11th Conference on Atmospheric Radiation and the 11th Conference on Cloud Physics

Tuesday, 4 June 2002: 11:44 AM
Properties of tropical and midlatitude ice cloud Particle Ensembles: Observations and parameterizations
Andrew J. Heymsfield, NCAR, Boulder, CO; and A. Bansemer and P. R. Field
Poster PDF (651.3 kB)
In this study, several bulk properties of ice particle populations sampled in midlatitude and tropical cirrus and deep stratiform ice clouds are characterized. Size distribution data and particle imagery were collected during Lagrangian spiral descents and balloon-borne ascents through cloud layers that formed in association with deep convection (tropics) and synoptic-scale lifting (midlatitude). Temperatures for the midlatitude clouds ranged between --20 and --63C and for the tropical clouds between 0 and -50C.

Gamma distributions are fitted to the measured particle size distributions (PSDs) over sizes from as small as tens of microns to as large as 1.5 cm. Exponential distributions are also fitted to the PSD. The intercept parameter and the slope of the PSD decreased monotonically with height. The value of mu (dispersion) for the gamma fits tended from positive values of 2 to 4 at large spectral slopes to --1 to --2 at small slopes. The maximum measured diameter increased with decreasing spectral slope. These trends are consistent with aggregational growth. The intercept parameter (N_0) from the midlatitude clouds were about an order of magnitude lower than those for the tropical PSDs. The slope parameter showed a weak dependence on temperature, ranging from about 300/cm at temperatures near -50C to about 10/cm near 0C.

Bulk properties are derived from the fitted PSDs. The ice water content (IWC) ranged from 0.0005 to 1 g/m3 and were about an order of magnitude higher for the tropical than for the midlatitude data set, because the midlatitude intercept parameter (and total particle concentration) values are lower. The median mass diameter is inversely related to the spectral slope to the 0.6 power) with little scatter because these values do not depend on the intercept parameter. The median mass diameter is also highly correlated with the effective diameter, a measure of the radiative size of a PSD; the effective diameter does not depend directly on the intercept parameter. The median mass and effective diameter values increase with temperature. The median mass diameter reaches values above 1 mm at temperatures above 0C. The median mass diameter increases with IWC.

On average, the median mass-weighted terminal velocity increases from about 10 to 100 cm/s between --60 and 0C. The median mass diameter is directly related to the median mass diameter. Weak correlations are noted between the median mass diameter and temperature, with variability resulting from the aggregation process, which depends on height below cloud top; the proximity to convection; and the variability of habits. Correlations between median mass diameter and IWC are also weak because the intercept parameter of the PSD influences IWC but not the median mass diameter. Empirical and analytic expressions are developed to represent the data and to allow the results to be extended to other situations.

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