Identification of snowfall microphysical processes from Eulerian vertical gradients of polarimetric radar variables

Abstract

Abstract. Polarimetric radar systems are commonly used to study the microphysics of precipitation. While they offer continuous measurements with a large spatial coverage, retrieving information about the microphysical processes that govern the evolution of snowfall from the polarimetric signal is challenging. The present study develops a new method, called process identification based on vertical gradient signs (PIVSs), to spatially identify the occurrence of the main microphysical processes (aggregation and riming, crystal growth by vapor deposition and sublimation) in snowfall from dual-polarization Doppler radar scans. We first derive an analytical framework to assess in which meteorological conditions the local vertical gradients of radar variables reliably inform about microphysical processes. In such conditions, we then identify regions dominated by (i) vapor deposition, (ii) aggregation and riming and (iii) snowflake sublimation and possibly snowflake breakup, based on the sign of the local vertical gradients of the reflectivity ZH and the differential reflectivity ZDR. The method is then applied to data from two frontal snowfall events, namely one in coastal Adélie Land, Antarctica, and one in the Taebaek Mountains in South Korea. The validity of the method is assessed by comparing its outcome with snowflake observations, using a multi-angle snowflake camera, and with the output of a hydrometeor classification, based on polarimetric radar signal. The application of the method further makes it possible to better characterize and understand how snowfall forms, grows and decays in two different geographical and meteorological contexts. In particular, we are able to automatically derive and discuss the altitude and thickness of the layers where each process prevails for both case studies. We infer some microphysical characteristics in terms of radar variables from statistical analysis of the method output (e.g., ZH and ZDR distribution for each process). We, finally, highlight the potential for extensive application to cold precipitation events in different meteorological contexts.

MASCDB, a database of images, descriptors and microphysical properties of individual snowflakes in free fall

“…MASC data have been exploited to propose three dimensional reconstruction techniques 22 , 23 and have been more in general proven useful to better describe, quantify and interpret the microphysical properties of falling snow 3 , 24 – 27 .…”

A 7-year record of vertical profiles of radar measurements and precipitation estimates at Dumont d’Urville, Adélie Land, East Antarctica

“…6, the equivalent reflectivity variability is 3.5 dBz (around 40 % of the median profile) at 300 m and decreases slightly with altitude ; the mean Doppler Both interannual and seasonal variability are larger near the surface than in altitude. In fact, we expect differences between precipitation events to emerge during the aggregation and riming processes (Planat et al (2021)) and thus be more pronounced in the lower gates. Moreover, turbulence in the katabatic layer (below 1 km or so) is likely to impact the hydrometeors fall speed, and to increase the mean Doppler velocity variability.…”

“…The median equivalent reflectivity in Fig. 4a ranges from 5.5 dBz in altitude, then increases as ice crystals grow through deposition and aggregation and densify through riming (Planat et al (2021)) towards a maxima of 10.2 dBz at 800 m. Then, reflectivity decreases slightly to 9.7 dBz at 300 m due to snowflake sublimation by low-level dry air blowing from the plateau (katabatic flow). The physics of this process is discussed in Grazioli et al (2017b).…”

Innovative systems for the detection of air particles in the quarries of the Western Rif, Morocco

“…Particle size and form are regarded as crucial factors; in the study of the effect of particulates on human health and the the environment, small particles present a greater health threat than large ones [19]. As the industrialization progresses and the car park increases, the air pollution problem from gas spray and airborne matter has become more challenging with Morocco's economic progress in the past few years, and the problem is bound to increase in the future [20].…”