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Matteo Colli
,
Luca G. Lanza
,
Roy Rasmussen
, and
Julie M. Thériault

Abstract

The aerodynamic response of snow gauges when exposed to the wind is responsible for a significant reduction of their collection performance. The modifications induced by the gauge and the windshield onto the space–time patterns of the undisturbed airflow deviate the snowflake trajectories. In Part I, the disturbed air velocity field in the vicinity of shielded and unshielded gauge configurations is investigated. In Part II, the airflow is the basis for a particle tracking model of snowflake trajectories to estimate the collection efficiency. A Geonor T-200B gauge inside a single Alter shield is simulated for wind speeds varying from 1 to 8 m s−1. Both time-averaged and time-dependent computational fluid dynamics simulations are performed, based on Reynolds-averaged Navier–Stokes (RANS) and large-eddy simulation (LES) models, respectively. A shear stress tensor k–Ω model (where k is the turbulent kinetic energy and Ω is the turbulent specific dissipation rate) is used for the RANS formulation and solved within a finite-volume method. The LES is implemented with a Smagorinsky subgrid-scale method that models the subgrid stresses as a gradient-diffusion process. The RANS simulations confirm the attenuation of the airflow velocity above the gauge when using a single Alter shield, but the generated turbulence above the orifice rim is underestimated. The intensity and spatial extension of the LES-resolved turbulent region show a dependency on the wind speed that was not detected by the RANS. The time-dependent analysis showed the propagation of turbulent structures and the impact on the turbulent kinetic energy above the gauge collecting section.

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Matteo Colli
,
Luca G. Lanza
,
Roy Rasmussen
, and
Julie M. Thériault

Abstract

The use of windshields to reduce the impact of wind on snow measurements is common. This paper investigates the catching performance of shielded and unshielded gauges using numerical simulations. In Part II, the role of the windshield and gauge aerodynamics, as well as the varying flow field due to the turbulence generated by the shield–gauge configuration, in reducing the catch efficiency is investigated. This builds on the computational fluid dynamics results obtained in Part I, where the airflow patterns in the proximity of an unshielded and single Alter shielded Geonor T-200B gauge are obtained using both time-independent [Reynolds-averaged Navier–Stokes (RANS)] and time-dependent [large-eddy simulation (LES)] approaches. A Lagrangian trajectory model is used to track different types of snowflakes (wet and dry snow) and to assess the variation of the resulting gauge catching performance with the wind speed. The collection efficiency obtained with the LES approach is generally lower than the one obtained with the RANS approach. This is because of the impact of the LES-resolved turbulence above the gauge orifice rim. The comparison between the collection efficiency values obtained in case of shielded and unshielded gauge validates the choice of installing a single Alter shield in a windy environment. However, time-dependent simulations show that the propagating turbulent structures produced by the aerodynamic response of the upwind single Alter blades have an impact on the collection efficiency. Comparison with field observations provides the validation background for the model results.

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Julie M. Thériault
,
Roy Rasmussen
,
Kyoko Ikeda
, and
Scott Landolt

Abstract

Accurate snowfall measurements are critical for a wide variety of research fields, including snowpack monitoring, climate variability, and hydrological applications. It has been recognized that systematic errors in snowfall measurements are often observed as a result of the gauge geometry and the weather conditions. The goal of this study is to understand better the scatter in the snowfall precipitation rate measured by a gauge. To address this issue, field observations and numerical simulations were carried out. First, a theoretical study using finite-element modeling was used to simulate the flow around the gauge. The snowflake trajectories were investigated using a Lagrangian model, and the derived flow field was used to compute a theoretical collection efficiency for different types of snowflakes. Second, field observations were undertaken to determine how different types, shapes, and sizes of snowflakes are collected inside a Geonor, Inc., precipitation gauge. The results show that the collection efficiency is influenced by the type of snowflakes as well as by their size distribution. Different types of snowflakes, which fall at different terminal velocities, interact differently with the airflow around the gauge. Fast-falling snowflakes are more efficiently collected by the gauge than slow-falling ones. The correction factor used to correct the data for the wind speed is improved by adding a parameter for each type of snowflake. The results show that accurate measure of snow depends on the wind speed as well as the type of snowflake observed during a snowstorm.

