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Daniel P. Tyndall
and
John D. Horel

Abstract

Given the heterogeneous equipment, maintenance and reporting practices, and siting of surface observing stations, subjective decisions that depend on the application tend to be made to use some observations and to avoid others. This research determines objectively high-impact surface observations of 2-m temperature, 2-m dewpoint, and 10-m wind observations using the adjoint of a two-dimensional variational surface analysis over the contiguous United States. The analyses reflect a weighted blend of 1-h numerical forecasts used as background grids and available observations. High-impact observations are defined as arising from poor observation quality, observation representativeness errors, or accurate observed weather conditions not evident in the background field. The impact of nearly 20 000 surface observations is computed over a sample of 100 analysis hours during 25 major weather events. Observation impacts are determined for each station as well as within broad network categories. For individual analysis hours, high-impact observations are located in regions of significant weather—typically, where the background field fails to define the local weather conditions. Low-impact observations tend to be ones where there are many observations reporting similar departures from the background. When averaged over the entire 100 cases, observations with the highest impact are found within all network categories and depend strongly on their location relative to other observing sites and the amount of variability in the weather; for example, temperature observations have reduced impact in urban areas such as Los Angeles, California, where observations are plentiful and temperature departures from the background grids are small.

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Brian K. Blaylock
and
John D. Horel

Abstract

The ability of the High-Resolution Rapid Refresh (HRRR) model to forecast the location of convective storms is of interest for a variety of applications. Since lightning is often present with intense convection, lightning observations from the Geostationary Lightning Mapper (GLM) on GOES-East are used to evaluate the performance of the HRRR lightning forecasts from May through September during 2018 and 2019. Model skill is presented in terms of the fractions skill score (FSS) evaluated within circular neighborhoods with radial distances from 30 to 240 km. Case studies of individual events illustrate that the HRRR lightning forecasts FSS varies from storm to storm. Mean FSS is summarized for the months with peak lightning activity (June–August) for the west, central, and east United States. Our results suggest that forecasters should use HRRR lightning forecasts to indicate general tendencies for the occurrence, region, and timing of thunderstorms in a broad region rather than expect high forecast accuracy for lightning locally. For example, when FSS is evaluated within small neighborhoods (30-km radius), mean FSS drops sharply after the first two hours of model integration in all regions and during all hours of the day. However, when evaluated within larger neighborhoods (60-km radius and larger), FSS in the western United States and northern Mexico remains high for all lead times in the late afternoon and early evening. This result is likely due to the model capturing the tendency for convection to break out over higher terrain during those hours.

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Taylor A. Gowan
and
John D. Horel

Abstract

Large wildfire outbreaks in Alaska are common from June to August. The Canadian Forest Fire Danger Rating System (CFFDRS) is used operationally by Alaskan fire managers to produce statewide fire weather outlooks and forecast guidance near active wildfires. The CFFDRS estimates of fire potential and behavior rely heavily on meteorological observations (precipitation, temperature, wind speed, and relative humidity) from the relatively small number of in situ stations across Alaska with precipitation being the most critical parameter. To improve the spatial coverage of precipitation estimates across Alaska for fire weather applications, a multisatellite precipitation algorithm was evaluated during six fire seasons (1 June–31 August 2014–19). Near-real-time daily precipitation estimates from the Integrated Multisatellite Retrievals for the Global Precipitation Mission (IMERG) algorithm were verified using 322 in situ stations across four Alaskan regions. For each region, empirical cumulative distributions of daily precipitation were obtained from station observations during each summer, and compared to corresponding distributions of interpolated values from IMERG grid points (0.1° × 0.1° grid). The cumulative distributions obtained from IMERG exhibited wet biases relative to the observed distributions for all regions, precipitation amount ranges, and summers. A bias correction approach using regional quantile mapping was developed to mitigate for the IMERG wet bias. The bias-adjusted IMERG daily precipitation estimates were then evaluated and found to produce improved gridded IMERG precipitation estimates. This approach may help to improve situational awareness of wildfire potential across Alaska and be appropriate for other high-latitude regions where there are sufficient in situ precipitation observations to help correct the IMERG precipitation estimates.

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John D. Horel
and
James T. Powell

Abstract

While many studies have examined intense rainfall and flash flooding during the North American Monsoon (NAM) in Arizona, Nevada, and New Mexico, less attention has focused on the NAMS’s extension into southwestern Utah. This study relates flash flood reports and Multi-Radar Multi-Sensor (MRMS) precipitation across southwestern Utah to atmospheric moisture content and instability analyses and forecasts from the High-Resolution Rapid Refresh (HRRR) model during the 2021–23 monsoon seasons.

