Browsing by Subject "CLOUD"

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  • Vakkari, Ville; Manninen, Antti J.; O'Connor, Ewan; Schween, Jan H.; Van Zyl, Pieter G.; Marinou, Eleni (2019)
    Commercially available Doppler lidars have now been proven to be efficient tools for studying winds and turbulence in the planetary boundary layer. However, in many cases low signal-to-noise ratio is still a limiting factor for utilising measurements by these devices. Here, we present a novel post-processing algorithm for Halo Stream Line Doppler lidars, which enables an improvement in sensitivity of a factor of 5 or more. This algorithm is based on improving the accuracy of the instrumental noise floor and it enables longer integration times or averaging of high temporal resolution data to be used to obtain signals down to -32 dB. While this algorithm does not affect the measured radial velocity, it improves the accuracy of radial velocity uncertainty estimates and consequently the accuracy of retrieved turbulent properties. Field measurements using three different Halo Doppler lidars deployed in Finland, Greece and South Africa demonstrate how the new post-processing algorithm increases data availability for turbulent retrievals in the planetary boundary layer, improves detection of high-altitude cirrus clouds and enables the observation of elevated aerosol layers.
  • Manninen, A. J.; Marke, T.; Tuononen, M.; O'Connor, E. J. (2018)
    We present a method using Doppler lidar data for identifying the main sources of turbulent mixing within the atmospheric boundary layer. The method identifies the presence of turbulence and then assigns a turbulent source by combining several lidar quantities: attenuated backscatter coefficient, vertical velocity skewness, dissipation rate of turbulent kinetic energy, and vector wind shear. Both buoyancy-driven and shear-driven situations are identified, and the method operates in both clear-sky and cloud-topped conditions, with some reservations in precipitation. To capture the full seasonal cycle, the classification method was applied to more than 1year of data from two sites, Hyytiala, Finland, and Julich, Germany. Analysis showed seasonal variation in the diurnal cycle at both sites; a clear diurnal cycle was observed in spring, summer, and autumn seasons, but due to their respective latitudes, a weaker cycle in winter at Julich, and almost non-existent at Hyytiala. Additionally, there are significant contributions from sources other than convective mixing, with cloud-driven mixing being observed even within the first 500m above ground. Also evident is the considerable amount of nocturnal mixing within the lowest 500m at both sites, especially during the winter. The presence of a low-level jet was often detected when sources of nocturnal mixing were diagnosed as wind shear. The classification scheme and the climatology extracted from the classification provide insight into the processes responsible for mixing within the atmospheric boundary layer, how variable in space and time these can be, and how they vary with location. Key Points
  • Kalivitis, N.; Kerminen, Veli-Matti; Kouvarakis, G.; Stavroulas, I.; Bougiatioti, A.; Nenes, A.; Manninen, Hanna; Petäjä, Tuukka; Kulmala, Markku; Mihalopoulos, N. (2015)
    While cloud condensation nuclei (CCN) production associated with atmospheric new particle formation (NPF) is thought to be frequent throughout the continental boundary layers, few studies on this phenomenon in marine air exist. Here, based on simultaneous measurement of particle number size distributions, CCN properties and aerosol chemical composition, we present the first direct evidence on CCN production resulting from NPF in the eastern Mediterranean atmosphere. We show that condensation of both gaseous sulfuric acid and organic compounds from multiple sources leads to the rapid growth of nucleated particles to CCN sizes in this environment during the summertime. Sub-100 nm particles were found to be substantially less hygroscopic than larger particles during the period with active NPF and growth (the value of kappa was lower by 0.2-0.4 for 60 nm particles compared with 120 nm particles), probably due to enrichment of organic material in the sub-100 nm size range. The aerosol hygroscopicity tended to be at minimum just before the noon and at maximum in the afternoon, which was very likely due to the higher sulfate-to-organic ratios and higher degree of oxidation of the organic material during the afternoon. Simultaneous with the formation of new particles during daytime, particles formed during the previous day or even earlier were growing into the size range relevant to cloud droplet activation, and the particles formed in the atmosphere were possibly mixed with long-range-transported particles.