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Jaclyn M. Ritzman
,
Terry Deshler
,
Kyoko Ikeda
, and
Roy Rasmussen

Abstract

Annual precipitation increases of 10% or more are often quoted for the impact of winter orographic cloud seeding; however, establishing the basis for such values is problematic for two reasons. First, the impact of glaciogenic seeding of candidate orographic storms has not been firmly established. Second, not all winter precipitation is produced by candidate “seedable” storms. Addressing the first question motivated the Wyoming state legislature to fund a multiyear, crossover, randomized cloud-seeding experiment in southeastern Wyoming to quantify the impact of glaciogenic seeding of wintertime orographic clouds. The crossover design requires two barriers, one randomly selected for seeding, for comparisons of seeded and nonseeded precipitation under relatively homogeneous atmospheric conditions. Addressing the second question motivated the work here. The seeding criteria—700-hPa temperatures ≤−8°C, 700-hPa winds between 210° and 315°, and the presence of supercooled liquid water—were applied to eight winters to determine the percent of winter precipitation that may fall under the seeding criteria. Since no observational datasets provide precipitation and all of the atmospheric variables required for this study, a regional climate model dynamical downscaling of historical data over 8 years was used. The accuracy of the model was tested against several measurements, and the small model biases were removed. On average, ~26% of the time between 15 November and 15 April atmospheric conditions were seedable over the barriers in southeastern Wyoming. These seedable conditions were accompanied by precipitation ~12%–14% of the time, indicating that ~27%–30% of the winter precipitation resulted from seedable clouds.

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Kyoko Ikeda
,
Roy M. Rasmussen
,
Edward Brandes
, and
Frank McDonough

Abstract

This study describes a freezing drizzle detection algorithm based on the Weather Surveillance Radar-1988 Doppler (WSR-88D) measured radar reflectivity. Although radar returns from freezing drizzle and light snow are similar—<5 dBZ and spatially uniform—freezing drizzle can be identified using feature parameters computed from radar reflectivity, such as local and global standard deviations and reflectivity texture weighted with a fuzzy-logic scheme. Algorithm results agree well with surface precipitation reports. The proposed algorithm can serve as one component of automated decision-support schemes for icing hazard detection and/or hydrometeor identification.

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Ben C. Bernstein
,
Roy M. Rasmussen
,
Frank McDonough
, and
Cory Wolff

Abstract

Using observations from research aircraft flights over the Great Lakes region, synoptic and mesoscale environments that appear to drive a relationship between liquid water content, drop concentration, and drop size are investigated. In particular, conditions that fell within “small drop” and “large drop” regimes are related to cloud and stability profiles, providing insight regarding whether the clouds are tied to the local boundary layer. These findings are supported by analysis of flight data from other parts of North America and used to provide context for several icing incidents and accidents where large-drop icing was noted as a contributing factor. The relationships described for drop size discrimination in continental environments provide clues that can be applied for both human- and model-generated icing forecasts, as well as automated icing algorithms.

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Gregory Thompson
,
Marcia K. Politovich
, and
Roy M. Rasmussen

Abstract

Recent advances in high-performance computing have enabled higher-resolution numerical weather models with increasingly complex data assimilation and more accurate physical parameterizations. With respect to aircraft and ground icing applications, a weather model’s cloud physics scheme is responsible for the direct forecasts of the water phase and amount and is a critical ingredient to forecasting future icing conditions. In this paper, numerical model results are compared with aircraft observations taken during icing research flights, and the general characteristics of liquid water content, median volume diameter, droplet concentration, and temperature within aircraft icing environments are evaluated. The comparison reveals very promising skill by the model in predicting these characteristics consistent with observations. The application of model results to create explicit forecasts of ice accretion rates for an example case of aircraft and ground icing is shown.

