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Michael J. Murphy Jr.
and
Steven Businger

Abstract

On 2 April 2006, Oahu’s Ko‘olau Mountain Range endured more than 6 h of heavy rain with accompanying flash flooding along its northeast-facing slopes. The storm responsible for the event left a pattern of precipitation characteristic of orographic anchoring of convection with extreme rainfall gradients along the slopes and maxima along the crest of the mountain range. In fact, this was the third flash-flood event to impact the Ko‘olau Mountains in just over 1 month, with each event occurring under conditions of moist southeasterly flow at low levels and moderate conditional instability. Under these conditions persistent convection and localized heavy rainfall often occur over the Ko‘olau Mountain Range. Mesoscale analyses of the thunderstorm complex responsible for the 2 April 2006 heavy rain event and the results of a high-resolution numerical simulation employing the Weather Research and Forecasting (WRF) model are described in this study.

Key features of the convection that contributed to the longevity of the event include repeat formation of convective cells along the eastern side of the central Ko‘olaus, minimal horizontal cloud motion, and strong updrafts that sloped toward the northwest in the lower levels. The westerly shear of the low-level flow determined the pattern of accumulated precipitation by aligning the slope of the convective updrafts nearly parallel to the southeast-to-northwest-orientated Ko‘olau Mountain Range. The microphysical structure of the convection was complex, with the vertical advection of hydrometeors originating below the freezing level facilitating high concentrations of ice particles and an environment conducive to charge separation and lightning.

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Kevin R. Kodama
and
Steven Businger

Abstract

The large area of responsibility covered by the Pacific Region of the National Weather Service provides a unique set of challenges to operational forecasters. Extratropical, subtropical, and tropical meteorological phenomena on a wide range of temporal and spatial scales must be considered on a daily basis. Compounding the problems of forecasting diverse weather for such a large area of responsibility is the fact that the Pacific Ocean is a data-sparse region. Recent improvements in data collection platforms and the continued progress made by researchers have helped increase the understanding of weather throughout the region, ultimately resulting in improved forecast services.

This article provides an overview of some of the weather phenomena encountered in the Pacific Region and helps set the stage for the accompanying articles that focus on specific weather forecasting problems. Some discussion is provided on the impact of the National Weather Service’s modernization program on operational forecasting in the region.

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Thomas E. Robinson
and
Steven Businger

Abstract

A new method for modeling the lowest model level vertical motion is described and validated. Instead of smoothing terrain heights, the new method calculates the terrain gradient on a high-resolution grid and averages the gradient values around a gridpoint location. In essence, the method provides a way to achieve some of the impact of very steep terrain on the flow without the computational overhead associated with the very high grid resolution needed to fully resolve complex terrain. The more accurate depiction of the terrain gradient leads to an increase in orographic vertical motion and causes rainfall to occur more often over the windward-facing mountain slopes, consistent with observations. Model results are compared with rain gauge data during the month of January 2016 as well as radar data from a case study on 9 March 2012. When implemented in the Weather Research and Forecasting (WRF) Model over the island of Oahu and compared with the current WRF method, the model precipitation forecast skill is improved. The new method produces more precipitation over the island during January 2016, which is closer to the observed value. On 9 March 2012, the new method clearly focuses the precipitation over the Ko‘olau Mountains, reducing the number of false alarm forecasts by nearly one-half. Although the changes to model precipitation skill were small, they were generally positive.