  • Kalske, Miika; Mäkitalo, Niko; Mikkonen, Tommi (Springer, Cham, 2018)
    Lecture Notes in Computer Science
    One of the more recent avenues towards more flexible installations and execution is the transition from monolithic architecture to microservice architecture. In such architecture, where microservices can be more liberally updated, relocated, and replaced, building liquid software also becomes simpler, as adaptation and deployment of code is easier than when using a monolithic architecture where almost everything is connected. In this paper, we study this type of transition. The objective is to identify the reasons why the companies decide to make such transition, and identify the challenges that companies may face during this transition. Our method is a survey based on different publications and case studies conducted about these architectural transitions from monolithic architecture to microservices. Our findings reveal that typical reasons moving towards microservice architecture are complexity, scalability and code ownership. The challenges, on the other hand, can be separated to architectural challenges and organizational challenges. The conclusion is that when a software company grows big enough in size and starts facing problems regarding the size of the codebase, that is when microservices can be a good way to handle the complexity and size. Even though the transition provides its own challenges, these challenges can be easier to solve than the challenges that monolithic architecture presents to company.
  • Harju, Jorma; Pineda, Jaime E.; Vasyunin, Anton; Caselli, Paola; Offner, Stella S. R.; Goodman, Alyssa A.; Juvela, Mika; Sipilä, Olli; Faure, Alexandre; Le Gal, Romane; Hily-Blant, Pierre; Alves, Joao; Bizzocchi, Luca; Burkert, Andreas; Chen, Hope; Friesen, Rachel K.; Guesten, Rolf; Myers, Philip C.; Punanova, Anna; Rist, Claire; Rosolowsky, Erik; Schlemmer, Stephan; Shirley, Yancy; Spezzano, Silvia; Vastel, Charlotte; Wiesenfeld, Laurent (2020)
    We present Atacama Large Millimeter/submillimeter Array maps of the starless molecular cloud core Ophiuchus/H-MM1 in the lines of deuterated ammonia (ortho-NH2D), methanol (CH3OH), and sulfur monoxide (SO). The dense core is seen in NH2D emission, whereas the CH3OH and SO distributions form a halo surrounding the core. Because methanol is formed on grain surfaces, its emission highlights regions where desorption from grains is particularly efficient. Methanol and sulfur monoxide are most abundant in a narrow zone that follows the eastern side of the core. This side is sheltered from the stronger external radiation field coming from the west. We show that photodissociation on the illuminated side can give rise to an asymmetric methanol distribution but that the stark contrast observed in H-MM1 is hard to explain without assuming enhanced desorption on the shaded side. The region of the brightest emission has a wavy structure that rolls up at one end. This is the signature of Kelvin-Helmholtz instability occurring in sheared flows. We suggest that in this zone, methanol and sulfur are released as a result of grain-grain collisions induced by shear vorticity.
  • Tuononen, Minttu; O'Connor, Ewan J.; Sinclair, Victoria A. (2019)
    The presence of clouds and their characteristics have a strong impact on the radiative balance of the Earth and on the amount of solar radiation reaching the Earth's surface. Many applications require accurate forecasts of surface radiation on weather timescales, for example solar energy and UV radiation forecasts. Here we investigate how operational forecasts of low and mid-level clouds affect the accuracy of solar radiation forecasts. A total of 4 years of cloud and solar radiation observations from one site in Helsinki, Finland, are analysed. Cloud observations are obtained from a ceilometer and therefore we first develop algorithms to reliably detect cloud base, precipitation, and fog. These new algorithms are widely applicable for both operational use and research, such as in-cloud icing detection for the wind energy industry and for aviation. The cloud and radiation observations are compared to forecasts from the Integrated Forecast System (IFS) run operationally and developed by the European Centre for Medium-Range Weather Forecasts (ECMWF). We develop methods to evaluate the skill of the cloud and radiation forecasts. These methods can potentially be extended to hundreds of sites globally. Over Helsinki, the measured global horizontal irradiance (GHI) is strongly influenced by its northerly location and the annual variation in cloudiness. Solar radiation forecast error is therefore larger in summer than in winter, but the relative error in the solar radiation forecast is more or less constant throughout the year. The mean overall bias in the GHI forecast is positive (8 W m(-2)). The observed and forecast distributions in cloud cover, at the spatial scales we are considering, are strongly skewed towards clear-sky and overcast situations. Cloud cover forecasts show more skill in winter when the cloud cover is predominantly overcast; in summer there are more clear-sky and broken cloud situations. A negative bias was found in forecast GHI for correctly forecast clear-sky cases and a positive bias in correctly forecast overcast cases. Temporal averaging improved the cloud cover forecast and hence decreased the solar radiation forecast error. The positive bias seen in overcast situations occurs when the model cloud has low values of liquid water path (LWP). We attribute this bias to the model having LWP values that are too low or the model optical properties for clouds with low LWP being incorrect.
