Intense Aggregation Above the Melting Layer Observed With Novel Triple-Frequency Radars

Ice aggregation is a key cloud microphysical process which is still not entirely understood. Laboratory studies and field observations reveal that particularly strong aggregation happens close to the dendritic growth zone (around -15°C) as well as close to the melting layer where temperatures close to 0°C enhance the stickiness of the ice particles.

In this study, we analyze ground-based observations from three vertically pointing radars (X, Ka and W band) obtained during a recent field campaign (TRIPEx, Nov. 2015 - Jan. 2016) at the Jülich Observatory of Cloud Evolution (JOYCE) site in Jülich, Germany. The transition from Rayleigh to Mie scatterers happens at different sizes depending on the radar frequency. Therefore, the radar reflectivity difference between two frequencies can provide informations about the mean snowflake size. Recent comparisons with in-situ data also revealed that triple frequency radar observations allow to distinguish between certain snow particle classes such as unrimed and rimed aggregates.

In this study, we show the analysis of several stratiform mid-latitude winter cases where the triple-frequency signatures give an indication of extremely intense aggregation starting already at 1 km above the melting layer. Particularly intense reflectivity differences are observed between X and Ka-Band (up to 20 dB) and rather moderate differences between Ka and W-Band (below 10 dB). Further, during the analyzed cases, only the X-band reflectivity vertical profile shows the classical bright band peak, whereas the other two frequencies (Ka and W) appear to decrease in absolute value towards the melting layer.

We have performed an extensive modeling study to investigate the potential sources of the observed reflectivity anomalies. When common inverse exponential distributions are assumed, we have been unable to reproduce either the measured reflectivity differences or the general vertical reflectivity profiles. We have been able to match the observed signatures only by assuming gamma size distributions including very large snow aggregates (up to several cm in size) which is in agreement with the onset of very intensive aggregation processes on top of the melting layer. These observational fingerprints of intense aggregation will be the basis for comparisons with detailed 1D Monte Carlo microphysical model simulations where we will test the impact of the different parameters influencing aggregation on the forward simulated radar signatures.