Search Results

You are looking at 1 - 7 of 7 items for

  • Author or Editor: J. Onton x
  • Refine by Access: All Content x
Clear All Modify Search
W. James Steenburgh
and
Daryl J. Onton

Abstract

The large-scale and mesoscale structure of the Great Salt Lake–effect snowstorm of 7 December 1998 is examined using radar analyses, high-density surface observations, conventional meteorological data, and a simulation by the Pennsylvania State University–National Center for Atmospheric Research fifth generation Mesoscale Model (MM5). Environmental conditions during the event were characterized by a lake–700-hPa temperature difference of up to 22.5°C, a lake–land temperature difference as large as 10°C, and conditionally unstable low-level lapse rates. The primary snowband of the event formed along a land-breeze front near the west shoreline of the Great Salt Lake. The snowband then migrated eastward and merged with a weaker snowband as the land-breeze front moved eastward, offshore flow developed from the eastern shoreline, and low-level convergence developed near the midlake axis. Snowfall accumulations reached 36 cm and were heaviest in a narrow, 10-km-wide band that extended downstream from the southern shore of the Great Salt Lake. Thus, although the Great Salt Lake is relatively small in scale compared to the Great Lakes, it is capable of inducing thermally driven circulations and banded precipitation structures similar to those observed in lake-effect regions of the eastern United States and Canada.

Full access
Daryl J. Onton
and
W. James Steenburgh

Abstract

The processes responsible for the Great Salt Lake–effect snowstorm of 7 December 1998 are examined using a series of mesoscale model simulations. Localized surface sensible and latent heating are shown to destabilize the boundary layer over the Great Salt Lake (GSL) and to produce mesoscale pressure troughing, land-breeze circulations, and low-level convergence that lead to the development of the primary band of convective clouds and precipitation. Model diagnostics and sensitivity studies further illustrate that

  • moisture fluxes from the lake surface were necessary to fully develop the snowband;

  • the hypersaline composition of the GSL did, however, decrease moisture fluxes compared to a body of freshwater, resulting in a 17% reduction of snowfall;

  • latent heat release within the cloud and precipitation band intensified overlake pressure troughing, convergence, and precipitation;

  • orographic effects were not responsible for snowband generation, but they did affect the distribution and intensity of precipitation in regions where the snowband interacted with downstream terrain; and

  • surface roughness contrasts across the GSL shoreline did not play a primary role in forming the snowband.

Simulations in which lake-surface temperature and upstream moisture were modified are used to illustrate how small errors in the specification of these quantities can impact quantitative precipitation forecasts, potentially limiting the utility of high-resolution mesoscale model guidance. Results are compared to those from studies of lake-effect precipitation over the Great Lakes, and the implications for operational forecasting and numerical weather prediction are discussed.

Full access
W. James Steenburgh
,
Scott F. Halvorson
, and
Daryl J. Onton

Abstract

Characteristics of lake-effect snowstorms associated with the Great Salt Lake are described. Using WSR-88D radar imagery, 16 well-defined and 18 marginal lake-effect events were identified from September 1994 through May 1998 (excluding June–August), with the former used for more detailed analysis. Precipitation during the well-defined events was frequently characterized by the irregular development of radar echoes over and downstream of the Great Salt Lake. The most commonly observed precipitation structures were solitary wind-parallel bands that developed along or near the major axis of the GSL and broad-area precipitation shields with embedded convective elements that formed near the southern shoreline.

Regional-scale composite analyses and rawinsonde-derived statistics showed that the lake-effect events occurred in post frontal westerly to northerly 700-hPa flow following the passage of an upper-level trough and associated low-level cold front. The lake-effect environment was characterized by limited steering layer (800–600 hPa) directional shear (generally 60° or less), moist- to dry-adiabatic low-level lapse rates, and small convective available potential energy (CAPE), although the CAPE may be locally greater over the Great Salt Lake. In all events, the lake–700-hPa temperature difference exceeded 16°C, which roughly corresponds to a dry-adiabatic lapse rate. The lake–land temperature difference was always positive and usually exceeded 6°C, indicating significant potential for the development of land-breeze circulations and associated low-level convergence over the lake. Radar-derived statistics suggest that lake enhancement is strongest during periods of northwesterly to northerly flow and large lake–land temperature differences. These characteristics are compared with those associated with lake-effect snowstorms of the Great Lakes and implications for operational forecasting are discussed.

