Abstract
An evaluation of the surface sensible weather forecasts using high-density observations provided by the MesoWest cooperative networks illustrates the performance characteristics of the ...Cooperative Institute for Regional Prediction (CIRP) Weather Research and Forecast (WRF) and the Eta Models over the western United States during the 2003 warm season (June–August). In general, CIRP WRF produced larger 2-m temperature and dewpoint mean absolute and bias errors (MAEs and BEs, respectively) than the Eta. CIRP WRF overpredicted the 10-m wind speed, whereas the Eta exhibited an underprediction with a comparable error magnitude to CIRP WRF. Tests using the Oregon State University (OSU) Land Surface Model (LSM) in CIRP WRF, instead of a simpler slab-soil model, suggest that using a more sophisticated LSM offers no overall advantage in reducing WRF BEs and MAEs for the aforementioned surface variables. Improvements in the initialization of soil temperature in the slab-soil model, however, did reduce the temperature bias in CIRP WRF. These results suggest that improvements in LSM initialization may be as or more important than improvements in LSM physics. A concerted effort must be undertaken to improve both the LSM initialization and parameterization of coupled land surface–boundary layer processes to produce more accurate surface sensible weather forecasts.
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DOBA, IZUM, KILJ, NUK, PILJ, PNG, SAZU, UILJ, UKNU, UL, UM, UPUK
2.
THE MATERHORN Fernando, H. J. S.; Pardyjak, E. R.; Di Sabatino, S. ...
Bulletin of the American Meteorological Society,
11/2015, Letnik:
96, Številka:
11
Journal Article
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Emerging application areas such as air pollution in megacities, wind energy, urban security, and operation of unmanned aerial vehicles have intensified scientific and societal interest in mountain ...meteorology. To address scientific needs and help improve the prediction of mountain weather, the U.S. Department of Defense has funded a research effort—the Mountain Terrain Atmospheric Modeling and Observations (MATERHORN) Program—that draws the expertise of a multidisciplinary, multiinstitutional, and multinational group of researchers. The program has four principal thrusts, encompassing modeling, experimental, technology, and parameterization components, directed at diagnosing model deficiencies and critical knowledge gaps, conducting experimental studies, and developing tools for model improvements. The access to the Granite Mountain Atmospheric Sciences Testbed of the U.S. Army Dugway Proving Ground, as well as to a suite of conventional and novel high-end airborne and surface measurement platforms, has provided an unprecedented opportunity to investigate phenomena of time scales from a few seconds to a few days, covering spatial extents of tens of kilometers down to millimeters. This article provides an overview of the MATERHORN and a glimpse at its initial findings. Orographic forcing creates a multitude of time-dependent submesoscale phenomena that contribute to the variability of mountain weather at mesoscale. The nexus of predictions by mesoscale model ensembles and observations are described, identifying opportunities for further improvements in mountain weather forecasting.
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BFBNIB, DOBA, IZUM, KILJ, NUK, PILJ, PNG, SAZU, SIK, UILJ, UKNU, UL, UM, UPUK
Simulations of local weather and air quality in urban areas must account for processes spanning from mesoto microscales, including turbulence and transport within the urban canopy layer. Here, the ...authors investigate the performance of the building-resolving Quick Urban Industrial Complex (QUIC) Dispersion Modeling System driven with mean wind profiles from the mesoscale Weather Research and Forecasting (WRF) Model. Dispersion simulations are performed for intensive observation periods 2 and 8 of the Joint Urban 2003 field experiment conducted in Oklahoma City, Oklahoma, using an ensemble of expert-derived wind profiles from observational data as well as profiles derived from WRF runs. The results suggest that WRF can be used successfully as a source of inflow boundary conditions for urban simulations, without the collection and processing of intensive field observations needed to produce expert-derived wind profiles. Detailed statistical analysis of tracer concentration fields suggests that, for the purpose of the urban dispersion, WRF simulations provide wind forcing as good as individual or ensemble expert-derived profiles. Despite problems capturing the strength and the elevation of the Great Plains low-level jet, the WRF-simulated near-surface wind speed and direction were close to observations, thus assuring realistic forcing for urban dispersion estimates. Tests performed with multilayer and bulk urban parameterizations embedded in WRF did not provide any conclusive evidence of the superiority of one scheme over the other, although the dispersion simulations driven by the latter showed slightly better results.
