Search Results

You are looking at 21 - 30 of 56 items for

  • Author or Editor: Roy M. Rasmussen x
  • All content x
Clear All Modify Search
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.

Full access
Kyoko Ikeda, Roy M. Rasmussen, William D. Hall, and Gregory Thompson

Abstract

Observations of supercooled drizzle aloft within two storms impacting the Oregon Cascades during the second Improvement of Microphysical Parameterization through Observational Verification Experiment (IMPROVE-2) field project are presented. The storms were characterized by a structure and evolution similar to the split-front model of synoptic storms. Both storms were also characterized by strong cross-barrier flow. An analysis of aircraft and radar data indicated the presence of supercooled drizzle during two distinct storm periods: 1) the intrafrontal period immediately following the passage of an upper cold front and 2) the postfrontal period. The conditions associated with these regions of supercooled drizzle included 1) temperatures between −3° and −19°C, 2) ice crystal concentrations between 1 and 2 L−1, and 3) bimodal cloud droplet distributions of low concentration [cloud condensation nuclei (CCN) concentration between 20 and 30 cm−3 and cloud drop concentration <35 cm−3].

Unique to this study was the relatively cold cloud top (<−15°C) and relatively high ice crystal concentrations in the drizzle region. These conditions typically hinder drizzle formation and survival; however, the strong flow over the mountain barrier amplified vertical motions (up to 2 m s−1) above local ridges, the mountain crest, and updrafts in embedded convection. These vertical motions produced high condensate supply rates that were able to overcome the depletion by the higher ice crystal concentrations. Additionally, the relatively high vertical motions resulted in a near balance of ice crystal fall speed (0.5–1.0 m s−1), leading to nearly terrain-parallel trajectories of the ice particles and a reduction of the flux of ice crystals from the higher levels into the low-level moisture-rich cloud, allowing the low-level cloud water and drizzle to be relatively undepleted.

One of the key observations in the current storms was the persistence of drizzle drops in the presence of significant amounts of ice crystals over the steepest portion of the mountain crest. Despite the high radar reflectivity produced by the ice crystals (>15 dBZ) in this region, the relatively high condensate supply rate led to hazardous icing conditions. The current study reveals that vertical motions generated by local topographic features are critical in precipitation processes such as drizzle formation and thus it is essential that microphysical models predict these motions.

Full access
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.

Full access
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.

Full access
Nicolas R. Leroux, Julie M. Thériault, and Roy Rasmussen

Abstract

The collection efficiency of a typical precipitation gauge-shield configuration decreases with increasing wind speed, with a high scatter for a given wind speed. The high scatter in the collection efficiency for a given wind speed arises in part from the variability in the characteristics of falling snow and atmospheric turbulence. This study uses weighing gauge data collected at the Marshall Field Site near Boulder, Colorado, during the WMO Solid Precipitation Intercomparison Experiment (SPICE). Particle diameter and fall speed data from a laser disdrometer were used to show that the scatter in the collection efficiency can be reduced by considering the fall speed of solid precipitation particles. The collection efficiency was divided into two classes depending on the measured mean-event particle fall speed during precipitation events. Slower-falling particles were associated with a lower collection efficiency. A new transfer function (i.e., the relationship between collection efficiency and other meteorological variables, such as wind speed or air temperature) that includes the fall speed of the hydrometeors was developed. The root-mean-square error of the adjusted precipitation with the new transfer function with respect to a weighing gauge placed in a double fence intercomparison reference was lower than using previously developed transfer functions that only consider wind speed and air temperature. This shows that the measured fall speed of solid precipitation with a laser disdrometer accounts for a large amount of the observed scatter in weighing gauge collection efficiency.

Open access
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.

Full access
Kevin E. Trenberth, Aiguo Dai, Roy M. Rasmussen, and David B. Parsons

From a societal, weather, and climate perspective, precipitation intensity, duration, frequency, and phase are as much of concern as total amounts, as these factors determine the disposition of precipitation once it hits the ground and how much runs off. At the extremes of precipitation incidence are the events that give rise to floods and droughts, whose changes in occurrence and severity have an enormous impact on the environment and society. Hence, advancing understanding and the ability to model and predict the character of precipitation is vital but requires new approaches to examining data and models. Various mechanisms, storms and so forth, exist to bring about precipitation. Because the rate of precipitation, conditional on when it falls, greatly exceeds the rate of replenishment of moisture by surface evaporation, most precipitation comes from moisture already in the atmosphere at the time the storm begins, and transport of moisture by the storm-scale circulation into the storm is vital. Hence, the intensity of precipitation depends on available moisture, especially for heavy events. As climate warms, the amount of moisture in the atmosphere, which is governed by the Clausius–Clapeyron equation, is expected to rise much faster than the total precipitation amount, which is governed by the surface heat budget through evaporation. This implies that the main changes to be experienced are in the character of precipitation: increases in intensity must be offset by decreases in duration or frequency of events. The timing, duration, and intensity of precipitation can be systematically explored via the diurnal cycle, whose correct simulation in models remains an unsolved challenge of vital importance in global climate change. Typical problems include the premature initiation of convection, and precipitation events that are too light and too frequent. These challenges in observations, modeling, and understanding precipitation changes are being taken up in the NCAR “Water Cycle Across Scales” initiative, which will exploit the diurnal cycle as a test bed for a hierarchy of models to promote improvements in models.

Full access
Jeffrey K. Lew, Derek C. Montague, Hans R. Pruppacher, and Roy M. Rasmussen

Abstract

Natural and artificial snowflakes have been rimed both in free fall and while suspended on a thin flexible fiber in the UCLA cloud tunnel. The results of these experiments show that during the early stage of riming, the motions exhibited by a riming aggregate do not affect the distribution of the rime accretion, in agreement with the observations of the riming behavior of porous ice disks, reported in Part I of this study. It was also found that the collection kernel of a 10-mm diameter porous aggregate increased with respect to porosity at the same rate as that in part I of this study.

A discussion is presented of the free-fall behavior and the time evolution of the terminal velocities of riming aggregates.

Full access
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.

Full access
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.

Full access