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John D. Horel

Abstract

The persistence of daily 500 mb geopotential height anomalies during the 17 winters from 1966 to 1982 is documented for a region over the central Pacific that is noted for its frequent blocking activity. Persistence is expressed here in terms of the degree to which a height value remains constant during the next several days. For a 9 grid-point region centered on 50°N, 165°W, blocks (large positive geopotential height anomalies) tend to be less persistent than negative height anomalies between −100 and −200 m. The greater persistence of negative height anomalies in this region is related to periods of stronger than normal zonal flow centered near 40°N (high zonal index). These periods of stronger than normal zonal flow are more persistent than expected of a first-order autoregressive process.

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John D. Horel

Abstract

The principal components derived by Wallace and Gutzler (1981) from a 500 mb height data set are linearly transformed using the varimax method. Their data set consists of 45 winter months of National Meteorological Center analyses of Northern Hemisphere 500 mb height. The linear transformation (or rotation) of the principal components emphasizes the strongest relationships within the 500 mb height data set; hence, spatial patterns associated with the rotated principal components are simpler to interpret than the spatial patterns associated with the unrotated components. The teleconnection patterns identified by Wallace and Gutzler (1981) on the basis of the negative extrema approach closely resemble several of the spatial patterns of the rotated principal components.

In order to show the seasonal dependence of the rotated principal components, an expanded data set consisting of 30 years of 500 mb height data is used. Most of the teleconnection patterns derived from the 90 winter month data set are “seesaws” with the southernmost center of high correlation located in the subtropics. In some cases, additional centers of high correlation are located downstream of the two primary centers. The spatial patterns associated with the rotated principal components based on 90 summertime months are analogous to those for the wintertime months but are displaced northward along with the displacement of the time mean jet streams and storm-track regions.

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John D. Horel

Abstract

The persistence of the planetary-scale circulation during the Northern Hemisphere winters from 1965/66 to 1981/82 is documented. National Meteorological Center analyses of 500 mb geopotential height for the Northern Hemisphere poleward of 20°N are used. Instead of constructing auto-correlation maps based on lime series at grid points, time series are constructed that show the pattern correlations between daily hemispheric maps. These time series provide information on the day-to-day changes in persistence of the winter circulation and allow comparisons of the intraseasonal and interannual variability of persistence.

The hemispheric circulation is usually quite persistent from day-to-day; correlations between successive maps usually fall within the range 0.7 to 0.9. As the time between the maps is increased the correlations between these maps decreases at a rate faster than that expected of an autoregressive (red-noise) process. The hemispheric circulation rarely resembles closely the circulation a few days before unless synoptic scale waves are removed.

Subjectively defined criteria are used to identify quasi-stationary regimes i.e. periods during which the planetary-scale circulation is more persistant than usual. A total of 58 regimes encompassing 25% of the 2040 days am identified. Multiple regimes are evident during several winter. The quasi-stationary regimes exhibit considerable diversity in their spatial configurations. Comparison of the regimes using principal component analysis suggests that the most frequently reoccurring regime consists of a superposition of a wavenumber 3 pattern at roughly 50°N upon zonally symmetric components at middle and polar latitudes of opposing signs. However, this principal component explains only 17% 6f the variance contained in the 58 regimes.

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John D. Horel

Abstract

The annual cycle in sea surface temperature (SST), surface wind and other atmospheric variables in the tropical Pacific are described. The primary data sets of SST and surface wind are derived from ship observations in the Pacific between 29°N and 29°S during the period 1946–76.

The annual cycle in SST away from the equator can be attributed to the annual cycle in solar heating. However, in the eastern equatorial Pacific, the annual cycle in SST undergoes systematic longitudinal changes in phase and amplitude. Near the coast of Peru, the warmest temperatures occur during March, while further west along the equator, the warmest temperatures occur progressively later and with diminished amplitude. The annual cycle in surface wind convergence along the equator displays similar changes with longitude.

The annual cycle in surface wind is dominated by the meridional migrations of the trade wind belts. Near the equator, the amplitude of the annual cycle in meridional wind is larger than that in zonal wind with the zonal flow (relative to the annual mean) directed into the summer hemisphere. The annual cycles in wind speed and pressure gradient are shown to be kinematically consistent. The annual cycles in rainfall, surface wind convergence, and satellite-derived outgoing infrared radiation and albedo exhibit many similarities in the regions dominated by tropical convection.

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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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Kenneth Sassen and John D. Horel

Abstract

Over an unusually brief three-day period in early August 1989, spectacular twilight effects indicative of a stratospheric volcanic cloud were seen at Salt Lake City, Utah. Concurrent polarization lidar observations detected an aerosol layer at altitudes between 14 and 16 km in the vicinity of the tropopause. Trajectory analyses indicate that the aerosol source was the relatively minor 19 July volcanic eruption of Santiaguito in Guatemala. Materials injected into the lower stratosphere by this eruption were transported initially by tropical easterlies and then by a subtropical jet stream to the locale. The sulfuric acid droplet cloud that formed during transport was affected locally by tropopause folds that promoted stratospheric-tropospheric exchanges. Although lidar depolarization analysis suggests that the ensuing cloud microphysical processes were usually dominated by acid droplet crystallization effects caused by ammonia gas absorption (yielding 0.1–0.2 linear depolarization ratios), there is also evidence for ice crystals at the coldest temperatures (∼−64°C) and for homogeneous droplets. Cloud optical thickness estimates are 0.01–0.02. Mesoscale cloud bands were observed visually near sunset and also occasionally during daylight, another unusual characteristic for volcanic clouds.

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John D. Horel and John M. Wallace

Abstract

No abstract available.

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John D. Horel and John M. Wallace

Abstract

Atmospheric phenomena associated with the Southern Oscillation are examined, with emphasis on vertical structure and teleconnections to middle latitudes. This paper is specifically concerned with the interannual variability of seasonal means for the Northern Hemisphere winter during the period 1951–78. Among the variables considered are sea surface temperature in the equatorial Pacific, precipitation at selected equatorial Pacific stations, a “Southern Oscillation Index” of sea level pressure, 200 mb height and tropospheric mean temperature at stations throughout the tropics, and Northern Hemisphere geopotential height fields. Selected statistics derived from surface data also are examined for the period 1910–45. Results are presented in the form of time series and correlation statistics for the variables listed above.

Results concerning the relationships between sea surface temperature, sea level pressure and rainfall are consistent with the major conclusions of previous studies by J. Bjerknes and others. Fluctuations in mean tropospheric temperature and 200 mb height are shown to vary simultaneously with equatorial Pacific sea surface temperature fluctuations, not only in the Pacific sector, but at stations throughout the tropics. The zonally symmetric component of these 200 mb height fluctuations is considerably larger than the Southern Oscillation in 1000 mb height, and the corresponding fluctuations in the mean temperature of the tropical troposphere are on the order of nearly 1 K.

The correlations between the tropical time series and Northern Hemisphere geopotential height fields exhibit well-defined teleconnection patterns. Warm episodes in equatorial Pacific sea surface temperature tend to be accompanied by below-normal heights in the North Pacific and the south–eastern United States and above-normal heights over western Canada.

Recent theoretical work by Opsteegh and Van den Dool (1980), Hoskins and Karoly (1981) and Webster (1981) on Rossby wave propagation on a sphere provides a basis for understanding the teleconnection in terms of the distribution of sea surface temperature and rainfall in the equatorial Pacific. The theory successfully explains several characteristics of the observed teleconnection patterns, including their horizontal scale and shape, their vertical structure and their seasonal dependence.

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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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