Ice microphysical processes in the dendritic growth layer: a statistical analysis combining multi-frequency and polarimetric Doppler cloud radar observations

Abstract

Abstract. The dendritic growth layer (DGL), defined as the temperature region between −20 and −10 ∘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) (sZDRmax), 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 sZDRmax 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 ∘C towards the bottom of the DGL. Aggregation increases more rapidly from −15 ∘C towards warmer temperatures. Surprisingly, KDP and sZDRmax are not reduced by the intensifying aggregation below −15 ∘C but rather reach their maximum values in the lower half of the DGL. Also below the DGL, KDP and sZDRmax remain enhanced until −4 ∘C. Only there, additional aggregation appears to deplete ice crystals and therefore reduce KDP and sZDRmax. 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.

Distinct secondary ice production processes observed in radar Doppler spectra: insights from a case study

“…The temperature range in the region where this mode is formed (-15 to -12 • C) is compatible with planar growth of crystals by vapor deposition. It is worth noting that the DL3 signature is, however, different from the ones typically observed in the dendritic growth layer, in which a small updraft, an increase in reflectivity, and a persistent spectral bimodality are often reported (von Terzi et al, 2022), and which is occasionally observed during this case study (see for instance, Fig. 3, around -15 • C between 15:00 and 16:30, or the spectrogram in Fig.…”

“…By examining not only (S)LDR but also the other radar variables, additional insight can be gained. For instance, cloud droplets are often identified by their signature in the form of a narrow, low-reflectivity peak with Doppler velocity close to zero (Li and Moisseev, 2019;Li et al, 2021;von Terzi et al, 2022).…”

“…The threshold values were chosen based on the literature (e.g., for SLDR: Oue et al, 2018;Luke et al, 2021, for Z e : Kogan et al, 2005, for MDV: Li andMoisseev, 2019;von Terzi et al, 2022) after adjustments based on a few individual spectra from our case study. In particular, for Doppler velocity, using a stricter threshold led to discarding some profiles that were affected by radial air motion (e.g., downdrafts).…”

Radar‐Derived Snowfall Microphysical Properties at Davis, Antarctica

“…This mode was visible in the Doppler spectra obtained by von Terzi et al. (2022). Unfortunately the BASTA radar did not have Doppler spectra capabilities during the Davis campaign, preventing us ascertaining if secondary ice processes were present at these temperatures and thus also whether this might explain the reduced Doppler velocities within the DGL over Davis.…”

“…Lower Doppler velocities within the DGL were investigated recently by von Terzi et al. (2022) in the northern mid‐latitudes, who noted that part of the reason is due to updrafts from latent heat released by enhanced depositional growth. However, the main explanation for the reduction of Doppler velocities within the DGL is the generation of high concentrations of small ice crystals associated with secondary ice processes, which acts to reduce the mean velocities.…”