Evaluating the capabilities and uncertainties of droplet measurements for the fog droplet spectrometer (FM-100)

摘要

Droplet size spectra measurements are crucial to obtain a quantitativemicrophysical description of clouds and fog. However, cloud droplet sizemeasurements are subject to various uncertainties. This work focuses on theerror analysis of two key measurement uncertainties arising during cloud dropletsize measurements with a conventional droplet size spectrometer (FM-100):first, we addressed the precision with which droplets can besized with the FM-100 on the basis of the Mie theory. We deduced errorassumptions and proposed a new method on how to correct measured size distributions for theseerrors by redistributing the measured droplet size distribution using astochastic approach. Second, based on a literature study, we summarizedcorrections for particle losses during sampling with the FM-100. We appliedboth corrections to cloud droplet size spectra measured at the high alpinesite Jungfraujoch for a temperature range from 0 °C to 11 °C.We showed that Mie scattering led to spikes in the droplet sizedistributions using the default sizing procedure, while the new stochasticapproach reproduced the ambient size distribution adequately. A detailedanalysis of the FM-100 sampling efficiency revealed that particle losses weretypically below 10% for droplet diameters up to 10 μm. For largerdroplets, particle losses can increase up to 90% for the largest dropletsof 50 μm at ambient wind speeds below 4.4 m s and even to>90% for larger angles between the instrument orientation and the windvector (sampling angle) at higher wind speeds. Comparisons of the FM-100 toother reference instruments revealed that the total liquid water content(LWC) measured by the FM-100 was more sensitive to particle losses than tore-sizing based on Mie scattering, while the total number concentration wasonly marginally influenced by particle losses. Consequently, for furtherLWC measurements with the FM-100 we strongly recommend to consider (1) theerror arising due to Mie scattering, and (2) the particle losses, especiallyfor larger droplets depending on the set-up and wind conditions.