  • Molteni, Ugo; Simon, Mario; Heinritzi, Martin; Hoyle, Christopher R.; Bernhammer, Anne-Kathrin; Bianchi, Federico; Breitenlechner, Martin; Brilke, Sophia; Dias, António; Duplissy, Jonathan; Frege, Carla; Gordon, Hamish; Heyn, Claudia; Jokinen, Tuija; Kürten, Andreas; Lehtipalo, Katrianne; Makhmutov, Vladimir; Petäjä, Tuukka; Pieber, Simone M.; Praplan, Arnaud P.; Schobesberger, Siegfried; Steiner, Gerhard; Stozhkov, Yuri; Tomé, António; Tröstl, Jasmin; Wagner, Andrea C.; Wagner, Robert; Williamson, Christina; Yan, Chao; Baltensperger, Urs; Curtius, Joachim; Donahue, Neil M.; Hansel, Armin; Kirkby, Jasper; Kulmala, Markku; Worsnop, Douglas R.; Dommen, Josef (2019)
    Terpenes are emitted by vegetation, and their oxidation in the atmosphere is an important source of secondary organic aerosol (SOA). A part of this oxidation can proceed through an autoxidation process, yielding highly oxygenated organic molecules (HOMs) with low saturation vapor pressure. They can therefore contribute, even in the absence of sulfuric acid, to new particle formation (NPF). The understanding of the autoxidation mechanism and its kinetics is still far from complete. Here, we present a mechanistic and kinetic analysis of mass spectrometry data from α-pinene (AP) ozonolysis experiments performed during the CLOUD 8 campaign at CERN. We grouped HOMs in classes according to their identified chemical composition and investigated the relative changes of these groups and their components as a function of the reagent concentration. We determined reaction rate constants for the different HOM peroxy radical reaction pathways. The accretion reaction between HOM peroxy radicals was found to be extremely fast. We developed a pseudo-mechanism for HOM formation and added it to the AP oxidation scheme of the Master Chemical Mechanism (MCM). With this extended model, the observed concentrations and trends in HOM formation were successfully simulated.
  • Juvela, Mika (2020)
    Context. Radiative transfer (RT) modelling is part of many astrophysical simulations. It is used to make synthetic observations and to assist the analysis of observations. We concentrate on modelling the radio lines emitted by the interstellar medium. In connection with high-resolution models, this can be a significant computationally challenge.Aims. Our aim is to provide a line RT program that makes good use of multi-core central processing units (CPUs) and graphics processing units (GPUs). Parallelisation is essential to speed up computations and to enable large modelling tasks with personal computers.Methods. The program LOC is based on ray-tracing (i.e. not Monte Carlo) and uses standard accelerated lambda iteration methods for faster convergence. The program works on 1D and 3D grids. The 1D version makes use of symmetries to speed up the RT calculations. The 3D version works with octree grids, and to enable calculations with large models, is optimised for low memory usage.Results. Tests show that LOC results agree with other RT codes to within similar to 2%. This is typical of code-to-code differences, which are often related to different interpretations of the model set-up. LOC run times compare favourably especially with those of Monte Carlo codes. In 1D tests, LOC runs were faster by up to a factor similar to 20 on a GPU than on a single CPU core. In spite of the complex path calculations, a speed-up of up to similar to 10 was also observed for 3D models using octree discretisation. GPUs enable calculations of models with hundreds of millions of cells, as are encountered in the context of large-scale simulations of interstellar clouds.Conclusions. LOC shows good performance and accuracy and is able to handle many RT modelling tasks on personal computers. It is written in Python, with only the computing-intensive parts implemented as compiled OpenCL kernels. It can therefore also a serve as a platform for further experimentation with alternative RT implementation details.