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Ian Morrison
,
Steven Businger
,
Frank Marks
,
Peter Dodge
, and
Joost A. Businger

Abstract

Doppler velocity data from Weather Surveillance Radar-1988 Doppler (WSR-88D) radars during four hurricane landfalls are analyzed to investigate the presence of organized vortices in the hurricane boundary layer (HBL). The wavelength, depth, magnitude, and track of velocity anomalies were compiled through analysis of Doppler velocity data. The analysis reveals alternating bands of enhanced and reduced azimuthal winds closely aligned with the mean wind direction. Resulting statistics provide compelling evidence for the presence of organized secondary circulations or boundary layer rolls across significant areas during four hurricane landfalls. The results confirm previous observations of the presence of rolls in the HBL. A potential limitation of the study presented here is the resolution of the WSR-88D data. In particular, analysis of higher-resolution data (e.g., from the Doppler on Wheels) is needed to confirm that data aliasing has not unduly impacted the statistics reported here. Momentum fluxes associated with the secondary circulations are estimated using the covariance between the horizontal and vertical components of the wind fluctuations in rolls, with resulting fluxes 2–3 times greater than estimated by parameterizations in numerical weather prediction models. The observational analysis presented here, showing a prevalence of roll vortices in the HBL, has significant implications for the vertical transport of energy in hurricanes, for the character of wind damage, and for improvements in numerical simulations of hurricanes.

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Ian Morrison
,
Steven Businger
,
Frank Marks
,
Peter Dodge
, and
Joost A. Businger
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Steven Businger
,
Selen Yildiz
, and
Thomas E. Robinson

Abstract

This study analyzes QuikSCAT surface wind data over the North Pacific Ocean to document the distribution of captured fetches in extratropical cyclones that produced hurricane force (HF) wind fields from January 2003 through May 2008. A case study is presented to introduce the datasets, which include surface wind analyses from the Global Forecast System (GFS) Global Data Assimilation System (GDAS), and wave hindcasts from the third-generation wave model (WAVEWATCH III; hereafter, WW3), in addition to the QuikSCAT surface wind data. The analysis shows significant interannual variability in the location of the captured fetches as documented by QuikSCAT, including a shift in the distribution of captured fetches associated with ENSO. GDAS surface winds over the ocean are consistently underanalyzed when compared to QuikSCAT surface winds, despite the fact that satellite observations of ocean surface winds are assimilated. When the WW3 hindcasts associated with HF cyclones are compared with buoy observations over the eastern and central North Pacific Ocean, the wave model significantly underestimates the large-swell events.

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Lacey Holland
,
Steven Businger
,
Tamar Elias
, and
Tiziana Cherubini

Abstract

Kīlauea volcano, located on the island of Hawaii, is one of the most active volcanoes in the world. It was in a state of nearly continuous eruption from 1983 to 2018 with copious emissions of sulfur dioxide (SO2) that affected public health, agriculture, and infrastructure over large portions of the island. Since 2010, the University of Hawaiʻi at Mānoa provides publicly available vog forecasts that began in 2010 to aid in the mitigation of volcanic smog (or “vog”) as a hazard. In September 2017, the forecast system began to produce operational ensemble forecasts. The months that preceded Kīlauea’s historic lower east rift zone eruption of 2018 provide an opportunity to evaluate the newly implemented air quality ensemble prediction system and compare it another approach to the generation of ensemble members. One of the two approaches generates perturbations in the wind field while the other perturbs the sulfur dioxide (SO2) emission rate from the volcano. This comparison has implications for the limits of forecast predictability under the particularly dynamic conditions at Kīlauea volcano. We show that for ensemble forecasts of SO2 generated under these conditions, the uncertainty associated with the SO2 emission rate approaches that of the uncertainty in the wind field. However, the inclusion of a fluctuating SO2 emission rate has the potential to improve the prediction of the changes in air quality downwind of the volcano with suitable postprocessing.

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Steven Businger
,
William H. Bauman III
, and
Gerald F. Watson

Abstract

An investigation was conducted of the mesoscale evolution of a quasi-stationary front, termed the Piedmont front owing to its location through the central Carolinas, and an associated outbreak of severe weather on 13 March 1986. Space-time relationships between mesoscale processes associated with the evolution of the surface front and the initiation of severe thunderstorms were studied utilizing the enhanced surface and upper-air observation networks deployed during the field phase of GALE. Surface streamline patterns, frontogenesis, and moisture-flux divergence were computed employing an objective analysis scheme.

