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Joost A. Businger and Steven P. Oncley

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A method is proposed to measure scalar fluxes using conditional sampling. Only the mean concentrations of updraft and downdraft samples, the standard deviation of the vertical velocity, and a coefficient of proportionality, b, need to be known. The method has been simulated from existing time series of the vertical wind component, temperature, and humidity in the surface layer. It is found that b has an almost constant value of 0.6 for both scalars and over a wide stability range. This result encourages application to other scalars and suggests that the method may be used beyond the atmospheric surface layer in the lower part of the planetary boundary layer.

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Thomas E. Robinson and Steven Businger

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

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

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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

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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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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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Robert A. Mazany, Steven Businger, Seth I. Gutman, and William Roeder

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The primary weather forecast challenge at the Cape Canaveral Air Station and Kennedy Space Center is lightning. This paper describes a statistical approach that combines integrated precipitable water vapor (IPWV) data from a global positioning system (GPS) receiver site located at the Kennedy Space Center (KSC) with other meteorological data to develop a new GPS lightning index. The goal of this effort is to increase the forecasting skill and lead time for prediction of a first strike at the KSC. Statistical regression methods are used to identify predictors that contribute skill in forecasting a lightning event. Four predictors were identified out of a field of 23 predictors that were tested, determined using data from the 1999 summer thunderstorm season. They are maximum electric field mill values, GPS IPWV, 9-h change in IPWV, and K index. The GPS lightning index is a binary logistic regression model made up of coefficients multiplying the four predictors. When time series of the GPS lightning index are plotted, a common pattern emerges several hours prior to a lightning event. Whenever the GPS lightning index falls to 0.7 or below, lightning occurs within the next 12.5 h. An index threshold value of 0.7 was determined from the data for lightning prediction. Forecasting time constraints based on KSC weather notification requirements were incorporated into the verification. Forecast verification results obtained by using a contingency table revealed a 26.2% decrease from the KSC's previous-season false alarm rates for a nonindependent period and a 13.2% decrease in false alarm rates for an independent test season using the GPS lightning index. In addition, the index improved the KSC desired lead time by nearly 10%. Although the lightning strike window of 12 h is long, the GPS lightning index provides useful guidance to the forecaster in preparing lighting forecasts, when combined with other resources such as radar and satellite data. Future testing of the GPS lightning index and the prospect of using the logistic regression approach in forecasting related weather hazards are discussed.

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Steven Businger, Michael E. Adams, Steven E. Koch, and Michael L. Kaplan
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