  • Pentikäinen, Pyry; O’Connor, Ewan J.; Manninen, Antti; Ortiz Amezcua, Pablo (2020)
    Doppler lidars provide two measured parameters, radial velocity and signal-to-noise ratio, from which winds and turbulent properties are routinely derived. Attenuated backscatter, which gives quantitative information on aerosols, clouds, and precipitation in the atmosphere, can be used in conjunction with the winds and turbulent properties to create a sophisticated classification of the state of the atmospheric boundary layer. Calculating attenuated backscatter from the signal-to-noise ratio requires accurate knowledge of the telescope focus function, which is usually unavailable. Inaccurate assumptions of the telescope focus function can significantly deform attenuated backscatter profiles, even if the instrument is focused at infinity. Here, we present a methodology for deriving the telescope focus function using a co-located ceilometer for pulsed heterodyne Doppler lidars. The method was tested with Halo Photonics StreamLine and StreamLine XR Doppler lidars but should also be applicable to other pulsed heterodyne Doppler lidar systems. The method derives two parameters of the telescope focus function, the effective beam diameter and the effective focal length of the telescope. Additionally, the method provides uncertainty estimates for the retrieved attenuated backscatter profile arising from uncertainties in deriving the telescope function, together with standard measurement uncertainties from the signal-to-noise ratio. The method is best suited for locations where the absolute difference in aerosol extinction at the ceilometer and Doppler lidar wavelengths is small.
  • Scolini, C.; Rodriguez, L.; Mierla, M.; Pomoell, J.; Poedts, S. (2019)
    Context. Coronal mass ejections (CMEs) are the primary source of strong space weather disturbances at Earth. Their geo-effectiveness is largely determined by their dynamic pressure and internal magnetic fields, for which reliable predictions at Earth are not possible with traditional cone CME models. Aims. We study two well-observed Earth-directed CMEs using the EUropean Heliospheric FORecasting Information Asset (EUH-FORIA) model, testing for the first time the predictive capabilities of a linear force-free spheromak CME model initialised using parameters derived from remote-sensing observations. Methods. Using observation-based CME input parameters, we performed magnetohydrodynamic simulations of the events with EU-HFORIA, using the cone and spheromak CME models. Results. Simulations show that spheromak CMEs propagate faster than cone CMEs when initialised with the same kinematic parameters. We interpret these differences as the result of different Lorentz forces acting within cone and spheromak CMEs, which lead to different CME expansions in the heliosphere. Such discrepancies can be mitigated by initialising spheromak CMEs with a reduced speed corresponding to the radial speed only. Results at Earth provide evidence that the spheromak model improves the predictions of B (B-z) by up to 12-60 (22-40) percentage points compared to a cone model. Considering virtual spacecraft located within +/- 10 degrees around Earth, B (Bz) predictions reach 45-70% (58-78%) of the observed peak values. The spheromak model shows inaccurate predictions of the magnetic field parameters at Earth for CMEs propagating away from the Sun-Earth line. Conclusions. The spheromak model successfully predicts the CME properties and arrival time in the case of strictly Earth-directed events, while modelling CMEs propagating away from the Sun-Earth line requires extra care due to limitations related to the assumed spherical shape. The spatial variability of modelling results and the typical uncertainties in the reconstructed CME direction advocate the need to consider predictions at Earth and at virtual spacecraft located around it.