MRMS quantitative precipitation estimates over southwestern Utah during summer depend largely on the areal coverage from the KICX WSR-88D radar near Cedar City, UT. Those estimates are generally consistent with the limited number of precipitation gauge reports in the region except at extended distances from the radar. A strong relationship is evident between days with widespread precipitation and afternoons with above average precipitable water (PWAT) and convective available potential energy (CAPE) estimated from HRRR analyses across the region.

Time-lagged ensembles of HRRR forecasts (initialization times from 03–06 UTC) that are 13–18 h prior to the afternoon period when convection is initiating (18–21 UTC) are useful for situational awareness of widespread precipitation events after adjusting for underprediction of afternoon CAPE. Improved skill is possible using random forest classification relying only on PWAT and CAPE to predict days experiencing excessive (upper quartile) precipitation. Such HRRR predictions may be useful for forecasters at the Salt Lake City National Weather Service Forecast Office to assist issuing flash flood potential statements for visitors to national parks and other recreational areas in the region.

Open access
John D. Horel
and
Angel G. Cornejo-Garrido

Abstract

Streamflow and historical records indicate that flooding in northern Peru was more severe during 1983 than during any year since 1891. A case study of the meteorological conditions along the northwest coast of South America from 10°S to 10°N during 1982–83 is presented. Station rainfall and satellite-derived outgoing infrared observations are used to deduce the structure and time evolution of convection in this region.

Substantial rainfall amounts were first observed along the western slopes of the Andes Mountains and coastal plain of southern Ecuador during November and December 1982, and it continued to rain in this region through June 1983. In northern Peru, the onset of the rains along the coastal plain was delayed until January 1983 and ended abruptly during mid-June 1983. Convective activity was much greater along the coastal strip than over the eastern equatorial Pacific to the cast of 100°W until May-June 1983. In addition, cloudiness was strongly modulated on the diurnal time scale, with more clouds during the night and early morning than during the afternoon.

The distribution of rainfall along the Peruvian littoral is compared to local changes in sea surface temperature, surface equivalent potential temperature, and surface wind. These comparisons suggest that abnormally high coastal ocean temperatures during the first half of 1983 aided the outbreak of convection during this period. However, the latitudinal extent and timing of the rainfall was quite different from that of sea surface temperature or surface equivalent potential temperature.

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John D. Horel
,
Andrea N. Hahmann
, and
John E. Geisler

Abstract

Outgoing longwave radiation (OLR) is used to describe the annual cycle of convection that resides over the Amazon Basin during austral summer and over Central America and the adjacent waters of the Pacific during austral winter. The preferred locations of the convective activity during the wet season in the respective hemispheres are determined, and the beginning and ending of these seasons is specified. The onset of the wet season over Amazonia usually occurs within a single month, while the onset of the wet season over Central America typically requires from one to three months. The annual cycle of convective activity in this regime is shown to exhibit a seasonal regularity and degree of symmetry with respect to the equator which exceeds those characterizing the other two annually varying regimes in the tropical belt. Analyses produced by the European Centre for Medium-range Weather Forecasts (ECMWF) are superimposed upon OLR fields to illustrate features of the atmospheric circulation in the vicinity of the tropical Americas that are associated with the annual cycle of convection. The onset and demise of the wet season in the Amazon Basin are further described by means of composites of these data. It is found that the Bolivian high inferred from the ECMWF data develops rapidly during the onset transition in a manner that is temporally and spatially consistent with the distribution of OLR.

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John D. Horel
,
Donna Ziegenfuss
, and
Kevin D. Perry

The Department of Meteorology (now Atmospheric Sciences) at the University of Utah faced reductions in state funding in 2008 that reduced support for nontenured instructors at the same time that the faculty were becoming increasingly successful obtaining federally supported research grants. A faculty retreat and subsequent discussions led to substantive curriculum changes to modernize the curriculum, enhance course offerings for undergraduate and graduate students, and improve the overall efficiency of the academic program. Maintaining discipline standards and existing teaching loads were important constraints on these changes.

Key features of the curriculum revisions for undergraduate majors included eliminating a very rigid course progression; shifting the emphasis from required courses to elective courses; offering many courses only every other year; and relying on half-semester short courses to survey subject areas rather than focusing in depth on fewer ones. The curriculum changes were evaluated through surveys and individual and focus group discussions of students and faculty. While the feedback suggests that the changes overall were beneficial, the transitional period during which the changes were implemented was difficult for faculty and students alike.

