Universität zu Köln

von Terzi, Leonie ORCID: 0000-0002-7054-4164, Dias Neto, Jose ORCID: 0000-0002-8488-8486, Ori, Davide ORCID: 0000-0002-9964-2200, Myagkov, Alexander and Kneifel, Stefan ORCID: 0000-0003-2220-2968 (2022). Ice microphysical processes in the dendritic growth layer: a statistical analysis combining multi-frequency and polarimetric Doppler cloud radar observations. Atmos. Chem. Phys., 22 (17). S. 11795 - 11822. GOTTINGEN: COPERNICUS GESELLSCHAFT MBH. ISSN 1680-7324

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Abstract

The dendritic growth layer (DGL), defined as the temperature region between -20 and -10 degrees C, plays an important role for ice depositional growth, aggregation and potentially secondary ice processes. The DGL has been found in the past to exhibit specific observational signatures in polarimetric and vertically pointing radar observations. However, consistent conclusions about their physical interpretation have often not been reached. In this study, we exploit a unique 3-months dataset of mid-latitude winter clouds observed with vertically pointing triple-frequency (X-, Ka-, W-band) and polarimetric W-band Doppler radars. In addition to standard radar moments, we also analyse the multi-wavelength and polarimetric Doppler spectra. New variables, such as the maximum of the spectral differential reflectivity (ZDR) (sZDR(max)), allows us to analyse the ZDR signal of asymmetric ice particles independent of the presence of low ZDR producing aggregates. This unique dataset enables us to investigate correlations between enhanced aggregation and evolution of small ice particles in the DGL. For this, the multi-frequency observations are used to classify all profiles according to their maximum average aggregate size within the DGL. The strong correlation between aggregate class and specific differential phase shift (KDP) confirms the expected link between ice particle concentration and aggregation. Interestingly, no correlation between aggregation class and sZDR(max) is visible. This indicates that aggregation is rather independent of the aspect ratio and density of ice crystals. A distinct reduction of mean Doppler velocity in the DGL is found to be strongest for cases with largest aggregate sizes. Analyses of spectral edge velocities suggest that the reduction is the combined result of the formation of new ice particles with low fall velocity and a weak updraft. It appears most likely that this updraft is the result of latent heat released by enhanced depositional growth. Clearly, the strongest correlations of aggregate class with other variables are found inside the DGL. Surprisingly, no correlation between aggregate class and concentration or aspect ratio of particles falling from above into the DGL could be found. Only a weak correlation between the mean particle size falling into the DGL and maximum aggregate size within the DGL is apparent. In addition to the correlation analysis, the dataset also allows study of the evolution of radar variables as a function of temperature. We find the ice particle concentration continuously increasing from -18 degrees C towards the bottom of the DGL. Aggregation increases more rapidly from -15 degrees C towards warmer temperatures. Surprisingly, KDP and sZDR(max) are not reduced by the intensifying aggregation below -15 degrees C but rather reach their maximum values in the lower half of the DGL. Also below the DGL, KDP and sZDR(max) remain enhanced until -4 degrees C. Only there, additional aggregation appears to deplete ice crystals and therefore reduce KDP and sZDR(max). The simultaneous increase of aggregation and particle concentration inside the DGL necessitates a source mechanism for new ice crystals. As primary ice nucleation is expected to decrease towards warmer temperatures, secondary ice processes are a likely explanation for the increase in ice particle concentration. Previous laboratory experiments strongly point towards ice collisional fragmentation as a possible mechanism for new particle generation. The presence of an updraft in the temperature region of maximum depositional growth might also suggest an important positive feedback mechanism between ice microphysics and dynamics which might further enhance ice particle growth in the DGL.

Item Type:	Journal Article
Creators:	Creators Email ORCID ORCID Put Code von Terzi, Leonie UNSPECIFIED orcid.org/0000-0002-7054-4164 UNSPECIFIED Dias Neto, Jose UNSPECIFIED orcid.org/0000-0002-8488-8486 UNSPECIFIED Ori, Davide UNSPECIFIED orcid.org/0000-0002-9964-2200 UNSPECIFIED Myagkov, Alexander UNSPECIFIED UNSPECIFIED UNSPECIFIED Kneifel, Stefan UNSPECIFIED orcid.org/0000-0003-2220-2968 UNSPECIFIED
URN:	urn:nbn:de:hbz:38-676046
DOI:	10.5194/acp-22-11795-2022
Journal or Publication Title:	Atmos. Chem. Phys.
Volume:	22
Number:	17
Page Range:	S. 11795 - 11822
Date:	2022
Publisher:	COPERNICUS GESELLSCHAFT MBH
Place of Publication:	GOTTINGEN
ISSN:	1680-7324
Language:	English
Faculty:	Unspecified
Divisions:	Unspecified
Subjects:	no entry
Uncontrolled Keywords:	Keywords Language W-BAND RADAR; MELTING LAYER; SNOW CRYSTALS; X-BAND; PARTICLE CONCENTRATIONS; VERTICAL PROFILES; HABIT PREDICTION; WINTER STORMS; FREQUENCY; AGGREGATION Multiple languages Environmental Sciences; Meteorology & Atmospheric Sciences Multiple languages
URI:	http://kups.ub.uni-koeln.de/id/eprint/67604