  • Ade, P. A. R.; Aghanim, N.; Armitage-Caplan, C.; Arnaud, M.; Ashdown, M.; Atrio-Barandela, F.; Aumont, J.; Baccigalupi, C.; Banday, A. J.; Barreiro, R. B.; Bartlett, J. G.; Battaner, E.; Benabed, K.; Benoit, A.; Benoit-Levy, A.; Bernard, J. -P.; Bersanelli, M.; Bielewicz, P.; Bobin, J.; Bock, J. J.; Bonaldi, A.; Bond, J. R.; Borrill, J.; Bouchet, F. R.; Boulanger, F.; Bridges, M.; Bucher, M.; Burigana, C.; Butler, R. C.; Cardoso, J. -F.; Catalano, A.; Chamballu, A.; Chary, R. -R.; Chen, X.; Chiang, H. C.; Chiang, L. -Y; Christensen, P. R.; Church, S.; Clements, D. L.; Colley, J. -M.; Colombi, S.; Colombo, L. P. L.; Juvela, M.; Keihanen, E.; Keskitalo, R.; Kurki-Suonio, H.; Poutanen, T.; Suur-Uski, A. -S.; Tuovinen, J.; Väliviita, Jussi (2014)
  • Eden, D. J.; Liu, Tie; Kim, Kee-Tae; Juvela, M.; Liu, S. -Y.; Tatematsu, K.; Di Francesco, J.; Wang, K.; Wu, Y.; Thompson, M. A.; Fuller, G. A.; Li, Di; Ristorcelli, I.; Kang, Sung-ju; Hirano, N.; Johnstone, D.; Lin, Y.; He, J. H.; Koch, P. M.; Sanhueza, Patricio; Qin, S. -L.; Zhang, Q.; Goldsmith, P. F.; Evans, N. J.; Yuan, J.; Zhang, C. -P.; White, G. J.; Choi, Minho; Lee, Chang Won; Toth, L. V.; Mairs, S.; Yi, H. -W.; Tang, M.; Soam, A.; Peretto, N.; Samal, M. R.; Fich, M.; Parsons, H.; Malinen, J.; Bendo, G. J.; Rivera-Ingraham, A.; Liu, H. -L.; Wouterloot, J.; Li, P. S.; Qian, L.; Rawlings, J.; Rawlings, M. G.; Feng, S.; Wang, B.; Li, Dalei; Liu, M.; Luo, G.; Marston, A. P.; Pattle, K. M.; Pelkonen, V. -M.; Rigby, A. J.; Zahorecz, S.; Zhang, G.; Bogner, R.; Aikawa, Y.; Akhter, S.; Alina, D.; Bell, G.; Bernard, J. -P.; Blain, A.; Bronfman, L.; Byun, D. -Y.; Chapman, S.; Chen, H. -R.; Chen, M.; Chen, W. -P.; Chen, X.; Chen, Xuepeng; Chrysostomou, A.; Chu, Y. -H.; Chung, E. J.; Cornu, D.; Cosentino, G.; Cunningham, M. R.; Demyk, K.; Drabek-Maunder, E.; Doi, Y.; Eswaraiah, C.; Falgarone, E.; Feher, O.; Fraser, H.; Friberg, P.; Garay, G.; Ge, J. X.; Gear, W. K.; Greaves, J.; Guan, X.; Harvey-Smith, L.; Hasegawa, T.; He, Y.; Henkel, C.; Hirota, T.; Holland, W.; Hughes, A.; Jarken, E.; Ji, T. -G.; Jimenez-Serra, I.; Kang, M.; Kawabata, K. S.; Kim, Gwanjeong; Kim, Jungha; Kim, Jongsoo; Kim, S.; Koo, B. -C.; Kwon, Woojin; Kuan, Y. -J.; Lacaille, K. M.; Lai, S. -P.; Lee, C. F.; Lee, J. -E.; Lee, Y. -U.; Li, H.; Lo, N.; Lopez, J. A. P.; Lu, X.; Lyo, A. -R.; Mardones, D.; McGehee, P.; Meng, F.; Montier, L.; Montillaud, J.; Moore, T. J. T.; Morata, O.; Moriarty-Schieven, G. H.; Ohashi, S.; Pak, S.; Park, Geumsook; Paladini, R.; Pech, G.; Qiu, K.; Ren, Z. -Y.; Richer, J.; Sakai, T.; Shang, H.; Shinnaga, H.; Stamatellos, D.; Tang, Y. -W.; Traficante, A.; Vastel, C.; Viti, S.; Walsh, A.; Wang, H.; Wang, J.; Ward-Thompson, D.; Whitworth, A.; Wilson, C. D.; Xu, Y.; Yang, J.; Yuan, Y. -L.; Yuan, L.; Zavagno, A.; Zhang, C.; Zhang, G.; Zhang, H. -W.; Zhou, C.; Zhou, J.; Zhu, L.; Zuo, P. (2019)
    We present the first release of the data and compact-source catalogue for the JCMT Large Program SCUBA-2 Continuum Observations of Pre-protostellar Evolution (SCOPE). SCOPE consists of 850 mu m continuum observations of 1235 Planck Galactic Cold Clumps (PGCCs) made with the Submillimetre Common-User Bolometer Array 2 on the James Clerk Maxwell Telescope. These data are at an angular resolution of 14.4 arcsec, significantly improving upon the 353 GHz resolution of Planck at 5 arcmin, and allowing for a catalogue of 3528 compact sources in 558 PGCCs. We find that the detected PGCCs have significant sub-structure, with 61 per cent of detected PGCCs having three or more compact sources, with filamentary structure also prevalent within the sample. A detection rate of 45 per cent is found across the survey, which is 95 per cent complete to Planck column densities of N-H2 > 5 x10(21) cm(-2). By positionally associating the SCOPE compact sources with young stellar objects, the star formation efficiency, as measured by the ratio of luminosity to mass, in nearby clouds is found to be similar to that in the more distant Galactic Plane, with the column density distributions also indistinguishable from each other.
