Two Layers of Melting Ice Particles Within a Single Radar Bright Band: Interpretation and Implications

Abstract

Dual‐frequency dual‐polarization radar observations of the melting of two ice populations in a stratiform rainfall event are presented. The observed phenomenon occurs as a two‐layer linear depolarization ratio (LDR) signature in a single radar bright band. Doppler spectra observations show that the upper LDR layer is caused by the melting of ice needles, potentially generated by the rime‐splintering process, while the lower one is mainly due to the melting of background ice particles formed at the cloud top. The melting signal of small needles acts as a unique benchmark for detecting the onset of melting and is used to verify the current methods for the identification of melting layer boundaries. The radar‐derived characteristics of the melting layer are found to be dependent on the radar variable and frequency used. The implications of the presented findings for radar‐based studies of precipitation properties in and above the melting layer are also discussed.

The prevalence of precipitation from polar supercooled clouds

“…We note that because the R CB retrievals, described by the third moment of the size distribution (IWC) weighted by V D , are based on radar reflectivity measurements (sixth moment), the uncertainties associated with the resultant R CB values are likely to be smaller than in equivalent Z e -based retrievals of ice crystal number concentration (zeroth moment) (see Ulbrich, 1983). The Arctic and Antarctic sites represent relatively contrasting polar conditions, with a drier, colder, and more pristine atmosphere over McMurdo Station (e.g., Bromwich et al, 2012;Lubin et al, 2020;Shupe et al, 2011;Silber et al, 2018a). Despite these atmospheric state differences between the two sites, the general similarity of Arctic and Antarctic precipitation occurrence reported here at cloud base strongly suggests that they are regionally representative at least to some degree.…”

“…To detect supercooled cloud layers, we use 6 or 12 hourly soundings acquired at Utqiaġvik (formerly Barrow), North Slope of Alaska (NSA; Verlinde et al, 2016), from November 2011 to April 2019 and 12 hourly soundings acquired at McMurdo Station, Antarctica, between December 2015 and January 2017 (Lubin et al, 2020). After linearly interpolating onto a 15 m grid, supercooled layers are identified where atmospheric temperature is between 0 and −40 • C and relative humidity (RH) exceeds 95 % over at least two adjacent grid cells, consistent with an RH uncertainty of 5 % (Holdridge et al, 2011).…”

“…The overall differences in detected Arctic versus Antarctic precipitation frequency ( Fig. 1) are likely influenced by geographical INP variability associated with both long-range transport and local source regions (e.g., Vergara-Temprado et al, 2018), as well as differing cloud temperatures (e.g., Lubin et al, 2020;Scott and Lubin, 2016), which impact INP activation (e.g., Kanji et al, 2017;Knopf et al, 2018).…”

Multifrequency radar observations of clouds and precipitation including the G-band

“…SKYLER, like a typical centimeter wavelength radar, observed a bright band marked by clear boundaries at both the top and the bottom. Inferring information about the ice-melting process from the properties of the radar-detected bright band is still an active area of research (e.g., Heymsfield et al, 2015;Li et al, 2020). The early work of Fabry and Zawadzki (1995) suggested that the magnitude and vertical extent of the radar reflectivity enhancement at centimeter-wavelength radars are influenced by precipitation rate, phase transitions (i.e., liquid coating ice), change in fall speed throughout melting, precipitation growth and changes in the particle size distribution linked to aggregation and breakup.…”

Assessing the Effect of Riming on Snow Microphysics: The First Observational Study in East China

“…Riming is an important snow growth process and has an impact on the physical properties of ice particles (Barthazy & Schefold, 2006; Erfani & Mitchell, 2016; Grazioli et al., 2015; Li et al., 2018; Moisseev et al., 2017). Riming can also impact the microphysical properties of the melting layer (Li & Moisseev, 2020; Li et al., 2020; Mroz et al., 2020) and rainfall below the melting layer (Lin et al., 2020; Sarma et al., 2016). Furthermore, riming is affected by anthropogenic aerosols (Borys et al., 2003; Jackson et al., 2012), which is also supported by the ECHAM4 general circulation model simulations (Lohmann, 2004).…”