Observations of Particle Size and Phase in Tropical Cyclones: Implications for Mesoscale Modeling of Microphysical Processes

Abstract

Mesoscale model simulations of tropical cyclones are sensitive to representations of microphysical processes, such as fall velocities of frozen hydrometeors. The majority of microphysical parameterizations are based on observations obtained in clouds not associated with tropical cyclones, and hence their suitability for use in simulations of tropical cyclones is not known. Here, representations of mass-weighted fall speed Vm for snow and graupel are examined to show that parameters describing the exponential size distributions and fall speeds of individual hydrometeors [through use of relations such as V(D) = aDb] are identically important for determining Vm. The a and b coefficients are determined by the composition and shape of snow and graupel particles; past modeling studies have not adequately considered the possible spread of a and b values. Step variations in these coefficients, associated with different fall velocity regimes, however, do not have a large impact on Vm for observed size distributions in tropical cyclones and the values of a and b used here, provided that coefficients are chosen in accordance with the sizes where the majority of mass occurs. New parameterizations for Vm are developed such that there are no inconsistencies between the diameters used to define the mass, number concentration, and fall speeds of individual hydrometeors. Effects due to previous inconsistencies in defined diameters on mass conversion rates between different hydrometeor classes (e.g., snow, graupel, cloud ice) are shown to be significant. In situ microphysical data obtained in Hurricane Norbert (1984) and Hurricane Emily (1987) with two-dimensional cloud and precipitation probes are examined to determine typical size distributions of snow and graupel particles near the melting layer. Although well represented by exponential functions, there are substantial differences in how the intercept and slope of these distributions vary with mass content when compared to observations obtained in other locations; most notably, the intercepts of the size distributions associated with tropical cyclones increase with mass content, whereas some observations outside tropical cyclones show a decrease. Differences in the characteristics of the size distributions in updraft and downdraft regions, when compared to stratiform regions, exist, especially for graupel. A new representation for size distributions associated with tropical cyclones is derived and has significant impacts on the calculation of Vm.