Following the arrival at the Carolina coast of a coastal front, the Piedmont front rapidly developed along an axis of dilatation over the eastern margin of the Piedmont, while the coastal front gradually dissipated over the nearshore waters. A differential cloud cover across the Piedmont front resulted in enhanced solar insolation on the warm side of the front that strengthened frontogenesis and acted to further destabilize the atmosphere. On the afternoon of 13 March four severe thunderstorms formed in the vicinity of the Piedmont front. Three of the storms were located in the vicinity of mesolows that formed on the front Subsequently, convection organized into a squall line along the front as synoptic-scale forcing associated with a short-wave trough and cold front aloft (CFA) overtook the Piedmont front from the west.

Stability analyses indicate that on the synoptic scale only a weak to moderate potential for severe weather existed over portions of eastern North and South Carolina. However, fields of moisture-flux divergence show a mesoscale pattern of enhanced convergence well correlated with the locations of the severe thunderstorm cells. A schematic is presented that summarizes the principal factors involved in the development of the severe weather in this complex case.

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Steven Businger
,
David I. Knapp
, and
Gerald F. Watson

Abstract

A storm-following climatology was compiled for the precipitation distributions associated with winter cyclones that originate over the Gulf of Mexico and adjacent coastal region. The goal of this research is to investigate the roles of the Gulf of Mexico and Atlantic Ocean as sources of moisture for these storms, and to investigate geographic/orographic influences on the precipitation distributions. A second objective of this research is to provide forecasters with a potential guide with which to evaluate numerical model forecasts of quantitative precipitation for these storms. A 24-y climatology (1960–1983) was compiled of storms that originated over the Gulf of Mexico and adjacent coastal region, and produced wide-spread areas of precipitation (total ≥ 25 mm). Sixty-six storms satisfied these criteria, and three dominant storm tracks were identified. Six-h totals of hourly precipitation data were objectively analysed for individual storm belonging to each of the three tracks, and grid-point values were composited in a storm-following coordinate system. Charts of mean precipitation distributions and frequency of occurrence were constructed to display the evolving precipitation fields surrounding storms belonging to each track. The resulting climatology is presented.

To provide an example of the application of the precipitation climatology, results from a GALE Case study am presented.

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William H. Bauman III
,
Michael L. Kaplan
, and
Steven Businger

Abstract

Space shuttle landings at the shuttle landing facility at Kennedy Space Center are subject to strict weather-related launch commit criteria and flight rules. Complex launch commit criteria and end-of-mission flight rules demand very accurate nowcasts (forecasts of less than 2 h) of cloud, wind, visibility, precipitation, turbulence, and thunderstorms prior to shuttle launches and landings.

During easterly flow regimes the onset of convective activity has proven to be particularly difficult to predict. Contrasting weather ranging from clear skies to thunderstorms occurs on days with seemingly similar synoptic environments. Four days of easterly flow during the Convection and Precipitation/Electrification (CaPE) Experiment were investigated in an effort to identify and simulate key features that distinguish convectively active and suppressed conditions. Data from CaPE and operational data, including satellite imagery and National Centers for Environmental Prediction model analysis output over the Florida peninsula and surrounding data-sparse Atlantic Ocean, are combined in the research. It is found that elevated moisture in the midtroposphere above the marine boundary layer helps distinguish convectively active and passive days. Moreover, analysis reveals that the moisture distribution is related to jet dynamics in the upper troposphere.

A series of simulations using the Mesoscale Atmospheric Simulation System (MASS) model was undertaken. The MASS model run with a coarse grid (45 km) correctly simulates the development of the upper-level jet streak and its general impact on convective activity over the Florida peninsula. The MASS model run with a nested (11 km) grid and moisture enhancement of the initial model state from radar, satellite, and surface data results in the best short-term (6 h) forecast of relative humidity and precipitation patterns over the Florida peninsula and proximate coastal environment. Implications of the research results for nowcasting convective activity over Cape Canaveral are discussed.

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