Dual-Polarization Radar Rainfall Estimation over Tropical Oceans

Abstract

Dual-polarization radar rainfall estimation relationships have been extensively tested in continental and subtropical coastal rain regimes, with little testing over tropical oceans where the majority of rain on Earth occurs. A 1.5-yr Indo-Pacific warm pool disdrometer dataset was used to quantify the impacts of tropical oceanic drop-size distribution (DSD) variability on dual-polarization radar variables and their resulting utility for rainfall estimation. Variables that were analyzed include differential reflectivity Zdr; specific differential phase Kdp; reflectivity Zh; and specific attenuation Ah. When compared with continental or coastal convection, tropical oceanic Zdr and Kdp values were more often of low magnitude (<0.5 dB, <0.3° km−1) and Zdr was lower for a given Kdp or Zh, consistent with observations of tropical oceanic DSDs being dominated by numerous, small, less-oblate drops. New X-, C-, and S-band R estimators were derived: R(Kdp), R(Ah), R(Kdp, ζdr), R(z, ζdr), and R(Ah, ζdr), which use linear versions of Zdr and Zh, namely ζdr and z. Except for R(Kdp), convective/stratiform partitioning was unnecessary for these estimators. All dual-polarization estimators outperformed updated R(z) estimators derived from the same dataset. The best-performing estimator was R(Kdp, ζdr), followed by R(Ah, ζdr) and R(z, ζdr). The R error was further reduced in an updated blended algorithm choosing between R(z), R(z, ζdr), R(Kdp), and R(Kdp, ζdr) depending on Zdr > 0.25 dB and Kdp > 0.3° km−1 thresholds. Because of these thresholds and the lack of hail, R(Kdp) was never used. At all wavelengths, R(z) was still needed 43% of the time during light rain (R < 5 mm h−1, Zdr < 0.25 dB), composing 7% of the total rain volume. As wavelength decreased, R(Kdp, ζdr) was used more often, R(z, ζdr) was used less often, and the blended algorithm became increasingly more accurate than R(z).

Effect of disdrometer type on rain drop size distribution characterisation: a new dataset for south-eastern Australia

“…An obvious relevant future investigation given the main outcomes of this study would be the study of the effect of small diameter droplets (< 0.6 mm) on integrated DSD variables and on the parameterisation of the DSD. As the present work has shown the capacity of the Thies LPM to capture this part of the drop size spectrum with high sensitivity, a study could build on this dataset and the recent findings from Thurai and Bringi (2018), and Raupach et al (2019) to study the parameterisation of the DSD, including the small droplets and the implication for retrievals such as Williams et al (2014). Another direction of research could be the comparison of the Thies LPM to other instruments that unlike the OTT Parsivel 1 or Parsivel 2 are able to measure concentration of small droplets.…”

“…Overall, co-located measurements using different instrument types, such as Thurai et al (2017), Angulo-Martinez et al (2018) and the present work, are one of the possible ways of at least analysing the discrepancies between different measurement sources and discussing possible biases of each method, therefore providing a more robust estimate of the DSD spectra and derived integrated variables. In particular, such long-term high-resolution datasets are of importance for testing existing and new parameterisations of the DSD (Raupach et al, 2019;Thurai and Bringi, 2018;Thurai et al, 2019), combine weakness and strengths of each sensor to derive more accurate DSD spectra or investigate the sampling and spatio-temporal characteristics of rainfall at the micro scale (Uijlenhoet et al, 2003a(Uijlenhoet et al, , b, 2006Jaffrain et al, 2011;Jaffrain and Berne, 2012a, b).…”

“…Typically, video disdrometers (two-dimensional video disdrometers or 2DVD) were initially considered the most reliable to accurately measure PSVD (Thurai et al, 2017), particularly for particles larger than 0.3 mm. Recent work has revisited this and now considers the 2DVD to significantly underestimate small particles in particular (Thurai and Bringi, 2018). Long-term deployments are often difficult and costprohibitive, and as a result these 2DVD instruments have typically been deployed for short-term research experiments.…”

“…Since all PSVD observations using sensing techniques are subject to biases and errors, one of the most relevant approaches to accurately determine DSD is to use co-located instruments, preferably of different types and brands, to compare and evaluate the estimates from different instrumental sources. Such approaches have been used to evaluate DSD measured simultaneously by Parsivel 1 only (Tapiador et al, 2010;Jaffrain and Berne, 2011;Jaffrain et al, 2011;Jaffrain and Berne, 2012a), Parsivel and 2DVD (Raupach and Berne, 2015), Meteorological Particle Spectrometer (or Sensor) and 2DVD (Thurai et al, 2017;Thurai and Bringi, 2018), Parsivel 1 and LPM (Petan et al, 2010) and Parsivel 2 and LPM (Angulo-Martinez et al, 2018). Work assessing the accuracy and biases of Parsivel has been widespread, while evaluation of the Thies LPM has received comparatively little attention (Brawn and Upton, 2008;Adirosi et al, 2018;Angulo-Martinez et al, 2018).…”

“…In Australia, DSD observations have been conducted for the tropical region near Darwin during extensive campaigns over the past 20 years (Dolan et al, 2013;Thompson et al, 2018). Furthermore, extensive campaigns have been conducted to monitor PSVD (Maki et al, 2001;Penide et al, 2013;Thurai et al, 2010), and recent dedicated research vessel campaigns over multiple years have led to characterisation of the Australian sector of the Southern Ocean, which influences Australian weather and climate (between latitudes of 40 and 65 • S; Mace and Protat, 2018a, b).…”