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Understanding these lake-effect snowstorms has been limited by several factors including a lack of detailed radar and surface observations. ...1994, when a National Weather Service (NWS) Weather ...Surveillance Radar-1988 Doppler (WSR-88D) was installed, local forecasters relied entirely on hand-drawn radar analyses from a local Federal Aviation Administration radar. ...significant potential for advancing the understanding and short-range prediction of GSL lake-effect snowstorms will develop over the coming years as observations from new surface stations over and around the Great Salt Lake are collected as part of the Utah Mesonet (Stiff1997).
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DOBA, IZUM, KILJ, NUK, PILJ, PNG, SAZU, UILJ, UKNU, UL, UM, UPUK
This paper investigates the kinematic flow and precipitation evolution of a winter storm over and upstream of the Wasatch Mountains Intermountain Precipitation Experiment third intensive observing ...period (IPEX IOP3) using a multiply nested version of the fifth-generation Pennsylvania State University (PSU)--National Center for Atmospheric Research (NCAR) Mesoscale Model (MM5). Validation using in situ aircraft data, radiosondes, ground-based radar, and surface observations showed that the MM5, which featured four domains with 36-, 12-, 4-, and 1.33-km grid spacing, realistically simulated the observed partial blocking of the 8-12 m s-1 ambient southwesterly flow and development of a convergence zone and enhanced lowland precipitation region upwind of the initial Wasatch slope. The MM5 also properly simulated the advance of this convergence zone toward the base of the Wasatch during the passage of a midlevel trough, despite not fully capturing the westerly wind shift accompanying the trough. Accurate simulation of the observed precipitation over the central Wasatch Mountains (within 25% of observed at all stations) required a horizontal grid spacing of 1.33 km. Despite close agreement with the observed surface precipitation, the Reisner2 bulk microphysical scheme produced too much supercooled cloud water and too little snow aloft. A model microphysical budget revealed that the Reisner2 generated over half of the surface precipitation through riming and accretion, rather than snow deposition and aggregation as implied by the observations. Using an intercept for the snow size distribution that allows for greater snow concentrations aloft improved the snow predictions and reduced the cloud water overprediction. Sensitivity studies illustrate that the reduced surface drag of the Great Salt Lake (GSL) enhanced the convergence zone and associated lowland precipitation enhancement upstream of the Wasatch Mountains. The presence of mountain ranges south of the Great Salt Lake appears to have weakened the along-barrier flow and windward convergence, resulting in a slight decrease in windward precipitation enhancement. Diabatic cooling from falling precipitation was also important for maintaining the blocked flow.
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Narrow corridors of water vapor transport known as atmospheric rivers (ARs) contribute to extreme precipitation and flooding along the West Coast of the United States, but knowledge of their ...influence over the interior is limited. Here, the authors use Interim European Centre for Medium-Range Weather Forecasts (ECMWF) Re-Analysis (ERA-Interim) data, Climate Prediction Center (CPC) precipitation analyses, and Snowpack Telemetry (SNOTEL) observations to describe the characteristics of cool-season (November-April) ARs over the western United States. It is shown that AR frequency and duration exhibit a maximum along the Oregon-Washington coast, a strong transition zone upwind (west) of and over the Cascade-Sierra ranges, and a broad minimum that extends from the "high" Sierra south of Lake Tahoe eastward across the central Great Basin and into the deep interior. East of the Cascade-Sierra ranges, AR frequency and duration are largest over the interior northwest, while AR duration is large compared to AR frequency over the interior southwest. The fractions of cool-season precipitation and top-decile 24-h precipitation events attributable to ARs are largest over and west of the Cascade-Sierra ranges. Farther east, these fractions are largest over the northwest and southwest interior, with distinctly different large-scale patterns and AR orientations enabling AR penetration into each of these regions. In contrast, AR-related precipitation over the Great Basin east of the high Sierra is rare. These results indicate that water vapor depletion over major topographic barriers is a key contributor to AR decay, with ARs playing a more prominent role in the inland precipitation climatology where lower or less continuous topography facilitates the inland penetration of ARs.