Full access
Kenneth A. Hart
,
W. James Steenburgh
, and
Daryl J. Onton

Abstract

Forecasts produced for the 2002 Olympic and Paralympic Winter Games (23 January–25 March 2002) by a multiply nested version of the fifth-generation Pennsylvania State University–National Center for Atmospheric Research (PSU–NCAR) Mesoscale Model (MM5) are examined to determine if decreasing horizontal grid spacing to 4 km improves forecast accuracy over the finescale topography of the Intermountain West. The verification is based on high-density observations collected by the MesoWest cooperative networks, including approximately 200 wind and temperature sites and 100 precipitation sites across northern Utah.

Wind and precipitation forecasts produced by the 4-km MM5 domain were more accurate (based on traditional measures) than those of its parent 12-km domain. The most significant improvements in wind speed forecasts occurred at night in valleys and lowland locations where the topography of the 4-km domain produced more accurate nocturnal flows. Wind direction forecast improvements were most substantial at mountain sites where the better topographic resolution of the 4-km domain more accurately reflected the exposure of these locations to the free atmosphere. The 4-km domain also produced quantitative precipitation forecasts that were either equally (small events) or more (large events) accurate than the 12-km domain. Precipitation bias errors varied substantially between the two domains since the representation of the region’s narrow, steeply sloped, basin-and-range topography improved dramatically at 4-km grid spacing.

Curiously, the overall accuracy of temperature forecasts by the 4-km domain was not significantly better than that of the 12-km domain. This was due to an inability of the MM5 to properly simulate nocturnal and persistent cold pools within mountain valleys and the lowlands upstream of the Wasatch Mountains. Paradoxically, the added resolution of the 4-km domain, coupled with the failure of this version of the MM5 to fully capture the nocturnal and persistent cold pools, resulted in poorer skill scores. At upper elevations, which are typically above the cold pools, the 4-km domain was substantially more accurate.

These results illustrate that decreasing horizontal grid spacing to less than 10 km does improve wind and precipitation forecasts over finescale Intermountain West topography. It is hypothesized that model improvements will ultimately enable the advantages of added model resolution to be fully realized for temperature forecasts over the Intermountain West.

Full access
J. Horel
,
T. Potter
,
L. Dunn
,
W. J. Steenburgh
,
M. Eubank
,
M. Splitt
, and
J. Onton

The 2002 Winter Olympic and Paralympic Games will be hosted by Salt Lake City, Utah, during February–March 2002. Adverse weather during this period may delay sporting events, while snow and ice-covered streets and highways may impede access by the athletes and spectators to the venues. While winter snowstorms and other large-scale weather systems typically have widespread impacts throughout northern Utah, hazardous winter weather is often related to local terrain features (the Wasatch Mountains and Great Salt Lake are the most prominent ones). Examples of such hazardous weather include lake-effect snowstorms, ice fog, gap winds, down-slope windstorms, and low visibility over mountain passes.

A weather support system has been developed to provide weather information to the athletes, games officials, spectators, and the interested public around the world. This system is managed by the Salt Lake Olympic Committee and relies upon meteorologists from the public, private, and academic sectors of the atmospheric science community. Weather forecasting duties will be led by National Weather Service forecasters and a team of private weather forecasters organized by KSL, the Salt Lake City NBC television affiliate. Other government agencies, commercial firms, and the University of Utah are providing specialized forecasts and support services for the Olympics. The weather support system developed for the 2002 Winter Olympics is expected to provide long-term benefits to the public through improved understanding, monitoring, and prediction of winter weather in the Intermountain West.

