar-lidar observations from the Atmospheric Radiation Measurement (ARM) Program. The ice cloud properties analysed are the frequency of ice cloud occurrence, the morphological properties (cloud top height and thickness), and the microphysical and radiative properties (ice water content, visible extinction, effective radius, and total concentration). The variability of these tropical ice cloud properties is then studied as a function of the large-scale cloud regimes derived from the International Satellite Cloud Climatology Project (ISCCP), the amplitude and phase of the Madden-Julian Oscillation (MJO), and the large-scale atmospheric regime as derived from a long-term record of radiosonde observations over Darwin.Correspondence to: A. Protat (alain.protat@latmos.ipsl.fr)The vertical variability of ice cloud occurrence and microphysical properties is largest in all regimes (1.5 order of magnitude for ice water content and extinction, a factor 3 in effective radius, and three orders of magnitude in concentration, typically). 98 % of ice clouds in our dataset are characterized by either a small cloud fraction (smaller than 0.3) or a very large cloud fraction (larger than 0.9). In the ice part of the troposphere three distinct layers characterized by different statistically-dominant microphysical processes are identified. The variability of the ice cloud properties as a function of the large-scale atmospheric regime, cloud regime, and MJO phase is large, producing mean differences of up to a factor 8 in the frequency of ice cloud occurrence between large-scale atmospheric regimes and mean differences of a factor 2 typically in all microphysical properties. Finally, the diurnal cycle of the frequency of occurrence of ice clouds is also very different between regimes and MJO phases, with diurnal amplitudes of the vertically-integrated frequency of ice cloud occurrence ranging from as low as 0.2 (weak diurnal amplitude) to values in excess of 2.0 (very large diurnal amplitude). Modellers should now use these results to check if their model cloud parameterizations are capable of translating a given atmospheric forcing into the correct statistical ice cloud properties.

Ice cloud microphysical trends observed by the Atmospheric Infrared Sounder

“…The largest values of r ei are found for light easterly winds around 5-10 m s −1 for opaque, consistent with convectively active regimes generating larger r ei (e.g. Protat et al, 2011). Smaller values of a few µm are associated with westerly winds during suppressed convection.…”

“…Using ground-based retrievals at Darwin, Protat et al (2011) found that r ei is ∼ 1-3 µm larger during active deep convection compared to suppressed conditions. Using a simultaneous retrieval of ice cloud properties from the MODIS and POLDER instruments, van Diedenhoven et al (2014) found that r ei is larger in stronger convective events com- (2015) show similar results with DARDAR data for the thickest convective clouds above 10 km.…”

“…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, 2011;van Diedenhoven et al, 2014). Values of AIRS r ei are plotted against T cld for opaque, non-opaque, and multi-layer clouds separately; only 1.9 % of ice cloud tops are identified as opaque.…”

Ice cloud microphysical trends observed by the Atmospheric Infrared Sounder

“…The AMSR low frequency (LF) wind is 25 used to quantify the response of r ei to variability in near surface wind speeds in the presence of convection. regimes generating larger r ei (e.g., Protat et al, 2011). Smaller values of r ei are associated with westerly winds during suppressed convection.…”

“…Using ground-based retrievals at Darwin, Protat et al (2011) found that r ei is ~1-3 µm larger during active deep convection compared to suppressed conditions. 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.…”