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DOBA, IZUM, KILJ, NUK, PILJ, PNG, SAZU, UILJ, UKNU, UL, UM, UPUK
Mesoscale-model simulations are used to examine the structure and dynamics of a gap-outflow event over the Gulf of Tehuantepec, Mexico, that was associated with a surge of cold air along the eastern ...slopes of the Sierra Madre. The simulated gap-outflow winds emerged from Chivela Pass, reached a maximum speed of 25 m s super(-1), and turned anticyclonically as they fanned out over the gulf. Northerly winds were also able to ascend the mountains east, and to a lesser extent west, of Chivela Pass, indicating that the movement of cold air across the Sierra Madre was not confined to the pass. A mesoscale pressure ridge was aligned along the axis of the gap-outflow jet, which was flanked to the west by an anticyclonic eddy, and to the east by a weaker cyclonic eddy. A model-derived trajectory along the axis of the outflow jet traced an inertial path, with anticyclonic curvature produced primarily by the Coriolis acceleration. The cross-flow pressure-gradient acceleration along this trajectory was negligible because it followed the axis of the mesoscale pressure ridge. Trajectories west (east) of the jet axis experienced stronger (weaker) anticyclonic curvature than expected from inertial balance because the cross-flow pressure-gradient acceleration produced by the mesoscale pressure ridge reinforced (opposed) the anticyclonic deflection by the Coriolis acceleration. As a result of these directional variations in the cross-flow pressure-gradient acceleration, a fanlike wind pattern was observed rather than a narrow jet. Because of the large changes in SST and surface roughness that are observed during these gap-outflow events, better representation of these effects might improve future mesoscale-model simulations. Such improvements could be accomplished through coupled atmosphere-ocean mesoscale modeling, which could also be used to advance understanding of the oceanography of the gulf.
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DOBA, IZUM, KILJ, NUK, PILJ, PNG, SAZU, UILJ, UKNU, UL, UM, UPUK
Carbon dioxide is a stable constituent of the atmosphere that has no major terrestrial sinks other than atmospheric transport in the absence of photosynthetic activity by plants. In urban ...atmospheres, CO2 mixing ratios are often elevated above ambient by large local sources from combustion. We measured CO2 mixing ratios and the isotopic composition of CO2 at four locations in the Salt Lake Valley, Utah, during a persistent cold pool event in the winter of 2004. The results showed a strong influence of atmospheric stability and the height of the capping inversion on CO2 mixing ratios and suggested that during persistent cold pool events the air mass beneath the capping inversion can be relatively well mixed. Spatial and temporal patterns in the isotopic composition of CO2 and the relationship between particulate concentrations and CO2 mixing ratio support this interpretation. These results suggest that CO2 mixing ratio, which is abundant and relatively easily measured in urban atmospheres, can provide information about complex wintertime atmospheric transport and mixing as well as carbon cycling in urban mountain basins.
Although atmospheric rivers (ARs) typically weaken following landfall, those that penetrate inland can contribute to heavy precipitation and high-impact weather within the interior of western North ...America. In this paper, the authors examine the evolution of ARs over western North America using trajectories released at 950 and 700 hPa within cool-season ARs along the Pacific coast. These trajectories are classified as coastal decaying, inland penetrating, or interior penetrating based on whether they remain within an AR upon reaching selected transects over western North America. Interior-penetrating AR trajectories most frequently make landfall along the Oregon coast, but the greatest fraction of landfalling AR trajectories that eventually penetrate into the interior within an AR is found along the Baja Peninsula. In contrast, interior-penetrating AR trajectories rarely traverse the southern "high" Sierra. At landfall, interior-penetrating AR trajectories are associated with a more amplified flow pattern, more southwesterly (vs westerly) flow along the Pacific coast, and larger water vapor transport (q upsilon ). The larger initial q upsilon of interior-penetrating AR trajectories is due primarily to larger initial water vapor q and wind speed upsilon for those initiated at 950 and 700 hPa, respectively. Inland- and interior-penetrating AR trajectories maintain large q upsilon over the interior partially due to increases in upsilon that offset decreases in q, particularly in the vicinity of topographical barriers. Therefore, synoptic conditions and trajectory pathways favoring larger initial q upsilon at the coast, limited water vapor depletion by orographic precipitation, and increases in upsilon over the interior are keys to differentiating interior-penetrating from coastal-decaying ARs.
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DOBA, IZUM, KILJ, NUK, PILJ, PNG, SAZU, UILJ, UKNU, UL, UM, UPUK
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.
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DOBA, IZUM, KILJ, NUK, PILJ, PNG, SAZU, UILJ, UKNU, UL, UM, UPUK
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