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Roy M. Rasmussen
,
Andrew Crook
, and
Cathy Kessinger

Abstract

The formation and evolution of convective rain and snow bands prior to and during the crash of Continental Airlines flight 1713 on 15 November 1987 at Denver Stapleton Airport are discussed. Convective rain occurred during the early stages of the storm in association with the approach of an upper-level trough from the west. Snow bands were observed following the passage of a shallow Canadian cold front from the north. These bands formed above the cold front and moved from southeast to northwest at 7 m s−1 with a horizontal spacing of 10–30 km. The winds within the cloud layer were southeasterly from 5 to 10 m s−1, suggesting that the bands were advected by the mean, cloud-layer flow. The most likely mechanism producing these bands was a convective instability in the shear layer above the cold front.

As the upper-level trough moved to the east, the winds in the cloud layer shifted to northerly, causing the bands to move southward with the major axis of the band oriented north–south. The high snowfall rate just prior to the takeoff of flight 1713 occurred as a result of one of these north–south–oriented bands moving over Denver Stapleton Airport from the north during the latter stages of the storm.

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

Abstract

Accurate snowfall measurement is challenging because it depends on the precipitation gauge used, meteorological conditions, and the precipitation microphysics. Upstream of weighing gauges, the flow field is disturbed by the gauge and any shielding used usually creates an updraft, which deflects solid precipitation from falling in the gauge, resulting in significant undercatch. Wind shields are often used with weighing gauges to reduce this updraft, and transfer functions are required to adjust the snowfall measurements to consider gauge undercatch. Using these functions reduces the bias in precipitation measurement but not the root-mean-square error (RMSE). In this study, the accuracy of the Hotplate precipitation gauge was compared to standard unshielded and shielded weighing gauges collected during the WMO Solid Precipitation Intercomparison Experiment program. The analysis performed in this study shows that the Hotplate precipitation gauge bias after wind correction is near zero and similar to wind corrected weighing gauges. The RMSE of the Hotplate precipitation gauge measurements is lower than weighing gauges (with or without an Alter shield) for wind speeds up to 5 m s−1, the wind speed limit at which sufficient data were available. This study shows that the Hotplate precipitation gauge measurement has a low bias and RMSE due to its aerodynamic shape, making its performance mostly independent of the type of solid precipitation.

Open access
Douglas A. Wesley
,
Roy M. Rasmussen
, and
Ben C. Bernstein

Abstract

The Longmont anticyclone, a region of low-level anticyclonic turning and convergence during episodes of northerly winds along the Front Range of the Rocky Mountains, is documented for a snow event that occurred during the Winter Icing and Storms Project. The complex terrain in this region, especially the barrier to the west and the sloping Cheyenne Ridge to the north, is critical for the formation of this mesoscale feature. Upward motions related to this persistent convergent region downstream of the Cheyenne Ridge can strongly influence local snowfall distributions. The particular event studied in this manuscript was weakly forced on the synoptic scale. Through close examination of Doppler radar, special sounding and surface mesonetwork data, the effects of the Longmont anticyclone on snowfall were determined. The results of the analyses suggest that the convergence triggered convective snowbands in a region of delayed postfrontal cold advection at low levels. A series of mesoscale model simulations predicted the behavior of low-level northerly flow along the Front Range and demonstrated the role of the terrain during the development of the Longmont anticyclone. The results of these simulations were compared to the case study results.

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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
George D. Modica
,
Scot T. Heckman
, and
Roy M. Rasmussen

Abstract

A hydrostatic regional prediction model is modified to permit the existence of both liquid and ice hydrometeors within the same grid volume. The modified model includes an efficient ice-water saturation adjustment and a simple procedure to create or remove cloud water or ice. The objective was to determine whether such a model could provide deterministic forecasts of aircraft icing conditions in the 6–36-h period. The model was used to simulate an orographically forced icing event (the Valentine's Day storm of 12–14 February 1990) that occurred during the 1990 phase of the Winter Icing and Storms Project (WISP-90). Output from a 24-h nested-grid integration of the model was compared to observations taken during WISP-90. The model produced a thin (∼1-2 km deep) supercooled liquid water (SLW) cloud that was in good agreement with observations in terms of initiation, duration, liquid water content, and location. Results of the simulation also suggest that slantwise ascent can be an important component in the production of SLW.

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Roy M. Rasmussen
,
Jothiram Vivekanandan
,
Jeffrey Cole
,
Barry Myers
, and
Charles Masters

Abstract

The relationship between liquid equivalent snowfall rate and visibility is investigated using data collected at the National Center for Atmospheric Research Marshall Snowfall Test Site during two winter field seasons and using theoretical relationships. The observational data include simultaneous liquid equivalent snowfall rate, crystal types, and both automated and manual visibility measurements. Theoretical relationships between liquid equivalent snowfall rate and visibility are derived for 27 crystal types, and for “dry” and “wet” aggregated snowflakes. Both the observations and theory show that the relationship between liquid equivalent snowfall rate and visibility depends on the crystal type, the degree of riming, the degree of aggregation, and the degree of wetness of the crystals, leading to a large variation in the relationship between visibility and snowfall rate. Typical variations in visibility for a given liquid equivalent snowfall rate ranged from a factor of 3 to a factor of 10, depending on the storm. This relationship is shown to have a wide degree of scatter from storm to storm and also during a given storm. The main cause for this scatter is the large variation in cross-sectional area to mass ratio and terminal velocity for natural snow particles.

