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Kevin M. Craft
and
John D. Horel

Abstract

Desert playas, such as those in northern Utah, form a landscape often in stark contrast to surrounding mountain ranges due to their minimal topographic relief, lack of vegetation, and saline soils. Dry highly reflective halite surfaces, which make up many of the desert playas in northern Utah, are generally characterized by a surface albedo over 40%. However, their albedo can be reduced abruptly to less than 20% by flooding due to rainfall, runoff from surrounding higher terrain, or surface winds transporting shallow water across the playas. A weather station installed during September 2016 to study the Bonneville Salt Flats (BSF) in northern Utah provides estimates of surface albedo that can be related to cycles of flooding and desiccation of the halite surface. The normalized difference water index (NDWI) derived from the MODIS MOD09A1 land surface reflectance product estimates the fractional coverage of surface water over the BSF. NDWI values computed over 8-day periods from 2000 to 2018 highlight year-to-year and seasonal variations in playa flooding events over the BSF. Periods of playa flooding were observed with both ground-based observations and NDWI with sharp reductions in albedo when the surface is flooded.

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John D. Horel
and
Xia Dong

Abstract

This study estimates whether surface observations of temperature, moisture, and wind at some stations in the continental United States are less critical than others for specifying weather conditions in the vicinity of those stations. Two-dimensional variational analyses of temperature, relative humidity, and wind were created for selected midday hours during summer 2008. This set of 8925 control analyses was derived from 5-km-resolution background fields and Remote Automated Weather Station (RAWS) and National Weather Service (NWS) observations within roughly 4° × 4° latitude–longitude domains. Over 570 000 cross-validation experiments were completed to assess the impact of removing each RAWS and NWS station. The presence of observational assets within relatively close proximity to one another is relatively common. The sensitivity to removing temperature, relative humidity, or wind observations varies regionally and depends on the complexity of the surrounding terrain and the representativeness of the observations. Cost savings for the national RAWS program by removing a few stations may be possible. However, nearly all regions of the country remain undersampled, especially mountainous regions of the western United States frequently affected by wildfires.

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Timothy W. Barker
and
John D. Horel

Abstract

The persistence of the planetary-scale circulation in a perpetual January experiment of the NCAR Community Climate Model is investigated. Pattern correlations between maps of 500 mb geopotential height are used to identify periods in which the large scale flow remains quasi-stationary for periods of a week or longer. Thirty- one distinct periods are dominated by quasi-stationary flow patterns encompassing 22% of the model experiment. The time between quasi-stationary periods is typically longer than their duration. On the basis of subjective similarities among some of these events, we classified many of them into four distinct types. The characteristics of the model quasi-stationary regimes are contrasted with those observed and those found in simpler models.

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John D. Horel
and
Carlos R. Mechoso

Abstract

The persistence of the planetary-scale simulation over the North Pacific Ocean is investigated during 18 Northern Hemisphere winters from 1965/66 to 1982/83. Quasi-stationary flow patterns dominate 20 periods during the 6 El Niño winters. In contrast, 29 such periods are observed during the remaining 12 winters. Nearly all of the quasi-stationary episodes during El Niño winters exhibit negative 500 mb geopotential height anomalies in the Gulf of Alaska-Aleutian Island region. During the other 12 winters, episodes characterized by positive height anomalies in that region occur as frequently as those exhibiting negative height anomalies.

The observed persistence of the planetary circulation is contrasted to that simulated by the UCLA general circulation model. Ten winters of model output are analyzed: during five winters, sea surface temperatures (SSTs) are prescribed to evolve through their climatological seasonal cycle while during the other five winters, SST anomalies corresponding to idealized or observed El Niño conditions are added to the climatological field. The model atmosphere has less intraseasonal variability, and quasi-stationary events are less frequent than observed. However, the model is successful in simulating the observed preponderance of quasi-stationary regimes which exhibit below-normal 500 mb geopotential height anomalies in the Gulf of Alaska during winters with positive SST anomalies in the equatorial Pacific. The evolution of the model's quasi-stationary events suggests that they result directly from dynamical processes in midlatitudes, but their characteristics are apparently affected by SST conditions.

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Neil P. Lareau
and
John D. Horel

Abstract

The position and variability of storm tracks across western North America are examined during the October–April cool seasons spanning 1989–2010. The location and intensity of storms are represented by strong synoptic-scale ascent, which is diagnosed by the alternative balance omega equation applied to ECMWF Re-Analysis Interim data. This dynamically filtered method removes poorly resolved updrafts arising from subsynoptic-scale phenomena such as convection and mountain waves. The resulting vertical motions are illustrated for the case of a strong storm traversing the western United States.

Summary statistics of synoptic-scale ascent are compiled over months, seasons, and the entire 21-yr period. Locations exhibiting high mean values of ascent are deemed to represent storm tracks. The climatological-mean storm track exhibits a sinusoidal shape across the eastern Pacific and western North America. The composite evolution of strong storms moving along specific segments of the storm track show regional differences (e.g., storms poleward of 50°N tend to result from progressive low-amplitude troughs progressing through the mean planetary ridge, while storms over the western United States are initiated by digging troughs that temporarily suppress the mean ridge).

Seasonal shifts in the storm track are pronounced and exhibit coherent regional patterns. Interannual variations in synoptic-scale ascent indicate meridional shifts in position as well as changes in the degree of amplification within the dominant sinusoidal storm track. These changes in structure are related to the phase of ENSO: El Niño (La Niña) winters favor zonal (amplified) and southern (northern) storm tracks.

