Characterization of video disdrometer uncertainties and impacts on estimates of snowfall rate and radar reflectivity

[摘要] Estimates of snow microphysical properties obtained by analyzingcollections of individual particles are often limited to short timescalesand coarse time resolution. Retrievals using disdrometerobservations coincident with bulk measurements such as radarreflectivity and snowfall amounts may overcome these limitations;however, retrieval techniques using such observations requireuncertainty estimates not only for the bulk measurements themselves,but also for the simulated measurements modeled from the disdrometerobservations.Disdrometer uncertainties arise due to sampling andanalytic errors and to the discrete, potentially truncated form of thereported size distributions. Imaging disdrometers such as theSnowflake Video Imager and 2-D Video Disdrometer provide remarkablydetailed representations of snow particles, but view limitedprojections of their three-dimensional shapes. Particle sizesdetermined by such instruments underestimate the true dimensions ofthe particles in a way that depends, in the mean, on particle shape,also contributing to uncertainties. An uncertainty model that accountsfor these uncertainties is developed and used to establish theircontributions to simulated radar reflectivity and snowfall rate.Viewing geometry effects are characterized by a parameter, ϕ,that relates disdrometer-observed particle size to the true maximumdimension of the particle. Values and uncertainties for ϕ areestimated using idealized ellipsoidal snow particles. The model isapplied to observations from seven snow events from the CanadianCloudSat/CALIPSO Validation Project (C3VP), a mid-latitude cold-seasoncloud and precipitation field experiment.Typical total uncertaintiesare 4 dB for reflectivity and 40–60% for snowfall rate, arehighly correlated, and are substantial compared to expecteduncertainties for radar and precipitation gauge observations. Thedominant sources of errors are viewing geometry effects and thediscrete, truncated form of the size distributions. While modeledZe–S relationships are strongly affected by assumptions about snowparticle mass properties, such relationships are only modestlysensitive to ϕ owing to partially compensating effects on boththe reflectivity and snowfall rate.

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