| Popis: |
While a majority of precipitating clouds in the Alps are mixed-phase clouds (MPCs), a lot of microphysical processes behind ice formation and precipitation initiation are still not fully understood. In this thesis, orographic precipitation events were analysed with measurement data from the RACLETS field campaign in Davos during winter 2019 in order to investigate microphysical properties of precipitation and MPCs. Precipitation was analysed with Parsivel laser disdrometers, while in-situ cloud measurements were taken with the holographic imager HOLIMO. Remote sensing and meteorological observations furthermore provided information about atmospheric conditions during the measurement period. In a first case study, a cold front precipitation event was analysed. Comparing measurements from two Parsivel disdrometers at different locations, observed differences in accumulated precipitation could be explained with microphysical properties of precipitation. Increased snowfall rates at one measurement site were mostly the result of higher precipitation particle number concentrations and higher particle fall velocity in combination. In MPC measurements mostly ice crystals were found, compared to only small cloud droplet number concentrations. Both, ice crystal and cloud droplet number densities increased over time. High cloud particle number concentrations furthermore were also consistent with high radar reflectivities measured with a cloud radar. During a second precipitation event, a transition from rainfall to snowfall was investigated. Using Parsivel disdrometer measurements, a transition from typical raindrop size-velocity distributions to typical snow particle distributions was observed. Furthermore, co-existing raindrops and snow particles were found during the short transition period, resulting in bimodal particle size-velocity distributions. Additionally, a melting layer could be observed with radar in the lowest few hundred metres above ground, consistent with Parsivel measurements on a mountain above the melting layer, where only snowfall was detected. |