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David T. Myrick
and
John D. Horel

Abstract

Federal, state, and other wildland resource management agencies contribute to the collection of weather observations from over 1000 Remote Automated Weather Stations (RAWS) in the western United States. The impact of RAWS observations on surface objective analyses during the 2003/04 winter season was assessed using the Advanced Regional Prediction System (ARPS) Data Assimilation System (ADAS). A set of control analyses was created each day at 0000 and 1200 UTC using the Rapid Update Cycle (RUC) analyses as the background fields and assimilating approximately 3000 surface observations from MesoWest. Another set of analyses was generated by withholding all of the RAWS observations available at each time while 10 additional sets of analyses were created by randomly withholding comparable numbers of observations obtained from all sources.

Random withholding of observations from the analyses provides a baseline estimate of the analysis quality. Relative to this baseline, removing the RAWS observations degrades temperature (wind speed) analyses by an additional 0.5°C (0.9 m s−1) when evaluated in terms of rmse over the entire season. RAWS temperature observations adjust the RUC background the most during the early morning hours and during winter season cold pool events in the western United States while wind speed observations have a greater impact during active weather periods. The average analysis area influenced by at least 1.0°C (2.5°C) by withholding each RAWS observation is on the order of 600 km2 (100 km2). The spatial influence of randomly withheld observations is much less.

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David T. Myrick
and
John D. Horel

Abstract

Experimental gridded forecasts of surface temperature issued by National Weather Service offices in the western United States during the 2003/04 winter season (18 November 2003–29 February 2004) are evaluated relative to surface observations and gridded analyses. The 5-km horizontal resolution gridded forecasts issued at 0000 UTC for forecast lead times at 12-h intervals from 12 to 168 h were obtained from the National Digital Forecast Database (NDFD). Forecast accuracy and skill are determined relative to observations at over 3000 locations archived by MesoWest. Forecast quality is also determined relative to Rapid Update Cycle (RUC) analyses at 20-km resolution that are interpolated to the 5-km NDFD grid as well as objective analyses obtained from the Advanced Regional Prediction System Data Assimilation System that rely upon the MesoWest observations and RUC analyses. For the West as a whole, the experimental temperature forecasts issued at 0000 UTC during the 2003/04 winter season exhibit skill at lead times of 12, 24, 36, and 48 h on the basis of several verification approaches. Subgrid-scale temperature variations and observational and analysis errors undoubtedly contribute some uncertainty regarding these results. Even though the “true” values appropriate to evaluate the forecast values on the NDFD grid are unknown, it is estimated that the root-mean-square errors of the NDFD temperature forecasts are on the order of 3°C at lead times shorter than 48 h and greater than 4°C at lead times longer than 120 h. However, such estimates are derived from only a small fraction of the NDFD grid boxes. Incremental improvements in forecast accuracy as a result of forecaster adjustments to the 0000 UTC temperature grids from 144- to 24-h lead times are estimated to be on the order of 13%.

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John D. Horel
and
Chris V. Gibson

Abstract

The evolution of a major winter storm over Utah during 6–7 January 1992 is analyzed using surface and upper-air observations and satellite imagery. A mesoscale model is used to deduce the dynamical processes that took place during the storm. Output from the Nested Grid Model of the National Meteorological Center is used to specify the initial conditions and the lateral boundary conditions of the mesoscale model. Two numerical simulations that each last 12 h in duration are studied here. The first begins at 1200 UTC 6 January, while the second starts at 0000 UTC 7 January. Attention is placed on a secluded zone of warm, moist air that is located along the northern and western boundaries of the midtropospheric cyclonic circulation as it moved across Utah. Output from the second mesoscale simulation is used to explain the processes by which air in the secluded zone is lifted. These processes include large-scale ascent west of the cyclonic circulation aloft, lift provided by a shallow cold front, and orographic ascent as the low-level flow encountered the mountain ranges of western Utah.

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Lawrence B. Dunn
and
John D. Horel

Abstract

The utility of numerical model guidance produced by the National Meteorological Center has been evaluated for the forecast of convection over central Arizona during the summer monsoon season. Model output from the Nested Grid Model (NGM) and Eta model has been compared to observations taken during the 1990 field experiment referred to as the Southwest Area Monsoon Project (SWAMP).

The NGM precipitation forecasts showed little skill for events in which heavy precipitation was observed over Phoenix, Arizona. Selected events during the SWAMP period were simulated using the Eta model. Qualitative comparisons of the Eta model's precipitation forecasts with lightning data and satellite imagery suggest that the model has little skill over Arizona during the warm season. Nocturnal heavy precipitation over the lower deserts of central Arizona is nearly always preceded by afternoon convection over the mountains to the north and east. The convection over the mountains was absent in the model.

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Lawrence B. Dunn
and
John D. Horel

Abstract

Output from simulations of the Eta model are compared to special observations collected during the 1990 Southwest Area Monsoon Project (SWAMP). The emphasis is on the model's prediction of the preconvection air mass over Phoenix, Arizona, and on the model's representation of the low-level jet and moisture surge observed over southwest Arizona.

At times the model fails to capture the rapid increase in low- and mid-level moisture that is observed in the hours prior to the onset of convection. Subsequent convection is not predicted by the Eta model. In one event the model very accurately predicts the evolution of the air mass over Phoenix during the period just prior to the outbreak of severe convection. However, no convection is predicted by the model. The model seems unable to generate convection over the high terrain or lower deserts of central Arizona regardless of whether the air mass is simulated correctly.

A low-level jet feature observed over southwest Arizona during SWAMP is not correctly simulated by the Eta model. The model produces a very strong sea-breeze circulation from the Gulf of California into western Arizona in each simulation. The moisture and stability profiles associated with the sea-breeze are inconsistent with observations over southwest Arizona, which leads to a misrepresentation of the low- and midlevel moisture field over the region. Poor initial conditions in the sea surface temperature field over the Gulf of California are, at least in part, responsible for the model error.

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