Ice microphysics observations in tropical cyclones from CAMEX-4 and NAMMA: Implications for hurricane models

Cloud parameterization schemes in mesoscale models make assumptions about the shape of assumed exponential and gamma size distributions for different ice species, such as graupel, snow and cloud ice. In-situ observations of cloud particles obtained during the Convection and Moisture Experiment (CAMEX-4) and the NASA African Monsoon Multidisciplinary Analyses (NAMMA) are used to determine the dependence of the slope, shape, and y-intercept of gamma distributions on temperature, total water content, vertical velocity and stage of tropical cyclone (TC) evolution. As CAMEX-4 sampled tropical storms, hurricanes, and unorganized convective systems during the Keys Area Microphysics Project (KAMP) and NAMMA sampled 2 tropical waves that developed into hurricanes and 3 that did not, microphysical observations in a variety of conditions provide a unique data set to elucidate these relations.

Ten second averages of size distribution functions were calculated from the two-dimensional cloud and precipitation probes (CAMEX-4) and from the cloud and precipitation imaging probes (NAMMA). Observed size distributions of ice particles (125 μm < D < 3 mm) were fit to gamma functions using a non-linear fitting routine. Variations of the gamma fit parameters were examined to assess contributions of different hydrometeor sizes to the total number, area and mass of the cloud as a function of the intensity of the tropical cyclone. Implications for the development of cloud microphysical parameterization schemes for mesoscale models are discussed.