  • Alina, D.; Ristorcelli, I.; Montier, L.; Abdikamalov, E.; Juvela, M.; Ferriere, K.; Bernard, J. -Ph.; Micelotta, E. R. (2019)
    We present a statistical study of the relative orientation in the plane of the sky between interstellar magnetic fields and filaments hosting cold clumps. For the first time, we consider both the density of the environment and the density contrast between the filaments and their environment. Moreover, we geometrically distinguish between the clumps and the remaining portions of the filaments. We infer the magnetic field orientations in the filaments and in their environment from the Stokes parameters, 1 assuming optically thin conditions. Thus, we analyse the relative orientations between filaments, embedded clumps, internal and background magnetic fields, depending on their environment and evolutionary stages. We recover the previously observed trend for filaments in low column density environments to be aligned parallel to the background magnetic field; however, we find that this trend is significant only for low-contrast filaments, whereas high-contrast filaments tend to be randomly orientated with respect to the background magnetic field. Filaments in high column density environments do not globally show any preferential orientation, although low-contrast filaments alone tend to have perpendicular relative orientation with respect to the background magnetic field. For a subsample of nearby filaments, for which volume densities can be derived, we find a clear transition in the relative orientation with increasing density, at n(H) similar to 10(3) cm(-3), changing from mostly parallel to mostly perpendicular in the off-clump portions of filaments and from even to bimodal in clumps. Our results confirm a strong interplay between interstellar magnetic fields and filaments during their formation and evolution.
  • Tonttila, J.; O'Connor, E. J.; Hellsten, A.; Hirsikko, A.; O'Dowd, C.; Jarvinen, H.; Räisänen, P. (2015)
    The turbulent structure of a stratocumulus-topped marine boundary layer over a 2-day period is observed with a Doppler lidar at Mace Head in Ireland. Using profiles of vertical velocity statistics, the bulk of the mixing is identified as cloud driven. This is supported by the pertinent feature of negative vertical velocity skewness in the sub-cloud layer which extends, on occasion, almost to the surface. Both coupled and decoupled turbulence characteristics are observed. The length and timescales related to the cloud-driven mixing are investigated and shown to provide additional information about the structure and the source of the mixing inside the boundary layer. They are also shown to place constraints on the length of the sampling periods used to derive products, such as the turbulent dissipation rate, from lidar measurements. For this, the maximum wavelengths that belong to the inertial subrange are studied through spectral analysis of the vertical velocity. The maximum wavelength of the inertial subrange in the cloud-driven layer scales relatively well with the corresponding layer depth during pronounced decoupled structure identified from the vertical velocity skewness. However, on many occasions, combining the analysis of the inertial subrange and vertical velocity statistics suggests higher decoupling height than expected from the skewness profiles. Our results show that investigation of the length scales related to the inertial subrange significantly complements the analysis of the vertical velocity statistics and enables a more confident interpretation of complex boundary layer structures using measurements from a Doppler lidar.
  • Li, Haoran; Moisseev, Dmitri (2020)
    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.