First applications of the Virga-Sniffer – a new tool to identify precipitation evaporation using ground-based remote-sensing observations

The dominant cloud type in the subtropical Atlantic is the trade wind cumulus with a cloud base located near the lifting condensation level (LCL) below 1 km. Other common clouds in this region with their base above 1 km are stratiform cloud layers or cloud edges near the trade wind inversion at 2-3 km. Precipitation in all these clouds mainly forms at temperatures above freezing point by collision and coalescence. Therefore, precipitation generally occurs as light rain/drizzle from stratiform cloud layers or as showers from well-developed trade wind cumuli. Precipitation underneath a cloud base is often visible as fall streaks. If the precipitation evaporates before reaching the ground, these fall streaks are called virga.

Combined continuous long-term ground-based remote-sensing observations with vertically pointing cloud radar and ceilometer are well-suited to identify these precipitation evaporation fall streaks. Here we show the first application of a new open-source tool, the Virga-Sniffer which was developed within the frame of RV Meteor observations during the ElUcidating the RolE of Cloud–Circulation Coupling in ClimAte (EUREC⁴A) field experiment in Jan–Feb 2020 in the Tropical Western Atlantic. In the simplest approach, it detects virga from time-height fields of cloud radar reflectivity and time series of ceilometer cloud base height. The Virga Sniffer was applied to RV Meteor observations during EUREC⁴A and statistical results as well as an evaporation case study are presented. Spectral W-band radar data from a fall streak, identified as virga by the Virga-Sniffer, was used to calculate evaporative cooling rates. Sensitivity studies were performed to investigate the influence of vertical wind and relative humidity uncertainties.  Possible future applications of the Virga-Sniffer within the frame of EUREC⁴A include detailed studies of precipitation evaporation with a focus on cold pools or cloud organization, or distinguishing moist processes based on water vapor isotopic observations.