Continuous secondary-ice production initiated by updrafts through the melting layer in mountainous regions

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-Mos sop process is active between -3 and -8 degrees C and has been implemented into several models, while all other secondary-ice processes are poorly constrained and lack a well-founded quantification. During 2h of measurements taken on a mountain slope just above the melting layer at temperatures warmer than -3 degrees 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 mu m 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.

Automated summary

Accurate prediction of ice crystal number concentration in clouds is crucial for determining radiation budget, cloud lifetime, and precipitation formation. Secondary-ice production is believed to cause observed discrepancies between ice crystal number concentration and ice-nucleating particle concentration. Studies have shown continuous high concentration of secondary ice, likely caused by drizzle drop freezing and recirculation theory introducing melted snowflakes as drizzle drops in clouds. A parametrization of droplet fragmentation at slightly sub-zero temperatures has been introduced, estimating higher splinter production from fragmenting droplets.