Polarimetric radar and in situ observations of riming and snowfall microphysics during CLACE 2014

Abstract

Abstract. This study investigates the microphysics of winter alpine snowfall occurring in mixed-phase clouds in an inner-Alpine valley during January and February 2014. The available observations include high-resolution polarimetric radar and in situ measurements of the ice-phase and liquidphase components of clouds and precipitation. Radar-based hydrometeor classification suggests that riming is an important factor to favor an efficient growth of the precipitating mass and correlates with snow accumulation rates at ground level. The time steps during which rimed precipitation is dominant are analyzed in terms of temporal evolution and vertical structure. Snowfall identified as rimed often appears after a short time period during which the atmospheric conditions favor wind gusts and updrafts and supercooled liquid water (SLW) is available. When a turbulent atmospheric layer persists for several hours and ensures continuous SLW generation, riming can be sustained longer and large accumulations of snow at ground level can be generated. The microphysical interpretation and the meteorological situation associated with one such event are detailed in the paper. The vertical structure of polarimetric radar observations during intense snowfall classified as rimed shows a peculiar maximum of specific differential phase shift K dp , associated with large number concentrations and riming of anisotropic crystals. Below this K dp peak there is usually an enhancement in radar reflectivity Z H , proportional to the K dp enhancement and interpreted as aggregation of ice crystals. These signatures seem to be recurring during intense snowfall.

High‐Resolution Large Eddy Simulation of Snow Accumulation in Alpine Terrain

“…This finding is consistent with the observations using polarimetric radar data reported by Mott et al () who identified local enhancement of snowfall around mountain ridges and summits due to the occurrence of rimed snow aggregates and graupel. Evidence of riming in alpine snowfall has been also reported by Grazioli et al () even if they do not discuss in details the spatial variability of precipitation generated by riming. The differences in terms of spatial variability between graupel and snow aggregates in our study depend partially on assumptions made in the cloud microphysical scheme used in Meso‐NH (Pinty & Jabouille, ).…”

Quantifying the effect of riming on snowfall using ground‐based observations

“…Harimaya and Sato [] estimate that this value is higher for the coastal region of Japan and ranges between 50% and 100%. Grazioli et al [] have shown that there seems to be a correlation between occurrence of riming and precipitation accumulation.…”

“…Mitchell et al [] and Harimaya and Sato [] have shown that riming could explain 30% to 100% of surface snowfall mass. Furthermore, Grazioli et al [] have found that there is also an apparent positive correlation between a precipitation rate and riming occurrence during winter storms.…”

McSnow: A Monte‐Carlo Particle Model for Riming and Aggregation of Ice Particles in a Multidimensional Microphysical Phase Space

“…Precipitation in midlatitudes originates mostly from mixed‐phase cloud systems and growth of ice particles by the collection of supercooled liquid drops. This process, known as riming, contributes significantly to surface precipitation, see e.g., Grazioli et al () or Moisseev et al () for some observational evidence based on state‐of‐the‐art measurement techniques. For example at the coastal areas of Japan, heavily rimed snow particles are in fact present in more than 70% of the snowfall events and contribute more than 50% to wintertime surface precipitation (Harimaya & Sato, ).…”