Wanda Szyrmer

Biogenic and Anthropogenic Sources of Ice-Forming Nuclei: A Review

Abstract To develop theories and numerical models of the formation and microstructure of clouds and precipitation, it is necessary to identify the potential sources of ice nuclei in the atmosphere. However, the subject remains an area of debate. According to the most accepted theory, the great majority of atmospheric ice nuclei constitute soil mineral particles. But some evidence appears to favor the hypothesis of a nonnegligible contribution to the population of effective ice nuclei made by biogenic material, living or dead. Moreover, some specific human activities have been identified as prolific sources of particles on which ice crystals can be generated. In contrast, it has also been suggested that some anthropogenic effluents deactivate nuclei naturally occurring in the atmosphere. This paper summarizes present knowledge about the biogenic and anthropogenic sources of atmospheric ice nuclei. Recent research reveals an increasingly greater variety of sources and activities of ice nuclei. However intri...

Modeling of the Melting Layer. Part II: Electromagnetic

To complement the meteorological modeling of the melting layer, a model of the scattering properties at microwave frequencies for snow, melting snow, and rain is implemented. The scattering model, running in tandem with a meteorological model, generates the reflectivity fields associated with the hydrometeors in the model to facilitate comparisons with available observations. Several existing and a few new approaches for the scattering of melting snow are attempted. In addition, the models are run using several relationships for the density of snowflakes as a function of their size. A large variability in the prediction of the brightband intensity is observed as a function of the scattering model. However, the scattering model whose melting snow morphology resembles most the one of real snowflakes reproduces the available observations with the highest accuracy. Sensitivity to the snowflake density relationship used is found to be less important. Other features like the melting-layer thickness, brightband peak position, and Doppler velocity are also correctly predicted.

Modeling of the melting layer. Part I : Dynamics and microphysics

Abstract To obtain the full description of the dynamical and microphysical finescale structures required for the computation of the radar-derived brightband parameters, a numerical model has been developed. A bulk microphysics module was introduced into a nonhydrostatic, fully compressible dynamic framework. A microphysical parameterization scheme, with five water categories (vapor, cloud water, snow, melting snow, and rain), describes the interactions related to the evolution of the melting layer (melting and diffusional exchanges of mass of each hydrometeor category). Dynamic, thermodynamic, and microphysical processes are fully coupled. The main characteristics of the bulk parameterization scheme for melting of snow are the following: 1) wet snow is described by its water content and by an additional prognostic variable, namely, the diameter of the smallest snowflake not yet completely melted; 2) the fall velocity of the melting snowflakes is based on the laboratory observations; and 3) a size-dependen...

A General Approach to Double-Moment Normalization of Drop Size Distributions

Normalization of drop size distributions (DSDs) is reexamined here. First, an extension of the scaling normalization that uses one moment of the DSD as a scaling parameter to a more general scaling normalization that uses two moments as scaling parameters of the normalization is presented. In addition, the proposed formulation includes all two-parameter normalizations recently introduced in the literature. Thus, a unified vision of the question of DSD normalization and a good model representation of DSDs are given. Data analysis of some convective and stratiform DSDs shows that, from the point of view of the compact representation of DSDs, the double-moment normalization is preferred. However, in terms of physical interpretation, the scaling exponent of the single-moment normalization clearly indicates two different rain regimes, whereas in the double-moment normalization the two populations are not readily separated. It is also shown that DSD analytical models (exponential, gamma, and generalized gamma DSD) have the same scaling properties, indicating that the scaling formalism of DSDs is a very general way of describing DSDs.

Modeling of the Melting Layer. Part III: The Density Effect

A model of the melting snow and its radar reflectivity is presented here. The main addition to previous description of the melting layer is the explicit introduction of snow density as a variable. The model is validated with radar observations. Differences in brightband intensity for comparable precipitation rates are related here to the coexistence of supercooled cloud water (SCW) with snow above the melting level leading to riming and change in snow density. Cases where riming was suspected were selected according to the characteristics of the vertical profile of reflectivity flux above the melting layer and vertical Doppler velocities faster than expected from low-density snow. For stratiform precipitation with a melting layer, high snow-to-rain velocity ratio indicates high-density snow and consequently a small peak-to-rain reflectivity difference is expected. This relationship was computed from the model and confirmed with vertically pointing radar observations. In spite of the complexity of the physical processes present in the melting layer the model appears to capture the essential elements.