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Brooks E. Martner
,
Robert M. Rauber
,
Roy M. Rasmussen
,
Erwin T. Prater
, and
Mohan K. Ramamurthy

A winter storm that crossed the continental United States in mid-February 1990 produced hazardous weather across a vast area of the nation. A wide range of severe weather was reported, including heavy snowfall; freezing rain and drizzle; thunderstorms with destructive winds, lightning, large hail, and tornadoes; prolonged heavy rain with subsequent flooding; frost damage to citrus orchards; and sustained destructive winds not associated with thunderstorms. Low-end preliminary estimates of impacts included 9 deaths, 27 injuries, and $120 million of property damage. At least 35 states and southeastern Canada were adversely affected. The storm occurred during the field operations of four independent atmospheric research projects that obtained special, detailed observations of it from the Rocky Mountains to the eastern Great Lakes.

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Roy M. Rasmussen
,
Ben C. Bernstein
,
Masataka Murakami
,
Greg Stossmeister
,
Jon Reisner
, and
Boba Stankov

Abstract

The mesoscale and microscale structure and evolution of a shallow, upslope cloud is described using observations obtained during the Winter Icing and Storms Project (WISP) and model stimulations. The upslope cloud formed within a shallow arctic air mass that moved into the region east of the Rocky Mountains between 12 and 16 February and contained significant amounts of supercooled liquid water for nearly 30 h. Two distinct layers were evident in the cloud. The lower layer was near neutral stability (boundary layer air) and contained easterly upslope flow. The upper layer (frontal transition zone) was thermodynamically stable and contained southerly flow. Overlying the upslope cloud was a dry, southwesterly flow of 20–25 m s −1, resulting in strong wind shear near cloud top. Within 10 km of the Rocky Mountain barrier, easterly low-level flow was lifted up and over the mountains. The above-described kinematic and thermodynamic structure produced three distinct mechanisms leading to the production of supercooled liquid water: 1) upslope flow over the gently rising terrain leading into the Colorado Front Range, up the slopes of the Rocky Mountains and over local ridges, 2)upglide flow within a frontal transition zone, and 3) turbulent mixing in the boundary layer. Supercooled liquid water was also produced by 1) upward motion at the leading edge of three cold surges and 2) vertical motion produced by low-level convergence in the surface wind field. Large cloud droplets were present near the top of this cloud (approximately 50-µm diameter), which grew by a direct coalescence process into freezing drizzle in regions of the storm where the liquid water content was greater than 0.25 g m −3 and vertical velocity was at 10 cm s −1

Ice crystal concentrations greater than 1 L−1 were observed in the lower cloud layer containing boundary layer air when the top of the boundary layer air when the top of the boundary layer was colder than −12°C. The upper half of the cloud was ice-free despite temperatures as low as −15°C, resulting in long-lived supercooled liquid water in this region of the cloud.

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Edward A. Brandes
,
Kyoko Ikeda
,
Guifu Zhang
,
Michael Schönhuber
, and
Roy M. Rasmussen

Abstract

Winter-storm hydrometeor distributions along the Front Range in eastern Colorado are studied with a ground-based two-dimensional video disdrometer. The instrument provides shape, size, and terminal velocity information for particles that are larger than about 0.4 mm. The dataset is used to determine the form of particle size distributions (PSDs) and to search for useful interrelationships among the governing parameters of assumed distribution forms and environmental factors. Snowfalls are dominated by almost spherical aggregates having near-exponential or superexponential size distributions. Raindrop size distributions are more peaked than those for snow. A relation between bulk snow density and particle median volume diameter is derived. The data suggest that some adjustment may be needed in relationships found previously between temperature and the concentration and slope parameters of assumed exponential PSDs. A potentially useful relationship is found between the slope and shape terms of the gamma PSD model.

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