Abstract
Abstract. An accurate prediction of the ice crystal number concentration in clouds is important to determine the radiation budget, the lifetime, and the precipitation formation of clouds. Secondary-ice production is thought to be responsible for the observed discrepancies between the ice crystal number concentration and the ice-nucleating particle concentration in clouds. The Hallett–Mossop process is active between −3 and −8 ∘C and has been implemented into several models, while all other secondary-ice processes are poorly constrained and lack a well-founded quantification. During 2 h of measurements taken on a mountain slope just above the melting layer at temperatures warmer than −3 ∘C, a continuously high concentration of small plates identified as secondary ice was observed. The presence of drizzle drops suggests droplet fragmentation upon freezing as the responsible secondary-ice mechanism. The constant supply of drizzle drops can be explained by a recirculation theory, suggesting that melted snowflakes, which sedimented through the melting layer, were reintroduced into the cloud as drizzle drops by orographically forced updrafts. Here we introduce a parametrization of droplet fragmentation at slightly sub-zero temperatures, where primary-ice nucleation is basically absent, and the first ice is initiated by the collision of drizzle drops with aged ice crystals sedimenting from higher altitudes. Based on previous measurements, we estimate that a droplet of 200 µm in diameter produces 18 secondary-ice crystals when it fragments upon freezing. The application of the parametrization to our measurements suggests that the actual number of splinters produced by a fragmenting droplet may be up to an order of magnitude higher.
Highlights
Accurate weather forecasting in mountainous regions is more challenging than over flat topography as orography has a strong influence on the local weather; e.g., it creates local up- and downdrafts, which strongly impacts the development of clouds (Roe, 2005; Henneberg et al, 2017)
The appearance of cloud droplets larger than about 40 μm is often connected to secondary-ice production (SIP) by droplet fragmentation (Korolev et al, 2020)
A recirculation theory proposed by Korolev et al (2020) can explain these observations and can in general be applied to mountainous regions when a melting layer is present, and sufficiently large updrafts are produced on the windward side by the local topography
Summary
Accurate weather forecasting in mountainous regions is more challenging than over flat topography as orography has a strong influence on the local weather; e.g., it creates local up- and downdrafts, which strongly impacts the development of clouds (Roe, 2005; Henneberg et al, 2017). The correct phase partitioning and concentration of cloud particles is important for the determination of the radiation budget, the lifetime, and the precipitation amount of clouds (e.g., Lohmann, 2002; Henneberg et al, 2017) This can be especially important for mixed-phase clouds (MPCs) consisting of ice crystals and liquid droplets since they have a major contribution to the total precipitation in the midlatitudes (Mülmenstädt et al, 2015). The particle growth reduces the water vapor pressure and eventually leads to the evaporation of cloud droplets when the water vapor pressure drops below water vapor saturation over liquid This process is called the Wegener–Bergeron–Findeisen process
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