Observations of supercooled water and secondary ice generation by a vertically pointing X-band Doppler radar

When a vertically pointing Doppler radar (VPDR) observes two groups of hydrometeors having distinct fall velocities, the observed Doppler spectra are bimodal. This bimodality has been repeatedly observed in snow. At altitudes higher than the level at which the bimodality appears, the reflectivity of the snow rapidly increases at the same time as its fall velocity reaches values beyond those expected from the aggregation of snow crystals. This is indicative of riming by supercooled cloud droplets. We show here that the spectral bimodality observed in the snow after indication of riming may be associated with secondary ice generation or with supercooled drizzle. Higher snow content favors the formation of secondary ice, while in situations of low snow content, the presence of the supercooled cloud may lead to the formation of drizzle. In both cases, the bimodality indicates the significant content of the supercooled cloud. This type of observation can be used operationally to detect the supercooled water coexisting with the snow.

Snow Studies. Part II: Average Relationship between Mass of Snowflakes and Their Terminal Fall Velocity

Abstract This study uses a dataset of low-density snow aggregates measurements collected by a ground-based optical disdrometer that provides particle size and terminal fall speed for each size interval from which the velocity–size and area ratio–size relationships can be derived. From these relationships and relations between the Best and Reynolds numbers proposed in the literature, the mass power-law coefficients are obtained. Then, an approximate average relation between the coefficients in the experimentally determined velocity–size power law (with exponent fixed at 0.18) and the coefficients in the estimated mass power law (with exponent fixed at 2) is obtained. The validation of the retrieved relation is made by comparing, for each snowfall event, the time series of the reflectivity factor calculated from the derived mass–size relationship for a snowflake and from the size distribution measured by the optical disdrometer, with the reflectivity obtained from measurements. Using the measured snow size ...

Radar signatures of snowflake riming: A modeling study

The capability to detect the state of snowflake riming reliably from remote measurements would greatly expand the understanding of its global role in cloud‐precipitation processes. To investigate the ability of multifrequency radars to detect riming, a three‐dimensional model of snowflake growth was used to generate simulated aggregate and crystal snowflakes with various degrees of riming. Three different growth scenarios, representing different temporal relationships between aggregation and riming, were formulated. The discrete dipole approximation was then used to compute the radar backscattering properties of the snowflakes at frequencies of 9.7, 13.6, 35.6, and 94 GHz. In two of the three growth scenarios, the rimed snowflakes exhibit large differences between the backscattering cross sections of the detailed three‐dimensional models and the equivalent homogeneous spheroidal models, similarly to earlier results for unrimed snowflakes. When three frequencies are used simultaneously, riming appears to be detectable in a robust manner across all three scenarios. In spite of the differences in backscattering cross sections, the triple‐frequency signatures of heavily rimed particles resemble those of the homogeneous spheroids, thus explaining earlier observational results that were compatible with such spheroids.

Diagnostic of Supercooled Clouds from Single-Doppler Observations in Regions of Radar-Detectable Snow

Liquid water is produced in the updraft regions of subfreezing clouds when the generation of vapor excess over the water saturation value exceeds the vapor depletion through the depositional growth of the solid particles. A diagnostic technique for the presence of supercooled cloud in the presence of snow is presented here. The data required are single-Doppler observations of reflectivity and radial velocity as well as a nearby sounding of temperature. From these data, the 3D wind field is retrieved by a variational method. From the retrieved vertical motion, the supercooled water is derived from the steady-state balance relation between snow content and cloud liquid water. The method is tested with a kinematic model that includes the main microphysical processes expected to occur in stratiform subfreezing conditions. A comparison between aircraft in situ measurements of supercooled water content and the diagnosed as well as model-generated values shows good agreement.

A Microphysical Bulk Formulation Based on Scaling Normalization of the Particle Size Distribution. Part I: Description

Abstract The authors address the problem of optimization of the microphysical information extracted from a simulation system composed of high-resolution numerical models and multiparameter radar data or other available measurements. As a tool in the exploration of this question, a bulk microphysical scheme based on the general approach of scaling normalization of particle size distribution (PSD) is proposed. This approach does not rely on a particular functional form imposed on the PSD and naturally leads to power-law relationships between the PSD moments providing an accurate and compact PSD representation. To take into account the possible evolution of the shape/curvature of the distribution, ignored within standard one- and two-moment microphysical schemes, a new three-moment scheme based on the two-moment scaling normalization is proposed. The methodology of the moment retrieval included in the three-moment scheme can also be useful as a retrieval algorithm combining different remote sensing observati...