Faculty members have opportunities now to adjust courses based on their experiences gained teaching these courses in their new format. The feedback from students and faculty suggests that building improved relationships and interactions among co-enrolled undergraduate and graduate students is the greatest need in order to improve the classroom learning environment.

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Kim M. Waldron
,
Jan Paegle
, and
John D. Horel

Abstract

Numerical filters required to control spatial computational modes in a limited-area model (LAM) that uses the unstaggered. A grid are developed and tested over the complex topography of the Great Basin of the western United States. The filters are founded upon Fourier expansions of forecast deviation fields and function equally effectively for both periodic and aperiodic local structures. Unlike other spatial filters, the approach used here avoids any direct contamination of larger scales. Provided that the shortest resolved wavelength of two grid intervals is removed, the results do not depend strongly on the range of filtered short waves or on the type and order of horizontal space difference approximations.

This approach leads naturally to methods in which the large scales predicted by an ambient outer model can be directly incorporated within the complete domain of the inner LAM, rather than just through conditions applied at the lateral boundaries of the LAM. This technique has some similarities to methods used both in operational regional models in Japan and in recent regional research models at the National Centers for Environmental Prediction (formerly National Meteorological Center) of the United States. Several methods to incorporate the large scales into the LAM are evaluated in a winter storm case study and in an ensemble of seven forecasts.

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Craig B. Clements
,
C. David Whiteman
, and
John D. Horel

Abstract

The evolution of potential temperature and wind structure during the buildup of nocturnal cold-air pools was investigated during clear, dry, September nights in Utah's Peter Sinks basin, a 1-km-diameter limestone sinkhole that holds the Utah minimum temperature record of −56°C. The evolution of cold-pool characteristics depended on the strength of prevailing flows above the basin. On an undisturbed day, a 30°C diurnal temperature range and a strong nocturnal potential temperature inversion (22 K in 100 m) were observed in the basin. Initially, downslope flows formed on the basin sidewalls. As a very strong potential temperature jump (17 K) developed at the top of the cold pool, however, the winds died within the basin and over the sidewalls. A persistent turbulent sublayer formed below the jump. Turbulent sensible heat flux on the basin floor became negligible shortly after sunset while the basin atmosphere continued to cool. Temperatures over the slopes, except for a 1–2-m-deep layer, became warmer than over the basin center at the same altitude. Cooling rates for the entire basin near sunset were comparable to the 90 W m−2 rate of loss of net longwave radiation at the basin floor, but these rates decreased to only a few watts per square meter by sunrise. This paper compares the observed cold-pool buildup in basins with inversion buildup in valleys.

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Ernesto H. Berbery
,
Julia Nogués-Paegle
, and
John D. Horel

Abstract

The dynamical basis of intraseasonal oscillations of the Southern Hemisphere summer and winter seasons is studied with a combination of observed diagnostics and simplified prognostic models. High-frequency oscillations, zonal mean variations, and seasonal and interannual variabilities are removed from the six-year dataset in an effort to reduce the effect of high-frequency dynamical instabilities and long-period forced fluctuations. The diagnoses focus upon those processes that have most frequently been explained in terms of Rossby-wave propagation through atmospheres with variable refractive indices. It is useful to study both winter and summer seasons simultaneously because of the large changes in the seasonally averaged state and large consequent changes in atmospheric waveguides between these seasons. A nonlinear shallow-water model slowly relaxed toward the time-averaged winter and summer observed mean fields is used to describe the characteristics of wave propagation in a horizontally varying basic state. Perturbations are introduced in four different regions corresponding to points where observed atmospheric teleconnectivities are relatively large, and the signal propagation is analyzed using averaging procedures similar to those employed for the observational study. Furthermore, differences between stationary and nonstationary patterns are also discussed.

The four general regions selected for the observational study are Australia, New Zealand, South America, and the Atlantic Ocean. Differences from winter to summer are related to concomitant changes of the background latitudinal gradient of absolute vorticity. During winter and summer meridional propagation is toward the tropics. Winter wave patterns have mainly zonal paths and show a slow phase velocity on the order of 3 m s−1, while during summer, patterns tend to be geographically fixed. During winter, regions of imaginary refractive index flank the subtropical and polar jet streams. These jet streams seem to act as waveguides for disturbances emanating from the southern Indian Ocean and western Australia, where two wave trains exist. Wave activity flux vectors suggest that these disturbances originate in the subtropical southern Indian Ocean and that equatorward propagation prevails at the exit region of the subpolar jet stream and over South America and the Atlantic Ocean. During summer, observed wave patterns tend to have a more meridional component, again in agreement with the background latitudinal gradient of absolute vorticity.

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