Linking rain into ice microphysics across the melting layer in stratiform rain: a closure study

Abstract

Abstract. This study investigates the link between rain and ice microphysics across the melting layer in stratiform rain systems using measurements from vertically pointing multi-frequency Doppler radars. A novel methodology to examine the variability of the precipitation rate and the mass-weighted melted diameter (Dm) across the melting region is proposed and applied to a 6 h long case study, observed during the TRIPEx-pol field campaign at the Jülich Observatory for Cloud Evolution Core Facility and covering a gamut of ice microphysical processes. The methodology is based on an optimal estimation (OE) retrieval of particle size distributions (PSDs) and dynamics (turbulence and vertical motions) from observed multi-frequency radar Doppler spectra applied both above and below the melting layer. First, the retrieval is applied in the rain region; based on a one-to-one conversion of raindrops into snowflakes, the retrieved drop size distributions (DSDs) are propagated upward to provide the mass-flux-preserving PSDs of snow. These ice PSDs are used to simulate radar reflectivities above the melting layer for different snow models and they are evaluated for a consistency with the actual radar measurements. Second, the OE snow retrieval where Doppler spectra are simulated based on different snow models, which consistently compute fall speeds and electromagnetic properties, is performed. The results corresponding to the best-matching models are then used to estimate snow fluxes and Dm, which are directly compared to the corresponding rain quantities. For the case study, the total accumulation of rain (2.30 mm) and the melted equivalent accumulation of snow (1.93 mm) show a 19 % difference. The analysis suggests that the mass flux through the melting zone is well preserved except the periods of intense riming where the precipitation rates were higher in rain than in the ice above. This is potentially due to additional condensation within the melting zone in correspondence to high relative humidity and collision and coalescence with the cloud droplets whose occurrence is ubiquitous with riming. It is shown that the mean mass-weighted diameter of ice is strongly related to the characteristic size of the underlying rain except the period of extreme aggregation where breakup of melting snowflakes significantly reduces Dm. The proposed methodology can be applied to long-term observations to advance our knowledge of the processes occurring across the melting region; this can then be used to improve assumptions underpinning spaceborne radar precipitation retrievals.

Melting Behavior of Rimed and Unrimed Snowflakes Investigated With Statistics of Triple‐Frequency Doppler Radar Observations

“…Our results are overall in agreement with previous studies using the ZFR approach (Drummond et al., 1996; Gatlin et al., 2018; Mróz et al., 2021). The only previous statistical analysis of ZFRs by Gatlin et al.…”

“…(1996) and Mróz et al. (2021) indicated a tendency for growth processes to dominate. Only for intense precipitation events and unrimed particles, breakup of large melting snowflakes can overtake the initial additional aggregation inside the ML.…”

“…The corrected data set has been used and described before by Mróz et al. (2021), who also studied ML processes on a single day from the campaign, and Vogel et al. (2021), who applied a neural network to identify riming events on several selected days.…”

Coincident in situ and triple-frequency radar airborne observations in the Arctic

“…The 3 dB beamwidths and vertical sampling of the Ka-and X-band radar are 4.2 • and 30 m as well as 4.5 • and 30 m, respectively, whereas those of the W band are 0.75 • The habits correspond to state-of-the-art scattering models: the first habit is a mixture of aggregates from the database described in Kuo et al (2016); the next 14 habits are extracted from the ARTS scattering database (Eriksson et al, 2018); the last three habits are from the models of Leinonen and Szyrmer (2015). For the first two classes of models, scattering properties are computed via discrete dipole approximation for gamma PSD with the shape parameter µ equal −2, 0, and 8 (same symbols); for the last class the self-similar Rayleigh-Gans approximation (for the corresponding coefficients see details in Mróz et al, 2021a) is used with exponential PSDs. The characteristic mean mass-weighted maximum size of the particle size distribution increases with the curve moving out from the origin (that corresponds to Rayleigh particles with all DFRs being equal to zero).…”

Triple frequency radar retrieval of microphysical properties of snow

“…Because different frequency radars respond differently to the microphysical properties of snow (once their wavelengths become comparable with the size of snow aggregates) multi-frequency algorithms were recognised as a potential tool for solid phase precipitation studies (Hogan et al, 2000;Kneifel et al, 2011). Over the years, the availability of data from complex multi-frequency Doppler radar systems has fostered the development of algorithms based on dual frequency reflectivity (e.g., Matrosov, 1998), triple frequency reflectivity (e.g., Leinonen et al, 2018;Tridon et al, 2019;Battaglia et al, 2020b), dual frequency reflectivity and Doppler measurements (e.g., Mason et al, 2018) and even full Doppler spectral information (e.g., Mroz et al, 2021a). An increase in number of observables included in the inversion schemes went hand in hand with an increase in number of retrieved microphysical parameters.…”