The Virga-Sniffer – a new tool to identify precipitation evaporation using ground-based remote-sensing observations

Abstract

Abstract. Combined continuous long-term ground-based remote-sensing observations with vertically pointing cloud radar and ceilometer are well-suited to identify precipitation evaporation fall streaks (so-called virga). Here we introduce the functionality and workflow 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. The Virga-Sniffer Python package is highly modular and configurable and can be applied to multilayer cloud situations. In the simplest approach, it detects virga from time-height fields of cloud radar reflectivity and time series of ceilometer cloud base height. In addition, optional parameters like lifting condensation level, a surface rain flag as well as time-height fields of cloud radar mean Doppler velocity can be added to refine virga event identifications. The netcdf-output files consist of Boolean flags of virga- and cloud detection, as well as base- and top heights and depth for the detected clouds and virga. The performance of the Virga-Sniffer was assessed by comparing its results to the Cloudnet target classification resulting from using the CloudnetPy processing chain. 88 % of the pixel identified as virga by the Virga Sniffer correspond to Cloudnet classifications of precipitation. The remaining 12 % of virga pixel correspond to Cloudnet-classifications of aerosols and insects (about 7 %), cloud droplets (3 %), or clear-sky (about 1 %). Some discrepancies of the virga identification and the Cloudnet target classification can be attributed to applied temporal smoothing. Additionally, it was found that Cloudnet mostly classified aerosols and insects at virga edges which points to a misclassification caused by CloudnetPy internal thresholds. For the RV Meteor observations during EUREC4A, about 50 % of all detected clouds with bases below the trade inversion were found to produce precipitation that evaporates before reaching the ground. The most important virga-producing clouds were either anvils of convective cells or stratocumulus clouds. 36 % of the detected virga originated from trade wind cumuli. Small virga with depths below 200 m most frequently occurred from shallow clouds with depths below 500 m, while virga depths above 1 km were mainly associated with clouds of larger depths, ranging between 500 and 1000 m. Virga depth showed no strong dependency on column-integrated liquid water path. The presented results substantiate the importance of low-level precipitation evaporation in the Atlantic lower winter trades. Possible 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. However, we envision extended use of the Virga-Sniffer for other cloud regimes or scientific foci as well.

Ground- and ship-based microwave radiometer measurements during EUREC4A

“…Aboard Meteor, the Leipzig Institute for Meteorology (LIM) of Leipzig University operated an MWR-type RPG-HATPRO Generation 5 (here referred to as LIMHAT) and a radar-radiometer system of type RPG-FMCW-94 dual polarization (DP), operating actively in the W-band (94 GHz) and containing a passive radiometer channel at 89 GHz (Küchler et al, 2017;Kalesse-Los et al, 2023; here referred to as LIMRAD). Both instruments were placed 4.5 m apart on the navigation deck of the ship at 15.8 m a.s.l.…”

“…LIMRAD was operated with two different radar settings as specified in Kalesse-Los et al (2023). Between 17 and 29 January 2020 as well as between 31 January and 28 February 2020, the temporal resolution of LIMRAD was 2.9 and 1.6 s, at a vertical resolution between 22 and 42 m, respectively.…”

“…In order to elucidate the underlying processes of the interactions, high-quality and fine-resolution observations were gathered during the EUREC 4 A field study in January and February 2020 (Stevens et al, 2021) over the tropical Atlantic, east of Barbados. A range of complementary atmospheric and oceanic measurements were performed by four different research aircraft (Konow et al, 2021;Bony et al, 2022;Pincus et al, 2021), as well as by ground-and shipbased observations (e.g., Acquistapace et al, 2022;Kalesse-Los et al, 2023). Microwave radiometric measurements were conducted at Barbados Cloud Observatory (BCO; Stevens et al, 2016), as well as aboard RV Meteor (hereafter referred to as Meteor) and the RV Maria S Merian (hereafter referred to as Merian).…”

“…During EUREC 4 A, BCOHAT measurements were complemented by HATPRO measurements aboard Meteor performed by the Leipzig Institute for Meteorology (LIM), here referred to as LIMHAT. Aboard Meteor, a 94 GHz cloud radar (Küchler et al, 2017) was installed, equipped with a passive radiometer channel measuring T B at 89 GHz (Kalesse-Los et al, 2023), here referred to as LIMRAD. A similar instrument was operated aboard Merian (Acquistapace et al, 2022), here referred to as MSMRAD.…”

The Virga-Sniffer – a new tool to identify precipitation evaporation using ground-based remote-sensing observations

“…The data measured on board RV Meteor during EUREC 4 A are hosted by the AERIS Portal https://observations.ipsl.fr/aeris/eurec4a/ (last access: 12 August 2022). This includes data products from the onboard meteorological station operated by DWD, the ceilometer (https://doi.org/10.25326/53, Jansen, 2020) operated by MPI for Meteorology Hamburg, and the Doppler cloud radar LIMRAD94 (https://doi.org/10.25326/164, Kalesse-Los et al, 2021), and the microwave radiometer LIMHAT (https://doi.org/10.25326/77, Kalesse-Los et al, 2020) operated by Leipzig University. CloudNet processing was done using CloudnetPy (https://doi.org/10.5281/zenodo.4011843, Tukiainen et al, 2020b;version 1.33.0, Tukiainen et al, 2020a).…”