The impact of varying definitions of particle maximum dimension on calculations of cloud properties from optical imaging probe data

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Description

Many algorithms exist for processing data from in-situ optical probes that acquire two-dimensional images of cloud particles as particles traverse an array of photodiode detectors clocked by rapid response electronics during aircraft motion. For deriving number distribution functions in terms of maximum particle length (Dmax), N(Dmax), many algorithms apply different definitions of Dmax. Common definitions include maximum length in photodiode array direction (DP), maximum length in time direction (DT), and their combinations, such as ½(DP+DT) and √(〖D_P〗^2+〖D_T〗^2 ). The diameter of the minimum circle enclosing the particle has also used. Due to the use of different Dmax, the concentrations of particles in different size ranges can vary considerably, with differences of even up to an order of magnitude in some size ranges. Differences in the N0, μ and λ values used to characterize N(Dmax) as gamma functions are also seen. The use of different definitions of Dmax also affects the calculation of bulk cloud and optical properties, such as ice water content (IWC), mass-weighted terminal velocity, mass-weighted mean particle dimension, precipitation rate, extinction and effective radius. These differences occur because fundamental relationships used to derive the bulk properties, such as relations for particle velocity and mass with particle dimension, can be based on different Dmax definitions. For example, IWC and extinction calculated by mass- and area-dimensional relations can vary by 2-3 times, and resulting effective radius can also differ by the same magnitude. Further, the mass-weighted terminal velocity can differ by 60%, and precipitation rate by one order of magnitude due to different definitions. These differences show the need to use consistent definitions of Dmax for defining N(Dmax) and the functional dependence of particle microphysical properties on Dmax. Implications for numerical modeling studies and remote sensing retrievals are discussed.