Testing the DSD-Based Stratiform-Convective Rain Separation for Ten Events in Greeley, Colorado

Bringi et al. (2009) had proposed a DSD-based separation of stratiform and convective (S-C) rain. The method has been tested for case events in Darwin, Australia (Thurai et al. 2010), as well as Ontario, Canada and Huntsville, Alabama (Thurai et al. 2016). In this paper, we consider events in Greeley, Colorado, (1.4 km MSL) where a rain measurement campaign was conducted from April to October 2015. The campaign entailed a 2D video disdrometer (2DVD), a meteorological particle spectrometer (MPS), a pluvio gauge, all installed inside a double fence (DFIR) located 13 km SSE of the CSU-CHILL radar (S and X bands, dual polarization). While the MPS, with its 50-micron resolution, enabled measurements of small drops, typically for drop diameters below about 1.1 mm, the 2DVD provided accurate measurements for drop diameters above 0.7 mm (Thurai et al., 2017). The combined spectra are analyzed in terms of the DSD parameters, namely, the normalized intercept parameter N_W, the mass-weighted mean diameter D_m. CHILL RHI scans are used to classify rain types as well as KFTG volume scans for some cases.

Testing of the S-C separation has been carried out for 10 events, with various durations, lasting from several minutes to several hours. The events include stratiform rain with varying bright-band thickness, drizzle, weak and strong convective rain. 2DVD contour images were used to exclude any DSDs which had any melting hydrometeors.

One minute DSDs, averaged over 3 minutes, are used to derive the N_W versus D_m based S-C classification. When compared with CHILL-based classification, 9 cases were found to agree with the Bringi et al. (2009) separation method. The one exception was a weak convective event which had stratiform rain in surrounding areas.

We will present a summary of all 10 events. The N_W - D_mvariation will also be stratified by CHILL S-band reflectivity and differential reflectivity over the instrument site as well as rain rates from the collocated Pluvio.

References

Bringi, V. N., C. R. Williams, M. Thurai, and P. T. May, 2009: Using dual-polarized radar and dual-frequency profiler for DSD characterization: A case study from Darwin, Australia. J. Atmos. Oceanic Technol., 26, 2107–2122.

Thurai, M., V. N. Bringi, and P. T. May, 2010: CPOL radar-derived drop size distribution statistics of stratiform and convective rain for two regimes in Darwin, Australia. J. Atmos. Oceanic Technol., 27, 932–942.

Thurai, M., P.N. Gatlin, and V.N. Bringi, 2016: Separating stratiform and convective rain types based on the drop size distribution characteristics using 2D video disdrometer data. Atmospheric Research 169, 416-423.

Thurai, M. P. Gatlin, V. N. Bringi, W. Petersen, P. Kennedy, B. Notaroš, and L. Carey, 2017: Toward Completing the Raindrop Size Spectrum: Case Studies Involving 2D-Video Disdrometer, Droplet Spectrometer, and Polarimetric Radar Measurements, Journal of Applied Meteorology and Climatology, Volume 56 No. 4, 877–896.