It also is shown that the visibility at night can be over a factor of 2 greater than the visibility during the day for the same atmospheric extinction coefficient. Since snowfall intensity is defined by the U.S. National Weather Service using visibility, this day/night difference in visibility results in a change in snowfall intensity category caused by only whether it is day or night. For instance, a moderate snowfall intensity during the day will change to a light snowfall intensity at night, and a heavy snowfall intensity during the day will change to a moderate snowfall intensity at night, for the same atmospheric extinction coefficient.

Thus, the standard relationship between snowfall intensity and visibility used by many national weather services (1/4 mile or less visibility corresponds to heavy snowfall intensity, between 5/16 and 5/8 mile corresponds to moderate intensity, and greater than 5/8 mile corresponds to light intensity) does not always provide the correct indication of actual liquid equivalent snowfall rate because of the variations in snow type and the differences in the nature of visibility targets during day and night. This false indication may have been a factor in previous ground-deicing accidents in which light snow intensity was reported based on visibility, when in fact the actual measured liquid equivalent snowfall rate was moderate to heavy.

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Roy M. Rasmussen
,
István Geresdi
,
Greg Thompson
,
Kevin Manning
, and
Eli Karplus

Abstract

This study evaluates the role of 1) low cloud condensation nuclei (CCN) conditions and 2) preferred radiative cooling of large cloud drops as compared to small cloud drops, on cloud droplet spectral broadening and subsequent freezing drizzle formation in stably stratified layer clouds. In addition, the sensitivity of freezing drizzle formation to ice initiation is evaluated. The evaluation is performed by simulating cloud formation over a two-dimensional idealized mountain using a detailed microphysical scheme implemented into the National Center for Atmospheric Research–Pennsylvania State University Mesoscale Model version 5. The height and width of the two-dimensional mountain were designed to produce an updraft pattern with extent and magnitude similar to documented freezing drizzle cases. The results of the model simulations were compared to observations and good agreement was found.

The key results of this study are 1) low CCN concentrations lead to rapid formation of freezing drizzle. This occurs due to the broad cloud droplet size distribution formed throughout the cloud in this situation, allowing for rapid broadening of the spectra to the point at which the collision–coalescence process is initiated. 2) Continental clouds can produce freezing drizzle given sufficient depth and time. 3) Radiative cooling of the cloud droplets near cloud top can be effective in broadening an initially continental droplet spectrum toward that of a maritime cloud droplet size distribution. 4) Any mechanism that only broadens the cloud droplet spectra near cloud top, such as radiative cooling, may not act over a sufficiently broad volume of the cloud to produce significant amounts of freezing drizzle. 5) Low ice-crystal concentrations (<0.08 L−1) in the region of freezing drizzle formation is a necessary condition for drizzle formation (from both model and observations). 6) Ice nuclei depletion is a necessary requirement for the formation of freezing drizzle. 7) The maximum cloud water mixing ratio and threshold amount for the onset of drizzle in stably stratified clouds was shown to depend strongly on the CCN concentration. 8) A key factor controlling the formation of freezing drizzle in stratified clouds is the lifetime of the mesoscale and synoptic conditions and the thickness and length of the cloud.

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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.

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Kyoko Ikeda
,
Edward A. Brandes
, and
Roy M. Rasmussen

Abstract

An unusual multiple freezing-level event observed with polarimetric radar during the second phase of the Improvement of Microphysical Parameterization through Observational Verification Experiments (IMPROVE-2) field program is described. The event occurred on 28 November 2001 when a warm front moved over the Oregon Cascade Mountains. As the front approached, an elevated melting layer formed above a preexisting melting layer near ground. Continued warming of the lower atmosphere eventually dissipated the lower melting layer.

The polarimetric measurements are used to estimate the height of the freezing levels, document their evolution, and deduce hydrometeor habits. The measurements indicate that when the two freezing levels were first observed melting was incomplete in the upper melting layer and characteristics of particles that passed through the two melting layers were similar. As warming progressed, the character of particles entering the lower melting layer changed, possibly becoming ice pellets or frozen drops. Eventually, the refreezing of particles ended and only rain occurred below the elevated melting layer.

The Doppler radial winds showed a well-defined wind maximum apparently associated with a “warm conveyor belt.” The jet intensified and descended through the elevated melting layer with time. However, the increase in wind speed did not appear connected with melting or result in precipitation enhancement.

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Piotr K. Smolarkiewicz
,
Roy M. Rasmussen
, and
Terry L. Clark

Abstract

This study focuses on basic island scale forcing mechanisms for the formation and evolution of a band cloud typically present upwind of the island of Hawaii. By means of numerical experiments and verification of our results against observations and laboratory experiments reported in the literature, we show that the band cloud is a complex three-dimensional phenomenon which is inseparable from the airflow around the island. In particular, we demonstrate that the event needs to be analyzed in terms of the basic fluid dynamics of strongly stratified flow past a three-dimensional obstacle. The band cloud is found to arise primarily from the dynamic interaction of the trade winds with the island. The upwind surface flow forms a separation line with an associated stagnation point. A low-level convergence zone forms along this line, resulting in an updraft line. If the updrafts are strong enough, a band cloud forms. Formation and characteristics of such a system are mostly controlled by the environmental stability and strength of the trade wind. A simple criterion for the occurrence of a strong band cloud is offered in terms of the height of the island, trade-wind speed, environmental stability, and the lifted condensation and/or free convection level.

A series of controlled experiments addresses questions on the role of the thermal forcing in the formation and evolution of the band cloud. In particular, we show that the band cloud is not primarily related to the diurnal cycle (as was anticipated in the literature), but that the diurnal effects are relatively weak modulations of the primary effects of a strongly fluid flow past the island.

The possibility of vortex shedding in the lee of the island and its implications for the band cloud are also discussed.

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