Observations and Parameterizations of Particle Size Distributions in Deep Tropical Cirrus and Stratiform Precipitating Clouds: Results from In Situ Observations in TRMM Field Campaigns

This study reports on the evolution of particle size distributions (PSDs) and habits as measured during slow, Lagrangian-type spiral descents through deep subtropical and tropical cloud layers in Florida, Brazil, and Kwajalein, Marshall Islands, most of which were precipitating. The objective of the flight patterns was to learn more about how the PSDs evolved in the vertical and to obtain information of the vertical structure of microphysical properties. New instrumentation yielding better information on the concentrations of particles in the size (D) range between 0.2 and 2 cm, as well as improved particle imagery, produced more comprehensive observations for tropical stratiform precipitation regions and anvils than have been available previously. Collocated radar observations provided additional information on the vertical structure of the cloud layers sampled. Most of the spirals began at cloud top, with temperatures (T) as low as −50°C, and ended at cloud base or below the melting layer (ML). The PSDs broadened from cloud top toward cloud base, with the largest particles increasing in size from several millimeters at cloud top, to 1 cm or larger toward cloud base. Some continued growth was noted in the upper part of the ML. Concentrations of particles less than 1 mm in size decreased with decreasing height. The result was a consistent change in the PSDs in the vertical. Similarly, systematic changes in the size dependence of the particle cross-sectional area was noted with decreasing height. Aggregation—as ascertained from both the changes in the PSDs and evolution of particle habits as observed in high detail with the cloud particle imager (CPI) probe—was responsible for these trends. The PSDs were generally well-represented by gamma distributions of the form N = N0ΓDμe−λΓD that were fitted to the PSDs over 1-km horizontal intervals throughout the spirals. The intercept (N0Γ), slope (λΓ), and dispersion (μ) values were derived for each PSD. Exponential curves (N = N0e−λD; μ = 0) were also fitted to the distributions. The λΓ values for given spirals varied systematically with temperature as did the values of λ (exponential), and the data generally conformed to values found in previous studies involving exponential fits to size distributions in midlatitude frontal and cirrus layers. Considerable variability often noted in the PSD properties during the loops of individual spirals was manifested primarily in large changes in N0Γ and N0, but μ, λΓ, and λ remained fairly stable. Temperature is not found to be the sole factor controlling λΓ or λ, but is a primary one. Direct relationships were found between λΓ and N0Γ, or λΓ and μ, for the gamma distributions, and λ and N0 for the exponential. The latter relationship was not found as distinctly in earlier studies; observed PSDs in this study had better fidelity with less scatter. The μ values changed monotonically with T over the range of temperatures and were directly related to N0Γ or λΓ, thereby reducing the number of variables in the PSD functional equation to two. In the upper part of the ML, N0, and λ continued to decrease, and in the lower part these values began to increase as the largest particles melted. General expressions relating various bulk microphysical, radar, and radiative-transfer-related variables to N0Γ and λΓ were developed; they are useful for both tropical and midlatitude clouds. These relationships facilitate the specification of a number of bulk properties in cloud and climate models. The results presented in this paper apply best to temperatures between 0° and −40°C, for which the measured radar reflectivities fall in the range of 0 to 25 dBZe.