Full access
Kenneth A. Hart
,
W. James Steenburgh
,
Daryl J. Onton
, and
Andrew J. Siffert

Abstract

The skill of a mesoscale-model-based model output statistics (MOS) system that provided hourly forecasts for 18 sites over northern Utah during the 2002 Winter Olympic and Paralympic Games is evaluated. The MOS system was developed using three winters (November–April 1998/99, 1999/2000, and 2000/01) of forecasts by the fifth-generation Pennsylvania State University–National Center for Atmospheric Research Mesoscale Model (MM5) and observations from Olympic venues and transportation corridors.

MOS temperature, relative humidity, wind speed, and wind direction forecasts were considerably more accurate than those produced by the 12- and 4-km MM5 grids. A primary contributor to MM5 temperature and relative humidity errors was a systematic overprediction of surface temperature (i.e., a warm/dry bias) during persistent and nocturnal cold-pool events when corresponding errors in MM5 dewpoint temperature forecasts were not observed. MOS largely corrected for this temperature bias. MOS wind speed forecasts outperformed the 12- and 4-km MM5 forecasts by the largest margin at locations with the lowest wind speed variability. Raw model and MOS performance exhibited minimal sensitivity to variations in model initial and lateral boundary conditions (derived from the forecasts of either the National Centers for Environmental Prediction's Eta Model or the Aviation run of the Global Spectral Model). MOS temperature, relative humidity, and wind speed forecasts were equal to or more skillful than human-generated forecasts produced by the Olympic Forecast Team.

The results illustrate that statistical techniques continue to improve upon purely numerical predictions even at high resolution. This is particularly true in a region of complex terrain where detailed characteristics of local topography and microclimates remain unresolved. It is recommended that traditional MOS or other statistical techniques based on high-density surface observations available from the MesoWest cooperative networks be used to improve gridded forecast products created by the National Weather Service Interactive Forecast Preparation System (IFPS) and other applications.

Full access
Bryan G. White
,
Jan Paegle
,
W. James Steenburgh
,
John D. Horel
,
Robert T. Swanson
,
Louis K. Cook
,
Daryl J. Onton
, and
John G. Miles

Abstract

The short-term forecast accuracy of six different forecast models over the western United States is described for January, February, and March 1996. Four of the models are operational products from the National Centers for Environmental Prediction (NCEP) and the other two are research models with initial and boundary conditions obtained from NCEP models. Model resolutions vary from global wavenumber 126 (∼100 km equivalent horizontal resolution) for the Medium Range Forecast model (MRF) to about 30 km for the Meso Eta, Utah Local Area Model (Utah LAM), and Pennsylvania State University–National Center for Atmospheric Research Mesoscale Model Version 5 (MM5). Forecast errors are described in terms of bias error and mean square error (mse) as computed relative to (i) gridded objective analyses and (ii) rawinsonde observations. Bias error and mse fields computed relative to gridded analyses show considerable variation from model to model, with the largest errors produced by the most highly resolved models. Using this approach, it is impossible to separate real forecast errors from possibly correct, highly detailed forecast information because the forecast grids are of higher resolution than the observations used to generate the gridded analyses. Bias error and mse calculated relative to rawinsonde observations suggest that the Meso Eta, which is the most highly resolved and best developed operational model, produces the most accurate forecasts at 12 and 24 h, while the MM5 produces superior forecasts relative to the Utah LAM. At 36 h, the MRF appears to produce superior mass and wind field forecasts. Nevertheless, a preliminary validation of precipitation performance for fall 1997 suggests the more highly resolved models exhibit superior skill in predicting larger precipitation events. Although such results are valid when skill is averaged over many simulations, forecast errors at individual rawinsonde locations, averaged over subsets of the total forecast period, suggest more variability in forecast accuracy. Time series of local forecast errors show large variability from time to time and generally similar maximum error magnitudes among the different models.

Full access